Radium's Super Useful Guide to SLR Cameras

This document is written mainly for the beginner who wants to dive right into the good stuff, and the point-and-shooter who's ready to move beyond the basic modes and find out what those Av, Tv, and M markings on the other side of their settings dial mean. This is not a document about composition but more about how to use your camera.

The information is divided into two parts: the first introduces the math, science, and terminology behind everything, and the second, much shorter part, uses that information to create some simple rules of when to use what modes. You can skip to the later part if you like, I suppose, but I will think less of you. And I know where to find you... (I will, however, forgive you for skipping the Advanced Explanation sections.) Finally, while non-SLR users may find useful information here, it's written primarily for SLR users.

Part I: The Math and Science

Let's begin with the Focal Length of your lens. This is a number you may have learned about in a physics class at one time or another. If you take any magnifying lens (including the very lens on your camera itself) and hold it near a wall or a piece of paper across from a window, you will find a point where it focuses a clear, inverted image of the window on the wall. Go ahead and try it (if you do use your camera lens, be careful about keeping dust out of the lens and the camera body once you separate them).

The distance from the lens to the wall (or the "film" when our lens is mounted on our camera) is called the Focal Length and is measured in millimeters. 50mm is considered a "Normal" focal length (we'll see why later). Lenses with shorter focal lengths (referred to as "wide-angle" or "short" lenses) give wide views and lenses with longer focal lengths ("telephoto" or "long" lenses) give narrow, close up views. Also, keep in mind that when a photographer describes a lens as a "50mm lens," they are almost certainly referring to the focal length (and not, say, the diameter).

Typical focal lengths are anywhere from 28mm to 300mm (you'll find plenty of lenses outside of that range, but most general purpose lenses fall within this range). The apparent size of your image is directly proportional to your focal length: a 25mm lens will make images half the size of a 50mm, and a 300mm lens will make images 6 times the size of a 50mm. It just so happens that 50mm yields an image size that looks "about right" to human perceptions.

A zoom lens is simply a lens with a variable focal length. They will likely be labeled 28-70mm or some such range and the ring you rotate to zoom the lens will be marked with different numbers within this range.

The Focal Length and the Maximum Aperture (see later) are the two major properties of any lens and you should make sure you're familiar with what they mean.

Advanced Explanation

Technically, to measure focal length, the object you image (our window, in this case) should be infinity meters away to get the exact focal length, but so long as it's at least several meters, your measurement will be close enough. ...Of course, that's not quite true either: if you actually were to measure the distance, it would come up short. This is because the focal length is actually defined as the length from the focal plane (the wall/paper/film) to the center of the lens. For a thin lens, it's about right, but most camera lenses are very complex, have several elements in them, and are not very thin at all.

You may also have noticed that, as objects are placed closer to the lens, you have to move the lens farther from the wall to maintain a sharp image. This is how your focus ring works. Rotate the focus ring (make sure you're in manual focus mode so as to disengage the auto-focus motor, otherwise you can damage the auto-focus) and notice the lens moves slightly farther or closer to the "film" as you focus on objects that are closer or farther from the camera. How far your focus ring can move the lens from the film determines how close your subject can be to the camera. Too close, and it can't be focused. As it turns out, if you hold the lens double the focal length away from the film, your object can be that same distance from the lens. For example, if you take a 50mm lens and focus it so that it's 100mm from the film, subjects that are 100mm from the lens will be in focus. This is an extremely close focus and is called a Macro. This is how the Macro mode on cameras works. And as you might guess, it works better with lower focal length lenses. Macro lenses need to be able to move to near double their focal length. It's easier to move a 28mm lens out to 56mm than it is to move a 120mm lens all the way out to 240mm.

