Quick Tip: Straighten Up!

One of the easiest photo flaws to fix is a tilted picture. A horizon that's not quite horizontal or a vertical line that's not quite vertical can distract the eye away from the subject of a photo. So fix it!

Yikes. (Intentionally tilted in software for purposes of illustration.)

Most image editing programs will have at least a rotate function. You can use this to eyeball a correction. Or--my favourite--if your editing software allows, you can simply find a line in the photo that should be horizontal or vertical and, using the appropriate tool, drag your mouse along this line. The software will then do the rotating for you. Here's how to do it in my favourite software, darktable:

Open your image. In the right hand panel, click over to the basic group (the circle button), and then find the crop and rotate function. Select this function. Place your cursor at one end of a line that should be either horizontal or vertical. Right-click and hold, then (while still holding the right-click button) drag your cursor along the line to the other end. Release the button. darktable should automatically rotate the image for you.

If you're having trouble getting it just right, you can try zooming in to 100% or 200%. I find this often affords a more accurate correction. In this example, I'm using the shore as a guide, zoomed in to 100%:

Straightening a horizon in darktable. Click to enlarge.

Don't let your photos suffer from crooked horizons or diagonal verticals! Your viewers will thank you.

Behold! A (more or less) straight horizon!
18mm (~28mm equiv); 1/250 @ f/5.0, ISO 100

Quick Tip: Instantly Improve Your Cell Phone Photo Quality

Do you use your cell phone camera, either for quick snaps, or as your primary camera? If so, we need to talk about zoom.

One thing most dedicated cameras have that cell phone cameras lack is optical zoom. Optical zoom uses--you guessed it--optics to zoom in or out of your scene. In other words, the lens itself adjusts the zoom.

A photo from my cheap cell phone camera. No zoom used. Click to enlarge.

Digital zoom, on the other hand, is essentially a simulation. Digital zoom enlarges the central portion of the image and cuts away the rest. Basically, it's doing in-camera editing--cropping and enlarging the photograph as you take it, without actually magnifying the scene.

Optical zoom will not result in a loss of quality in your image (or if it does, it will generally not be a significant loss of quality, and any loss in quality will be related to the quality of the lens itself). Digital zoom, on the other hand, will irreparably degrade the quality of your photograph.

Similar photo, same cheap camera. This time, I used digital zoom. Click to enlarge. Yikes!

So what should you do? Avoid zooming in with your cell phone camera! If possible, get closer to your subject. If you can't get closer, consider cropping in post-processing--at least you'll have more control over the final result.

If you've ever wondered why your cell phone pictures just don't look very good, digital zoom may be the culprit.

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.

Introduction to Exposure: The Exposure Triangle

 Why Do I Care About Exposure?

Which would you choose? This...

...or this?

You care about exposure (or should care, anyway) because the way your camera exposes your photo has a dramatic effect on your photo's final appearance--its brightness, its sense of motion, its depth of field (whether or not it has that nice blurred background, for instance), and its noise level (a type of visual distortion). If you are shooting in full auto, your camera will make all of these decisions for you, based on some guesses. If you are shooting in full manual, you will make all of these decisions for your camera (for better or for worse!). Other modes will be somewhere in between. When you understand how your camera exposes, you can start to take more control over the appearance of your images. Even if you don't have access to full manual controls, you can start to understand that, for example, portrait mode will give you a blurrier background by opening up your aperture. This will in turn let you have much more of an influence over the final appearance of your photographs. So let's get to it!

About the Example Images

For the examples in this post, I will be using the following image as a control image. It appears as the middle image in all vertically-stacked comparisons. (Image is slightly underexposed. My fault. It's not quite as bad as it looks because the background was beige, not white, but I probably should have brought it up about 2/3 of a stop. My bad. C'est la vie. You'll get the idea anyway, and maybe I'll replace the images some day.)

Shutter speed 4/5 sec, Aperture f/5.6, ISO 400 (these numbers will mean something to you soon)

Images were taken of a basic still life scene from a tripod, using a two-second self-timer (to eliminate any camera shake) under identical lighting conditions. I used manual focus to keep the focus as consistent as possible throughout. Focus point was the number 3 on the sock conversion guide (close up available in the Aperture section of this post). All images have been minimally post-processed from raw to have the same white balance. All noise reduction was turned off.

The Exposure Triangle

No matter what type of camera you use, your photography can only benefit from an understanding of the elements of exposure. These are the elements that control how your digital camera records light--and, therefore, how it records an image. Broadly speaking, your (digital) photo's exposure depends on three factors (sort of): shutter speed, aperture, and ISO (technically, ISO isn't actually an element of exposure since it doesn't change the amount of light that enters the camera, but to simplify things, I'm including it as part of exposure). You'll often see these three elements referred to as the exposure triangle.

What exactly is exposure? In simple terms, it refers to how much light reaches the image sensor (or film) in your camera. Exposure will dramatically affect how your photograph looks--most obviously in how bright or dark it is, but also in the appearance of the colours and the amount of digital noise in the image. The more light that reaches your image sensor, the brighter the resulting image will be.

