Sensor Size Issues

Have a look at any lens on a 35 mm camera. Its maximum aperture is likely to be f/22. There is a good reason for this. At about that aperture, the effects of diffraction become quite pronounced. Try it on TrueDoF-Pro. Set the sensor size to 36 mm (35 mm format is actually 36 mm wide) and move the aperture slider. If you have enabled the Diffraction option, you will notice that the depth of field drops to zero at about f/22.

Now try setting the sensor size to 6x6 format, or 6x7. You will note that you can use larger f-numbers before the effects of diffraction become too serious. It is no wonder that some medium format lenses stop down even as far as f/45. (Large format lenses go down to f/64.)

But now see what happens when you set the sensor size to something quite small, e.g. a CF 1.5 sensor, as commonly found on DSLRs. How large an f-number can you use? Not very high. Quite disturbing, isn’t it? (And you haven’t even had a look at a 4/3” sensor yet!) Of course, you still get plenty of depth of field at the smaller apertures that are perfectly usable, but you just need to be careful that you don’t overdo it with regard to using large f-numbers (that’s one reason why you purchased TrueDoF-Pro).

So why do lens manufacturers, who have been so responsible in the past and correctly matched lenses to camera formats, now produce lenses that are not perfectly suited to the vast majority of cameras? Well, partly it’s because many of these lenses are designed to be used on 35 mm (“full-frame”) cameras as well as cameras of smaller format. On the other hand, where lenses are specifically made for exclusive use on smaller formats, there is no excuse, technically speaking, for including large f-numbers (I suspect that marketing reasons come into it — consumers might think a lens inferior if it only stopped down to, say, f/11).

Clearly, with smaller formats, one needs to use wider apertures than one does with larger formats. Unfortunately, lens makers have not taken advantage of progress in optical engineering in order to create faster lenses. Instead, they’ve taken advantage of the greater sensitivity of digital sensors compared to film and thus today produce lenses that are not, on the whole, any faster than they were decades ago. This is a pity.