christine gacharna | B L o G » a Northern Virginia photographer specializing in black and white images

Exposure = ISO + Aperture + Shutter Speed. Learn it, know it, live it.

Let me start by saying I don’t buy the top of the line camera body Nikon makes, especially since I’ve gone digital. Digital camera bodies are like computers; in a couple of years, newer, better technology will render today’s bodies almost useless. (Almost. But that’s a discussion for another day.) Lenses, on the other hand, hold their use and value. Lenses contribute more toward making a great photo than a camera body does.

I’d rather have a $1500 lens on a $500 body than a $3000 body with a $500 lens.

One of the most important considerations in buying and/or using a lens is aperture.

This is how I think of aperture: imagine a sheet of glass between you and your subject. That sheet of glass represents aperture when shooting wide open. There’s only that tiny stretch of depth across your frame that will be in sharp focus. This is known as shallow depth of field. I did my best to photograph an example of shallow depth of field using critical aperture (keep reading) where you can literally see the plane of focus:

Aperture, by definition, is the variable opening by which light enters a camera through a lens. Aperture is the lens opening or f/stop in a photographic equation. Aperture is the amount of light made available to the sensor to record an image.

So let’s review focal length just for a moment and return to the numbers printed on a lens: for example, my Nikon 105mm 1:2.8 lens. We know that the 105mm means focal length, but what does the ratio-looking 1:2.8 mean?

This number designates the widest aperture available for that lens and it’s written as a ratio because aperture is in fact a fraction (stay with me.) For example, my 105mm 1:2.8 lens can be shot wide open at f/2.8. Some zoom lenses will have numbers that look like this: 1:4-5.6 This number means that f/4 is the largest aperture available at the widest (wide angle) end but only f/5.6 at the longest (telephoto) end (say, for example, it’s a 70-200mm zoom, f/4 at 70mm and f/5.6 at 200mm is the widest aperture available for that lens.) My 70-200mm telephoto zoom lens has a 1:2.8 ratio; I can shoot wide open at f/2.8 no matter what focal length I’m at with this lens.

This number is the crux of the issue of lens choice. All of my lenses are f/2.8 or f/1.2 — all of them. Lenses slower than f/2.8 are deal-breakers for me. It’s just the way I shoot. Perhaps you shoot differently; say you shoot landscapes at mostly f/22 and don’t have much use for a lens wide open. Perhaps you never shoot pictures indoors, so a lens that can perform in low-light situations isn’t of much use to you. That’s exactly my point, why I’m going to all of this effort to show you how to use your camera; it’s why I can’t tell you one way or another what camera-lens combo would be the best gear for you to buy. Remember Rilke?

Look at this aperture scale to help you visualize aperture as the amount of light splashing in through your lens toward your sensor:

Each stop on the aperture scale = one full stop of light.

Take our exposure equation that we learned last post:

Exposure = ISO (sensitivity to light) + Aperture (amount of light made available to record an image) + Shutter Speed (the length of time the light is made available to record an image)

How sensitive you’re making your camera to light + how much light you’re letting in and for how long. That’s it! That’s your exposure equation. How much light you’re letting in is controlled by aperture. Think of aperture as pouring light onto your sensor through your lens; at f/2.8, you’re just dumping it in wide open but by the time you stop down to f/22, you’re pouring it quite carefully through a small funnel.

Photography is, by definition, painting with light. Not enough light and you might as well take your ball and go home. Light gives a photographer options and options are the paint in a photographer’s hands.

Moving from f/2.8 to f/5.6 on the aperture scale is two full stops of light. In ISO terms, which you might be a little more familiar with if you’re using a point-and-shoot and ISO is really the only adjustment you make, that’s the equivalent of moving from ISO 400 to ISO 1600. Think of the difference in the quality of your images when you shoot on ISO 1600 as compared to ISO 400. (If you don’t already know this offhand, don’t worry; we’ll cover it.) If you’re starting at f/5.6, you’re going to have to make up that loss of light somewhere. Looking back at our exposure equation, there are only two other options: ISO or shutter speed. If you’re working with a long telephoto lens, you might not have as much wiggle-room to slow your shutter speed. That leaves you bumping up your ISO which might give you noisy or not so crisp and clear images.

There’s also the issue of performance. A “critical aperture” is the aperture at which a lens can be expected to best perform, and is generally two stops from the maximum (or largest) lens aperture. So if your largest aperture is f/1.2, then you can expect very good, clean, sharp images from shooting that lens at f/4; but if f/5.6 is your maximum aperture, you’ll have to stop that lens down to f/11 to achieve “critical aperture.” Scroll back up to the aperture chart and look at the circles, particularly at f/11 — visually compare the amount of light available at f/11 with the amount of light available at f/4 or even f/1.2. See my concern?

Generally speaking, the larger the f/stop (think of the size of the circle, not the number!) the better the lens. The larger the f/stop, the “faster” the lens.

Q: Why is f/22 smaller than f/2.8 when 2.8 is a smaller number than 22? It’s very confusing!

