10D ............................. f/11.8 20D ............................. f/10.3 40D ............................. f/9.3 50D ............................. f/7.6Here we have for several APS-C 1.6 crop factor cameras, an f/stop beyond which diffraction effects from the lens will limit resolution more than sensor resolution.
There are lots of things to explain here. First off though, it is worth mentioning that the word Aperture is a bit of a misnomer, we are actually specifying f/stop (we have to be pedantic about this because some nit-picker will point out that these are f/stops not an aperture which would be a lens diameter in millimeters). Now that we have cleared this up, we will go ahead and call this "aperture" like any sensible photographer.
It is important to point out that diffraction is just the laws of physics. A lens that was only limited by diffraction would be a perfect lens. Most real lenses have other issues that limit their performance. Only truly superb lenses can produce images limited only by diffraction. Diffraction increases as a lens is stopped down.
The idea behind the above table is that if we stop down (use bigger f/stop numbers) than those in the table above, our lens will be generating images which cannot make full use of the resolution of the sensors in the specified cameras. This has some people worried. As cameras are sold with more and more pixels on the same sized sensor, they require better and better images from the lenses, or the pixels are just providing "empty magnification".
Those are the basic ideas, I will provide more details at the end of this page. One other thing is worth pointing out at this stage. Lenses are usually stopped down because the photographer wants greater depth of field. This leads to a compromise between diffraction issues and desired depth of field. Or maybe it doesn't if your goal is the final image at a certain print or image size. A lot of these issues only become apparent if you study images at unrealistic levels of magnification. This leads us to ask the following question:
There are any number of philosophies about what makes a good photograph.
Some of the people who focus on technical excellence become "pixel peepers" loosing track of the forest while studying the trees. They study images at 1:1 screen resolution analyzing lens artifacts, sensor resolution, noise, and so forth. We can throw the baby out with the bath water though if we neglect what they say entirely. Otherwise fine images can be ruined by technical problems.
I got interested in this whole business when I read the "50D milesone" article (link below). Apparently the Canon 50D sensor was the straw that broke the camels back for many people.
DLA = 1600 / pdHere "pd" is pixel density in pixels per millimeter. I find it more useful to think of sensors in terms of the pixel size (or pixel pitch), given in microns. So we can transform the above equation using the relationship pd = 1000 / pp where pp is the pixel pitch in microns. This gives the equation I like, which is:
DLA = 1.6 * ppWhere pp is the pixel pitch in microns.
Let's look at pixel pitch for some cameras of interest:
Canon 70D xx.0 MP, 22.3 by 14.9 mm sensor, xxxx x xxxx pixel, 4.1 micron pixels (244 per mm) DLA = f/6.6 Canon 7D 18.0 MP, 22.3 by 14.9 mm sensor, 5184 x 3456 pixel, 4.3 micron pixels (232 per mm) DLA = f/6.9 Canon 50D 15.1 MP, 22.3 by 14.9 mm sensor, 4752 x 3168 pixel, 4.7 micron pixels (213 per mm) DLA = f/7.5 Canon 20D 8.2 MP, 22.3 by 14.9 mm sensor, 3520 x 2344 pixel, 6.3 micron pixels (158 per mm) DLA = f/10.2 Canon 5Dii 21.1 MP, 36.0 by 24.0 mm sensor, 5618 x 3744 pixel, 6.4 micron pixels (156 per mm) DLA = f/10.2 Canon 1Dsiii 21.1 MP, 36.0 by 24.0 mm sensor, 5618 x 3744 pixel, 6.4 micron pixels (156 per mm) DLA = f/10.2 Canon 1Diii 10.1 MP, 28.1 by 18.7 mm sensor, 3888 x 2592 pixel, 7.2 micron pixels (138 per mm) DLA = f/11.5Clearly if the DLA limit is any kind of a crisis for you, you should pick up a used 1D Mark III and avoid the 7D like poison. The Canon 70D with 20.2 megapixels on an APS-C sensor will be even more of a catastrophe. Notice that the pixel pitch on the 5Dii is essentially the same as the APS-C 20D camera.
Effective f/stop = Marked f/stop * (1 + magnification)This formula is not entirely correct, and for longer macro lenses it will underestimate the effective f/stop. For details see:
Using the above formula at 1:1, the magnification is 1 and as an example and f/stop of f/4 would become f/8, which is a 2 stop change. If we were using the MPE-65 lens at 5x, then we would change the f/stop by a factor of 6 (from f/4 to f/24). Note that if we stop down to f/16 to gain depth of field, we would actually be working with an effective f/stop of f/96. These huge f/stop numbers (and tiny effective apertures) produce drastic diffraction effects, seriously compromising image quality. To obtain quality images at these magnification levels, it is almost essential to use lenses wide open (or nearly so), and resort to focus stacking to obtain depth of focus.
Tom's Photography Info / tom@mmto.org