Warning: there will be a lot of numbers in the post. Do not fear! You do not have to be good at math to understand these numbers. If you have a basic understanding of fractions (e.g., that 1/100 is bigger than 1/500), you'll be just fine.

Even if you do not have access to manual modes on your camera (Program, Aperture Priority, Shutter Priority, and Manual), an understanding of exposure will help you to understand how to use the modes that are available to you in order to come closer to your photographic vision.
 
Shutter Speed

This is probably the easiest of the three elements to understand.  The shutter speed controls, as the name suggests, the amount of the time that the shutter stays open (and, therefore, how long the sensor is exposed to light). All other settings being the same, a faster shutter speed will allow less exposure to light and a slower shutter speed will allow more exposure to light.

The shutter speed can also be used to freeze or convey motion. A slower shutter speed will blur a moving subject, while a fast shutter speed will freeze a subject in motion. Too slow a shutter speed also introduces the risk of camera shake if you are shooting handheld. If your photos are blurry, it may be because you are using too slow of a shutter speed for your situation.

Typically, a shutter speed is expressed as a fraction of a second, such as 1/125 or 1/250. Some cameras may only show the denominator (125 or 250, in this example). A shutter speed of 1/125, or 125, will let in more light than a shutter speed of 1/250, or 250. If you see a number such as 1.5", the " indicates an exposure time in seconds, rather than in fractions of a second.

The effect of shutter speed on exposure.
Top: Shutter speed 8/5sec, Aperture f/5.6, ISO 400
Middle: Shutter speed 4/5 sec, Aperture f/5.6, ISO 400
Bottom: Shutter speed 2/5 sec, Aperture f/5.6, ISO 400

The general rule of thumb for avoiding camera shake when shooting handheld is to have a shutter speed no slower than 1/(your focal length) in the case of a full frame camera, or 1/(your focal length * the crop factor) in the case of a crop-sensor camera. If you don't know if you have a full frame sensor, you probably don't. The crop factors of Nikon and Canon cameras are 1.5 and 1.6 respectively, but I generally just multiply by two to be on the safe side. You can decrease shutter speed somewhat if you have a stabilised lens (look for abbreviations such as VR or IS). Using this rule will not, of course, impact subject motion blur.

In fibre art photography, you are almost always going to want a well-focused image without subject motion blur (or camera shake). A photograph with a blurred subject will obscure the details of the knit/crochet/other item, typically rendering it useless as a photo of the fibre object itself. Therefore, you will want to maintain an adequately fast shutter speed to compensate for camera shake (when shooting handheld) and subject motion (when shooting a moving subject, which could be as slow as a sock on a slightly twitching foot, or as dramatic as a shawl billowing out behind a running human; each of these would typically require a different minimum shutter speed to freeze motion).

Aperture

Aperture describes the width of the front opening inside your lens (the entrance pupil). A wider aperture will let in more light, and a narrower aperture will let in less light.

Aperture is expressed in f-stops. What tends to trip people up with this is that a smaller number indicates a wider opening. For example, an aperture value of 2.8 lets in much more light than an aperture value of 11. This is because the aperture value represents a ratio or fraction, where numbers such as 2.8 and 11 are the result of dividing the focal length of the lens by the diameter of the entrance pupil of the lens. The wider the opening of the entrance pupil, therefore, the smaller the aperture value. You may see aperture values preceded by f/, such as f/2.8 or f/11.

The effect of aperture on exposure.
Top: Shutter speed 4/5 sec, Aperture f/4.0, ISO 400
Middle: Shutter speed 4/5 sec, Aperture f/5.6, ISO  400
Bottom: Shutter speed 4/5 sec, Aperture f/8.0, ISO 400

Your chosen aperture will also affect the depth of field in your photo. Simply put, the depth of field is how much of your photo appears to be in focus. The wider your aperture (remember, a wider aperture is a smaller number), the shallower your depth of field--that is, less of your photo will appear to be in focus. If you've ever seen a photo with a beautifully blurred background, it probably used a wide aperture (although there are other factors that influence depth of field, which I will not cover in this post). Conversely, if you've seen a photo with a lot of detail from front to back, it probably used a narrow aperture.

Close up of different depths of field. Notice how much sharper the numbers in the chart are on the right-most image compared to the middle and left images.
From left to right, aperture values are f/4.0, f/5.6, and f/8.0

Camera lenses have maximum apertures, which represent the widest possible opening of the entrance pupil (in relation to the focal length). A 50mm lens with a maximum aperture of 2.8, for example, can open wider than a 50mm lens with a maximum aperture of 4. This will affect the camera's ability to let in the most light possible in low-light situations, as well as the degree to which you can decrease your depth of field.

The aperture you want to use in fibre art photography will largely depend on the intent of your photo (although it may also be limited by the constraints of your environment). If you want to blur an unsightly background while keeping the fibre object largely in focus, you will probably want to use a moderately wide aperture. If you want to focus on just one detail of the fibre object and blur the rest of the image, you will probably want to use a very wide aperture. If, however, you want to tell a story by keeping your surroundings in focus, as well as the fibre art object, you will want to use a narrow aperture.

