Focal length discrepancy with D200

[ms10]

While playing around with bokeh with two of my lenses and my D200 I came across a substantial focal length discrepancy which I am at a loss to explain. I'm really hoping someone can help here.

The two lenses are the Nikon 18-200mm 3.5-5.6 VR and Nikon 70-300mm 4-5.6 G (not the ED version).

Basically, I was attempting to compare the bokeh of them at 200mm. However, I noticed that to create comparable images the 70-300mm had to be set to 105mm.

The 18-200 at 200mm appeared to have the same focal length as the 70-300 at 105mm. I recorded the focal length in both cases from the exif data and the focal ring position.

I realise the 18-200 is officially a DX lens, but surely this shouldn't affect the effective magnification.

Any ideas, anyone?

 

[Ilkka Nissilä]

Try comparing them at infinity. Some lenses shorten their FL when focused close. The 18-200mm appears to be one such lens.

 

[Just Shoot Me]

The 18-200 is only 200mm at infinity. At any closer distance the lens is not a true 200mm. I notice this with my 105 VR Micro and the 18-200. With the 18-200 set at 200 ata distance of about 2-3 feet the 105 give a closer/smaller/more maginfied image then the 18-200. So at that distance the 18-200 is really a 85-95mm lens. That's just the way it is.

 

[ms10]

Thanks, I didn't know that.

True, I was testing with close object, so what you say will most likely be the explanation.

 

[Joseph S Wisniewski]

The 18-200 is only 200mm at infinity. At any closer distance the
lens is not a true 200mm.

Based on 30+ years experience with zooms, I'd hazard a guess that it's not even 200mm at infinity. That's typical of zooms, the manufacturers "round off" to get better numbers. So a 18.3-195mm is labeled 18-200mm.

I notice this with my 105 VR Micro and
the 18-200. With the 18-200 set at 200 ata distance of about 2-3
feet the 105 give a closer/smaller/more maginfied image then the
18-200. So at that distance the 18-200 is really a 85-95mm lens.
That's just the way it is.

Yup...

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Ciao! Joe

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