Raw Processing: Introduction to darktable

As I mentioned in a recent post, I use the program darktable for the bulk of my raw file development (and I plan to use it for file management, when I get around to actually, y'know, managing my files). I thought I'd give a brief overview of the program for those who might be interested in getting a taste of it without committing to an installation, or those who have recently installed it and find themselves completely lost.

 

darktable for Windows: lighttable

This is something like what you'll see when you open darktable, provided you have imported files into the program already. This is the lighttable section of the program.

On the left panel, you have options to import files and folders, view imported images (filtered in a variety of ways depending upon user instructions), see recently used collections of images, and see the image information (such as EXIF data and file location) for the photo you have selected in the central panel.

In the centre panel, you'll see a variety of images. The particular images you will see will depend upon the selections you make in the left panel. You can further filter the displayed images with options at the top and bottom of the panel. Also in this centre area are display options and access to global preferences (click the wheel icon).

(Quick tip: if you hover your mouse over a photo in the centre panel and hold down z on your keyboard, the photo will be enlarged until you release z. If you hover your mouse over photo and hold ctrl+z, the photo will be enlarged with indications over all areas that darktable detects as being in focus.)

In the right panel you'll find a variety of functions, such as tagging options (for file management), styles (similar to Adobe Lightroom presets; these provide preconfigured edits to selected images), metadata editing, and export options (to convert edited raw files to image files).

The lighttable is used mainly for file management and export; you won't typically perform edits in this area except, perhaps, for batch processing with styles (but I don't recommend this approach).

If you want to edit an individual raw file, you have two options: you can double-click on it, or you can select it and then click on "darkroom" to the top right of window. Either way, you'll be taken to the darkroom.

darktable for Windows: darkroom

The darkroom is where the real magic happens. Here you can take control over the appearance of your final image through a seemingly endless (and, therefore, initially intimidating) choice of tool modules.

At the centre of the screen, you'll see the image you are editing. At the bottom of the screen, you'll see thumbnails of other images, which can be double-clicked for quick editing access. Between the main screen and the film strip of images is a narrow bar with several icons; these include, on the left, quick access to presets and styles, and on the right, over- and under-exposure warning toggles, and soft-proofing and gamut checking toggles (full disclosure: I have only the faintest idea what the latter two toggles are used for).

The left panel of the screen holds a number of drop-down modules. Snapshots allows you take a quick grab of your image at a given point in the editing process, and later compare that snapshot to the edited image as it currently stands. The history module shows all of the edits you have made to the raw file. These edits are stored in an XMP sidecar file--what this means, in simple terms, is that no edits are made to the original file itself; this is why darktable is described as a nondestructive editor. You will probably notice that even if you have opened a file for the very first time, there will be some items listed in the history module; this is because darktable applies certain basic adjustments automatically. Under the history module are a global colour picker module, a tagging module, an image information module, and a mask editor module. The mask editor module allows you to manage masks that you have used to apply local edits to particular areas of your image (versus applying the edits to the entire image).

The right panel of the darktable is where most of your work takes place. At the top of the panel, you'll see the histogram for your image, as well as some basic image information. There are various options for how the histogram is displayed, and if you wish, you can perform some very basic edits on your image by dragging either side of the histogram. Under the histogram are seven icons, linking you to various groups of editing modules. In the above example, you can see the basic group (accessed by clicking the plain circle icon), with the base curve module expanded. At the bottom of the panel you will see an area labeled 'more modules'; here you can control which modules are displayed for easy access, which modules are hidden, and which modules are included in your favourites section.

So that is a very basic overview of darktable, quite possibly using some incorrect terminology (the darktable user manual, for example, uses panels to refer to areas such as the history stack and the mask manager rather than to the different areas of the screen). Interested in learning more? Keep an eye on this site, or delve further into the manual by clicking here.

darktable is nondestructive, free, and open-source, and available for a variety of operating systems (chances are pretty good that yours is one of them). It was designed by photographers, for photographers--and in my experience, the photographers' influence is easily seen in all of its functions. Check it out if you like how it looks!

