Diffraction-limited spot size

The following procedure will not give you exact lines per mm resolution, but will allow you to test one lens against another, or compare different f-stops on the same lens.

Diffraction at the DLA is only barely visible when an image is viewed at full-size (100%, 1 pixel = 1 pixel) on a monitor or when output to a very large print. As sensor pixel density increases (on any brand camera), the narrowest aperture we can use to get perfectly pixel-sharp images gets wider.

Put a removable sticky label on the chart for each lens and aperture in use. Change labels when changing lenses or apertures. This eliminates confusion later when looking at slides or negatives and wondering which frames were taken with which lens and at what aperture.

DLA does not mean that narrower apertures should not be used – it is simply the point where image sharpness begins to be compromised for increased DOF and longer exposures.

Abbediffractionlimit derivation

You will find "DLA" referenced in many of the DSLR camera reviews on the site. DLA is an acronym for Diffraction Limited Aperture. This aperture value is the result of a mathematical formula that approximates the aperture where diffraction begins to visibly negatively affect image sharpness at the pixel level.

Set your digital camera to ISO 100 at its highest capture resolution. If you are shooting film, load your camera with a high resolution film like Tech Pan (black and white) or Fuji Velvia slide film.

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If you are shooting film, when you get your processed film back, look at it with a microscope of about 50X magnification. Simple, inexpensive microscopes are sold by Edmund Scientific. You want the Scientifics catalog.

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Diffraction limitedmeaning

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If you are shooting film, you can not accurately judge the results on film with a standard film loupe or by projecting the images with a slide projector. You will need a simple microscope that will give you a magnification of about 50x.

Lenses for Axis cameras with M12 mount.

The above chart is too small to download and print. For actual lens testing, a higher resolution version is here.  This is a large file. Right click and save the file to your computer, then print it on photographic paper at about 3x3 inches in size at your printer's highest resolution photo setting.

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Many cameras have been tested using the lens shown in that example. Learn how diffraction affects your image quality from your specific camera (or select a camera that has similar pixel density to your camera). When faced with a narrow aperture situation, you will have the knowledge to make the right decision.

The results of your test will not correlate with anyone's published lines per mm results. This procedure is only for comparing results from your own lenses.

Diffraction-limited resolution

Use two flash units at 45 degree angles to the chart as your light source. It is best not to use ambient light due to possible errors from vibration due to mirror slap. If you must use ambient light, make sure the light level is high enough or low enough that you aren't in the 1/60 to 1/2 second shutter speed range if you are testing a lens longer than 100mm.

Put your camera and lens on a tripod about one inch in distance from the chart for every mm of focal length on the lens being tested. For a 50mm lens, you will be 50 inches away. For a 300mm lens, you will be 300 inches away.

Diffractionlimit formula

Take your printed (or photocopied) copies of the chart and attach them to the center, corners, and edges of a flat piece of foam core, or other flat board, about 24 inches by 36 inches in size. Add more copies of the chart between the center and edges.

And, higher resolution sensors generally continue to deliver more detail than lower resolution sensors at apertures narrower than the DLA – until the "Diffraction Cutoff Frequency" is reached. The progression from sharp the soft as the aperture narrows beyond DLA is not an abrupt one – and the change from immediately prior camera models to new models is usually not dramatic.

Don't be surprised if the center chart has sharper lines than the edge charts. Most lenses are sharper in the center of the frame, especially at wide apertures. At the smallest apertures, all lenses get less sharp due to diffraction (bending of the light as it passes through the small aperture).

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diffraction-limited spot size formula

You can see from the small version of the chart below that the lines are grouped in sets of six, three horizontal and three vertical. Each set of six lines is slightly smaller than the preceding set.  The smaller the lines that are are distinguishable on film, the greater the resolution, or in other words, the sharper the lens is at that aperture.

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One way to test your own lenses is with the USAF 1951 lens testing chart. The best way to do this is to download a high resolution file of the chart from the link at the bottom of the page, and print the file out at about 3x3 inches in size on photo paper at your printers highest resolution (with many printers this is 1440 or 1880 ppl). You will need to print several copies of the test chart

If you can't print your own chart, you can also find it on page 105 in the book SPECIAL PROBLEMS, one of the Time-Life series of photography books. You can get this book at your library, or have your library get it on inter-library loan. After obtaining a copy of the chart from the above book (or from the link at the bottom of the page), photocopy the chart. Resize or shrink the test chart "square" down to about 3 inches by 3 inches in size and print several copies

You will be able to see important differences in sharpness between different lenses set at the same aperture, and differences in apertures with the same lens.

Check out this specific diffraction comparison example using the ISO 12233 chart comparison tool. The mouseover feature will show you the image quality degradation at f/11 by comparing to f/5.6.

Focusing errors are the main problem you will face. After the most careful focusing possible through an eyepiece magnifier, it still helps to bracket focus a little and go by the sharpest frame/photo for that lens and aperture.