Softness and aperture.

[Summerstar]

This is probably a silly question, but I'm quite good at thinking them up...

Shooting film on 35mm it was axiomatic that one needed to stop the lens down from its maximum aperture to get it to work at its sharpest. But of course, the circular image back-focussed by the lens was fully utilised by the frame dimensions.

Now I'm shooting with a Nikon D7000 which has a DX sensor which utilises only the central portion of the back-focussed image. So does it still hold true that the lens will perform better when stopped down, or does stopping down have no effect on sharpness because the outer part of the back-focussed image is redundant?

 

[Dale_tem]

Most lenses loose a little sharpness when fully open, so people usually recommend stepping down a bit, test with your own lenses see.

 

[Nawty]

The same still holds true, you're just not using the edges of the frame. Of course the edges are the first to go so you are getting the better part of the image but you still gain sharpness from stopping down.

Of course, if the lens is a DX lens then you are utilising most of the lens anyway.

 

[Forbiddenbiker]

The actual diameter of the FF or DX back focused image, does not change size when altering the aperture. Stopping down will dim and reduce the amount of light within the disc of back-focused image, because we're filtering the wider rays out of our exposure in favour of more parallel rays, but during this adjustment the back focused image diameter will remain constant to its FF or DX design standards. The aperture controls only how much of the front lens surface size is used to collect light. a smaller portion means less of wider light rays are allowed to be recorded during exposure, leaving more shutter time needed to collect the straighter rays to make correct exposure, this combination as we know goes onto providing an image with more depth of field in focus.

 

[Krisstiffer]

Two things to think about:
Digital camera sensor designs mean that if you stop down beyond a certain point you will gradually start to lose sharpness through diffraction effects. This is inherent in ALL digital systems - the point at which this starts to take effect depends on the pixel density of the sensor - generally the higher the pixel density of the sensor then the sooner this starts to raise it's ugly head as you stop down. As an idea (don't know Nikon but here's Canon AS A GUIDE) - an 18MP APS camera has a diffraction limit around f6.7, the full frame 20ish MP sensors have a limit of around f10 or f11. Diffraction won't neccesarily be a problem - the limit is usually a theoretical calculation. But generally think of it being the potenital for an image to start appearing soft beyond those apertures.

Then there's the lens - these usually have a "sweet spot" that is somewhere in the middle of the aperture range. Your typically lens will not give of it's best at either the wide open or fully closed extremes.

 

[HoppyUK]

Two things to think about:
Digital camera sensor designs mean that if you stop down beyond a certain point you will gradually start to lose sharpness through diffraction effects. This is inherent in ALL digital systems - the point at which this starts to take effect depends on the pixel density of the sensor - generally the higher the pixel density of the sensor then the sooner this starts to raise it's ugly head as you stop down. As an idea (don't know Nikon but here's Canon AS A GUIDE) - an 18MP APS camera has a diffraction limit around f6.7, the full frame 20ish MP sensors have a limit of around f10 or f11. Diffraction won't neccesarily be a problem - the limit is usually a theoretical calculation. But generally think of it being the potenital for an image to start appearing soft beyond those apertures.

Then there's the lens - these usually have a "sweet spot" that is somewhere in the middle of the aperture range. Your typically lens will not give of it's best at either the wide open or fully closed extremes.

Not really. Diffraction is an optical characteristic, nothing to do with pixel density, but is also affected by sensor size. Most lenses will hit the diffraction ceiling for maximum sharpness, in the centre, around f/8 on full frame, and f/5.6 on crop formats. Check any respected lens test and they will bear this out.

A really good lens might peak at a slightly lower f/number, and at a correspondingly higher level. Less good lenses struggle around the edges and they can still be getting better at higher f/numbers, but at a correspondingly lower level.

There is a theory, and it's not one I have much time for, that pixel density is also a factor. It's based on the point when Airy disc size gets smaller than the pixel size, therefore capping resolution at sensor level, but the flaw in this argument is that sharpness is about contrast as well as resolution - and arguably contrast is the more significant component - and that can go on rising regardless of the resolution limit. I have never seen any convincing evidence of this pixel density theory, but plenty that says it's not a meaningful limitation.