Zoom & Focal Length

Nazbit

Nazbit

Nazbit

Soldato
Joined
11 Dec 2004
Posts
3,871
Hi all,

Just 2 questions

Is it right that you can take the focal length of a lense and divide the second number by the first to get the zoom?

Eg 300/70 = 4x Zoom?


Secondly I was in a certain camera shop today looking at a 400D and the guy I was talking to told me that putting a sigma or tamron lense on a 400D is effectivley a 480m lense.

How does that work?
 
What in gods name?

There's a 1.6 crop factor with digital cameras (well, most). So a 50mm lens is 50mm*1.6.
 
I know

I dont have a clue about that formula, I think he knew even less than me (Which aint much!) So wanted to check here.
 
Yes, if a zoom lens has a range of 70-300, you can divide the longest length (300) by the shortest (70) to see the magnification factor, in this case 300 / 70 = approx 4.3x magnification. However this figure doesn't let you compare how much reach a lens has as a 100-400mm lens also has a 4x magnification, but it has a longer focal length, measured in mm.

The issue of equivalent focal length (efl) complicates things a little.

Most modern film cameras have a film size of 35mm (diagonal).

Most digital cameras (aside from those such as the Canon 1DS MkII, which are marked as 'full frame' cameras) do not have a digital sensor this size. The sensor is smaller. This produces the following effect (image courtesy of Canon):
angle-main04.jpg


The full frame sensor captures more of the scene, but when the image files of a APS-C camera are printed/displayed with the same physical dimensions, the APS-C camera shot appears to have further reach. (Of course, the same effect could be achieved by cropping the full-frame camera's image).

Thus (and sorry for the long-winded explanation) you can multiply the focal length of a lens (eg 300mm) by the crop factor (eg, 1.6 for Canon SLRs) to get the efl, 480mm.


EDIT - dammit, I went to the trouble of typing all that and then I find this link
http://en.wikipedia.org/wiki/Crop_factor
 
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