>Posted by Don Williams (Ou1954) on Tuesday, October 28, 2003 - 2:12 am: > > >Posted by David Fung (Davidfung) on Monday, October 27, 2003 - 11:36 >pm: Hi. It is a given that when using the very small apertures >(f/22 or less) in most 35mm lenses the resolution and contrast is not >ideal. It seems from general concensus that it is sometimes >performing worse then wide open. I gather the problems mainly stems >from diffraction effects, and the difficulty in producing a good >aperture opening that is so small. Especially for shorter focal >lengths. It is also assumed that regardless of the quality of a >filter, it will add negative effects to resolution and contrast to an >image. The question is this. If the shutter speed had to remain the >same (that is, it cannot be faster), is using a ND filter to reduce >exposure from (say, a maximum aperture of) f/22, to f/11 be better >then using it at f/22? Another words, is the effect of the filter >worse then the effects of using a lens at its maximum aperture? >Anyone done any research or tests on this matter? Thanks for your >comments. >DAW writes: >I've heard the same thing all my life, that is that lenses seem to do >best at f/5.6, f/6.3, or f/8. (I know that's not a normal sequence but >I've had cameras with at least 1 or two of those settings.) > >On the other hand, there is a French optics professor on the >RolleiGroup who, I think, said that f/22 was not small enough to cause >diffraction problems. I'll Bcc: him on this and see how he responds >(being too lazy to work it out for myself). Besides, who can work in >this smoke and heat? > >
> >DAW
Here is his response regarding f-stops:
Don
The question is rather simple. Each lens designed to cover a given format has a best aperture. In 35 mm photography the best aperture is what you say, i.e. 5.6/6.3/8.
However for larger formats, the best aperture is smaller. In 6x6 it is 8->11, in 6x9 11-16, in 4"x5" 16-22.
Where does this come from ?
Residual geometrical aberrations define a minimum spot size. Engineers have to make severe compromises between cost, performance, max aperture, maximum image circle etc.. and this yields a minimum aberration spot. This spot scales in proportion of the focal length or with the diagonal of the format, for a given type of lens. For ex&le consider a 5-6 element standard lens, say a 50mm, or a 80mm planar/xenotar or a 100mm or a 150 standard for 4"x5". If we do not take into account the fact view camera lenses have to cover a wider angle than a 50mm for 24x36mm pictures, the minimum aberration spot size roughly increases proportionally to the diagonal of the format.
The best aperture is the one for which residual aberrations and diffraction contribute equally to a combined aberration/diffraction spot. The diffraction spot does not scale with the focal length, it depends only on the relative f-number : something like d_diff(in microns) ~= N microns where N is the f-number. At f/16 the diffraction spot is about 16 microns whether you use a 35mm focal length or a 350 mm telephoto. So for larger formats you can stop down more until diffraction is equal to the residual aberration spot, since you'll not enlarge the image as much as in 35mm when you use 4"x5" film.
In 35 mm photography as soon as you stop down beyond f/11 you degrade the quality of your image, but the degradation is very slow.
Now if you insist on having the maximum image quality, forget about 35 mm photography and use 6x6cm or larger formats on a tripod ;-);-)
I home that this is clear enough. People should not be afraid of stopping down a good lens for 35mm photography at f/16 ; simply with a Rollei 6x6 TLR stopped down at f/16 and a final image enlarged 5x (instead of 10x from a 35mm neg) will be much better. About 4 times in terms of equivalent number of pixels.
-- Emmanuel BIGLER