Nikon 24-85 f3.5-4.5: Why isn't 85mm 85mm?

[mbeebee]

Hi Everyone,

 

I just received a Nikon 24-85 f/3.5-4.5G from B&H. Seems like a great

lens. Then I noticed my intuition about focal lengths conflicting

with the numbers on the lens barrel, and some testing confirmed it.

At the wide end (24-35mm), everything appears fine: the field of view

for the 24mm setting corresponds to that of a 24mm prime (Nikon 24mm

f/2.8), and the 35mm setting corresponds to that of the Nikon 18-35

f/3.5-4.5. But things get off from there. The 50mm setting is a bit

wider than the Nikon 50mm f/1.8, and the 85 mm setting is no where

near 85mm: tested against the Nikon 80-200 f/2.8 set at 80mm, the

85mm setting is still wider�-perhaps corresponding to ~70mm. (All

tests were done on an F100, if anyone cares.) I�m afraid I don�t live

in an area in which I can pop down to the camera store and try out a

few others.

 

Questions: Can anyone who owns the Nikon 24-85mm f/3.5-4.5G confirm

these tests for me? Is this how these lenses are, or is this somehow

a �bad sample� that I need to send back?

 

Thanks for your help,

 

Martin

[bruce_rubenstein___nyc]

Zooms are often less than their marked focal length by a small percentage. What you are probably seeing, though, is the effect of internal focusing. At anything less than infinity focus the focal length is shorter. (This is even true for none zooms like the AF 85/1.8.) Try checking the field of view out doors. The lens is probably working as designed.

[h._p.]

As Bruce has indicated, the focal length marked on a lens is just an indication. Some lenses are off from their marked focal length by as much as 10%. I once had an ancient Zeiss Jena Biometer which was marked as 5cm and on a test bench (my excuse is that I had a quiet day) it turned out to be 58mm, which, curiously, is the focal length marked on later versions of the lens.

[mbeebee]

Thanks, guys--that was it. I was comparing focal lengths at close range in my home office. Outside, at distances close to infinity, the focal lengths matched perfectly (at least as far as I could tell). So the discrepency appears to be an issue only at close range, at which point the 85mm setting isn't quite 85mm. Perhaps--as Bruce mentioned--because of the internal focusing.

 

Thanks again,

 

Martin

[dan_brown4]

The focal length is only accurate at infinity focus. On many of

these zooms (the 28-105 comes to mind) the focal length gets

significantly shorter as the focus gets closer. And I don't merely

mean the usual magnification change of closer focusing, the

lens design trades focal lenth accuracy and distortion for

sharpness. Note that the 24-85G has real bad distortion too. I

can't use mine for architecture, even with the smallish D100

sensor.