brainiac wrote: brainiac wrote:
The simple way to understand the matt screen/d.o.f. question is to remember that the matt screen is a textured surface which necessarily has peaks and troughs. By virtue of the fact that the peaks are nearer the lens and the troughs are futher away (on a tiny scale), matt screens artificially increase the perceived d.o.f. of the lens at any aperture. Remember that at the film plane d.o.f. is much smaller than it is on the far side of the lens, so that\'s why the amplitude of focus screen texture makes such a dramatic difference. The sensor itself doesn\'t have those peaks and troughs, so focus screens always seem to have a little more d.o.f. than the photo ends up with. But an S-type screen is significantly closer to the flatness of the sensor, so it makes it much easier to judge focus accurately at wide apertures when the d.o.f. at the film plane is significantly less than the amplitude of the peaks and troughs on the focus screen. It is impossible to judge accurate manual focus on the standard screens with very wide apertures, and so all that money spent on the 35 f1.4 instead of the 35 f2 is liable to go to waste on the 7D since AF is seldom accurate enough, and manual focus with a well-calibrated S-type screen is the best way to guarantee that your razor thin d.o.f. occurs where you want it.
paulfeng has kindly pointed out that I am talking a lot of crap here, and he may well be right. I simply don\'t know enough about the optics of focus screens to say whether surface texture amplitude is the actual mechanism by which d.o.f. increases in focus screens, so let\'s call what I wrote a gross oversimplification. Ultimately the point is that standard focus screens are brighter and show more d.o.f. than the picture will have. That\'s verifiable by observation of a quick test. It\'s also true that you see less d.o.f. in the vf when you use an S-type screen, and it\'s also much easier to place the razor thin d.o.f. of wide apertures at the desired distance accurately. So call the above a fancy parable with a real and practical message: it may well be very hard to manually focus wide aperture lenses on a 7D since it doesn\'t show a good approximation to the d.o.f. that the sensor will record.
For me the jury\'s still out on this, until I compare a 7D against an Eg-S screen. But from the reports so far, it looks like the 7D has a standard Canon screen, previous examples of which hide the true d.o.f. from the manual focusser.
There are some related thoughts on this topics in this dedicated thread:
