Aperture, also known as f-stop, is one of THE primary creative tools in photography. The wider the lens is open, the more light gets through, but that is not the critical factor — the critical factor is how depth of field changes when you change aperture.
Bottom Line: If you want to learn the craft of photography, you can NOT spend enough time working with aperture and depth of field — and how it is affected by focal length and distance.
Confusion #1: Sadly, the original math behind the optics dictated that the WIDER the aperture, the SMALLER the f-stop number — this has confused photographers since 1843, and still causes confusion when someone asks for a larger aperture: Does that mean larger number or wider open lens?
Confusion #2: While smaller apertures (higher numbers) ALWAYS increase the depth of field, the effect is vastly more obvious with longer lenses than with wide angles. So you can't generalize about what aperture and depth of field without reference to focal length.
Confusion #3: While depth of field always increases as we go from wider (f2) to smaller (f16) apertures, and this is true for ALL lenses under all conditions, the SHARPNESS of the image actually decreases a bit at very small aperture openings, like f22, f32, and so forth. This happens because at tiny aperture sizes light acts very weird and diffracts so the image softens, all over.
Confusion #4: Many complex, modern multi-element zoom lenses are actually less sharp wide open than at a few stops down from wide open, particularly at the edges. This is not a depth of field issue — it is just that the lenses are pushing the envelope of technology so far that at wide open they are often just not sharp. Stopping down just one or two f-stops will usually make the image vastly sharper, as well as bring in that extra depth of field.
I highly recommend goofing around with a depth of field calculator — is often really surprising what the critical depth of field really is for a given camera, lens, aperture, and distance. Here is a nice online calculator.
Left
Right
At f1.2 with this Canon 85mm lens, only the edge of Nicole's eye, and some of her cheek are in focus. It takes a bit of skill to learn to work with a lens as fast as a 1.2, and when I shot this, I didn't realize how tricky it was. I thought I focused on the pupil of the eye, but wasn't that accurate.
The depth of field calculator says: Near focus limit = 29.9 in Far focus limit = 30.1 in Total depth of field = 0.2 in (not very much)
At f1.8 there is much more in focus — note that I skipped over f1.4: the difference was pretty subtle.
The depth of field calculator says: Near focus limit = 29.9 in Far focus limit = 30.2 in Total depth of field = 0.3 in (still not very much)
At f2 the whole face is pretty much in focus, and nicely isolated from the out-of-focus background. This brings up an important point: there is a tendency to shoot either wide open, for that narrow-depth-of-field look, or stopped all the way down, for the deep-focus look. But this approach, which I am often guilty of, is very lazy. Usually there is an f-stop that does the best job of isolating the subject from the background and it is usually not quite wide open: you need to use a calculator or depth of field preview to see what you are getting.
The depth of field calculator says: Near focus limit = 29.8 in Far focus limit = 30.2 in Total depth of field = 0.34 in (still not very much)
At f2.8 there is more depth of field than f2, but the background is starting to come into focus in a way that might be too much, depending on how big the photo is viewed, plus the subject matter.
The depth of field calculator says: Near focus limit = 29.8 in Far focus limit = 30.2 in Total depth of field = 0.48 in (wow, ALMOST a half inch!)
At f4 the calculator sez we have just over one half inch of depth of field, but at this size it looks a bit deeper.
The depth of field calculator says: Near focus limit = 29.8 in Far focus limit = 30.3 in Total depth of field = 0.67 in (wow, OVER a half inch!)
At f5.6 the in-focus parts are probably at their sharpest because many lense are "happiest" at f5.6.
The depth of field calculator says: Near focus limit = 29.5 in Far focus limit = 30.5 in Total depth of field = 0.95 in (almost a whole freaking inch!)
At f8 her whole face is in focus, but the impression of focus isolation from the background is not as strong as at wider apertures.
The depth of field calculator says: Near focus limit = 29.3 in Far focus limit = 30.7 in Total depth of field = 1.35 in (well over an inch, or about from check to eye)
At f16 it feels like everything is in focus, but only compared to the wider apertures — see what the calculator sez:
The depth of field calculator says: Near focus limit = 28.7 in Far focus limit = 31.4 in Total depth of field = 2.7 in (2.7 inches is not nearly enough to get from Nicole's eye all the way to the background, which looks sharper than in the other shots, but is not really in focus.)
Note that the Canon 85mm f1.2 lens does not stop down to a smaller aperture than f16!
At f1.2 with this Canon 85mm lens, only the edge of Nicole's eye, and some of her cheek are in focus. It takes a bit of skill to learn to work with a lens as fast as a 1.2, and when I shot this, I didn't realize how tricky it was. I thought I focused on the pupil of the eye, but wasn't that accurate.
The depth of field calculator says: Near focus limit = 29.9 in Far focus limit = 30.1 in Total depth of field = 0.2 in (not very much)
At f1.8 there is much more in focus — note that I skipped over f1.4: the difference was pretty subtle.
The depth of field calculator says: Near focus limit = 29.9 in Far focus limit = 30.2 in Total depth of field = 0.3 in (still not very much)
At f2 the whole face is pretty much in focus, and nicely isolated from the out-of-focus background. This brings up an important point: there is a tendency to shoot either wide open, for that narrow-depth-of-field look, or stopped all the way down, for the deep-focus look. But this approach, which I am often guilty of, is very lazy. Usually there is an f-stop that does the best job of isolating the subject from the background and it is usually not quite wide open: you need to use a calculator or depth of field preview to see what you are getting.
The depth of field calculator says: Near focus limit = 29.8 in Far focus limit = 30.2 in Total depth of field = 0.34 in (still not very much)
At f2.8 there is more depth of field than f2, but the background is starting to come into focus in a way that might be too much, depending on how big the photo is viewed, plus the subject matter.
The depth of field calculator says: Near focus limit = 29.8 in Far focus limit = 30.2 in Total depth of field = 0.48 in (wow, ALMOST a half inch!)
At f4 the calculator sez we have just over one half inch of depth of field, but at this size it looks a bit deeper.
The depth of field calculator says: Near focus limit = 29.8 in Far focus limit = 30.3 in Total depth of field = 0.67 in (wow, OVER a half inch!)
At f5.6 the in-focus parts are probably at their sharpest because many lense are "happiest" at f5.6.
The depth of field calculator says: Near focus limit = 29.5 in Far focus limit = 30.5 in Total depth of field = 0.95 in (almost a whole freaking inch!)
At f8 her whole face is in focus, but the impression of focus isolation from the background is not as strong as at wider apertures.
The depth of field calculator says: Near focus limit = 29.3 in Far focus limit = 30.7 in Total depth of field = 1.35 in (well over an inch, or about from check to eye)
At f16 it feels like everything is in focus, but only compared to the wider apertures — see what the calculator sez:
The depth of field calculator says: Near focus limit = 28.7 in Far focus limit = 31.4 in Total depth of field = 2.7 in (2.7 inches is not nearly enough to get from Nicole's eye all the way to the background, which looks sharper than in the other shots, but is not really in focus.)
Note that the Canon 85mm f1.2 lens does not stop down to a smaller aperture than f16!
