F-Stop 'N Go

At long last, the promised post on “f-stops,” but first: a quick multiple-choice quiz to see whether another technical post like this one is really necessary.

f-stop means:

(a) a term used to describe a situation when one is tempted to say a very bad word but doesn’t;

(b) a traffic maneuver typical of Utah drivers in which the driver does not bring the vehicle to a complete stop at a stop sign, but rather slows down a little and then rolls right on through (as in “that f-dude executed a perfect f-stop.”);

(c) the ratio of the focal length of a lens or lens system to the effective diameter of its aperture;

(d) a measure of the air speed velocity of an unladen swallow.

The answer, of course, is “d,” and, if you selected any other answer, you’d better read on lest you disgrace yourself as a photographer … ;0)

Let’s make this easy: it really doesn’t matter what f-stop means from a technical standpoint. Really. Ratios be danged. We do need to understand, however, what f-stop values mean for a particular photograph.

And what do those values mean as a practical matter?

- The lower the f-stop value, the wider the aperture.
- The higher the f-stop value, the more narrow the aperture.
- A wide aperture = a narrow depth-of-field.
- A narrow aperture = a wide depth-of-field.
- A wide aperture allows faster shutter speeds.
- A narrow aperture requires slower (longer) shutter speeds.
- The lower the maximum f-stop value, the better the lens.

Let’s unpack each of those in turn. Recall from the “’M’ is for Manual’” post that f-stop measures the size of the aperture (or hole) through which light passes to expose a photograph. For some mathematical ratio yukitty-yuk reason we needn’t concern ourselves with, the smaller the f-stop value, the wider the aperture. So, for example, f2.8 means shooting “wide open” (that’s a big aperture, relatively speaking) while f32 is really “stopped down,” in other words, a really, really small hole.

Why do we care? For two reasons: first, the size of the aperture determines the depth of field. Narrow aperture = wide depth of field, and vice versa. Second, the size of the aperture also affects the shutter speeds we can get away with.

With that in mind, consider two photography “challenges” and how to address them using what we now know about f-stops and shutter speeds.

Situation #1: Late evening—the sun’s just gone down—and the kids are playing in the yard. You want to photograph them without using a nasty flash.

We know the light is low here, so we have to do everything possible (short of using that nasty flash) to get as much light as possible. So, first, look for a place in the yard with more light: where, for example, the glow from the sun still reflects from the sky into the yard and onto your subject. Second, set the film speed high (say, 800) or on automatic. Lastly, set the f-stop low to get a wide aperture and more light into the camera. What does that mean for depth-of-field? Well, it’s going to be shallow, but this is portrait photography so that’s a good thing: generally we only care if the subject remains in focus. What does it mean for shutter speed? Well, the wider we can get that aperture, the faster shutter speeds we can use to freeze motion.

Here’s where we start to see why lenses with a lower maximum f-stop value perform better (and, not coincidentally, often cost more). A cheap zoom lens, for example, may, depending on focal length, have a maximum aperture of 5.6, which isn’t all that wide. In low light, that means that the photographer has to use a slower shutter speed to compensate for the smaller aperture, and slower shutter speeds mean movement—blur—because the camera shutter is open longer and that records everything from the moving kid to your shaky hands. Result? Bad, blurry photo (or, you have to use flash, which is almost as bad).

Compare that photographer to my sister-in-law, Jenny, who took my advice and spent $80 to buy a 50mm fixed lens with a maximum aperture of f1.8--the lens used to take the low light, hand-held shot of my niece at the top of this post. That lens will take infinitely better pictures in that late evening or other low-light situations than a lens with a maximum aperture of f5.6 because it can capture a lot more light through that wider aperture and, as a result, allows her to get away with much faster shutter speeds.

Situation #2 – Early morning, Zion’s National Park, and you want to photograph the blooming cactus and the Great White Throne at the same time.

Here again: a low light situation, and one where we need a wide depth-of-field, as wide as we can get it so that both the cactus and the Great White Throne remain in focus. So, you stop down the camera by setting the lens at or close to its smallest possible aperture: say, f32. That creates an incredible depth of field, so everything’s cool, right? Not quite. With that itty bitty hole, how long do you think we have to leave open the shutter to get enough light to expose a photograph properly. A loooong time. And if we do so, what happens with movement: yep, records it. And what does that mean? Blur. So, now you know why any self respecting landscape photographer will have that camera mounted on a big, heavy tripod: to hold it still.

Check out this landscape by Scottish photographer Simon Butterworth: http://photo.net/photodb/photo?photo_id=8371957. Lovely, isn’t it? Under the “details” tab, you’ll see the following shorthand: Canon 1DS MKIII 16-35mm 16s f16, which, translated, means the photograph was taken with a Canon 1-DS Mark III camera, a 16-35mm lens (wide angle—remember?), with a shutter speed of 16 seconds, and an aperture setting of 16 (the “f” in “f16” is shorthand for f-stop).

Beyond knowing the settings the photographer used, it’s useful to think about why he chose those settings. He has a photo with water, a stone, a mountain, and the moon—all in one frame—so he needs a wide depth of field to keep everything in focus front to back: hence, he chose an f-stop of 16 (pretty small). What did that mean for his exposure time given the little amount of available light? A very looooong exposure time of, count ‘em, 16 seconds. (To put that in perspective, 99.9% of photographs are taken in exposures measured in hundredths of a second.) Could he have set his f-stop to 32 instead for an even greater depth of field? Sure, but that would’ve meant an even slower shutter speed, with greater risk of movement (even wind creates problems at long exposures) and more “noise” generated by his camera’s built in sensor. (He could get away with 16—as opposed to 32—here because he doesn’t have anything immediately in front of him that needs to remain in focus.)

See how a photographer has to make conscious decisions about f-stop settings and how the available light and desired final image affect those decisions?

That, my friends, is why we need to understand f-stops. Leave it up to the camera, and the camera will make compromises to get a proper exposure—compromises you may not like.