Since we are working step by step through the question of camera image sharpness and resolution. I thought that it would be useful to review what we have already learned and where we have to go next.

1.  We considered the pinhole camera and showed how  we can form an image by isolating single rays from each point of an object.  We also defined f-number and found that the bigger the aperture the worse the depth of field.
2. The problem with the pinhole camera is that it collects very little light and therefore requires a very long exposure.  This limitation can be overcome by using a lens to collect many rays from the same point and bring them to a focus at a corresponding point of the image.
3. We then considered what would happen if we had a “perfect lens.”  That is if the resolution was limited by the separation between and number of pixels, what would the resolution be?
4. This enabled us to determine what the number of pixel requirement is for displaying images on a computer monitor.
5. Similarly we determined the number of pixels requirement for high resolution printing.

So now we have to consider the other factors which govern image sharpness the quality and properties of the lens and stability of lens positioning.  This will lead us into some very fundamental concepts including:

1. The point spread function.
2. The Rayleigh criterion for image resolution.
3. The relationship between contrast and resolution – the so called modulation transfer function.

Ultimately our goal needs to be a very practical one, how do you cut beyond the qualitative hype of manufacturer’s ads and really assess a lens?  How do you find and understand real quantitative lens specifications?  And beyond that is there a way to critically assess and compare your own lenses?