2x300mm=500mm? lens mystery


Link Posted 16/04/2005 - 11:36
Hi all,

I have a Tamron SP 500mm f8 mirror lens.
I also have a Pentax F* 300mm f4.5 EDIF lens.
I also have a teleplus Pz-AF 2x convertor.

The thing is, when I use the 2x convertor on the 300mm lens, the resulting focal length seems to be exactly the same as the 500mm! So one of them doesn't do quite what it says on the tin. What's going on?



George Lazarette

Link Posted 16/04/2005 - 17:59
Many lenses are not exactly the focal length claimed on the box. Manufacturers round up or down to the nearest commonly found focal length, and it is possible in your case that the 300 is nearer to 275 and the 500 nearer to 550. And when you double the 275, lo and behold, you get 550!

Of course, it may be that the convertor is a bit optimistic too.



Link Posted 17/04/2005 - 16:45
I'd just like to add that this phenomena is often even more noticeable between zoom lenses that claim to offer the same focal lengths.
Die my dear doctor, that's the last thing I shall do!

George Lazarette

Link Posted 17/04/2005 - 17:37
You can see why they do it so as to get nice round numbers, but I wonder if the 77mm Ltd is really 77mm. Perhaps it's really 85mm, but Pentax thought they'd sell more by differentiating it.



Link Posted 17/04/2005 - 20:35
If the measurements made in tests of the 43mm are anything to judge by, then it is likely to be a rare example of accuracy and honesty. The 43mm is almost exactly that, bar the decimal points. The 31mm and 77mm are no doubt close to those figures as well.

I did read some years ago that most 24mm lenses were in fact closer to 25mm, but not many manufacturers label their lenses 25mm....

I think we could safely assume that many 28-80mm lenses could be 29-78mm ones, or something similar!
Best regards, John
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