A: It is confusing. It’s math. “f/stop” is literally (speaking in mathematical terms) a fraction that indicates the diameter of the aperture (or lens opening). “f” stands for the “focal length” of the lens, “/” is “divided by” and the number indicates the stop. So a 28mm lens set at f/2.8 would be [focal length] 28 divided by 2.8 = 10. The diameter of that lens opening would be 10. A 28mm lens set at f/22 would yield a diameter of 1.27. So in this case, a circle with a diameter of 10 would be larger than a circle with a diameter of 1.27 — whew!

Or, you can simply commit to memory the size of the circles in the chart I drew, f/2.8 being much larger than f/22, each numbered stop along the way halving the light reaching the sensor. That’s what I did.

Q: Why does it go f/1.2 and then f/2.8? Shouldn’t it be f/1.4?

A: That’s a good question. Sometimes it does, some lenses are f/1.4. The numbers don’t quite line up perfectly with halving and doubling. Sometimes cameras factor in half-stops or thirds, producing far more numbers than I included on the scale above. You’ll see the same things happen next post when we get to shutter speed, moving from 1/60 to 1/125. Don’t get too hung up on it.

The numbers on the charts I’m giving you are considered the “whole stops.” It’s a good frame of reference for a beginner. Once you memorize those numbers, you’ll find it becomes second nature to move from whole to whole in calculating exposure. This is useful because if you are shooting f/5.6 at [shutter speed] 1/250 but you’d like to be shooting at f/2.8 to blur your background more, all you have to do is adjust your shutter speed two stops to the right to make up for shifting your aperture two stops to the left. So the exposure of f/2.8 at 1/1000 is equal to the exposure of f/5.6 at 1/250 for this particular photo. Both photos will look the same as far as brightness and contrast, but the image shot at f/2.8 will have less depth of field.

Here’s an example of changing shutter speed and aperture combinations to produce basically the same exposure. In this case, I wanted to illustrate the difference in depth of field (notice the backgrounds) with different apertures. I moved my aperture roughly 5 steps to the right on the scale above, so I had to move my shutter speed roughly 5 steps to the left to compensate for the loss of light in order to achieve the same basic exposure. I was using my 28-70mm lens and probably should have shot 4 photos, two at 28mm and two at 70mm but my model was in a hurry to get to her meet so I shot two frames at roughly “normal” focal length, the first to illustrate shallow depth of field and the second to illustrate great depth of field:

See those pretty little circles in the background above? That’s bokeh. Wide open apertures produce blurry backgrounds and beautiful bokeh and shallow depth of field (blurry background.)

F/18 renders great depth of field (focus plane extends much farther throughout the background.) However, depth of field does not always insure sharp focus. A corresponding shutter speed of 1/20 to make up for the loss of light when I moved to f/18 was clearly against the rules we learned in focal length — it’s not a “first learn the rules, then learn to break them” example, it’s an example to illustrate why paying attention to focal length and its relationship to shutter speed is important. Remember, I was handholding my lens (the body-lens combo weighs about 10 pounds) at 48mm with a shutter speed of 1/20 and result is screaming camera shake or blur. Look at these close-up crops of the same images from above:

Extra Credit: Let’s say that second image was shot at (I’ll put it into whole stops to make it easier for you) [aperture] f/16 and [shutter speed] 1/30. iTunes treat goes to the first person who can tell me (before I post the answer in the next topic) what my exposure on that second image SHOULD have been, assuming a shutter speed of 1/60.

Bueller? Bueller? Anyone? Anyone?

Remember, we agreed that you’re looking to create images that are first and foremost in-focus and not too bright (overexposed) or too dark (underexposed.) Simple. First learn the rules, then learn to break them.

Lenses come in various focal lengths, the length or distance from the front of the lens to the camera’s sensor measured in millimeters.

Some lenses are primes (one focal length) and others are zooms (various focal lengths). Many photographers swear by prime lenses to achieve tack-sharp focus; other photographers swear by zoom lenses for versatility. I am both. If I’m looking to take a close up portrait of an 8-year-old, I use my fixed-focal-length prime 105mm (which is considered a telephoto lens; more on that in a minute).

My workhorse lens, the one I grab most often, is a 28-70mm zoom lens (zoom because it covers all focal lengths from 28 through 70, wide angle to telephoto zoom because of the field of view it covers. Keep reading.) This lens doesn’t produce quite the tack-sharp images as my primes, but the tradeoff I get is versatility. If I’m shooting portraits of a 1-year-old, who by definition movesveryfast, it’s incredibly helpful to be able to change the focal length of my lens in one quick movement from a 70mm to a 28mm and everywhere in between as she starts to run toward me. Can’t do that so easily with a telephoto lens or a prime lens. When working with children or in fast-changing environments, switching lenses can mean missing the shot. The flexibility and versatility of a zoom can be a gift to photographers.

