What is the accurate zoom spec?

[NCflyfisher]

I am considering a new Tamron 16-300mm for my digital Canon T2i. I currently have a Canon 75-300mm. When comparing the two at 300mm, the image size (horizontal dimension) captured varies significantly, and depends on subject difference. Four tests I took show the Tamron target image smaller than the Canon image by the following amounts: 8 feet (subject distance) - 40%, 25 feet - 20%, 66 feet - 11%, 200 feet - 3%. Clearly the two begin to merge only at long (100+ foot) subject distances. Tests were with camera on a tripod and repeated twice. What is the reason for this, and is it a defect in the type of design? This Tamron appears to have less of a maximum zoom amount than the cheaper Canon, especially with 'closer' subjects. By the way, I tested also a Canon 55-250mm at max zoom (250), and all images were the exact relative size (16.7% smaller or 250/300 the size), which I would expect. Any ideas?

[Manfred M]

Actually, your expectations are likely not quite on.

The answer is that the zoom distance is measured with the subject at the "infinity" position, and the actual focal length at distances closer to the camera can indeed be shorter. The degree of magnification is one of many parameters that the lens designer has to manage when designing a lens; image quality, speed (f-stop), distortion, aberration correction, etc. all come into play. If the lens is close to 200mm throughout the range from closest to most distant focusing distance, something will have to give.

The top of the line Nikkor f/2.8 70-200mm VR II lens has a focal length of around 135mm at the closest focusing distance rather than the 200mm shown on the focal length scale.

[dasmith232]

+1 to MM's comments above.

As a similar example, a friend of mine had the same Tamron lens. I had the Canon 75-300 (which I no longer have) and the Canon 70-300. We checked all three at the 300mm mark and the two Canon lenses were about the same (probably didn't measure quite as precisely as you) and the Tamron looked wider/shorter when we were focusing on a nearby target.

So I think what you're seeing is expected.

[NCflyfisher]

+1 to MM's comments above.

As a similar example, a friend of mine had the same Tamron lens. I had the Canon 75-300 (which I no longer have) and the Canon 70-300. We checked all three at the 300mm mark and the two Canon lenses were about the same (probably didn't measure quite as precisely as you) and the Tamron looked wider/shorter when we were focusing on a nearby target.

So I think what you're seeing is expected.

What you guys are telling me makes sense, I just never thought about it. I assumed that all 300mm lens settings would give the same image results at the same distance. But apparently this is only true at infinity. In fact, I can see the image size change as I manually focus the lens, so I guess that is another way of confirming this result. Are there other specs or info that would tell me how the lenses might compare at a closer (say, 100') distance? Or is it a mystery only to be determined via experiments?

[Manfred M]

Or is it a mystery only to be determined via experiments?

Correct!

In fact, I can see the image size change as I manually focus the lens, so I guess that is another way of confirming this result.

This is called "focus breathing". Something that video shooters hate, as in video, pulling focus without changing the focal length of the lens is a fairly common technique. Video lenses are designed so that they do not exhibit this, but it is generally not an issue in still photography. I understand that macro shooters do have this issue too when focus stacking.