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What are the theoretical and/or practical differences between shooting with different lenses at same focal length? For example, if we are shooting with a 24-70mm f/2.8 lens at 70mm focal length vs same focal length with a 70-200mm f/2.8 lens, assuming identical quality of optics and camera settings, will there be any difference in the image produced?

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I think you are looking for image quality/sharpness/rendering and less about focal length change, correct? All of these answers so far are examining focal length difference between the 2 lenses based on focus breathing, quality control, etc and I don't feel this is the thrust of the question. –  camflan Dec 9 '12 at 14:52

3 Answers 3

up vote 2 down vote accepted

A perfect lens would give the same exact results but there is no such thing as a perfect lens. Even so, this would only be true when focused at infinity. At closer distances, there is shift in focal-length that depends on the lens design. So with two different lenses, one may shift differently than the other.

Practically, you will certainly see a difference, not just between a 24-70 and 70-200 but two 24-70 or two 70-200. There is sample variation in lenses far more than in sensors. I wish they could build lenses with the same precision as sensors but they cannot yet or not at prices anybody but NASA can afford.

The labelled focal-length is also often rounded. Some manufacturers go with even numbers, multiples of five while some have used half-millimeters. So a 24-70mm, could easily be a 23-69mm or 25-72mm when measured, again with the focus at infinity.

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For anyone interested, Hasselblad has always put two numbers on their lenses: a nominal focal length (usually what they were trying to achieve with the design) and an actual, tested infinity focal length. For instance, their current H-series 120mm f/4 macro (the mark II) has a published actual focal length of 118.7mm, and the normal 80 is 82.3mm. Since part of their market is scientific/forensic, the exact number matters. It really doesn't for most of us. –  user2719 Dec 7 '12 at 16:26
    
Thanks Itai Now I understand more –  lawphotog Dec 12 '12 at 6:52

You might find some information by looking at a recent video on YouTube by ThatNikonGuy. Here he specifically addresses "focus breathing", but you may be able to see some other differences as well.

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Thanks Tenmiles :) –  lawphotog Dec 12 '12 at 6:53

The focal length of the lens is typically rounded to a nice number. If memory serves, Nikon's 24-70 is actually 24-68 mm. Many (most?) lenses are not exactly the focal length they claim to be, but they are very close to it.

"Focus breathing" is another consideration -- where the focal length of the lens changes depending upon the distance to the subject. Macro lenses always exhibit this behavior when focused at close range, superzooms typically do, too. Nikon's 70-200 f2.8 VRII suffers from this pretty severely, with the 200mm end of the lens coming down to about 135mm.

So, yes, there is a practical difference between what the lens claims to be and what it actually is. Usually, the difference is pretty small. Also, if you can determine where your preferred focal lengths are you may find that missing some range could be something you don't miss.

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Actually, the focal length change of an internally-focused lens is the cure for focus breathing (which is the effect of subject size and field of view change as you change focus). It's a cine/video term, and you'll see it clearly on good old-fashioned unit-focus lenses when the focus shifts from one subject to another (usually during an across-the-table conversation); the image enlarges and the field of view narrows as the lens if focused on the nearer subject. The image appears to "breathe" over the course of the conversation. –  user2719 Dec 7 '12 at 16:15
    
Interesting; thanks for the correction! –  Dan Wolfgang Dec 7 '12 at 16:20

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