Lenses, bokeh, diffraction and optical effects.
We are now coming to the part about lenses, bokeh, diffraction and optical effects.
Earlier in this thread I compared a Leica S2 to an hypothetical Nikon D800+, which is a D800 blown up so that its sensor size matches the one of the S2. That was including the lens, so the D800+ had a 62mm f/1.8 lens (a blown up 50mm f/1.8). The important part is that the f number did not scale at all, because f numbers are dimensionless. And this is very important, because many things are dependent on the f number.
For small sensors, the important part is diffraction. You will find a nice article about diffraction
here. What is important from us in this article, is that the size of the sensels dictates the minimum aperture of a lens. For example, for a sensor with 6µm sensels, diffraction first effects will be barely noticeable when stopped down beyond about f/11-f/16. This is not a practical limitation, unless you are interested in macrophotography. For smaller sensors, however, the limitation is more serious. Typically, for the tiny sensors used in P&S or cellphones with pixels under 2µm, f/2.8 may be the slowest aperture that does not degrade the picture. Typically, these cameras do not have a diaphragm at all, but use gray attenuation filters (as is also customary practice for video cameras). Typically as well, they use zoom lenses with sliding apertures and the long end can be as slow as f/5.6 or f/8. Since the lens barely resolve the sensels at f/2.8, you will have divided your linear resolution by 2 and your pixel count by 4 at the long end. And you have no depth of field control, since you don't have a real diaphragm.
For medium and larger sensors, the main difference is in the bokeh. Older photographers may remember the saying that large and medium format cameras allowed better depth of field control. But this is not quite true: due to the availability of very fast lenses (f/1.4 or faster), "Kleinbild" (24x36) cameras are actually the cameras which produce the thinner depth of field. So where did this belief come from?
The belief first comes from the fact that medium and large format cameras were used to produce larger prints. The formulas for calculating depth of field are dependent on the apparent size of the prints and we have seen that large prints seen close have been particularly attractive to the average viewer since the time of classical paintings.
But even if we do not want to produce larger prints, depth of field is, in practice, dependent of the sensor size: smaller sensors need a faster aperture to produce the same apparent depth of field all other things being equal. But aperture does not scale and a faster aperture, with any sensor size, comes with more optical aberrations. Spherical aberration, chromatic aberrations, coma, etc… are all dependent on aperture and increase considerably faster than the scaling power. Moreover, these aberrations also tend to be more difficult to control with smaller focal lengths, so smaller sensors are at a further disadvantage.
What does this mean in practice for different formats?
For tiny electronic sensors with tiny pixels, we would need apertures must faster than f/1.0 if we wanted small depth of field. The optical engineer can't do these at the standard focal length of these sensors and, in practice, the best they can do is f/1.8 (and much less for zooms at the long end). The f/1.8 lens is complex, need aspherical surfaces and special glass, mechanical tolerances are a nightmare since everything is so small (especially at the price the user is ready to pay) and the lens is plagued by aberrations, most noticeably chromatic aberration. Software corrections are often the only solution.
For 24x36 cameras, fast lenses are doable around 50mm, produce a very thin depth of field, but are also difficult to correct. When the photographer wants a depth of field small enough to emphasize the subject with a lens around the standard focal length, apertures around f/2.0-f/2.8 are chosen and we are in a zone where the aberrations are still responsible for bad bokeh: donut shape of out of focus highlights / split highlights (spherical aberration) or colored out of focus highlights (longitudinal chromatic aberration). Sweet spot of the lenses is around f/5.6-f/8, but depth of field is fairly large at these apertures.
For large format digital cameras, very fast lenses are usually not available. The reason is that these cameras use a central shutter and that limits the practical maximum aperture of the lenses. Still, when one wants depth of field control, apertures around f/5.6-f/8 are used and we are in the sweet spot: the lens is almost perfect and bokeh is neutral.
For much larger sensors: large format cameras, we have so much resolution on the sensor than we can afford to waste some and close down beyond the limits of diffraction. f/64 is a value for aperture rendered famous by large format photographers. Even when the photographer wants small depth of field, f/11-f/16 or slower is common (*). Not only aberrations are negligible, but the out of focus highlights take a shape produced by diffraction. This shape, approximately a bell curve, is just what we need for very pleasing bokeh.
(* optimal depth of field is very much an acquired taste, but correspond in practice to fast lenses on 24x36 cameras because this is what we are used to. Very fast lenses on large format have been emulated, check the Brenizer method in google, and the results are strange. The viewer interprets the results as if the subject were a miniature.)
