View Full Version : Diffraction Question

22-Mar-2007, 22:32
Is the degradation in sharpness due to diffraction simply a function of the wavelength of light and the physical size of the aperture opening?

Will two lenses of the same focal length at the same aperture be subject to the same resolution limits posed by diffraction? In other words, does the design of the lens matter?


Alan Davenport
22-Mar-2007, 22:49
Will two lenses of the same focal length at the same aperture be subject to the same resolution limits posed by diffraction? In other words, does the design of the lens matter?

Theoretically, every lens of any focal length, if at the same aperture, should produce the same physical amount of diffraction. IOW, the Airy disc for any 50mm lens, at f/22, will be the same diameter. The Airy discs for all 100mm, 300mm (or any other FL) at f/22 will be the same size as the Airy disc from a 50mm.

In practice, lens construction plays a part, mostly due to the number of blades in the aperture. A lens with a large number of blades in the aperture (I once had one with 19) will maintain a more circular aperture as it is stopped down, while fewer blades will form increasingly acute angles as the number of blades decreases. The angles where the blades meet create more diffraction as the angle becomes more acute (this is the cause of the "star" effect around bright lights in night photos.)

Leonard Evens
23-Mar-2007, 06:12
Let me add to Alan's response.

It is the relative aperture rather than the diameter of the aperture that matters. But, since the relative aperture is the ratio of the focal length to the diameter of the physical aperture, it makes no difference in the answer because of the way the question is posed.

In addition, the 'diameter of the aperture' is not quite a simple a concept as you might imagine. Technically, there is an entrance pupil and an exit pupil and how these are related to what is considered the diameter of the aperture will depend on the lens construction. Since you should be using the relative aperture or f-number in any case, this complication can usually be ignored. See Jacobson's Tutorial which can be found at photo.net for more details.

Also, it is the actual distance from the lens to the film that is relevant rather than the focal length. This is so close to the focal length for ordinary photography that it doesn't matter. But for close-up photography, you should use the effective f-number instead of the f-number. To get the effective number, you mulitply the f-number by the ratio of the bellows extension to the focal length, which is the same as one plus the magnification. In effect, the Airy disc is magnified if the film is further from the lens.

Finally, it should be noted that the effect of diffraction in the final image depends on the degree of enlragement. The usual rules refer to the size of the Airy disc at the image plane in the camera. This will of course be magnified if the camera image is enlarged.

Brian Ellis
23-Mar-2007, 07:56
In simple layman's terms, diffraction results from the bending (or "diffracting") of light as it strikes the edges of the aperture blades while passing through the lens and moving on to the film. If the aperture is wide open only a very small proportion of the total light striking the film has been bent because most of the light strking the film won't have hit the aperture blades. With a very small aperture a high proportion of the light striking the film will have been bent and so a higher proportion of diffracted light will have passed through to the film. Thus diffraction is more significant with smaller apertures than with large.

However, IMHO with 4x5 film and prints enlarged to about 5x or less diffraction shouldn't be a genuine concern in selecting an aperture. Diffraction is significant with smaller formats, especially 35mm, because the negative has to be enlarged so much to make a decent sized print (e.g. about 8x or more with 35mm). I think much too much is made of diffraction with large format photography (and for this purpose we'll ignore the question of what consitutes "large format" and whether stitched photographs are sufficiently close to "large format" to allow mention of them here without catastrophic consequences ensuing).

23-Mar-2007, 08:55

Thanks for your replies. I found them helpful.


23-Mar-2007, 09:07
There was a two part article in a couple of recent View Camera magazines by Bob Hallock that discussed this in some detail. In particular he talked about the trade offs in increasing sharpness by increasing the f number and DOF and when it no longer made sense because of diffraction effects. He had some simple rules of thumb for real world photographers to use. I recommend it to those interested in the subject.
Dave B.

Helen Bach
23-Mar-2007, 09:35
In a diffraction-limited case (ie there are no aberrations of any significance) one of the lens-dependent factors that may slightly affect the loss of sharpness caused by diffraction could be contrast. If the diameter of the Airy disk is constant (and unaffected by aberrations), the resulting resolution limit will depend on contrast – the point at which the peaks of two adjacent diffraction patterns can be discerned, which in turn means a discernible difference in image density on the film. Does that make sense? I don't think it is of any practical use, except to add to the sum of evidence of the imprecision of such calculations.