One Skein Photo Challenge: 13/52

This officially marks the completion of one-quarter of my one skein photo challenge! Hard to believe, since that means we're also about a quarter of the way through 2018.

This week I wanted to do a different take on my photo from week 2. Struggling with new ideas? Who, me? Maybe a little. I also pushed the development a bit in darktable to get something more interesting. Nothing radical, mind you, just retrieving some of the colour and feel.

24mm, 1/5 sec @ f/5.6, ISO 1600

Happy Easter (a bit early)!

Raw Processing and Conversion Software

So perhaps you've decided to shoot in raw. You'll need to process your raw files, and to do that, you'll need some specialised software. But what software should you use? Here are some options (not a complete list; merely some of the more popular programs. For more options, click here). N.b.: None of these links are affiliate links. I make no money if you click on them, nor if you make any purchases.

One file. Four versions. No destruction. No change to the original. That's the power of raw.

Paid Software
N.b.: I have not personally used any paid raw processing software, so I cannot comment on the quality of any of these programs.

Adobe Lightroom - this is the software that you will most commonly hear about. I would guess that most professional photographers use Lightroom for their raw file processing. Adobe currently offers a photography subscription package for US$9.99 per month for a year, which will get you both Lightroom and Photoshop (another powerful editing program) plus 20GB of cloud storage, as well as a Lightroom plan for the same price, which will get you access to Lightroom plus 1TB of cloud storage. Lightroom also offers file management functions. If you want access to lots of books, tutorials, and presets, and you're willing to pay a bit of money, this is probably your best bet.

Capture One - this is another piece of software used by many photography professionals. Unlike Lightroom, it does not operate on a subscription model. A free 30-day trial is available.

DxO PhotoLab - again, this software is used by many photography professionals and does not operate on a subscription model. It offers two editions, Essential Edition and ELITE Edition, as well as a free trial.

Free Software

darktable - this is my current choice of software for raw development. darktable is a powerful program that allows for file management and image processing, as well as select other functions. darktable has a significant learning curve, but I personally find the interface to be reasonably intuitive, and there is a wide variety of available tools. darktable lacks somewhat in existing tutorials; nonetheless, I have found that Lightroom tutorials will often translate reasonably well into darktable. Note that darktable offers global as well as local editing (i.e., you can make changes to the entire image or to particular areas of it); I find this flexibility very important in my own workflow. darktable will probably be the raw software I most often discuss on this blog.

RawTherapee - another popular free file management and raw development program. RawTherapee is powerful and fairly intuitive, and it supports most raw formats, as well as common image formats (e.g., JPEG and TIFF). Unlike darktable, RawTherapee does not support local editing; edits are applied to the entire image (with a few exceptions).

LightZone - an interesting piece of software based on photography master Ansel Adams's Zone System. LightZone operates a bit differently than most other raw processors, and may therefore suit some users who are unhappy with other options. I used LightZone until I had difficulties with how it rendered some of my photographs; I was unable to find support for my particular issue (I suspect it was camera-related) and reluctantly abandoned it in favour of darktable. LightZone also supports JPEG editing and offers batch-processing options for efficient workflow.

Other

Many camera companies offer their own raw processing and conversion programs. These are certainly worth looking at. They may not be as powerful as some of the other options, but they are typically free (provided you own one of their cameras) and are often sufficient for the average user's needs. I personally used Canon's Digital Photo Professional until I decided I wanted something more powerful, and I was reasonably pleased by the results (which include many of the earlier images posted on this blog).

Other editing programs such as Adobe Photoshop (paid) and GIMP (free) offer limited raw support. If you prefer to do all of your image editing in one program, and/or you are already familiar with one of these programs, you may wish to determine whether its raw editing functions are adequate for your needs.

Go forth and raw!

