Silly CV 21/4 question?

[Rafael]

I have what may be a silly question about the CV 21/4: Why does it have a minimum aperture of f/22? The minimum focusing distance of the lens is 0.5m and at f/16 the depth of field is already 0.5m - infinity (f/22 does not even appear on the depth of field scale). So going from f/16 to f/22 would not affect the depth of field at all. It seems that f/22 on this lens is really just the equivalent of f/16 with a 1-stop neutral density filter. Is that it?

 

[BJ Bignell]

I never thought about it, 'cause I rarely use f22, but I suppose you could look at it that way. However, if you use f22 at closest focus, you will get more depth of field up close than f16, getting you down to something really crazy (like 0.2 meters?). I've used the lens like this for a few shots, and the technique has its possibilities.

 

[erikhaugsby]

That's it, as far as I know.

 

[fgianni]

Don't you start to get a visible diffraction effect at f22 with such a wide lens?

 

[BJ Bignell]

Yes, there would be some diffraction, but not too severe. Besides, it's not all about sharpness, is it? Oh my god, maybe it is all about sharpness?

 

[VinceC]

There are shooting situations where you might want a slower shutter speed, and f/22 gives you an extra stop. For example, airplane propellers and helicopter blades look better blurred. It can also helpful for fill-flash situations. Heck, several of my old telephoto lenses go down to f/32, and I've used them at those speeds for the above-stated reasons.

 

[Ronald M]

Diffraction starts setting at the 1/4 the focal length in F stops. That makes it about 5.2. A 50 mm starts about 11. I`m not saying the image is trash smaller than that, but the effects get worse as it is closed more.

I have an 50mm elmar that does really well at 22 and others that are newer not so well. It depends on the lens.

It also depends on the physical restrictions of the closing mechanisms being able to close that much.

 

[fgianni]

Heck, several of my old telephoto lenses go down to f/32.

I don't expect an 80mm or longer lens to have any significant diffraction problem at f32.

Diffraction is inversely proportional to the physical diameter of the aperture, not to the f number, so in terms of f number a wide angle lens will start to show softness due to diffraction much earlier than a telephoto.

I can see visible softness on my CV 12mm past f8, and probably even at its widest (f5.6) some diffraction may be observed under the right condition.

 

[Rafael]

Don't mean to steal your thread, Rafael

You're not stealing my thread at all. This is a very useful discussion.

 

[rxmd]

http://www.erik-krause.de/schaerfe.htm says that at f/22 and distance setting 0.5 everything from 30.8 cm to 130 cm will be sharp (diffraction already accounted for; without accounting for diffraction the distances are 29.1 cm to 177 cm!), and that at hyperfocal distance 79.2 cm everything from 39.6 cm to infinity will be in focus.

 

[VinceC]

>>I don't expect an 80mm or longer lens to have any significant diffraction problem at f32.<<

That's good to know.

 

[Rafael]

Nice avatar, Rafael

Thanks, I figured it was about time I uploaded one!

The color of the velvet shows it is a "she" 😀

Well, you know...:angel:

 

[Fred]

The 21 is a good lens, I tend to use it at f8 to f11 most of the time, huge DOF at those apertures anyway, its even RF coupled although I tend not to use it that way preferring hyperfocal measurement.

 

[Dougg]

Diffraction is inversely proportional to the physical diameter of the aperture, not to the f number, so in terms of f number a wide angle lens will start to show softness due to diffraction much earlier than a telephoto.

This is my understanding, too, recalling from a long-ago reading of a photographic optics text. 🙂

It's the edges of the diaphragm that do it, and the smaller the hole, the larger the proportion of light that is affected by edge effects, scattering the rays. With a long focal length lens such as for a large-format camera, the opening is still pretty large even at f/32, so diffraction isn't as noticeable. The f-number is a relative aperture, might be useful to keep in mind; it's the focal length divided by the aperture diameter. So f/2 in a 50mm lens is a 25mm diameter aperture, but has to be 50mm in diameter for a 100mm lens. And f/22 in a 22mm lens would mean a 1mm aperture, small enough for quite noticeable loss of sharpness due to diffraction. Still, you might choose this anyway if depth-of-field is more important, for instance.

All lenses become limited in sharpness by diffraction at some aperture; The lens sharpens as you stop down until its ability to render fine detail is limited by increasing diffraction effects. Some call this cross-over point the lens's "sweet spot", where it's as sharp as it's capable.

 

[kully]

Are there any other factors, such as aperature blade smoothness or something?

I ask because I used to have a Pentax-M 50mm,f/1,7 and the same design made by Centon (Seagull, I think). The Centon would be noticeably worse with aperatures f/11 and f/16 - and I don't pixel peep or loupe.

 

[fgianni]

Are there any other factors, such as aperature blade smoothness or something?

I ask because I used to have a Pentax-M 50mm,f/1,7 and the same design made by Centon (Seagull, I think). The Centon would be noticeably worse with aperatures f/11 and f/16 - and I don't pixel peep or loupe.

I wouldn't expect a 50mm lens to show any significant diffraction problem at those apertures. But don't quote me on this 🙂