Digital Photography II – October 26th

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Upcoming seminar – basic to intermediate digital photography. Some from the first seminar have requested the conversation continue.

Understanding digital photography is a bit more complex than you might think. Among other things you may be in charge of your images in ways you never considered. Knowing how to use the camera settings and tools is a step in the right direction.

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Primary among good digital capture is understanding the difference between raw and jpg capture. Do you know how the jpg style settings in your camera can influence how you expose a raw file? What does the histogram actually represent and how can you use it to get the best capture in terms of exposure? Continue reading “Digital Photography II – October 26th”

Diffraction Limitation (why f/22 is a bad idea)

We all know that reducing the aperture reduces the size of the circle of confusion created by the lens which gives the effect of increased depth of field. What many photographers do not realize is that there is a point after which the small aperture begins to diffract or divert he light rays creating a larger rather than a smaller point of light. This point, known as the airy disc, rarely had an impact on conventional film photography.

In digital capture, the size and position of the pixel elements are fixed and if the airy disc becomes larger than the size of the pixel the diffraction begins to degrade the effective resolution. This is a simplified explanation, of course, and the diffraction effect varies with the size of the sensor and the number of pixels on the sensor. The smaller the pixel “sites” the faster this becomes an issue.

In a typical camera diffraction limitation will occur more quickly on a higher resolution sensor as the pixels sites are smaller. Small sensor cameras like point and shoot cameras suffer much more quickly from diffraction. This is why you see cameras with small sensors limit the smallest apertures to numbers like f/5.6 or even less. Since smaller sensors have effectively greater depth of field at larger apertures, this is not a problem in normal photography.

I suggest you visit Cambridge in Colour’s website for a thorough explanation of diffraction and a calculator that will help you determine the optimum apertures for your camera.
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

Also check out their page on depth of field calculation to see just how small your aperture needs to be on your camera to effectively render sharp focus. You may be surprised at the results.
http://www.cambridgeincolour.com/tutorials/dof-calculator.htm

Images made at typical landscape distances rarely require apertures smaller than f/8 or f/11 for effective sharp focus over the entire field. That means that apertures like f/16 or f/22 are not helping you make sharper images, they are actually creating diffraction that is making your images less sharp. Add to that the longer exposures needed you are adding more elements to your capture that can degrade rather than enhance your photography.

Diffraction Limit Sharpness vs Depth of FIeld

There is a limit to the resolution of any optical system. That limit is called diffraction and I first ran into the term with astronomical telescopes. In photography, according to optical theory, any lens devoid of aberrations should perform best wide open. In the real world our lenses generally perform best (sharpest) stopped down slightly from wide open, performing best in the range of f/4 to f/8.

In film days we typically closed down our lenses to achieve added depth of field. The reduction of the circle of confusion created by the out of focus image on either side of the point of focus makes the image appear sharper over a greater area. This is still true, providing we do not exceed the diffraction limitation of the lens. The effect of diffraction limitation has a more obvious effect in digital capture than it did in film for reasons I will not pretend to fully understand. The result is an image that loses critical sharpness even at the point of focus. This is because the disc resolved by the lens exceeds the size of the capturing pixel resulting in loss of resolution. That means that optimum sharpness and depth of field are a trade off after you reach the limiting aperture.

The pixel size on a DSLR is a result of the capture resolution, and the size of the pixels related to the lens determines the effective diffraction limitation, not the lens itself. Therefore, the limits change as you put a lens on different camera bodies with differing sensor resolutions. The diffraction limitation of a particular lens of mine is nearly the same on a 10Mp APS-C body as it is on a full frame 22Mp body because of the difference in the size of the pixels themselves.

While it may be helpful to know what your limitations are for a particular camera there are many other factors that will have a greater destructive effect on your images. All optical maximums depend on solid support, shutter speed, ISO noise, and other factors even including the characteristics imparted by the raw converter used to process the file.

For further information I recommend visiting http://www.cambridgeincolour.com/ and searching for diffraction limitation. There is more information there than you probably really want, but the interactive charts can be useful in ballparking the optimum apertures for your camera.