Finally, a last word on why 50mm is considered "normal." Normal is defined by what human eyes perceive. Human eyes take in an approximately 90-degree field of view. A 50mm lens on a standard film camera also yields about 90 degrees of view, and so 50mm is considered normal. At least, that used to be true... (are we seeing why this is all in the "Advanced Explanation" section???) It's not quite true anymore because the digital sensors in SLR cameras are smaller than the film that was used in film cameras. After doing some boring math, we find that a 32mm lens should be considered "normal" on a digital SLR. Hence a typical kit lens that comes with a dSLR has a range of 18-55mm (notice 32mm is more or less in the middle there--not arithmetically, but geometrically). 50mm on a dSLR actually appears slightly zoomed, but for this document, I will continue treating 50mm as normal. Incidentally, the size difference between a standard digital sensor and a frame of 35mm film is a factor of about 1.6. This is the so-called "cropping factor" you may have heard about when referring to digital SLR's. It means that a picture taken on a dSLR with a particular lens will appear to be cropped (zoomed in) by that factor when compared to the same picture shot with the same lens on a film SLR.

Shutter Speed and Aperture

To be correctly exposed, film (or in our case, the digital sensor) needs a certain amount of light to hit it. There are two ways to control how much light gets to the film: change the size of the hole it goes through, or change how long the film is exposed to the light. These are interchangeable and in fact, we use both methods, depending on the visual effect we are looking for in a particular photo.

Shutter Speed is measured in fractions of seconds. On your camera, the number 250 commonly means 1/250th of a second and 15 means 1/15th of a second. Fractions larger than 1 are commonly denoted with a quote beside the digits: 1 second will be denoted as 1" and 15 seconds as 15." You likely won't be using shutter speeds that slow anyway. The shutter speeds are fairly straightforward: 1/125th second lets in twice the light as 1/250th because the shutter is open for twice as long.

Aperture (the "iris" of the camera) is measured in strange numbers called "F-stops" (or "F-ratios.") These typically range from 1.8 to 22. Some lenses go higher or lower, and some don't go nearly that low. A typical progression of f-stops may look like:

        1.8    2.8    4   5.6    8    11    16    22

A higher number indicates a smaller hole, which means less light. Also, each of these settings lets in half the light as the previous. For example, 5.6 lets in half the light 4 does, and 11 lets in four times the light 22 does.

If nothing else, always remember, higher f-number equals smaller aperture!

Unfortunately, it gets a little more confusing, as most cameras are capable of setting the aperture to more sizes between these. The sizes listed above are each considered a full f-stop. Most cameras have aperture settings that are measured in half-stops or even third-stops.

As mentioned earlier, there are two very important properties of any lens that you should be familiar with. Focal Length was the first and Maximum Aperture is the other one. Your lens's maximum aperture is the lowest f-stop you can set it to and will usually be labeled somewhere on the front of the lens (and it will actually be variable on a zoom lens). This will give you an approximate idea of how much light your lens can pass and will be important later on. Basically, the lower the f-stop number, the better. A 50mm f/1.8 is generally better than a 50mm f/4.0.

Advanced Explanation

The f-numbers are actually the ratio of focal length / lens diameter. Closing the aperture (which, incidentally, is referred to as "stopping down the lens"--don't ask me why) is the equivalent of using a lesser diameter lens, so the ratio gets bigger as you close the aperture. A 50mm diameter lens with a 50mm focal length would be an f/1.0 lens. A very common lens is Canon's 50mm f/1.8 lens, which has about a 28mm diameter. Photographers refer to this as a "fast" lens because, since it lets so much light in compared to a smaller lens (say, a 50mm f/4.0), faster shutter speeds can be used with it. More on that later...

You may have noticed that longer lenses (and zoom lenses) tend to have a smaller maximum aperture (keep in mind, by "smaller" I mean the actual size of the aperture, not the f-stop number, which is actually larger for a smaller aperture... isn't this stuff great???). And further, the maximum aperture changes as you zoom a zoom lens. As an example, the standard 18-55mm zoom that comes with many cameras typically only goes down to f/3.5 -- and that's when it's zoomed full wide. Zoom it to its max of 55mm, and the minimum f-stop becomes an abysmal f/5.6 (compare this to the Canon lens I mentioned above at f/1.8).

So why do they do that? Zooming a zoom lens, as we know, changes its focal length... so keeping in mind that the f/ratio is defined as diameter/focal length, and we're changing the focal length (but not the diameter), we can see why our f-ratio would have to change as we zoom. If we could somehow increase the diameter of the lens as we zoomed it, we could actually make a lens for which f-ratio remains constant across the zoom range.