One Skein Photo Challenge: 12/52

Almost a quarter of the way through my one skein photo challenge! This week, I wanted to play with backlighting to give myself more practice with exposing for backlit subjects. Behold, my filthy basement window (some horrific dirt has been edited out)!

36mm, 1/40 @ f/8.0, ISO 400

Happy knitting! Happy shooting!

What the Heck is Raw, and Should I Use It?

If you spend much time exposed to photography talk, you'll soon start to hear the term "raw" bandied about (and you'll probably hear some strong opinions about how shooting raw is the best thing since sliced bread--or perhaps since plied yarn--or, alternatively, how it's a big waste of time and hard drive space). But what is raw?

A raw file, unlike a JPEG, is not an image. Rather, a raw file contains the information necessary to create an image. The information it contains was obtained at the time the particular photograph was made. Raw is often referred to as a digital negative since it is not usable without processing. The great advantage of a raw file is that it contains all of the information collected by the sensor when the photograph was taken. A JPEG, on the other hand, has compressed and discarded a significant portion of this data. This makes the raw file much more flexible than its processed counterpart. Raw files are also edited non-destructively, which means that you can do pretty much whatever you want to them without causing a decrease in the file quality (and you can't accidentally do something awful to your image and then save over the original, forever ruining it!). Basically, when you edit a raw file, you're just telling it how to display (and eventually create) the image it holds information about, without actually altering the file data. JPEGs, on the other hand, tend to degrade whenever you edit them, and because they have less information to begin with, you are more limited in the edits you can make.

Developing a raw file using darktable for Windows

More data, more flexibility, and non-destructive editing--sounds great, right? Unfortunately, there are a few disadvantages to shooting raw. First, raw files are significantly larger than JPEG files, so they will take up more space on your memory card (in camera) and your computer (after transfer) than the corresponding JPEG. Second, raw files require processing with specialised software. Processing will require additional time in your workflow (although once you are familiar with your raw processor, it can be quite fast), and many raw processors have a significant learning curve. If you don't get to know your raw processor well, your images may not look as good after processing as they would have if you'd just let your camera do all the work in the first place. Third, if you want to shoot many images in quick succession, shooting raw will limit you, because the larger files take up more space in the camera buffer.

So should you, as a fibre artist, shoot in raw? Ultimately, it's your decision, but here are some questions to consider:

• Does your camera even let you shoot raw? Many--and perhaps even most--do, but check your manual if you're not sure. If not, you're stuck with JPEG.
• Are you willing to learn to use a new piece of software? If not, stick with JPEG.
• Do you have very limited storage space for your image files? If so, you probably want to stick with JPEG.
• Are you interested in photography beyond your fibre art needs? If so, consider raw.
• Do you typically edit your JPEG files? If so, consider raw.
• Do you often forget to set your white balance in camera? If so, consider raw--you can change it when you develop your raw file without degrading your image.
• Do you frequently mess up your exposure, or shoot in uneven lighting conditions or with a broad dynamic range? If so, consider raw--you'll be able to recover a lot of extra image data.
• Do you want the highest possible image quality? If so, shoot raw.
• Are you just curious what all the fuss is about? Try raw!

Still undecided, not so confident in your processing abilities, or generally happy with your out-of-camera images yet occasionally wish you had more flexibility? If you have enough storage space, consider shooting raw + JPEG. This gives you the best of both worlds--JPEG files that are ready to go, but raw files that can be edited to your heart's content if you so desire. When I first started shooting raw, this was the method I chose. Now that I'm more comfortable with raw processing, I typically shoot only in raw because I like to develop my own images, although I do sometimes make exceptions for various reasons.

Earlier in my foray into raw processing, I used Canon's proprietary raw development software, Digital Photo Professional [DPP]. One of its benefits is that (as far as I can tell), your starting point for editing your raw file is the same as the JPEG you would have gotten out of your camera. Personally, I now find DPP rather limiting, but it was a nice way for me to ease into developing raw files. If you don't shoot a Canon camera you won't be able to use DPP, but you may want to look into whether your camera brand has its own raw processing software (most do), and see if you like it.

