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Apertures, Pixel Sizes, and Diffraction Limits.
Contrary to popular belief, smaller lens apertures (with their greater depth of field) don't necessarily result in sharper pictures. When the negative effects of aperture diffraction begin to outweigh the positive effects of increased depth of field, sharpness falls off and fuzziness sets in. Smaller apertures also cause slower shutter speeds, often requiring higher ISO settings to make a shot. Understanding how aperture diffraction works and relating how diffraction applies to your camera will help you to take sharper pictures.
Cambridge in Colour has an outstanding tutorial & essay about diffraction limited photography. Once I took the time to read & understand it, experiment with the interactive diffraction table, and work with the diffraction calculator, I saw how critically important diffraction limits were.
Below: A representation of Cambridge in Colour's interactive visual diffraction table. The grid on the left side represents the relative pixel size of the selected camera's image sensor, the column in the center is an aperture selector, the next column is a selection list of assorted digital cameras, and the column on the far right shows the listed camera sensor's individual pixel area in square microns.
This will all make more sense when you go to the tutorial, read the article, and try out the real interactive table for yourself. The interactive table is a bit shy on instructions, here's some help:
- Hover your mouse pointer over the camera of choice (hover, don't click). Java powered magic will select the camera and resize the pixel grid display automatically .
- Then go to the left and move the mouse pointer up & down over the aperture selections (again - hover, don't click).
- A beam of light will shine on one pixel. Larger apertures (smaller f/numbers) produce less diffraction and the beam will stay within the pixel boundaries. Smaller apertures (larger f/numbers) produce more diffraction and diffuse the beam of light out into adjacent pixels. That's not good, because when you fill all the pixels with beams of over diffracted light, they fuzz themselves up all over each other.
- The aperture setting where the diffraction starts to spill into adjacent pixels is the smallest aperture recommended (the diffraction limit) for that particular camera. Anything smaller will tend to create diffraction blurring. Once you reach the camera's aperture diffraction limit, the smaller the aperture, the more the blurring effect.
Visual Table Examples:
Below: Using a Nikon D2X camera's pixel size as an example, an f/8 aperture passes a clean light beam that stays within pixel boundaries, safely below the edges of diffraction limits.
Picture this: A nine pixel pattern at f/8 would look like the sketch shown below. The central peaks and secondary rings of the light beams fall within the D2X's pixel boundaries. No diffraction blurring or softening will occur.
At f/11 diffraction begins to spill into adjacent pixels.
The nine pixel pattern at f/11 would now look like the sketch shown below. The central peaks of light are within the edges of the pixel boundaries but the secondary rings of have begun to overlap. Some diffraction blurring & softening will occur.
f/22 is well over the D2X's diffraction limit. Serious image quality degradation will result.
And the nine pixel pattern at f/22 would now look something like the sketch below. The central peaks of light strongly overlap each other, the secondary light beams have become mush.
Below: Cambridge in Colour's Diffraction Limit Calculator (screenshot) with Nikon D2X values entered.
The visual tables and the diffraction limit calculator both show that one should probably avoid anything higher than f/8 with a Nikon D2X. Looking through the shooting data on pictures I've taken with my own D2X, I can confirm that my sharpest, cleanest looking pictures have all been at f/8 or lower.
With this in mind, I do my best to limit my maximum aperture to f/8 or lower on my D2X, and f/11 or lower on my D70 . And I'm not at all reluctant to shoot wide open.
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