Jumping from there, you might start to see why longer lenses tend to have smaller maximum apertures. A longer lens, in order to have a wider aperture, would need to be physically bigger. To create a 300mm f/1.0 lens, we'd have to have a piece of glass nearly a third of a meter in diameter. This would be huge, heavy, and extremely costly. At that size, lens makers start using other geometries similar to what telescope makers use (you can see these in the hands of professional photographers at sporting events--more on that later...).

Another way to look at all of this Aperture/Focal Length business is this: there's only so much light coming in through that window we're trying to take a picture of. When you use a long lens to create a bigger image, you are spreading that same, finite amount of light across a bigger area--your image will have to be dimmer. A very similar effect can be seen with an overhead projector, or a slide projector (or any projector for that matter). The farther you pull it from the screen, the bigger the image gets, but also the dimmer it gets because you only have a finite amount of light to work with.

Getting Correct Exposure

With this knowledge of Shutter Speed and Aperture, you are prepared to learn what I'd consider the most important principle in all this. So don't stop now, this is the best part!

You can simply set your camera to the "P" mode (which stands for "Program") and it will follow its own programming, select an appropriate aperture and shutter speed, and take the picture for you. The problem is, in Program mode, you never know what your camera will do. Sure, the exposure will come out right, but what if your subject was moving quickly but your camera decided to use a slow shutter speed? (You'd getter a blurry subject, that's what!)

Enter what I like to call the "Semi-Automatic" modes. It turns out that for any given lighting situation, you can use a wide range of combinations of aperture and shutter speed (unless your lighting is extreme, more on that in a moment). Let's say your camera is in Program mode and you decide to take a picture of that window. Let's say the camera decided to use 1/60th of a second shutter speed and an f/16 aperture. That picture (unless your camera is broke) should come out looking fine, but we actually have the option of tweaking one setting and letting the camera re-adjust the other to compensate.

As mentioned earlier, each full f-stop lets in half the light of the previous, and halving the shutter speed lets in twice the light. So, we should be able to change the camera's settings from 1/60th and f/16 to, say, 1/125th and f/11 (note that cameras like to use nice, round numbers for shutter speed--to be technically correct, we should use 1/120th, not 1/125th, but the difference is negligible). Likewise, we can push one number and adjust the other as much as we like until we run into some hard limit such as the bottom of the Aperture range, or a shutter speed that's too slow, or whatever.

This is it, the big one, so know it well:

You can always trade off between aperture and shutter speed.

So there are some questions remaining--why would you want to? And what are the limits? This is where SLR cameras really start showing their worth...

Shutter Speed Priority Mode

Shutter Priority mode (labeled Tv on Canons and probably similarly on yours) is a mode that lets you pick the shutter speed and lets the camera pick the aperture. You simply lock in the setting you want, and as lighting changes, the camera reads how much light there is and automatically calculates the aperture.

Shutter speed, once again, is considerably more straightforward than Aperture. The basic rule is anything taken on a slow shutter speed will look blurry if it's moving. Using Shutter mode almost always means you are trying to capture (or avoid capturing) some kind of motion in your photo.

Sporting events often call for fast shutter speeds in order to make sure the subject (who is often moving quickly) remains sharp. Freezing a balloon in mid-pop requires a very fast shutter speed (not to mention some darn good timing).

You might use a slow shutter speed on the ocean or a waterfall to create an interesting blur effect on the moving water. Or, you might freeze the water in mid-air with a fast shutter speed.

Here is a good place to learn an important rule of thumb:

Try to avoid shutter speeds slower than the length of your lens

(Unless you have a tripod. And yes, I know the units don't agree in that statement!). What this means is that if you're using a 28mm lens, you can get away with handholding the camera down to about 1/28th of a second. But if you're zoomed to 300mm, even 1/250th of a second may be too slow to compensate for the slight wobble of the camera in an unsteady human hand. You'd need to use at least 1/300th (preferably 1/500th) to avoid hand-shake blur. Notice how much the image wobbles when you're trying to hold a camera still at high zoom? The film sees that too.

You may now feel pity for sports photographers. They often need fast shutter speeds to capture their fast-moving subjects. Fast shutter speeds require wide apertures. Wide apertures require lots of expensive glass. They also need to be able to zoom in close, which requires long focal lengths, which requires even more expensive glass to keep aperture wide and shutter speeds fast... The lesson here is never become a sports photographer unless you have large piles of money that you're bored with.

