F4 vs F22

[dan_shoe]

Is a perfectly focused subject sharper at F4 or F22? I have heared F22

is always sharper or just as sharp as F4, in addition to being deeper.

But I have also heard that in the center of its focal range, F4 is

sharper. Here is one such opinion cut from another thread...

 

Mark James , apr 15, 2006; 11:03 a.m.

Something I didn't see above is that the two cameras you mentioned

place somewhat different demands on the leses used. The 30D has

smaller pixels, and thus needs sharper glass to maximize image

quality. But, when using lenses designed for full frame it uses the

center of the frame, so it uses the "sweet spot" of the lens. This

generally gives better uniformity across the resulting image. The 5D

has larger pixels than the 30D, but they are spread out over a larger

area, which really taxes lenses at the edge of the frame. All of this

is dependent on how you use the camera and lenses. I suspect you

already know that if the lenses are used at f22, then sharpness will

be about equally bad for any lens because of diffraction.

 

What is correct - 2nd opinions? Thanks -Dan Shoe

[oceanphysics]

There's no theoretically correct answer -- it depends on the lens. For a high-quality telephoto prime, there is little doubt that f/4 would be sharper anywhere in the frame. For most other lenses I'd go with f/4 as well, except possibly at the edges. Diffraction does eat into sharpness at f/22.

[adam rosser]

For a 1.6 crop camera like the 30D diffraction starts to have serious effects by about f/16. A

properly focussed shot at f/4 should be noticeably sharper.

 

Bob Atkins has an article about this issue, but I couldn't find it quickly. Do a search over at

www.bobatkins.com.

[neild]

As I understand things (could be mistaken), the "best" aperture varies with format size. For instance, with 35mm film f/16 and smaller is getting into diffraction, with more diffraction effects the smaller the aperture becomes. DOF increases, but diffraction gives the effect of an overall 'softening' of the whole image (look at pinhole photos - they have lots of diffraction softening).

 

OTOH, large format photographers routinely employ f/64 in their work with apparently no problems. Not sure what camera you have, but if it's a 1.6x crop camera like the 20D then your diffraction limit will be even less than 35mm film because the format is smaller than 35mm film - that is, you might see diffraction effects by f/11.

 

So - to answer your question - f/4 would be better than f/22 I would expect.

[bobatkins]

You might want to read this:

 

http://www.bobatkins.com/photography/technical/diffraction.html

 

A really good lens (like a fast prime) should be better at f4, but some lenses might not be very different and some lenses (like inexpensive, slow, consumer telephoto zooms at their longest focal length) could be better at f22.

[robert_hall2]

Would Mr. Atkins tell us where he got the test target for his first set of tests in the link? Thanks!

[dave_trayers]

Bob,

 

I read your article on diffraction and have a question... is the limit based on the f-stop of the aperture or the actual physical size of the opening? As the focal length increases the corresponding physical open increases for a given f-stop. I would think that the actual phyical diaphram opening is what governs the diffraction limit, so would diffraction be more of a problem at shorter focal lengths than longer?

[mark_ryan2]

Everyone knows an F22 is better than an F4. The F4 was used in Vietnam and the F22 is 21st Century, baby! F16 is a good middle ground.

 

Too much Military Channel on Cable?

[luis alvarez torres]

F8 and your there!

[gary_jean1]

"F8 and your (sic) there!"

 

Until they add the teleportation feature, I still have to get myself there.

 

"f/8 and be there!"

[lester_wareham]

My 50/1.4 is sharpest at f5.6 and is much sharper at f4 than f22. The 10-22 seems to be sharpest between f5.6-f11 after which it drops off, but not as much.

So it depends on the lens, f5.6-f8 is normall a good compromise.