Where is a lens generally the sharpest?

fish

Yeah, I know...it's tough to generalize about lenses, but let's say a given lens of decent quality stops down to f22. While I'm pretty sure max DOF is at f22, is it also true that the lens will be the sharpest there? Or are lenses generally sharper at some aperture in the middle of the range?

Reason I ask, is I've got a wonderful Canon 17-40/4 L that has a tremendous DOF at f22, but it doesn't seem to be fantastically sharp at the infinity side of things there. Would I do better to stay at f16 instead?

Of course it may be the cheapo Hoya filter I had on there at the time...since replaced by a Heliopan SH-PMC as of today (bhphoto RAWKS!).

thanks in advance.

patch29

As a rule of thumb a lens is almost always sharpest at apertures between f8 and f11. Large apertures have too many optical problems, and small apertures too much diffraction.

Source

I pulled the quote from the article "Resolution, contrast, MTF". The other articles will also give a lot of info on lenses.

They have many lens comparisons, not too many newer lenses but it shows which types tend to be better than others. Look under products.

pathfinder

patch29 wrote:

Source

I pulled the quote from the article "Resolution, contrast, MTF". The other articles will also give a lot of info on lenses.

They have many lens comparisons, not too many newer lenses but it shows which types tend to be better than others. Look under products.

Fish - this is a link to a discussion of just this topic - the relationship of a smaller aperature and increased depth of field versus the increasing loss of sharpness due to diffraction effects at small aperatures - eg what is the absolute sharpest aperature - http://www.kenrockwell.com/tech/focus.htm - there is enough information here to keep you busy all day - take a look and tell me what you think.

Pathfinder

hutchman

Fish,

I don't know the answer to your question, but I have one of my own.

I'm looking to buy either the lens you purchased or the 16-35L. Just wondering why you chose the one you did?

Hutch

pathfinder

pathfinder wrote:

Fish - this is a link to a discussion of just this topic - the relationship of a smaller aperature and increased depth of field versus the increasing loss of sharpness due to diffraction effects at small aperatures - eg what is the absolute sharpest aperature - http://www.kenrockwell.com/tech/focus.htm - there is enough information here to keep you busy all day - take a look and tell me what you think.

Pathfinder

Another factor that you didn't state was your camera on a tripod? What was your shuutter speed? Poor Lens sharpness can be confused with camera movement of course.......And likke you alluded - some people prefer not to use filters on ultrawide angle lenses also....

cmr164

pathfinder wrote:

Fish - this is a link to a discussion of just this topic - the relationship of a smaller aperature and increased depth of field versus the increasing loss of sharpness due to diffraction effects at small aperatures - eg what is the absolute sharpest aperature - http://www.kenrockwell.com/tech/focus.htm - there is enough information here to keep you busy all day - take a look and tell me what you think.

Pathfinder

Be a aware that the Ken Rockwell discussion is about view cameras and that makes a huge difference in the usual focal lengths of the lenses and in the effects of every kind of distortion. It is an very interesting article but it also has the possibility of leading folks astray.

The articles at http://www.photodo.com/nav/artindex.html that Patch recommended are much more suitable, particularly the Resolution, contrast, MTF article that he mentions. It is important to realize that diffraction distortion is tied to the absolute physical diameter of the aperture and that the f-number is the ratio of that aperture to the focal length. Thus f8 on a 50mm lens might be optimum but on a digicam with a 8mm-24mm zoom, f8 is so bad that that the lens is not even allowed to stop down that far.

For the beginner, I highly recommend Canon's EF Lens 101 pages. The focal length comparisons, the lens care artcle and the glossary are all worth the visit.

fish

Thanks for the links, kids. I'll dig into it.

Hutch: I bought the 17-40L instead of the 16-35L primarily due to price (the difference is HUGE), but also because the 17-40 has gotten consistently good marks.

