Advantages of slower lenses?

[scott russell]

I have a few questions about lenses; which may be oversimplified since many lenses are different, but I was wondering if a slower lens would make a sharper image stopped down than a faster lens. If a lens is generally at its sharpest a few stops down from wide open; wouldn't an F8 stopped down to F16 provide more depth of field at its sharpest point than a 4-5.6 lens only stopped down to 8 or 11? I was in a camera store looking a an f8 super angulon (or maybe grandagon, sorry i can't remember) and the guy behind the counter said that that particular lens, is actually at its sharpest point stopped 3-4 stops down from its minimum aperture. Would this mean, that taking an f8 lens and stopping it down to f32 would put diffraction at a minimum while having as much DOF as possible?

[Daniel_Buck]

My guess is the main advantage to slower lenses is that they weigh less, and smaller filter rings. I would guess that diffraction (which is the main cause of lenses getting soft when stopped down??) at f45 would be the same on an f3.5 lens, as it is an f11 lens when both stopped down to f45. I could very well be wrong though :-)

[Ron Marshall]

I have a few questions about lenses; which may be oversimplified since many lenses are different, but I was wondering if a slower lens would make a sharper image stopped down than a faster lens. If a lens is generally at its sharpest a few stops down from wide open; wouldn't an F8 stopped down to F16 provide more depth of field at its sharpest point than a 4-5.6 lens only stopped down to 8 or 11? I was in a camera store looking a an f8 super angulon (or maybe grandagon, sorry i can't remember) and the guy behind the counter said that that particular lens, is actually at its sharpest point stopped 3-4 stops down from its minimum aperture. Would this mean, that taking an f8 lens and stopping it down to f32 would put diffraction at a minimum while having as much DOF as possible?

Diffraction is only a function of the f-stop, and does not depend on maximum aperture.

Lens test data:

http://www.hevanet.com/cperez/testing.html

[Alan Davenport]

Daniel is correct, that the main advantages are lighter weight and more affordable filters. Not to mention, more affordable lenses! Diffraction is dependent only on the effective aperture; any lens -- of any focal length -- will produce the same amount of diffraction blur at a given f/stop (when focused at infinity.) I'd have to check the math, but IINM the diffraction would also be the same at any given image magnification (when at the same f/stop) since the diffraction blur increases as the lens extension is increased from infinity focus.

The only disadvantage of slower lenses is that they are a bit more difficult to compose and focus. A good fresnel screen helps with the composition part. Unless you intend to actually shoot at the wider aperture of the faster lens, there isn't really much advantage to the extra speed, IMO...

[Mark Woods]

I think that the f/stop is dependent on the focal length. Generally the "sweet" spot is half way between the most open stop and the most closed.

[Dan Fromm]

Scott, it depends on the design type. Slow tessar types (f/6.3) have more coverage than fast (f/4.5, ... ).

Mark, I can't agree with your statement to the effect that a lens shoots best midway between wide open and fully stopped down. This because I use process lenses that stop down to anywhere from f/64 to f/260 out-and-about. Depending on coverage needed, they're best at f/16 or f/22. Midway? No, sir!

[Ken Lee]

It is harder to design and manufacture very fast lenses. To get them to cover a large area with excellent correction, is even harder and more costly. As we look back in time, we see that lenses were generally slower, for those reasons.

Also, wide lenses become quite large. Imagine a Large Format 250mm lens that is f/1.0: It would be 250mm or 10 inches across. That's a lot of glass - don't drop it on your foot !

[mrossano]

Another factor to consider is the complexity of the lens design. A good example would be a comparison of the characteristics of f5.6 and f8 Super Angulons, or similarly, f4.5 and f6.8 grandagons. The slower versions of these designs are 6 element lenses. The faster versions are 8 elements. The faster lenses have more coverage, better MTF as you move off-axis and lower distortion. In contrast, the (now discontinued) f8 Nikkor SW lenses are 8 element designs. They have the superior performance of the faster offerings from other makers, but in a smaller package.

[Brian Ellis]

" . . . wouldn't an F8 stopped down to F16 provide more depth of field at its sharpest point than a 4-5.6 lens only stopped down to 8 or 11?"

I don't know where the "sharpest point" is (and that isn't something anyone should worry much about anyhow with a 4x5 or larger camera) but all other things that affect depth of field being equal (i.e. same focal length lens, same camera position) any lens will provide greater depth of field at f16 than f8 or f11.

[Joseph O'Neil]

The numero uno reason I use F8 and F9 lenses is because while backpacking, they are smaller and lighter than F5.6 lenses. They also tend to be less expensive to buy, when directly comparing apples to apples, so to speak.