If you're interested in shooting raw, you can check out my post about raw processing software here. I hope you found this a helpful introduction to raw files. Happy shooting!

One Skein Photo Challenge: 11/52

I can't believe I'm more than 1/5 through my one skein photo challenge (and that means we're more than 1/5 through the year, I think...). This one isn't perfect, but it's close to what I was trying for last week, so I'll count it as a success. Would have done a better edit, but it is late!

37mm, 3.2" @ f/18, ISO 100, manually-fired flash

Cheers!

Ten Signs Your Photography is Improving

I removed a pole from my husband's head while I was shooting him the other day.

Okay, so what I actually did was change my position while taking pictures of him when I realised that there was a telephone pole centred behind his head in the photo, and that not moving would make the final photo look as though he was growing said pole from his head. But it happened on a day when I was feeling particularly discouraged. My photographs were boring, and I was still fumbling with my buttons, and I'd just tried to handhold a shot with too slow a shutter speed for my focal length, because I'd been lazy about changing my settings to match the lens I'd just put on my camera.

Removing that telephone pole gave me hope, because seeing it in the first place means that I'm seeing better now than I was a few weeks ago. A few weeks ago, all of my photos would have had that pole growing out of his head, rather than only the first few.

Progress in photography is sometimes, but not always, visible in your photographs. So what are some ways to know that you're growing in your craft? Here are ten (but there are many more).

1. Your photos look better.

This one doesn't need much explanation. Your photos look better because you're better, and you rely less on luck.

154mm; 1/250 @ f/5.6, ISO 1600
It's not perfect, but a few months ago, I would not have seen this shot.

2. Your photos look worse.

This isn't always a sign of growth, but it certainly can be. Maybe your photos look worse because you're starting to take more control over your camera settings. Maybe you're experimenting with angles instead of sticking to what's safe. Maybe you're using a new lens with a new focal length, and you're not used to its effects on your photos. Maybe you're shooting Raw and working on your Raw developing/post-processing skills. Whatever the reason, your photos might look a whole lot worse shortly before they start to look a whole lot better.

3. Your photos look like you expect them to look.

If you can envision your shot before you take it, and the shot you take looks like your vision, you are well on your way to mastering photography (but maybe it's time to experiment with a new focal length or a new technique some of the time).

4. You notice objects in the background and edges of your frame before you shoot.

We're really good at seeing only our subject, and ignoring detritus and distractions. Learning to see like a photographer includes looking at every corner of the frame and deciding which elements you want to include and exclude. Fewer shots with distractions in the corners or with trees growing out of people's heads demonstrates progress!

5. You crop less.

This is related to the previous two points. If you know what your photo will look like, and you have excluded unwanted elements from the frame, you'll find yourself having to crop your image much less frequently. You'll get the framing right in camera instead.

250mm; 1/320 @ f/5.6, ISO 2000
Framed exactly how I wanted it in camera.

6. You are able to offer more detailed critiques of photos.

Part of seeing is being able to see elements within the final shot--whether it's your photo or another person's. When you start to be able to see why a photo works or doesn't, including little things like a distracting blob of colour in the background or a nuanced use of supplementary light, you'll also be better at seeing these things through your lens and adjusting--or not--accordingly.

194mm; 1/500 @ f/5.6, ISO 2000
I am now able to see how much stronger this shot would have been if I'd shifted an inch or two to the right.

7. You think more before you shoot.

You don't just "spray and pray". You think about what you want to capture, what angle might best allow you to do so, and perhaps even whether it's worth pressing the shutter button. Basically, you weed out a lot of your shots before you even take them (but don't go overboard on this! Taking "sketch images", as David duChemin calls them, is usually an important part of the process).

171mm; 1/500 @ f/5.6, ISO 2000
Intentionally isolated against a simple background after carefully considering multiple angles.