Pathfinder: tripod, mirror lockup, remote release, loooooong exposure. I'm going to go back and try some f16 shots.

pathfinder

cmr164 wrote:

Be a aware that the Ken Rockwell discussion is about view cameras and that makes a huge difference in the usual focal lengths of the lenses and in the effects of every kind of distortion. It is an very interesting article but it also has the possibility of leading folks astray.

The articles at http://www.photodo.com/nav/artindex.html that Patch recommended are much more suitable, particularly the Resolution, contrast, MTF article that he mentions. It is important to realize that diffraction distortion is tied to the absolute physical diameter of the aperture and that the f-number is the ratio of that aperture to the focal length. Thus f8 on a 50mm lens might be optimum but on a digicam with a 8mm-24mm zoom, f8 is so bad that that the lens is not even allowed to stop down that far.

For the beginner, I highly recommend Canon's EF Lens 101 pages. The focal length comparisons, the lens care artcle and the glossary are all worth the visit.

Yes the Ken Rockwell article primarily refers to view cameras because those cameras are used by individuals already doing everything they can for maximum sharpness - eg tripods, braces, high shutter speeds if possible etc and the large format lenses move further from the film in focusing from near to far - But he does include a table for 35 mm camera lenses as well as medium format and large format.
He specifically says ""[font=Georgia, Times New Roman, Times, serif]This article is for the demanding and accomplished photographic artist who posses considerable technical and visual skill. [/font][font=Georgia, Times New Roman, Times, serif]If you are a beginner or just shooting a 35mm camera then this article really has nothing for you. Just use a tripod and choose the smallest aperture you have if you need depth of field." He even places the article at the very bottom of his page of articles under the heading of "Advanced Techniques." [/font]
[font=Georgia, Times New Roman, Times, serif] If you read several of Rockwell's articles you realize he is VERY assertive and overstates things sometimes for emphasis or sarcasm OR both. But he does have interesting things to say, even if I do not always agree with him. But I do agree that the sharpest aperature is not the smallest.[/font]
[font=Georgia, Times New Roman, Times, serif] With very wide angle lenses for 35mm cameras do you really need to stop down much below f11 or f16 due to their extreme depth of field available anway?[/font]

In regards to to digicam zooms - I do not understand the significance of this as Fish's question was specifically about the 17-40 L Canon lens which is not used on digicams to my knowledge.... Digicams zooms do not even have a depth of field markings anywhere on them - not do most 35mm lenses anymore - altho most 35mm manual focus primes always did.

The articles you referenced on Photo.do are excellent and should be perused by all photograhers who desire to improve their work. Thank you.

pathfinder

fish wrote:

Thanks for the links, kids. I'll dig into it.

Hutch: I bought the 17-40L instead of the 16-35L primarily due to price (the difference is HUGE), but also because the 17-40 has gotten consistently good marks.

Pathfinder: tripod, mirror lockup, remote release, loooooong exposure. I'm going to go back and try some f16 shots.

Fish - sounds very good - did you take off the filter too? Try f11 and f8 as well and tell us your results - I am very interrested in this discussion.

cmr164

pathfinder wrote:

...
In regards to to digicam zooms - I do not understand the significance of this as Fish's question was specifically about the 17-40 L Canon lens which is not used on digicams to my knowledge.... Digicams zooms do not even have a depth of field markings anywhere on them - not do most 35mm lenses anymore - altho most 35mm manual focus primes always did.

Probably because readers on dgrin.com are way more likely to be using a digicam than an 8x10 view camera.

Wonder what the specs on an 8x10" imager would be. :jealous

fish

patch29 wrote:

Source

I pulled the quote from the article "Resolution, contrast, MTF". The other articles will also give a lot of info on lenses.

They have many lens comparisons, not too many newer lenses but it shows which types tend to be better than others. Look under products.

Okay, I read the article. My brain hurts. I also read another article on that site, Improved Sharpness. What did I take away? f8 is the sharpest...it's where the optical sharpness and diffraction curves intersect.