Aperture Priority Mode

Aperture Priority mode (labeled Av on Canons) is the mode that lets you pick the aperture and lets the camera pick the shutter speed. You simply lock in the setting you want, and as lighting changes, the camera reads how much light there is and automatically calculates the shutter speed.

The focus "field" is defined simply as what's in focus. The term "depth-of-field" refers to how wide the range of subjects in focus is. A narrow depth of field means that very few objects are in focus: something 4 ft away may be sharp, but something 5 ft is noticeably blurry and the mountains in the background are completely blurred out. A wide depth of field can mean anything from 3 ft to infinity is in focus. And as it turns out, depth of field is controlled by aperture. Open apertures create narrow depth of field, and stopped down apertures create wide depth of field.

Portraits look fantastic with a wide-open aperture because the background is thrown completely out of focus allowing the subject to stand out and not have the background be a distraction. Incidentally, this is one reason why having a fast lens is better. If your lens only goes down to f/4.0, you simply won't be able to blur the background out as well as a faster lens could. I find f/2.2 on my 50mm is a nice aperture to work with. That particular lens will go down to f/1.8, but sometimes the focus is so shallow that if the face of my subject is turned, one eye can be in focus while the other, at only a slightly different distance from the camera, can be blurry.

Landscapes often call for the opposite as portraits. You may want everything from the rocks at your feet to the mountains on the horizon to be in focus. This requires a small aperture. Here, the pitfall can be that at high aperture, your shutter speed may be forced too low. Beware of this situation and keep a tripod on hand if you plan to use lots of small apertures.

Another use for Aperture mode is in low light situations. You can lock the aperture to its maximum and know that you'll be letting in the greatest amount of light and using the fastest possible shutter speed. A limit of this is that sometimes, the fastest is still not fast enough... again, this is what tripods are for (though there are a couple cheats, see next section).

One thing to know is that when taking wide depth of field shots, you need to focus between the borders of your field. For example, if you have a tree 10 ft away and mountains at the horizon (approximately infinity--yeah, that was for all you math people!), then you need to focus some place in between 10 ft and infinity at, say 40 ft. This one just requires experimentation, so go out and play with this if landscapes are your thing.

ISO Speed and Manual Mode

The term ISO speed used to refer to the "speed" of the film. Some film was more sensitive to light than others. If you were using ISO 400 film and your camera wanted to use 1/60th shutter speed in a particular situation, then the same exact picture would require 1/30th shutter speed with ISO 200 film (and ISO 800 would only need 1/125th). Obviously higher ISO sounds better, but for the exception that higher ISO film was more grainy. ISO 100 film takes fantastic pictures (which digital is only now surpassing) while ISO 800 film creates noticeably grainy pictures.

Now in the digital age, the situation has interestingly not changed as much as you might think. You can adjust the sensitivity of your sensor (which is still measured as an ISO speed typically between 100 and 1600) simply by changing a setting and you'll get the same results--you can use faster shutter speeds on higher ISO speed settings. But you'll also suffer the same consequences in the form of noise.

Experiment with the different ISO settings and look at the difference. I recommend sticking to 200 when you can and avoid going over 400. True, there are some low light conditions that you simply can't take a picture in without going to ISO 1600, but they come out looking awful due to noise (which can be partially fixed using noise filtering in Photoshop but it's not a miracle cure and not easy, either). If you like to shoot in low light and a tripod isn't an option, try getting a faster lens.

The last setting I'll discuss is Manual mode. This mode lets you set both the aperture and the shutter speed. This is useful in a few rare instances. One is when you know the light is so low that even at maximum aperture, the camera is going to select a shutter speed that's too slow. You can force the shutter speed higher. This will cause your picture to under-expose and be too dark, but at least it won't be blurry. Again, Photoshopping afterward can partially fix this, but not completely. Another use for manual mode is astro photography and fire works. I'm not covering those things here, but look'em up on the interweb and you'll probably find plenty. Most cameras don't leave you completely in the dark (sorry!) but offer some form of light metering which usually looks like:

        [-2 .. -1 .. 0 .. +1 .. +2]

The idea being that when the meter is at 0, you will be properly exposed. +2 means you'll be over-exposed by two full f-stops (or shutterspeed is 4x what it needs to be) and likewise, the minus direction means you'll be under-exposed.

