 Matthew M. Paulson |
mag factor with 2 sigma zoom lenses Here's an interesting issue (at least I think so). I have a Nikon D50. Up to a few days ago, one of lenses I've been using is a 10 year old Sigma 70-210 UC (from an older Nikon 6006). A few days ago, I purchased the new Sigma 18-200 DC. So the first thing I wanted to do is to take identical shots with both lenses and compare the image quality. Right? I mean, wouldn't you? What fun! So I try. Then I discover the magnifaction factor on the older 70-210 (at full zoom), is much, much greater than the new 18-200 (at full zoom). I mean a lot more than 210-200=10, if you get my drift. I know, you are going to say that the newer sigma DC for digital might be "adjusted, marketing wise" to be a true 200mm on a cameras with non-full frame sensors. But if that's the case, why don't I get the "true" 18mm on the wide angle side? I'm probably getting around 28-30mm worth. Any ideas?
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Jon Close |
Lenses (especially zooms) aren't necessarily exactly equal to the focal length marked. The industry generally allows +/- 5% so that an "18-200" lens may actually be 19mm-195mm, and a "70-210" may actually be 66mm-220mm. The nominal focal length of a lens is measured at infinity focus. At closer focus distances the focal length can change to be shorter or longer depending on the particular lens design, and at closest focus distance may be significantly different. So 2 different lenses like the 18-200 and 70-210 may not necessarily be the same focal length, even if set to the same focal length marked on the barrel. Re - not getting "true" 18mm on the wide side - You actually are getting 18mm (or at least 19mm). You are only seeing the angle of view that you are used to from a 28mm lens because the D50 has a sensor that is smaller than the 35mm film frame. 18mm lens on a D50 (15.6mm x 23.7mm sensor size) gives the equivalent view as 27mm lens on a 35mm film camera (24mm x 36mm film frame), which is the equivalent view as a 57mm lens on a 6x7 medium format camera.
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Michael H. Cothran |
The apparent focal length of the lenses will be significantly longer on the D50 (which has a 1.5 focal length factor) compared to the N6006. But both lenses, set to the same focal length, should offer near identical angle of views when mounted on the same camera. If not, somebody's cheating! Michael H. Cothran
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Michael H. Cothran |
PS - John makes a good point, in that the marked focal length of a lens is never really exact, but should still be within close tolerances - less than a few mm's one way or the other. Wide angle lenses tend to be slightly longer than marked, while telephoto lenses tend to be slightly wider than marked. Nonetheless, I still believe that your two lenses from the same manufacturer should display very similar angles of view when set to the same focal length.
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