8. You adjust your settings with intentionality.

You choose to shoot with a certain aperture, shutter speed, and ISO, rather than just letting them happen. You consider what metering mode might be the most helpful. You decide which autofocus mode best suits your needs, or you switch to manual focus if your camera is struggling. Your camera is there to serve your vision, rather than to confuse you.

9. You think about the light in your shot.

You consider where the light is coming from and how that will affect your subject. You judge whether the light is hard or soft, and you plan your shot accordingly. You consider whether the light suits your purpose, or if you need to change it in some way.

10. You modify the light in your shot.

You're in a situation with light that doesn't meet your needs--so you change it. You move your subject to shade. You add light with a reflector or a flash. You flag the light. Basically, you make the light do what you want it to do, instead of simply doing what it's already doing.

How have you known that your photography was improving? What improvement are you seeing right now?

One Skein Photo Challenge: 10/52

This week's one skein photo challenge was a bit of a failure, but I learned a lot about light in the process. I almost had it figured out when I was interrupted by a roaming toddler (not the one who poured juice on my laptop last night...). Maybe I'll get it right next time. I think it's as valuable to post my failures as it is to post my wins.

35mm; 5.0" @ f/7.1, ISO 400, manually pulsed flash

One Skein Photo Challenge: 8/52; 9/52

Still coughing up a storm, but back in the saddle for my one skein photo challenge. Not happy with any of the things I tried this week, but I suppose that's the nature of the beast. They generated some ideas, at least; I just didn't have time to execute the newer, possibly better, ideas. But maybe it's a valuable exercise anyway, to get from here to there.

Two photos to make up for last week's sick days. Playing with focus. Intentionally unfocused (manual) in the first shot. Intentional camera motion blur in the second shot--panning a still subject with a long exposure, although you can't see much of the motion.

50mm, 1/50, f/1.8, ISO 1600

50mm, 1", f/6.3, ISO 400

Two Camera Settings That Will Instantly Improve Your Fibre Photos (Probably)

I know, it's kind of a click-baity title, but it's also likely true. If you shoot in anything other than full manual and raw, you need to know about these two settings. And if my very cheap smart phone camera has them (it does--I checked), your camera probably does, too.

Ready?

1. White Balance

Our eyes and brains do a great job of adapting to whatever colour of light that we see in order to make an object look basically the same under a variety of light temperatures. Unfortunately, our cameras are not so smart. If you take a photo of a white swatch in daylight, it'll generally look about the same in the image as it does to your eyes. However, if you take the same photo under incandescent light, for example, your image will take on a distinct orange cast:

White yarn under tungsten (incandescent) lighting, auto WB.

 Thankfully, there is a solution: setting your white balance. Basically, the white balance setting lets you tell your camera what kind of light you're shooting under, so the camera can compensate for the colour of the light and give you a photo that doesn't have an ugly colour cast. Instead, your photo will look basically like the scene did to your eye. Auto white balance will often do a good job, but in some situations, you'll want to input it manually. Check your manual for how to set it correctly, because it's different on different cameras.

If you forgot to set your white balance and you have a photo with a significant colour cast, all is not lost. Many (most?) post-processing programs have a white balance editor. Often the easiest way to use this is with the eyedropper/click method. Find the eyedropper option under your white balance and then click an area of the photo that should be a neutral colour. I find that if I have a person in the photo, the white of one eye is often a good spot to select.

Same swatch. White yarn under tungsten lighting, custom WB applied.

What happens if you're shooting in mixed lighting--for example, with window light and incandescent light? Well, unfortunately, the colours don't just magically mix and average out to one colour (they might do this to a degree if they are coming from approximately the same location, but it will still be imperfect). So you can make a choice and choose your balance for one colour of light, but part of your image will likely have a (probably ugly) colour cast to it. Or you can eliminate one of the light sources and balance for the other--a much better option, but not always possible. Alternatively, you can shoot later, or somewhere else--or you can override the entire ambient light with an external flash unit, but I only recommend doing the latter if you're confident you can do it well.

