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04-18-2011, 08:09 AM   #1
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Optimal aperture for lens

Hey everyone,


So I understand what the "optimal aperture" is, my question is other than simple test shooting, how do you find it? If I remember correctly I read somewhere on here that there is a setting in the camera that will go to the optimal aperture as programmed into the lens by the manufacturer. Is that correct?

Thanks,

04-18-2011, 08:16 AM   #2
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Yes, it's "MTF" option in program line selection. I thought it just stops the lens down 1 stop.
04-18-2011, 08:20 AM   #3
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"Optimal aperture" is based on sharpness, and is usually about 2 stops down from maximum aperture. However, that may not be the best aperture to use for the shot itself for depth of focus reasons.
04-18-2011, 08:31 AM   #4
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The P mode on the Kx...
But it's like the Auto mode but it lets you make changes with the wheel. P mode usualy stops down the lens and unlike full auto mode you can modify lots of setting both on menu and by moving the wheel.
(i havent used P much..so i may be wrong...i shoot either M,Av or Tv depending on the needs).

04-18-2011, 09:30 AM   #5
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QuoteOriginally posted by SpecialK Quote
"Optimal aperture" is based on sharpness, and is usually about 2 stops down from maximum aperture. However, that may not be the best aperture to use for the shot itself for depth of focus reasons.

Thanks, yeah I understand the whole DOF thing, from what I've been reading now I should be looking at the MTF graphs for my lenses.
04-18-2011, 09:37 AM   #6
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According to the K5 manual, MTF Priority in the Program Line does indeed prioritize the best aperture setting for any DA, DA-L, DFA, FA-J, or FA lens. Would be interesting to go through all your lenses and see what Pentax thinks is the optimal aperture vs. your own observations.
04-18-2011, 09:45 AM   #7
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You're getting a variety of answers because some cameras allow control over this, and some don't. On your camera, P mode is your only choice. The camera will get information from the lens, look at the light and give you its best guess for aperture.

On the K-7, I can tell the camera more about what I want to do, by choosing the Program Line. MTF Priority tries for maximum sharpness, there's a Deep or Shallow DOF priority for the depth of field, Hi-speed Priority will choose the widest apertures, and there are two auto settings. The upper-tier cameras all have some kind of Program Line control like this, with different options.

Manual-focus lenses can't tell the camera much, and non-Pentax lenses may not communicate all of the data that a Pentax lens can. The camera will use the focal length information you input, especially for shutter speed. It tries to give you shutter speeds that minimize camera shake.

You can look at test data for newer lenses. With older lenses, test yourself or just guess. No lens is at its best wide open. A good starting point is two stops down from wide open. More expensive lenses are probably good just one stop down. Dedicated macro lenses are often so well-corrected that any aperture is fine, as long as you have enough depth of field. I had a 28-300mm superzoom once that was OK at f11. Along with sharpness, colors, contrast and vignetting improve one or two stops from wide open.
04-18-2011, 01:43 PM   #8
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Yep, though, there's that MTF program line: it's the one my camera's set to all the time cause it's not only going for the sweet spots of a given lens, but usually about the closest to the exposure I'd choose unless it's very bright. Cause we have this 'Hyperprogram' thing going on, it's just a matter of turning a dial to shift things around with a K20d, though.

For me, that's usually I just up the aperture dial a few stops: if it's bright and I'm in Program, I probably can't see too well, so more DOF is usually good. Other than that, it's a good program line, though, if you're steady when it's *not* very bright. (Me, I'm like, 'Wouldn't it be nice to be able to *program* the program line? )

Still, if you have that MTF option, it'll give you a good opinion on what your lens' sharpest aperture is, cause it'll tend to want to be there.

Once you know, you can use aperture priority.


(Cheat sheet, though: FA 50s are real nice from f4-f13: plenty of room there: I like to stay past 2.4 in general, and, well, it's a good lens. The kit lens, to my experience, is happiest between f8 and f11 and near the middle of its range. )

04-25-2011, 12:45 PM   #9
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QuoteOriginally posted by DogLover Quote
According to the K5 manual, MTF Priority in the Program Line does indeed prioritize the best aperture setting for any DA, DA-L, DFA, FA-J, or FA lens. Would be interesting to go through all your lenses and see what Pentax thinks is the optimal aperture vs. your own observations.
With the K-7 MTF Priority seems to get it about right most of the time (10-17 fisheye, 18-55 WR and 50-200 WR), but with the DA 35mm F2.8 Macro Limited it insists on f4. Annoying
04-25-2011, 02:49 PM   #10
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Yes, the camera's teensy weensy little brain will algorithmically select an aperture that Pentax engineers think is optimal. But do you have other goals? Can you outsmart the camera? Is an algorithmic 'best' what YOU want the picture to look like? What are YOUR constraints?

You probably know the rule: an APS-C camera's diffraction limit is around f/9, so you shouldn't stop down beyond there or you'll get horrible fuzziness and bad karma. WRONG. Diffraction effects are noticeable with tripod+mirror-lock-up+timer, pixel-peeped shots. In real-world application, with shaky photographers and subjects, it's not noticeable -- any camera movement swamps-out the diffraction effects.

So the 'optimal' aperture is the one that gives the effect you want, that produces the image you want. Maybe that's f/1.2 or f/22 or whatever. It depends on the DOF desired, the overall sharpness desired, the trade-off with shutter speed for capturing motion, all that fluffy stuff. Every photo is an equation to be solved, a function of aperture, speed, ISO, focal length, shooting distance, light, background, subject, attitude, etc. An 'optimal' aperture may be a minimax solution, or an extreme, or quite irrelevant. How to find what works best? Experiment!
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