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Forum: General Photography
06-02-2015, 04:25 AM
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Forum: General Photography
06-02-2015, 03:22 AM
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How about sharp starting at f4? S-M-C/Super Macro-Takumar 50mm F4 It will probably out resolve a lot of lenses at f4 but it still gets sharper as it is stopped down.
A good macro is designed to be sharp across the frame but nearly all lenses (even macros) will be sharper in the centre than at the edges. After all, it is mostly subjects towards the centre of the frame that we want to capture. Without getting too much into optics, if a lens is sharper at the centre, it will get sharper as it is stopped down because we are using more of the "better" part of the lens (That is putting it a bit crudely but one of the factors to consider is that a lens has a curved surface and it is projecting onto a flat surface. Sony are developing a curved sensor which will supposedly to address that. But how finicky do we need to get?). If we stop down further - usually past f11 but it depends on sensor size - diffraction effects start to degrade the perceived sharpness of the lens. That is why some lenses may be described as "diffraction limited". It means that the decline in sharpness past a certain point is due to diffraction and not the optical qualities of the lens. If you look at graphs of lens tests you will see that almost all lenses get sharper as the are stopped down and then start to decline after f8 or f11 (some even sooner). Diffraction is there at wider apertures too but it is not so noticeable.
The dpreview site allows different lenses to be compared - with caution, because there are only a couple of Pentax lenses there and other comparisons must be made on the same sensor to be meaningful. It is here: Lens review data: Digital Photography Review
The Zeiss Otus 55/f1.4 is about the sharpest lens around at the moment. It is at the bottom of the list. It is very sharp at f1.4, peaks at f2.8 and then starts to decline (while still being very sharp). It has 12 elements in 10 groups. They must have gone to all that trouble for a reason. It is undoubtedly a good lens, but costs about $5000. The SMC Pentax-DA* 55mm F1.4 SDM can be had for about one eighth of that price. It has nine elements in eight groups - less glass, less weight, but also a reputation for being very sharp, and getting sharper as it is stopped down. It is not so much a question of why manufacturers don't make all lenses as sharp as possible. They all work to a price and even an ordinary kit lens these days is produced to fine tolerances and can deliver a great image in the right hands.
If you want to get right into the optics side though, I propose this: If we had an optically perfect lens projecting onto a matched, curved sensor, It would exhibit maximum sharpness at maximum aperture because diffraction would be at the minimum. The moment we began to stop down, sharpness would decrease as diffraction increased. In our imperfect world, it doesn't quite work that way, but the Zeiss Otus optics are pushing towards that limit.
That should open a can of worms! :),
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Forum: General Photography
06-02-2015, 01:02 AM
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As a general rule, more expensive = better optics. There will always be exceptions. As a general rule, also, prime lenses will be sharper than zoom lenses. It is inevitable that there will be some trade-offs in zoom lenses, and a decrease in sharpness is usually what we get (and often expect). But the sharpest lens is not always the best for the job.
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