These tests are designed for enlarging lens, but can be modified to test camera lenses by either adapting the camera lens to the enlarger (epoxy an old, short, extension tube to a lens board) or running the test in-camera, on film instead of paper. To run these tests with an enlarger you must make sure that your enlarger is correctly aligned to begin with. Check your owners manual for instructions.
Light Falloff
This is one of the easiest tests to perform. A diffusion type enlarger will produce much more uniform illumination than a condenser enlarger, but either will work. Make prints with the bare enlarger light (no film in the carrier) on the contrastiest grade of black-and-white paper that you have. Make a set of prints for each lens at apertures ranging from wide open to three or four stops down, The exposure times must be adjusted to produce a medium gray print. Make sure the lens is in focus by temporarily putting in a negative. Don't be surprised at how much light falloff there is -- it's normal, especially at wide apertures. The use of a high contrast paper, and/or condenser enlarger, magnifies the results as in a "worst-case-scenario".
Aperture Accuracy
Exposures will be off if the aperture settings are not exact. That is, if you think you are stopping down one f-stop, but are actually only stopping down 1/2 of a stop, your exposure will be way off. This can be tested with most exposure meters. Many hand-held exposure meters have enlarger attachments available, but dedicated enlarging meters and color analyzers can be used for this task, as long as they have readouts in seconds. Do not use film or paper to test the apertures since reciprocity rule failure can cause bias in the results. Start by taking an exposure reading at the maximum f-stop. Vary the film speed setting to get a typical exposure time -- for example, 10 seconds at f 2.8. Now stop the lens down one f-stop. The meter should drop to 20 seconds. Continue to stop down and note the increase in exposure. If there is a deviation of more than 15% from what the exposure should be, it's best to avoid using that f-stop setting or remember to compensate appropriately when it is used. Top-notch lenses will usually only show significant deviations at very small apertures. Inexpensive lenses many show errors at all settings.
Distortion
Distortion is whether or not the lens can keep a straight-lined object in the negative straight on the paper. The affect is more visible near the edges of the format, so you can measure distortion by stretching a piece of thread across a negative carrier near one border of the film frame. Tape down the thread so that it is fairly tight. It does not have to be precisely placed, just straight. When this is done, project a full-framed image of the thread on the baseboard. Then align a straightedge with the projected image of the hair. No need to make a print. You can use another ruler to measure how much the image bends away from the straightedge at a particular distance from the center -- say, 5 inches. You can then compare this number with the results from other lenses. All lenses have some distortion. It's just that some are better than others. This aberration cannot be diminished by changing the f-stop.
Flare
All lenses exhibit flare, but it varies tremendously from lens to lens. It also varies according to the f-stop that the lens is used at. Attach a small piece of black tape to the underside of a piece of glass which you cut to fit into the negative carrier in the enlarger. Next insert a sheet of low contrast B&W paper into the easel and laid Kodak step tablet directly on the paper, off to one side of where the tape square would be projected. Then make a very heavily overexposed print of the tape square, with the lens set to its optimum aperture. The exposure time is not critical, but must be long enough to produce some density in the print of the taped-over area of the glass. Ideally, that area will print as white -- any density in that area is caused by flare.
Flare depends on your enlarger and the type of illumination it produces, as well as the lens. Make sure that all of your tests are done on the same enlarger. The step tablet acts as a calibration filter-each step in the tablet cuts the amount of 'white' light reaching the print paper in that area by a half-stop. Suppose, for example, that the density of the tape image equals the density of the 14th step in the tablet image. That means the amount of flare equals the intensity of the white light cut by seven (1/2 x 14) stops. If you were to change the print exposure, the density of the tape image would change, but since the tablet was contact-printed as part of the some exposure, its density would change also -- the tape image would still match the some step on the step tablet image. That is why you do not worry about exact exposure times. In practice, real negatives would not be anywhere as contrasty as the test plate, so your flare levels would be much less.
The following tests require a resolution target of some type that will provide the ultra-fine detail you need to check image quality. You also need a focusing magnifier which gives a clear, high-powered image and which will let you examine image quality and focus at the corners of the field, such as the Micromego Critical Focuser. Without this you can make prints for comparisons, but it is more time consuming. There are many high resolution test charts made but most are expensive. Conventional photographic negatives and low-cost enlarger test-slides won't help at all. At best you might get a fuzzy 100 lines/mm out of such items. That is nowhere near enough to tell you much about a good lens. While you don't need a plate that goes to 1,000 lines/mm, you will need at least 200 lines/mm, and the plate must be sharp and grainless. Fortunately, there is a low-cost alternative to a test plate and it only costs a few dollars. Get a sheet of 85-line 50 percent Zip-A-Tone from your local art supply store. Affix a piece of it to a small sheet of plate glass and burnish it down well. You now have a flat test target for enlarger alignment and lens testing. Each little dot on the screen is filled with fine, contrasty detail which is the pattern of the plate that was used to print the Zip-A-Tone. You will easily be able to see aberrations like lateral and longitudinal color as well as astigmatism. In the following test descriptions, you can substitute a Zip-A-Tone plate for a resolution test plate -- the difference is that you will not be able to assign precise numerical results to your observations.
Lens Element Centering
A perfectly aligned lens produces a symmetric image-all four corners will look alike, even if they look different from the central image quality. If one or more lens elements is slightly tilted or mounted off-center, the corner performance will be degraded of some corners more than others. The purpose of this test is to measure how much the performance varies around the edge of the field, not to measure the absolute quality of the lens.
Put the test plate in the enlarger at a position about 80 percent of the way to the corner of the format. Carefully focus the image with the lens wide open and note the finest bar pattern you can see. The rotated the lens mount by an eighth turn, refocused the image, and recorded its appearance. Continue all around the image. You should expect to see some variation in both contrast and resolution. If you see heavy smearing or distortion of lines, you have a misaligned lens element. If there is a sudden jump in image quality between two adjacent positions, that is also an indication of possible decentering. In general, a decentered lens element not only produces a lower resolution image in parts of the field, but one with noticeably less contrast and "smearing." Remember that a 25-50 percent variation is acceptable and normal.
Resolution and Contrast
The purpose of this test is to check the on- and off-axis image quality at various apertures. This test is a measure of the 'ideal' performance of the lens -- it doesn't take into account how well the lens focuses an image into the paper plane.
Set up the target and the test lenses so that the target is 80 percent of the way to the comer of the field, in one of the sharper areas determined in the preceding test. Focus the image with the lens wide open and note the resolution and contrast. Then stop the lens down, making these same observations at each f-stop. To check central performance, repeat these steps with the test plate in the exact center of the lens's field.
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