Question on diffraction

According to messages that I saw more than one year ago, DSLR are more prone to diffraction then film DSRL due to the relatively small CCD and pixels.

Somebody did a some study and declared that Canon will start having the problem at F8 while Nikon (due to the bigger CCD) will start at F11.

Do you face the problem?

I don't need to shoot at Fstop higher than F11 too often but in macro fotography, I've a 105mm and in order to get a decent DOF is necessary to work with high F # due to the short distance and DOF. I recently had to use F22 here:
http://www.pbase.com/sartorig/sigma_105mm_macro

what is your point of view on this? To use high F # or stay away from it? How do you do with macro ?

--
Regards
Gabriele Sartori

Gabriele,

You are confusing a couple of different concepts here.

Diffraction does limit the resolution of a lens at small apertures, but it' s a function of how the edge of the diaphram blades interferes with the light waves and thus is directly related to the physical size of the diaphram when the picture is taken. So a 50mm lens at f22 will have the same diffraction problem no matter what the film or sensor size.

A general rule for "normal" lenses is to take the focal length and divide by four. That's usually the approximate point where diffraction becomes a problem. So, for our 50mm lens, that would be f12.5. That's because a 50mm lens at f12.5 has an aperature only 4mm wide, so it starts to behave like a pinhole camera. A 35mm lens at f12.5 has an aperture of just 2.8mm.

(As most people know, the f stop represents the aperature as a fraction of the focal length, so an aperture of f2 is 1/2 the focal length, or with the 50mm lens, 25mm in diameter. F4 is 1/4 of the focal length, etc.)

Now, here's where film or sensor size comes in. With the S2 sensor and its 1.5x cropping, the "normal" lens would be 35mm. And that lens would become diffraction challenged at about f8.75. With 4x5 film, 150mm is a normal lens, and it would be fine until F37.5. (Unfortunately, my divide by 4 formula is only totally accurate with "normal" lenses, and not for wide angle retrofocus designs, but you get the idea.) So, for the same angle of view , smaller sensor cameras would be more defraction limited.

This is the reason that so many digital video cameras and digital still cameras with small 1/3" or smaller sensor restrict the user to f11 or f8. The "sweet spot" or sharpest aperuture on most 35mm lenses is f5.6-8, for most medium format lenses f11, and for most larger format lenses f16.

I don't believe there is any difference between film and digital of the same physical sensor or film size.

Gabriele Sartori said:

According to messages that I saw more than one year ago, DSLR are
more prone to diffraction then film DSRL due to the relatively
small CCD and pixels.
Somebody did a some study and declared that Canon will start having
the problem at F8 while Nikon (due to the bigger CCD) will start at
F11.

Do you face the problem?
I don't need to shoot at Fstop higher than F11 too often but in
macro fotography, I've a 105mm and in order to get a decent DOF is
necessary to work with high F # due to the short distance and DOF.
I recently had to use F22 here:
http://www.pbase.com/sartorig/sigma_105mm_macro

what is your point of view on this? To use high F # or stay away
from it? How do you do with macro ?

--
Regards
Gabriele Sartori

Click to expand...

I did a quick test with my Nikkor 85mm AF 1.8. I shot 3 consecutve images of a brick wall about 8 ft. away at f5.6 @1/250, f8.0 @1/125 and f11 @1/60 RAW STD, STD, Off, (tripod, cable release)

My subjective conclusion viewing the 69MB 16bit tiffs on a 19 Mitsubishi 930 SB CRT .24dp, 1280x1024, Matrox G550 videocard:

f5.6 is very sharp
f8.0 is a bit sharper but not by much
f11 noticeably less resolution

My plan is to use the 85mm at 5.6 or 8.0 unless I need lots of DOF. I also have architecture shots with the Nikkor 20mm AFD @ f11 and I can see the loss of resolution due to diffraction but I need the DOF

Gabriele,

I'm weak on the theory, but agree with you that I'm having macro problems in practice at smallish apertures.

I do a lot of heavy macro: either a 55mm or 60mm macro in front of the Vivitar 2X macro-focusing teleconverter, in front of one or two Nikon extension tubes (PN-11, with the PK-13 tube if I'm shooting film). When I'm using slides, I find that I can get acceptably sharp shots at f/11 on the macro lens, but that I get diffraction problems with the same setting and the S2. I usually use f/8 or f/8-11 with the S2. It's tough--at 2-5X life size, you need all the DoF you can get... I feel your pain.
Dan P