Test chart tamrom
Front and back focus tests are good to run. You need to do it for each prime lens (fixed focal length) you have, and at your favorite focal lengths for wide angle zoom lenses too. That means on this 17-50, you would pick wide 17mm and zoomed in 50mm, and several others between. If you pick 25mm and 35mm to test, you would end up with 4 potentially different adjustments. These would apply only at 17, 25, 35 & 50mm. If you shoot at say 20mm focal length, and assume it's the same as 17 or even 25mm, you may be wrong. Too much back or forward focusing is a problem with the camera body usually - not the lens.
I'd pay attention to the lens. Look for cloudiness on the edges of the glass, scratches and fungus on glass, wetness or oil on the aperture blades, specs of dust inside the lens, etc. All of these things take away value from the lens, as they can adversely affect the image or shorten the useful life of the lens. You check these things by looking directly through the lens (not on the camera) and look at a dark background - then a light colored background to help show dirt and dust inside.
The only other thing is to look at actual images taken with the lens. Enlarge the pictures to 100% or more. Look for fuzzy edges, purple fringe edges, colors. You will see all of these issues the more you zoom in. You have to decide what it acceptable to you and what is not. Expensive lenses capture great color, are very sharp and have minimal purple fringing on hard edges (high contrast areas) in pictures.
Jan 30, 2012 |
Tamron Test Chart For Tamron Af 28200mm...