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03-19-2012, 02:00 PM   #1
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Relationship between aperture and zoom

Although I am relatively new to digital photography, I do understand all the basic principles and am trying to become as proficient as possible, at least from a hobby standpoint.

When experimenting with indoor shooting in Av mode using the 18-55mm kit lens that came with my K-x, I notice that if I shoot wide angle at 18mm I can adjust the aperture setting anywhere from f5.6 up to f40. However, if I zoom in to 55mm, my aperture range tops out at f22. So, my technical question of the day is this: Scientifically speaking, why does a change in focal length have an affect on minimum aperture?

Can someone more intelligent than me explain the relationship between aperture and focal length (BTW, I only have a BA degree so keep it simple). I should know the answer to this, but I can't wrap my brain around it for some reason. Thanks.

03-19-2012, 02:30 PM   #2
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Are you sure that the numbers are not the other way around?

At 18mm it ranges from F3.5 to F22, at 55mm from F5.6 to F40. Essentially 1.5 stops are lost on both ends when it is zoomed to 55mm. The kit lens is designed as a variable aperture lens which is cheaper to make. That's why it is labeled as "SMC PENTAX - DA 1:3.5 - 5.6 18-55mm". The first number (3.5) applies to the 18mm end and the second (5.6) to the 55mm end. The minimum aperture (large number) is equally skewed in this design to F22 at 18mm and F40 at 55mm. There are other designs for zoom lenses that keep the aperture fixed regardless of the focal length in the zoom range.

F-stop is Focal Length divided by the diameter of the iris (opening of aperture blades). A given opening will result in a wider (small number) f-stop when applied to a shorter focal length That's why fast telephotos (at F2.8 or F4) have huge front glasses to accommodate the equally huge diameter of the needed iris for that f-stop at the long focal length.
03-19-2012, 03:18 PM   #3
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demp10 nailed it. An f-stop is a ratio. An f/2 aperture has an opening 1/2 the focal length. An f/4 is 1/4 the focal length. Et cetera. (F-stops run in the sequence 1.0--1.4--2.0--2.8--4.0--5.6--etc because of the square root of 2; don't worry about that now.) A fixed maximum-aperture zoom like a 17-50mm f/2.8 adjusts the size of the aperture during zooming to maintain that constant ratio. A floating max-aperture zoom like a 35-70mm f/3.5-4.5 has a fixed aperture size, so the ratio changes as the focal length is zoomed.

The DA18-55/3.5-5.6 is funny, aperture-wise. At the wide end (18mm) its aperture ranges from f/3.5 to f/22. At the long end (55mm) it ranges from f/5.6 to f/40. I have a few other zooms whose minimum apertures float like that, but most top-out at f/22 or f/32 throughout the entire zoom range.

Why the variance? Because many variable-focal-length designs exist and they don't work the same. Some internal-focusing zooms have quite different close-focus distances and magnifications in their zoom range. We get complaints every now and then about being 'cheated' thusly, but that's how optics work, alas.

A few basic rules:

* In any focal range, a fixed-max-aperture zoom is probably better than a floating-max-aperture zoom, and is probably heavier and costlier.
* A push-pull zoom probably has more constant focus distances than an internal-focus zoom, but it's more likely to suffer zoom creep.
* And, most vital for old-lens collectors like me: Most modern zooms are better optically than most older zooms, but exceptions abound.
03-19-2012, 03:51 PM   #4
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Back in the 1960s zoom lenses were constant aperture with zoom. That's because the cameras didn't have through-the-lens metering that would compensate as the aperture changed. Once cameras had automatic exposure with TTL metering they could make zooms smaller and cheaper by letting the effective aperture vary as it zoomed; and since they have gone to autofocus is isn't important to keep focus constant, as the camera focuses just as the shot is taken. With manual focus SLRs it was common to zoom in to fine focus, then zoom back out to frame the shot. That only works if the lens stays in focus while zoomed.

Also, back then there were no "prime lenses." There were lenses, and a few zoom lenses.

03-19-2012, 04:37 PM   #5
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That's why I am always afraid to ask questions, the answers may be too complicated. In reality, though, I understand what everyone is saying. I had never really thought about the issue before now, but when I was playing around with the camera just to experiment with various settings, to see how settings change when one aspect or another is tweaked, it caught my attention for some reason. Now I remember why I hated high school math and physics.
03-19-2012, 05:11 PM   #6
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QuoteOriginally posted by Stardog Quote
That's why I am always afraid to ask questions, the answers may be too complicated.
They only seem complicated at first. When you dig into them, they're even worse. Ay yi yi.

In reality, though, I understand what everyone is saying.
You're doing better than some of us then. Sometimes I don't even understand what *I'm* saying.

Now I remember why I hated high school math and physics.
The problem was the teachers, not the subjects. Too many teachers just don't have the enthusiasm.

But I digress. Photography is a technical field, as well as an arena for artistic expression. Lots of it doesn't make sense without some arithmetic and visualization and exposure to principles of composition. We can mostly skip the calculus and pre-calculus and quantum optics and chaos theory, but a little geometry and trig creeps in unannounced. At least we needn't worry much about chemistry now.

Any technical field is distinguished by its jargon, its specialized language. We learn the field by learning the language. We learn acronyms like FF, MF, DOF, FOV, AOV, YMMV, etc; and common words with specific meanings like normal, register, perspective, long, stop, bayonet, index, fanboi, etc.

PFC (PentaxForumsCom) is an ongoing seminar on photography. I've been shooting for 5+ decades, but I've learned a sh!tload right here in the last few years. And I know I have a sh!tload more to learn. It's an infinite process. Long may it wave.

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