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Lens Tests

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Lens Tests

A Simplified Method

This is a simplified subjective method of testing lenses optical sharpness. This should not be confused with tests for focus accuracy and discrimination in a 3D environment, that is a totally different issue although often confused by the inexperienced. A focus accuracy test is suggested here Focus Test.

The advantage over this method, compared to just taking a shot of a garden etc, is elimination of variables such as subject and camera movement, insufficient depth of field, flatness of field and variations in focus location. It uses a nice consistent high contrast subject to make comparisons easy; checking sharpness on general photographic views is not very easy.

As the method is subjective, it is worth testing against a known good lens for comparison.

1. Get a piece of broadsheet newspaper and stick this on a wall in your house somewhere where it will be evenly lit. It is probably best to use daylight although artificial light can be used if it is even.  Make sure it has a lot of detailed small type over the newsprint. This gives you a fine detailed high contrast target.

2. Set the camera and lens up on a sturdy tripod with the lens optical centre at the same height as the centre of the broadsheet. Set the tripod distance from the broadsheet to be enough so that the newsprint just fills the frame at the test focal length.

3. Make sure the camera back is exactly parallel to the wall and newsprint. A spirit-level will help in the vertical direction. A laser spirit level is also useful if used with a set square. Place that against the wall in the centre of the newsprint and check the laser is on the centre of the lens cap for both set-square orientations. Other methods are possible, take some time over this. You may need to make several position adjustments and tripod head adjustments until you are happy.

4. Set the camera to centre AF point only, it helps to use back button focus * for this procedure. AF the lens on the newsprint and set ISO 100 and Av mode. Set the camera to mirror lockup and use either a cable release or the timer to avoid vibrations. 

5. Check you have the display set to show channel clipping by video inversion and to display a histogram. Take a test exposure. Check the histogram is as far right as possible without any clipping on the whites. Adjust the exposure compensation until happy.

6. Set the lens to infinity focus and AF on the target, if not using back button focus turn off the lens AF switch. Start at the lens widest f-stop and take a test exposure. Repeat the test exposures at 1 stop intervals down to say f22.

7. Set the lens to closest focus and AF on the target (temporarily turn on the AF switch if not using the back focus button), if not using back button focus turn off the lens AF switch. Start at the lens widest f-stop and take a test exposure. Repeat the test exposures at 1 stop intervals down to say f22.

8. Set the lens to infinity focus and AF on the target (temporarily turn on the AF switch if not using the back focus button), if not using back button focus turn off the lens AF switch. Start at the lens widest f-stop and take a test exposure. Repeat the test exposures at 1 stop intervals down to say f22.

9. You now have three sets of test results each with an independent AF action and the same focus setting throughout the sequence. Check the sharpness of the three frames on the widest f-stop at 100% on you computer monitor and use the sequence with the sharpest first frame. This helps eliminate AF inaccuracies. Don't forget to sharpen the results first to compensate for the anti-alias filter in using RAW (Canon recommend USM 300% radius 0.3 threshold 0), if using JPG use standard sharpening setting. 

10. Repeat if needed for other focal length settings from 2.

You can now pixel peep the results to check for relative sharpness. Don't forget to sharpen if using RAW as in step 9. Look for the lens sharpness to peak between f5.6 to f11 depending on lens type. 

Check sharpness about the edge and corners in points of symmetry about the optical axis. They should be similar in sharpness. Some variation is to be expected for wide open lenses, particularly fast wide angles, reducing as the lens stops down. However gross variation may indicate element decentering.

To aid you you can work through my own lens test menus and look at compare against the sharpness of the 100% crops.

 

Last Updated 05/06/2008

All Content 2005-16 Lester Wareham All Rights Reserved     

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