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Old Feb 9, 2009, 9:39 PM   #1
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so could someone shed a little light . . .
shouldn't a lens be its sharpest at the smallest diaphragm (say f22)?
especially a good wide angle that has a deeper DOF.
if f11 is the sharpest for a lens, why even have f22?
you might get more dof but it will be softer throughout, right?

most confusing to me.

thanks for your considered answer.

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Old Feb 10, 2009, 3:54 AM   #2
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I notice you posted the same question in another running thread but to save taking over that persons thread I will answer here.

For a pretty detailed answer check outhttp://www.bobatkins.com/photography...ffraction.html which gives theory and examples.

The reason you might want to go past the optimum setting is to get more dof or a slower shutter speed.
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Old Feb 10, 2009, 4:14 AM   #3
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ohillary says:

so could someone shed a little light . . .
shouldn't a lens be its sharpest at the smallest diaphragm (say f22)?
especially a good wide angle that has a deeper DOF.
if f11 is the sharpest for a lens, why even have f22?
you might get more dof but it will be softer throughout, right?

most confusing to me.

Back in days of yore, when all I had to shoot with was a Minolta SRT-102 film camera, it was conventional wisdom that you shot at the lenses smallest aperture for the sharpest image. Then, somewhere down the line, this advice changed to what it is today due to a physical phenomenon called light diffraction which occurs when light scatters slightly as it passes through the aperture blades at very small apertures.

I'm not up on the technicalities of the situation, so I don't know when this became recognized as a problem. Maybe lens testing facilities weren't good enough to notice this small aperture diffraction back then...

This leads me to wonder just how much of a problem this is for people using a lens outside of an optical testing facility. I mean, could the average photographer, with all of the other factors that influence image sharpness in the real world, tell the difference in resolution between a shot made at f/8 or f/11 and one made at f/22 from the same equipment? You've got me interested to the point where I might have to investigate this further.

For the sake of argument, even if there was enough of a difference to be discernible to the eye, if lenses were limited to f/11 at the small end, this would reduce the exposure options and available depth of field to a level that would prove far more annoying to a photographer than the tiny advantage in optical resolution would be worth.

To properly expose very bright scenes, you would have to have a camera with a couple stops faster shutter speed than you'd need with a lens of smaller aperture -- or you'd have to routinely use a neutral density filter that would likely impact the optical resolution more than light diffraction around a smaller aperture would.

The disadvantages of a restricted depth of field are obvious.

The fact that lenses are not made with this restriction and that fantastic photographs are routinely made at apertures smaller than f/11 verifies that photographers feel that the benefits of smaller apertures outweigh any real world disadvantages.

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Old Feb 10, 2009, 7:06 AM   #4
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No, a lens shouldn't be its sharpest at its smallest aperture. In fact, the opposite is true. It _should_ be its sharpest wide open.

Unfortunately, nothing manmade is perfect and most flaws in design and execution, along with any design compromises, show more the wider you open a lens. In practice, most modern small format (35mmFF/FX to APS-c/DX) camera lenses perform their best one to three stops from maximum. Beyond that, sharpness decreases.
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Old Feb 10, 2009, 8:19 AM   #5
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I have been able to see the difference in sharpness due to aperture selection. Most are sharpest at f 8 - 11 and it seems to be as high as f16 on my D700. But I would rather have more in focus (DOF) and softer overall than tack sharp only at the focused point for many of my pictures. This is especially true for Macro where DOF is far more important to me than ultimate sharpness.
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Old Feb 10, 2009, 9:40 AM   #6
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thanks for your responses and the link.
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Old Feb 10, 2009, 10:23 AM   #7
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ReneB3 wrote:
Quote:
I have been able to see the difference in sharpness due to aperture selection. Most are sharpest at f 8 - 11 and it seems to be as high as f16 on my D700. But I would rather have more in focus (DOF) and softer overall than tack sharp only at the focused point for many of my pictures. This is especially true for Macro where DOF is far more important to me than ultimate sharpness.
That is what would be expected with a FF over APS-C. If you look at the table half way down the link I posted you can see how it is affected.
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Old Feb 10, 2009, 12:51 PM   #8
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I guess my real point is that you can see this effect in your photos. Unlike most technical articles about gear this is one you don't need charts and graphs to measure. It just jumps out at you.
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Old Feb 10, 2009, 12:52 PM   #9
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Ah, yes you certainly do.
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Old Feb 16, 2009, 9:31 PM   #10
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The analogy that Bob Atkins gave about water and the hose is not really correct. However, the rest of his article is good.

Diffraction has to do with the wavelength and the aperture (hole) size. Suppose you throw a big rock into a pond. The waves spread out in a circle around where the rock hit the water.But at the far end of a very big pond, the waves are almost straight lines(between each wave), because the circle the waves make are so big.

Let's say the wave length of those waves are 30cm (1 foot). You place a couple of boards across the end of the pond, with a 305cm (10 feet) gap. What happens? The wave continues on at pretty much straight lines, except right near the edges where the waves bend slightly "around the corner".

If you then narrow the gap to 30cm (1 foot, the same as the wavelength of the waves),the wave pattern the other side of the two boards is a circle, because the "disturbance" is a similar size to the wavelength.

Suppose we make the gap 61cm(2 feet). On the other side, it will be a little bit straight, but have a fair bit of circle around the edges. This is like a lens that has been stopped down to F32. It's not quite straightand it's not circular, but the edges no longer respresent the orginal wave at the edges, so it becomes "fuzzy".

This is why smaller apertures give you images that are less sharp, because the proportion of light that "bends" around the edges of the aperture blades compared to the unbent light, is higher.




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