chromvision's reviews

Sigma 18-200mm f/3.5-6.3 DC

Recently I compare the resolution of tis lens with the other lenses I have (Tamron 17-50 and Tamron 70-200). One of the measurement that I made was the amount of sensor pixels per 1 mm of actual target (same for all shots and lenses). I use Sony alpha 350 camera. To my surprise I found that Sigma actually is not 18 - 200 but rather 18 - 135 and in my version also has about a foot backfocusng. If backfocus may be the feature of my particular lens, the deviation of the focal distance from assigned scale is the property of lens construction.
I could not post actual graph here, so I will just post numbers (focal dist. (mm), resolution (pixel/mm)
18 -- 1.9
26 -- 2.6
35 -- 3.7
55 -- 5.8
70 -- 7.2
135 -- 12.2
200 -- 14.5
If anyone plot this line, you will see that it deviates from strait line down at already 100 mm focus
For 200 mm the resolution should be arount 20 and it is only 14.5 which makes tese lense to have focal region from 18 to 140 only.