If you're shooting in raw, I probably don't need to tell you this, but it doesn't matter what white balance you choose--you will set it anyway in post-processing. That said, setting it to approximately the temperature you want may give you a more pleasing in-camera image preview. I usually leave mine set to auto because it is adequate, and because I'm lazy, and then I correct it in post when necessary.


2. Exposure Compensation

Are your photos too dark or too bright? You've probably under- or overexposed. Although a bit counter-intuitive, photos with a lot of white or light-coloured areas will tend to be underexposed (too dark), and photos with a lot of black or dark-coloured areas will tend to be overexposed (too bright). This is often a problem in winter scenes containing a lot of snow, in close-up shots of particularly dark or light yarn, in images which include the sun or another a light source, or when there are a lot of deep shadows. It occurs because the camera sees in shades of grey, and when it is determining exposure, it wants to turn everything it sees into middle grey.

So if you take a picture of a white swatch using automatic exposure, it'll come out a dingy grey (underexposed) instead of a very light grey with white highlights (which the eye would 'read' as white):

No Exposure Compensation
(Aperture-Priority AE, 1.3s, f/7.1, ISO 100)
The actual swatch is white.

Conversely, if you let the camera automatically expose for a very dark blue swatch, it will instead render a more middle blue colour (overexposed), giving you an inaccurate representation of the item:

No Exposure Compensation
(Aperture-Priority AE, 15s, f/7.1, ISO 100)
The actual swatch is navy blue.

There is an easy fix for this. Check your camera manual and find out how to set your exposure compensation. If your photo is underexposed, move the exposure compensation toward the positive (+) side. If you are trying to portray a very light knit or crochet item accurately, you will probably need to use positive exposure compensation:

+2 1/3 Exposure Compensation
(Aperture-Priority AE, 6 sec, f/7.1, ISO 100)
Same white swatch; same location; same lighting.

If your photo is overexposed, move the exposure compensation toward the negative (-) side. If you are trying to portray a very dark knit or crochet item accurately, you will probably need to dial in negative exposure compensation:

-1 1/3 Exposure Compensation
(Aperture-Priority AE, 6 sec, f/7.1, ISO 100)
Same navy swatch; same location; same lighting.

So what happens if you want to shoot a scene with both very light and very dark areas? Well, you might not have a problem at all. If you examine the shooting information for the two accurate exposures above, you'll notice that they are identical--both the accurate white swatch and the accurate blue swatch, which were shot in identical locations under identical lighting, were exposed for 6 seconds, at f/7.1 and ISO 100. A test shot containing the two swatches confirms that the camera's dynamic range is fully capable of exposing correctly for both swatches and that in this case, its metering of a more balanced scene will be fairly accurate:

No Exposure Compensation
(Aperture-Priority AE, 5s, f/7.1, ISO 100)
Same swatches; same location; same lighting.

You may find yourself in a situation where you simply can't capture the full range of tones, however--in bright sunshine with deep shadows, when including a direct light source, with bright snow in the background--any number of scenarios. You have a few choices:

1. Expose for whichever is more important: either for the brights/highlights, using positive exposure compensation, or for the darks/shadows, using negative exposure compensation
2. Expose for the middle, (probably) using no exposure compensation, and possibly losing some detail in both your highlights and your shadows
3. Use bracketing (and a tripod, if at all possible) and combine the images in post-production (beyond the scope of this entry; you can start with a google search for "HDR photography" if you're curious
4. Use an in-camera HDR function, if your camera has one

What if you're shooting in full manual? You can manually compensate. If your photos are underexposed, instead of shooting with the light metre centred, shoot with it towards the positive side. If your photos are overexposed, shoot with negative exposure. Easy!

How do you know if you have exposed properly? Check the histogram when you review your images (I'm starting to sound like a broken record, but check your camera manual if you don't know how to do this). When in doubt, expose to the right--it's easier to darken the image without degrading quality than it is to lighten it. No idea what I'm talking about? Check out my post on histograms by clicking here.