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Part II: The Quick and Dirty Guide to When to Use What

So with all that information, we can create a laundry list of simple rules to help us decide when to use which settings.

Portraits / Only Subject In Focus - Av Aperture Mode, Around 2.2 If you would like a shallow range of focus, use a low Aperture setting like 2.2 or even lower. This is great for close-ups, especially of people, because it completely obscures the background.
Landscapes / Everything In Focus - Av Aperture Mode, Higher Setting If you want a wide range of focus, use a higher Aperture setting like 16 or 22. At 22, many cameras can put anything from 6 feet all the way out to infinity in crystal clear focus. Remember also to focus "between" the closest and farthest subjects (see earlier for details). Also, remember that a small Aperture will require a longer Shutter speed; you may need a tripod.
Stuck In Low Light - Av Aperture Mode, Lowest Setting If you are in low light, can't use a tripod, and don't want to use a flash, then you pretty much have no choice but to lock the aperture to its lowest (most-open) setting and hope there's enough light for a decent shutter speed. Also try a wider lens, stick to the shutter-speed-faster-than-lens-length rule mentioned earlier, and brace your arm against a post, if possible. Finally, you might try manual mode or a higher ISO speed as mentioned earlier. Stuck In Low Light 2 - Tv Shutter Speed Mode, "Safe" Setting I prefer to use Aperture mode for low light, but you may instead switch to shutter speed mode, lock it to a "safe" exposure (using the shutter-speed-faster-than-lens-length rule), and then hope there's enough light for a good exposure. The same advice applies about bracing yourself, ISO speeds, and manual mode.
Freezing Fast Moving Objects - Tv Shutter Speed Mode, Higher Setting To freeze motion, use a fast Shutter speed (or use a flash--both will freeze motion, but using a fast Shutter speed instead of a flash looks more natural since you won't have the unpredictable and artificial light of a flash to deal with).
Blurring Fast Moving Objects - Tv Shutter Speed Mode, Lower Setting To intentionally blur a moving object, use a slower Shutter speed like 1/30th or longer. You can also create an interesting effect by "chasing" the object with the camera as you snap the shot.

Wide Angle Lenses

Make things look smaller, farther away. Foreground objects look disproportionately larger than background objects. They can take in more of the scene so are well suited to sweeping landscape shots. They also can often focus into the macro ranges for extreme close up shots. Wide angles are less susceptible to hand-shake blur at low shutter speeds.

Telephoto Lenses

Make things look larger, closer. Background objects look disproportionately larger than foreground objects. Medium telephotos make decent portrait lenses as they can get close shots without having to be uncomfortably close to the subject. Telephotos are often large and heavy (and expensive!). They are very sensitive to hand-shake and often require a tripod even at higher shutter speeds.

The Flash

I personally can't stand using the flash and tend to instead use a low aperture lens like my trusty 50mm f/1.8 and attempt to hand-hold the shot. But with my bias out of the way, here's what you need to know about using the flash.

A flash adds an unnatural and often unpredictable light to a photo. They can cause reflections from eyeglasses, windows, and the notorious red-eye effect. If possible, angle the flash up and bounce it off a ceiling. This helps reduce red-eye and also gives more natural looking results. The farther the flash unit is from the lens of the camera, the less likely you are to get red-eye.

Flashes also have limited range--a typical built in flash only works to about 15 ft, and even my halfway decent one only goes to 50 ft. This makes them pretty useless with telephoto lenses, which are usually focused on objects farther than 15 ft away. All those people who snap pictures at the superbowl usually get under exposed shots because of this.

Flashes can be used in daylight to help fight against backlighting, which is when a subject becomes a silhouette due to a bright light source (often the sun) being behind them. Finally, when in low light and all else fails, you may want to resort to using a flash. I will admit that my strategy of trying to hand-hold shots on my 50mm f/1.8 has definitely yielded a fair amount blurry photos.

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How To Use Your SLR Camera. Januray 2008. Tom Sciortino