Herschel writes: > I can't see that there's much magical about stopping a lens down. The > optical configuration doesn't change, the refractive indices of the > glasses don't change... > You really need to be at a place where you're stopped down enough to > avoid the outer area of the glass (The edges) and NOT enough so that the > aperture becomes an optical device of its own (Pinhole) Also, if you're > shooting negs, They need to be FLAT and you need to be really > anal-precise about focus because at that magnification there's zero > depth of field. > > The old adage of closing down 3 stops from wide open should get you > close In any case. > > This is my opinion (Mythology?) and I can't refer to any research except > in MTF curves which support my theory to some extent. Karl, I'm sure, > has actually done the groundwork and could tell us how valid the -3 > stops thang is. Lenses have lots of aberrations and flaws and good ones designed to try to overcome many conflicting traits. None are perfect. The thing about the aperture we all rely on is that it also causes diffraction - the light blooms around the edges of the diaphragm, lowering the contrast and degrading the image sharpness - a fully stopped down lens may give the greatest DOF, but it also generally yields the softest, least sharp image too :( It's to do with blooming. Funny really - most of the new video cards go to great lengths to create blooming! (see below) But then a wide open lens will be less sharp too (generally speaking) because of problems like spherical aberration - the fact that a lens generally can't focus all the wavelengths to the same plane and subsequently the image will be fuzzy. Red light foicusses one place, green another, and blue somewhere else. If you can stop down a lens with this fault, you can make the angle the light travels smaller and subsequently the cones of light (focussing on the receiver) smaller and the image will look sharper. it's a depth of field type thing.. (smaller angles - apparent sharpness.. you know ;) there is usually ONE aperture at which a lens performs best regarding sharpness - it might be wide open, fully stopped or somewhere in between.. that's not to say the lens will perform sharpest across the whole frame either :/ Novoflex (German) lenses were/are highly sought after by wildlife photographers who tend to plonk their subjects dead centre in the frame because that particular manufacturer never tried to make the lenses sharp from edge to edge - the centre of those lenses is razorblade sharp while the edges were often *very* soft. Stopped down fully you could get an improvement on whole frame sharpness but you lost the centre due to diffraction. These were some of the sharpest lenses money could buy too.. then of course there is production variations to add to the problem. Just because the one provided to whomever did the lens testing showed certain traits doesn't mean another one will be the same. I personally am not a big fan of compulsive lens testing, nor am I a seeker of the fabled ultrasharp King of Lenses. Too many people spend too much time shooting test targets IMO and unless you really like collecting shots of enlargements of newsprint, targets, fine wire frames or the edges of razorblades then best not to get too obsessed with such things. However it *is* worth doing a few simple tests with your own personal lenses to find out where they perform best, then making a mental note of which lenses are best to use in which circumstances (and which are your sharpest lenses). I did do a lot of sweating in the darkroom over enlarger lenses though, for I felt a sharper print was of more importance than a sharp image. And finding the sharp enlarger lens revealed that a lot of my taking lenses were a heck of a lot better than I thought they were when I was in my early years, having prints made by others (who I discovered were incapable of focussing their d*mned enlarger lenses! ) I have one particular print that sells really really well, shot with Kodak 3200 B&W, there's very little in focus at all (it's actually kinda blurry) yet everyone can see the pin sharp grain and without exception the comments were 'wow, that's SO sharp'! perception is a funny thing ;) But when it comes to copy work you should pick the lens/aperture combination that offers the *best* you can get. gawd, I use a bellows lens hood and mask when i copy to cut stray light and reduce flare = increase sharpness. This actually reminded me of why I still use flash for some copy work (and why I should more) - diffraction again.. The focal plane shutter. As that slides past the film/sensor it provides another sharp edge for the light to bleed around, lowering contrast and sharpness. To get the sharpest images again with focal plane shutter equipped cameras, definitely go for flash. the shutter is completely open and the light coming through is not going to be influenced by the shutter (and turn down the ambient background light!! :) more on lenses worth a glance: examples of diffraction and how it softens images with example pics: http://www.luminous-landscape.com/tutorials/understanding-series/u-diffraction.shtml http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm blooming in graphics cards: http://en.wikipedia.org/wiki/Bloom_(shader_effect) spherical aberration: http://www.vanwalree.com/optics/spherical.html good pics and general explanations of stuff: http://en.wikipedia.org/wiki/Lens_(optics) k