Note: if you are looking through the viewfinder of a DSLR camera, you will see your scene through the widest possible aperture, and not through the aperture value you or your camera have set. This is so you can see your scene with as much available light as possible. Most (all?) DSLR cameras have a depth of field preview button which will show you the depth of field your scene will have with your chosen aperture; be aware, however, that your view through the viewfinder will become dimmer when you use this button (unless you are shooting at maximum aperture) due to the loss of light from the narrower opening in your lens. The preview button (and the subsequent darkening in your viewfinder) will not have an effect on the final exposure; only your actual shutter speed, aperture, and ISO settings will determine the exposure. Check your camera manual to locate your depth of field preview button.

ISO

ISO speed is one of those complicated things that has a simple main effect: to change the brightness of image. All you really need to know is that a higher ISO value will lead to a brighter image, whereas a lower ISO value will lead to a darker image.

The effect of ISO on image brightness.
Top: Shutter speed 4/5 sec, Aperture f/5.6, ISO 800
Middle: Shutter speed 4/5 sec, Aperture f/5.6, ISO 400
Bottom: Shutter speed 4/5 sec, Aperture f/5.6, ISO 200

A high ISO does come with a cost, however: as your ISO speed increases, the amount of digital noise (visual distortion consisting of random scattered pixels that do not represent the original appearance of the scene) in your photo will also tend to increase. Therefore, it is worth shooting with the lowest ISO possible within the parameters established by your minimum required shutter speed and aperture values. Underexposing your image in camera and then brightening it in post-processing will generally add more noise than shooting with a higher ISO, so that is not an especially viable alternative. Shoot with the ISO speed that your image requires--and no more! Also be aware that high ISO performance varies from one camera to the next, often dramatically; for instance, ISO 1600 on a newer DSLR may show very little noise, while the same ISO on an older camera may be almost unbearably noisy.

Close up shots of noise levels of varying ISO speeds with equivalent exposures. This camera handles high ISO levels reasonably well. Click on the image for a 100% comparison.
From left to right, ISO values are 400, 1600, and 6400.

In fibre art photography, using a lower ISO (provided your exposure is correct) will decrease the chance that digital noise will interfere with an accurate and pleasing image of your item. High ISO speeds risk obscuring detail and creating an aesthetically displeasing image by scattering random pixels throughout the photo.

Shutter Speed + Aperture + ISO = Image Brightness

These three elements work in concert to determine the brightness of your final image.  Each one is measured in stops of light, where each higher stop represents double the amount of light of the previous stop. For example, a shutter speed of 1/125 will let in twice as much light as a shutter speed of 1/250 (assuming aperture and ISO remain the same); likewise, an aperture of f/2.8 will let in twice as much light as an aperture of f/4 (assuming shutter speed and ISO remain the same) [note that simply multiplying an aperture by 2 does not indicate a doubling of light because the amount of light admitted through a circular opening does not double with a doubling of the circle's diameter; but don't worry about that, you only need to know that a stop means a doubling of light]; and likewise, an ISO speed of 400 will result in twice as much light as an ISO speed of 200 (assuming shutter speed and aperture remain the same).

This means that you can achieve an equivalent brightness in an image in many different ways. Suppose that you have an image that is correctly exposed at 1/250 sec, f/8, ISO 400. You could achieve the same exposure (brightness) at 1/125 sec, f/8, ISO 200 (a one-stop slower shutter speed and a one-stop slower ISO); or at 1/250 sec, f/5.6, ISO 200 (a one-stop wider aperture and a one-stop slower ISO); or at 1/125 sec, f/5.6, ISO 100 (a one-stop slower shutter speed, a one-stop wider aperture, and a two-stop faster ISO), or at a multitude of other settings. However, the appearance of the photo--its sense of motion, its depth of field, and its level of digital noise--will all be affected by your choice of settings.

In the original image in this example, depending on your focal length and the distance of your subject from the background you may have a moderately deep depth of field (so, while your background might be blurred, your fibre art object will probably be largely or entirely in focus), with a fast enough shutter speed to freeze a moderate level of subject movement (such as walking at a reasonably slow pace), and, depending on your camera, a small to moderate amount of digital noise. In the first modification (the slower shutter speed and slower ISO), your depth of field will remain the same and you will have less visible digital noise, but you may not be as successful at freezing motion, and you are more likely to suffer from blurring due to camera shake (depending on your focal length). In the second modification (a wider aperture and a slower ISO), you will have the motion-freezing capabilities of the original image but more of your photo will be blurry, and you will have less digital noise than in the original. In the final modification, you will have less motion-freezing capability than in the original (the same as in the first modification) and a shallower depth of field (the same as in the second modification) but you should have very little digital noise in your photo.

Three equivalent exposures.
Top: Shutter speed 2/5 sec, Aperture f/4.0, ISO 400
Middle: Shutter speed 4/5 sec, Aperture f/5.6, ISO 400
Bottom: Shutter speed 1/5 sec, Aperture f/8.0, ISO 3200

Conclusion

Still with me after all of that? Awesome! Now that you have a better understanding of exposure, you're all set to take some more control over the final appearance of your images. I can't wait to see what you can do!