Have a look at any lens on a 35 mm camera. Its maximum aperture is likely to be f/22. There's a good reason for this. As you stop down, and you get closer to f/22, increasing diffraction begins to overwhelm whatever gains you're making in terms of decreased defocus blur. By f/22, diffraction is king. In fact, you're not gaining much in terms of image sharpness beyond f/14 or so (although f/16 is also fine). If you go to f/22, you're doing damage to your image. You may have good reason to go to f/22, and you may feels that you can live with the consequenses. But you wouldn't want to go further any further. Experienced photographers know this. And so do lens manufacturers. And that's why they don't give you the option to go beyond f/22.
What about 6x6 format, or 6x7? Well, you can use larger f-numbers before the effects of diffraction become too serious. Some medium format lenses stop down to f/32, or even go as far as f/45. (Large format lenses go down to f/64.)
Having said all this, you might be surprsied to hear that diffraction is entirely independent of sensor size. The amount of diffraction depends only on your f-number. At any given f-number, the blur due to diffraction is exactly the same, no matter what sensor you have. So why is it ok to stop down beyond f/22 if you have a sensor that is larger than a 35mm sensor. Well, it's because an image on a large snesor does not need to be enlarged as much for viewing as does an image on a small sensor. If you don't enlarge as much, image blur does not show up as much.
Now this is where it gets interesting (and distubing): Let's see what happens when we go the other way — towards smaller sensors, e.g. a CF 1.5 sensor, as commonly found on DSLRs. How large an f-number can you use? Not very high. Try it on TrueDoF-Pro (with the diffraction option enabled). The app won't let you go beyond f/10 (it's ok — go ahead and set f/11 on your camera, but think very carefully about going beyond that)! Quite disturbing, isn’t it? And we haven’t even had a look at a 4/3” sensor yet! Want to know its limit? It's f/7.1 (or f/8 at the most). Of course, you still get plenty of depth of field at the smaller apertures, 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 purchase TrueDoF-Pro).
Remember: small sensor means a larger enlarment factor for viewing of the image. The more you enlarge, the more easily you can see image blur. You want the image to appear sharp? Well, don't enlarge much. Don't want to enlarge much? Well, start with a large image (i.e. on a large sensor).
So why do lens manufacturers, who have been so responsible in the past and correctly matched lenses to camera formats, now produce lenses that stop down to f/22 on cameras with small (sub-35mm) sensors? Why make lenses that are not suited to the cameras on which they are used? Well, partly it’s because many of these lenses are designed to be used not only on small format cameras but also on cameras with 35 mm (“full-frame”) sensors. 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. One can be confident that marketing reasons come into all this — consumers might think a lens inferior if it only stopped down to, say, f/11. Imagine a consumer comparing two lenses. One stops down to f/11, the other to f/22. Which do you think the consumer is likely to be drawn to? I find the primacy of marketing very sad. There was a time (and I'm old enough to remeber it) when sound engineering mattered most.
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, which are expensive to manufacture. Instead, they’ve taken advantage of the greater sensitivity of digital sensors compared to film, and thus today they produce lenses that are not, on the whole, any faster than they were decades ago. This is a pity. On cameras with small sensors, we are thus stuck with lenses that offer a very narrow usable aperture range. You can't use a very wide aperture, because one isn't available. You can't use a moderately wide aperture, because aberrations are too great at such an aperture. You can't stop down too far because diffraction makes a blury mess of things. You only have a few stops to play with.
Bottom line: If you can afford it, go for "full-frame" 35mm cameras or, even better, medium format cameras. See, the old thinking still hold — bigger is better.
© 2013-2018 George Douvos
Sensor Size Issues