View Full Version : does 50-500 mm lens mean 10X optical zoom?

08-05-2006, 01:09 AM
I have seen or heard tele lens for digital SLR such as Sigma 50-500mm or Tamron 200-500 mm

I am completely new to this 'mm' measurement terms. I think that means the focal length right? (but you know I'm not a native english speaker and I don't quite understand it).

Does 50-500 mm lens mean it will enlarge an object 10X optically ? because with my Panasonic DMC FZ7 the 36 mm - 432 mm lens mean (12x) zoom lens.

And, does 200-500 mm lens mean that it will enlarge an object 2.5 X ?

If I want to photograph birds that is very far, about 100 m far or so, what kind of tele lens that is good for that?

Thanks for any answers...

08-05-2006, 03:33 AM
The X? zoom is simply a ratio between the maximum focal length divided by the shortest and does not equal what we think of as magnification.

Ie, 10-100mm = x10 zoom, 20-200 = x10 zoom, 50-500 = x10 zoom

Another thing to consider is that dslrs have a smaller sensor than slrs where an slrs sensor was the piece of film. Thus dslrs capture the middle portion of the information given by the lens in comparison to the larger "sensor" of a film camera, which results in a cropped field of view.

A 35mm film camera would need a longer focal length lens to equal that cropped field on view on the smaller dslr sensor, so in effect the same sized lens switched from a 35mm film camera to a dslr would give what looks like a longer reach ..

So the reach of the lens if expressed in traditional terms must be multiplied by the focal length multiplier (FLM) of the particular dslr.

Most dslrs have FLMs of 1.5 or 1.6, so in other words a traditional 50-500mm lens length lens used on a dslr with a FLM of 1.5 gives a cropped field of view equal to a 50x1.5-500x1.6 , = 75-750mm

This may sound great, that short to middle reach lens becomes nice for longer range work, but it hits you if you want to work at wide angle.

That nice expensive 18mm lens becomes a not so nice 18xFLM lens ie 27mm for a FLM of 1.5

See http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_Multiplier_01.htm

(Note your camera has an actual focal length of 6-72 , thus it can be said that your sensor has a FLM of x6 , ie 6x6-72x6 = 36-432mm..)

A 50mm lens has by tradition been said to (roughly) equal the perspective of the human eye.

So, a 50-500mm lens would (roughly) be equal to x1 to x10 magnification compared to the human eye..