White balance and exposure compensation: your new photographic best friends!

Introduction to Exposure: The Luminosity Histogram (with Bonus Blinkies)

I have the plague (a.k.a. a particularly nasty head cold) so I'm taking a break from my one skein challenge. I'm itching to get the camera snapping, but I cough when I move, so the couch has my name on it. That means no new fibre-y photos for this post. Oh well. You'll have to make do with some recent shots of a nice single malt Scotch instead. That's almost as pretty as yarn, right?

So, now that you've learned about how the camera exposes images, you need to be able to judge the quality of your exposure. To a degree, simply looking at the image will give you a rough idea of how it has been exposed. The problem, however, is that camera screen brightness and the brightness of your surroundings can have a major impact on how the image appears during camera playback. An image on a bright camera screen might look overexposed when, in actual fact, it is dramatically underexposed. Fortunately, there is a better way. Enter the luminosity histogram. (Don't worry, this isn't high school math, nor do you need to know high school math. I promise, this is pretty easy stuff once you get the hang of it). There are also RGB (red, blue, and green) histograms available to you that give you information about the individual colour channels in your image, but here I'll only be discussing the luminosity histogram.

Here's the quick version:

The luminosity histogram is a depiction of the tones in your image. Tone basically describes how light or dark a given colour is, ranging from pure black to pure white. Midtones are, as expected, in the middle. The luminosity histogram shows you the distribution of tones in an image from dark (left side of the histogram) to light (right side of the histogram). If the histogram is tall in the middle, it means that there are a lot of areas of midtones in the image. If the histogram is bunched to the left, it means that the image is very dark. If the histogram is bunched to the right, it means the image is very light. If there are spikes at both ends of the histogram and a dip in the middle, it means that there are many pixels that are very dark in the image, and many pixels that are very light in the image, but very few pixels that are in the midtone range.

The ideal histogram is one that reflects how you want the image itself to look. It's that simple. If you have a lot of light tones in the image and you want them to look light, you want your histogram to skew to the right. If you have a lot of dark tones in the image and you want them to look dark, you want your histogram to skew to the left. And so on.

Most cameras will let you turn on the histogram in playback mode. Check your manual for how to do this. If you take a quick glance at the image after you shoot it, you'll get a pretty good idea of how it's exposed. Another option most cameras offer is a highlight clipping warning, more commonly called "blinkies". If you turn this on, any parts of the image that are pure white (and therefore contain no detail--blown or clipped highlights ) will flash on your screen. This leaves you free to judge whether you need to retake the image because you overexposed, or whether those highlights should, in fact, be clipped (such as if you shot into the sun or another bright light source).

That's the (relatively) short version. Still with me? Let's take a look at some examples in more detail.

1/100, f/2.8, ISO 400

Here we have a nice glass of Scotch, in a nice bright setting. Notice the lack of dark and black areas in the image, and the abundance of light areas. Now let's look at this image with its histogram (click to enlarge if you're having trouble seeing the histogram properly).

See how the histogram bunches to the right-hand side and is empty on the left-hand side? This reflects the absence of darker tones in the image. The large spike to right of the image reflects the light-coloured background. The small hump near the middle of the histogram reflects the midtones in the image--the Scotch and its glass.

Let's look at the histogram for the same image after converting it to greyscale to make the tones a bit easier to perceive.

Note that the histogram is essentially identical (some slight differences due, as far as I can tell, to the nature of the conversion process). Again, there is an absence of dark grey or black tones, and a preponderance of light grey tones, and, therefore, the histogram is skewed to the right. The glass and its contents (especially the right-hand side) is more of a light-middle grey but takes up less relative space in the image than the lighter background, so the hump on the histogram that represents the midtones is smaller than the peaks representing lighter tones.

Let's look at another example. Here's the same glass of Scotch, in a somewhat moodier setting.

