articles/Digital/onmattersofresolution-page4

On Matters of Resolution! - part 4 of 1 2 3 4 5

by Mike McNamee Published 01/02/2009

For the macro photographer, diffraction limiting is experienced faster and, in practice, more frequently. The effective aperture of the lens is reduced by increasing magnification, such that at 1:1 an aperture of f8 might seem to be safe but the effective aperture

depth of field we tend to use smaller and smaller apertures. However, after diffraction limiting is reached there will be no real increase on depth of field, just an overall softening of the image.

The foregoing has real implications for the choice of camera model, suddenly the pixel pitch is of paramount importance and the pixel count starts to look a little shaky - your Nikon D3x will reach diffraction limiting sooner than your Nikon D3. This is insignificant, perhaps, for social photography, but becomes a problem for scientific- and naturemacro work, when depth of field and resolution are important. The reality is not quite as gloomy as this might suggest but the difference between the D3x and D3 is almost exactly one stop, or, in depth of field terms, you can grab an extra 50% DoF before diffraction limiting kicks in - this might just be worth having!

The problem with this analysis is that the difference will not all be realised if the lens is not perfect (and they never are of course). Camera shake could totally overwhelm the effect anyway, residual aberrations are always lurking.

One outcome of the analysis is that even the landscape photographer should prefer the use of carefully considered hyperfocal distance focusing rather than slamming in f22 and hoping for the best. It boils down to old-fashioned craftsmanship and understanding what is going on with your lens and camera. The optimum performance of a lens is likely to be achieved just before you get to the diffraction limited aperture, so it is as well to know just where it is!

At close macro distances the optimum aperture gets to be disappointingly wide if retention of detail is paramount. You need to be thinking in terms of f5.6 on the lens barrel. If your subject is smooth and lacking in detail you might push the loss of critical detail against the increase in depth of field to advantage and stop down a little more. Testing is, however, quite important in case your system is pushed right over the limit.

The designers of the camera companies are, of course, alert to these effects but have always to strive for the optimum balance of properties, lens corrections and chip sizes. It is only when the advertising guys get involved that the water gets muddied, as they search for market advantage - that is where we came in, engraving the pixel count on the front of the camera. It is also why compact cameras have similar pixel counts to professional DSLRs even though they are wrecked beyond repair if used beyond f8 - there are pixels and there are pixels and not all pixels are created equal!

• On Matters of Resolution! page 1
  
• On Matters of Resolution! page 2
  
• On Matters of Resolution! page 3
  
• On Matters of Resolution! page 4
  
• On Matters of Resolution! page 5
  

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