My 28-70mm is a wide-angle to telephoto zoom lens. My 105mm is a prime telephoto lens. Focal length numbers range from fisheye to wide angle to normal to telephoto. A fisheye lens has a field of vision covering up to 180 degrees; wide angle is between fisheye and normal; a 50mm lens is considered “normal” because it sees roughly the same field of view as the human eye; and a telephoto lens has a narrower field of view and a magnified image (think spotting scopes or binoculars.) Here’s a chart to help you visualize focal lengths:

Here’s an example of my 28-70mm lens shot at 28mm and again at 70mm without changing the distance from me to the subject:

If you are a realtor looking to photograph the interior of homes, or a pilot looking to photograph your co-pilot inside a cramped flight deck, or a tourist looking to fit the Washington monument inside your frame, you’ll need a wide angle lens. 16mm or 28mm for example; the drawback to wide angle lenses is some visual distortion. (There is software that can fix this.) Traditionally, wide angle lenses are not considered ideal for portraits. (Although, we’ll discuss this later in “first learn the rules, then learn to break them.”)

Most portrait photographers start out at 105mm, partly because there is no unflattering distortion to facial features and partly because of this:

bokeh
noun Photography
the visual quality of the out-of-focus areas of a photographic image, especially as rendered by a particular lens : a quick, visual survey of the foreground and background bokeh of a variety of lenses.
ORIGIN from Japanese.

If you didn’t know what bokeh was but you know you love images where people are sharp and the background is really, really blurry, then you know you should be looking for a telephoto lens.

There are some instances in which you can’t physically move closer to your subject, so you need a lens that can take you there. If you are looking to photograph your son on the football field, or your daughter at a swim meet, or the Pope who drives by in the Pope Mobile while you stand behind police barricades from 25 feet away, you’ll need a telephoto lens to magnify the image you want to shoot. Examples of telephoto focal lengths are 85mm, 105mm, 200mm, 400mm. The numbers go even bigger than that — Sports Illustrated photographers use behemoths from the sidelines. If I were out shooting 300lb. men trying to kill one another, I’d use behemoth too, as my favored 28-70mm would put me right in the crossfire trying to get an image (if I didn’t get thrown out of the game first for walking onto the field.) Here’s an example of shooting from the edge of the pool with my 70-200mm telephoto zoom, first at 70 and then at 200:

Okay okay okay, I get it you say. I understand focal length. Next lesson.

But wait — one more thing. There a really, really important exposure equation that relates to focal length, and if you don’t learn it straight-up, you’ll spend a lot of money on a telephoto lens and come back asking why your pictures are consistently blurry. Here’s the magic equation to keep in mind:

Your shutter speed must be equal to or faster than your focal length to avoid camera shake. What does this mean? If you are hand-holding your camera with a 200mm telephoto lens, you must use a shutter speed of 1/200 or faster to avoid blurry images. Handholding a 400mm lens? Shutter speed must be 1/400 or faster. Handholding a 50mm lens? Shutter speed must be 1/50 or faster. Of course, you can use a tripod or a monopod to keep your camera still to compensate for this law (notice the Sports Illustrated photogs on the sidelines with their monopods, partly for this reason.) This might not make sense right away, but when we get to shutter speed it’ll start to click (pun intended?) For now, just remember this:

Focal length affects shutter speed choices.

Buying tips: Remember, you can’t put a Canon lens on a Nikon body or a Pentax lens on a Sony body. Lens mounts have electronic connectors that relay information from body to lens. There are some third party lenses that are built specifically for Nikon or Canon bodies, Sigma lenses, for example. But again, you can’t put a Sigma lens for Nikon mount on a Canon body. You can save money on third party lenses but be sure to do your research to make sure your lens is compatible with your camera body.

Some digital SLRs have smaller sensors than others. Check your manual. If your digital SLR has a “full frame” sensor, then the focal length of your lens will be equal to the focal length number printed on the lens and you can skip to the next paragraph. (85mm = 85mm on a full frame body, for example.) I wasn’t always shooting with a full frame digital SLR, and the way I got around this annoyance was to multiply the focal length of the lens by 1.5 to determine roughly how a lens would perform on my camera body. So my 85mm prime lens suddenly became equal to 127.5mm, which means my 200mm was suddenly 300mm. This is GREAT if you’re looking to make telephoto lenses more telephoto without spending money! However, this same law can prove bothersome if you work a lot in the fisheye or wide angle range. Some manufacturers make digital lenses designed specifically for digital bodies that are not full-frame. I don’t personally own any of these lenses, so I can’t show you any pictures from them.

Okay, so now you have some rough guidelines for choosing the focal length of your lens. You know you need a roughly wide angle/normal/telephoto lens for the kind of images you want to shoot, and you might try to find a zoom that could kill two birds with one stone. You start looking for lenses in respective categories. And now you’re confused again, because after the focal length numbers, there are ADDITIONAL numbers stamped on each lens, and you don’t know what they mean other than to make some of the lenses ghastly more expensive than others even though the focal length is exactly the same. What gives?