1/80, f/2.8, ISO 1600

I intentionally underexposed this image because I wanted all but the highlights of the Scotch to disappear into shadow. What do you think the histogram will look like? Let's see.

As you probably guessed, this histogram is heavily skewed to the left. In fact, you can see from the straight left edge that many of the pixels actually touch the left boundary of the histogram, meaning that they are pure black and have lost all detail. The small tail on the right of the histogram represents the parts of the image that are dark- to midtones--specifically, the whiskey, and the highlight on the rim of the glass. Again, let's take a peek in greyscale.

Sure enough, we have a mostly black image with small areas of dark- and mid-grey.

So what does the histogram of a correct exposure look like? It depends.

First of all, the only "correct" exposure is one that reflects what the photographer intended. So if I were trying to show all of the details of the Scotch's surroundings in the second image, the exposure would be incorrect--I lost much of the detail in shadow. However, because that was the effect I wanted, the exposure is correct for my purposes.

Let's take a look at a more classically-exposed image, with a larger range of colours and tones (not a great photo, but useful enough for the purposes of example).

1/100, f/5.0, ISO 12800 (no, I don't know what I was thinking either)

So what do we expect to see in the histogram? Well, we have a nice wide spread of tones. There are some very dark tones in the bottle behind the glass, and some highlights where the sun was peeking in through the curtain, but there are also a lot of tones in between the two extremes. Let's take a look at the histogram.

Sure enough, the histogram reflects the wide tonal range. This is something like what the histogram of a correctly-exposed image will often look like. But if your histogram doesn't look like this, that doesn't mean your image is wrong; it simply means it's different. Whether it fits your purpose is what matters. If, for instance, you have a lot of snow in the background of your picture, you will probably hope to see a nice spike at the right of your histogram--perhaps even a peak of pixels encountering the right boundary that are pure white. If you are showing off a nice black shawl, on the other hand, you will probably hope to see a nice spike at the left of your histogram. If you know the effect you're going for, and can approximately gauge the important tones in your scene, you can get a pretty good idea from your histogram of how your exposure looks, regardless of the brightness of your camera screen or the level of ambient light around you. Cool!

One last thing--let's talk a little bit more about clipping. I touched on it above, but basically, clipping is what happens when parts of your image are outside of the recordable range of tones or colours. This means that there is no image detail in those parts of the picture; in the case of clipped dark tones, for example, any pixels in the clipped area will appear as pure black (even if there was detail visible to the eye at the time of shooting); likewise, clipped highlights will appear as pure white. If you try to shift the exposure a bit in post-processing, or try to recover detail in highlights and shadows (a fairly common procedure, especially when processing raw files), there won't be any detail in those areas to recover.

If you have a flat edge on the left or right boundary of your histogram, it probably means that you have lost detail from the shadows or highlights. This doesn't necessarily mean there is anything wrong with your picture. As mentioned above, I intentionally underexposed and lost details in my shadows in the darker image of the Scotch. Clipping is, however, something to be aware of. To help you out, most cameras will offer you highlight clipping warnings--more colloquially known as "blinkies" or "zebra stripes". Check your manual to learn how to turn on this feature. If you turn on this warning and you have blown highlights in your picture, the playback image on your camera will look something like this:

Note the flashing red areas of this image. These are areas where the highlights have been clipped. Looking at the playback image, I could decide if I wanted to reshoot the image with a darker exposure to try to recover detail in those highlights, or if I was comfortable letting those highlights blow out. The highlights in this image are a few bright spots of light from the sun, so I was happy to leave them as they were. If, on the other hand, I were shooting a pair of white socks and I was getting blinkies somewhere on the socks, I would probably want to reshoot with a darker exposure so as not to lose important detail.

The luminosity histogram takes a bit of mental work to get the hang of, but once you understand it, it is an invaluable tool in your photographic arsenal. Now go forth and practice--and maybe drink some Scotch for me, because my bottle, as of earlier today, is sadly empty.