3.1) Nasim Mansurov
May 30, 2017 at 11:15 am
Donz, you can easily test any lens by doing two types of shooting – one to test AF and one to test for any decentering or other optical issues. The first step is to test AF at close and medium range distances. Simply defocus the lens at maximum aperture, then focus on a subject that has a lot of contrast with phase detect AF (a test chart would be ideal for this). Take a shot and repeat the process until you have at least 5-6 shots that you can use. Then use live view to focus precisely on the same target. Compare the previous images with the one from live view. If the images look very similar and sharpness is there, you are set. If the images look drastically different, your lens is off and needs to be either calibrated, or returned for a good copy that does not have the same AF issues. If you want to test for focus shift, repeat the same process with the lens stopped down to something like f/5.6 or f/8.
Then do a similar test with focus set to infinity. Ideally, stand on top of a hill or a tall building and shoot all the details below – make sure that everything is far way enough that you have maximum depth of field. Frame the image so that you have details on all four corners of the frame. Set your lens to desired aperture that you are planning to use the most, then use live view for precise focusing. Once focus is set, scroll from side to side of the frame and make sure that all the edges look roughly the same – you do not want to have one edge that is blurry due to shallow depth of field. Take some shots and look at them at 100% view and explore all the edges of the frame. If a lens does not have a decentered element, all four corners should look roughly the same. If a lens does have a decentered element, you will see one side looking much blurrier than the other.
These two tests should suffice in selecting a good candidate. I always recommend to test lenses in real environments vs a lab environment like mine, since there are too many challenges to deal with and one must have the right equipment and software to conduct proper testing. It can get expensive very quickly…