Question about extenders

Rgollar · May 30, 2015

I am shooting with a canon 7d mark II with CanonEF 70-200mm f/2.8L IS II USM Lens with a CanonExtender EF 1.4X III. If shooting at 200mm with the crop body puts it at (200mm x 1.6 crop body) = 320mm. Now with the extender would I go 320mm x 1.4 of the extender and get a total of 448MM? Am I doing this right? Thanks

Derrel · May 30, 2015

Well, yes in one common way, and NO in the actually correct way.

In the correct manner of listing things out, you list the ACTUAL lengths in mm of the lens. So, take the 200mm length, and multiply that by 1.4x., and you will get a 280mm focal length. So, the 70-200 with the 1.4x on it gives you a 280mm focal length lens. On ANY camera body. That is the correct way of looking at this. There is also a one f/stop loss of effective aperture, so wide-open, the f/2.8 maximum aperture drops to f/4 effectively.

The FOV factor on Canon APS-C bodies is 1.6x. So, take 280mm and multiply by 1.6, and you arrive at a "full-frame equivalent" field of view of a 448mm lens.

SO...you know one thing: the field of view at 200mm with the 1.4x converter on it is fairly narrow, fairly selective.

Rgollar · May 30, 2015

Thank you for summing this up for me aprreciate it.

Derrel · May 30, 2015

I structured my response so you'd have a clear understanding that the actual focal length number of 200 x 1.4 = 280mm because that affects 1) actual focal length and 2)the lens aperture. The thing is this: a 70-200mm lens with a 1.4x on it loses an f/stop of maximum light transmission, but is a 280mm lens; because it is only a 280mm lens, there is not "all that much" difference between what the 200mm shows and what the lens + converter shows...it is NOT the night and day difference many people might expect. if one takes that same 70-200 zoom and 1.4x converter pair and puts that on a full-frame body....the lens is STILL a 280mm focal length: it is not and never was a "360mm lens".

There is a great tendency to see the 70-200mm specification, and in some way, conjure up the idea that a 1.4x makes that into a "448mm lens"...it just does not work quite that way...the difference between a 70-200 with a 1.4x and a 350mm or 400mm prime lens is really a major, major difference. Because the body has its own built-in FOV factor of either 1.5x (Nikon,Pentax) or 1.6x (Canon), one gets used to the narowed field of view with any and all lenses; what REALLY MATTERS MOST is the actual, optical length of the lens, not the body's FOV factor.

When it really comes down to it, a prime 300mm f/4 lens feels and actually is, longer than the 70-200 + 1.4x, and a real 400mm telephoto lens feels, and is, much longer than a 300mm lens. When you put a "real" 400mm lens on a 1.5x camera, the effect is a very tight angle of view, with quite a high-magnification look to the images. The actual length of the lens, or a lens + an optical converter, is more-important in a practical sense than the body's FOV factor is.