DIY Lens Testing Help

[KyleCharles]

Hello All,

In the last few weeks I have managed to acquire three copies of the Voigtlander 50mm 2.5 Skopar (at well below market value) for use on my M2. I got the first one a few weeks ago for a good price, then just the other day I managed to find two more at a fantastic price ($500 for the pair!).

Now normally I wouldn't care about such things, but since I have three of them I figure I had might as well test them all and only keep the best one. I know that CV lenses are known for some sample variation and decentering.

I do not have a digital camera for these lenses. The finest grain film I currently have on hand are a few rolls of Tmax 100 and Tmax 400. I have a VERY sturdy tripod, a Plustek 7400 scanner, and the M2 has a self timer.

My questions are simple:
What should I be taking photos of to best illustrate the centering and sharpness of each sample? Brick walls at infinity? Newspaper at three feet? What?
Is Tmax 100 a good film for the task? Can this even be done with film?
Are things like centering defects more easily seen when the lens is used wide open or stopped down?

Any other advice would be greatly appreciated!

 

[jonmanjiro]

Here's what I'd do.

1. Infinity focus check - check that a building (or similar) really far away is actually sharp when the lens is set to infinity on the camera. Take a shot at maximum aperture, stopped down one stop, and then stopped down one more stop (three shots x three lenses = nine shots).

2. One meter focus check - check that an object one meter from the camera is actually in focus when focused on (focus test charts are good for this). Take a shot at maximum aperture, stopped down one stop, and then stopped down one more stop (three shots x three lenses = nine shots).

3. Decentering check - focus on a flat surface about 3 meters away with visible texture across it (brick wall etc.) with the camera as square on to the flat surface as possible, and again take a shot at maximum aperture, stopped down one stop, and then stopped down one more stop (three shots x three lenses = nine shots).

4. One more series of shots of your choice to fill the roll.

Keep the camera on a tripod in the exact same position for each series of shots so that the results for each lens can be directly compared. Use a slow enough film (with ND filter if needed) so that you can shoot wide open. Use the self timer to prevent camera shake. Scan the roll at the highest resolution on your scanner and compare the results. You never know, but I expect that you'll find little, if any, difference between them. Also factor in the tactile feel of the lenses (focus action, aperture action etc.) when making your decision, as there are often differences between lenses.

FYI a friend has checked quite a few Voigtlander lenses in a similar way and told me that while he found decentering to be uncommon, he did experience several cases of lenses where the rangefinder did not match the actual point of focus (slight back or front focusing).

 

[Gary Sandhu]

I was in a similar situation -- I ended up buying and then selling an M9 both to try out the camera but mostly to test the lenses. Sure, film vs digital lens performance will be different in some respects but my intention was to eventually get an M9 or similar.
The above suggested tests are good ideas. You might also want to take a series of regular photographs and I doubt you'll see a difference. Also inspect the lens for physical defects . Remember to record your results carefully!