Why is the Canon MP-E 65mm F/2.8 Macro Lens not called a zoom?

Question

The Canon MP-E 65mm works differently than my other macro lenses. Compare:

• With a standard 35mm macro, I can frame first and then adjust focus which only changes framing very slightly with.
• With the MP-E 65mm changing focus completely changes framing.

Assuming that the angle of view corresponds to a 65mm lens at 1X magnification, it seems that by 5X, the angle of view is about that of a 325mm lens! In other words, manual focusing (this lens does not autofocus at all) looks like it has the same effect as zooming on a zoom lens with the plane of focus fixed at the closest focus-distance.

So why is this not called a MP-E 65-325mm instead of simply 65mm? More importantly, what does the 65mm focal-length represent? And how can I use that to frame by shots with less guesswork?

Asked by Itai

Answer

It's called a 65mm because that's the combined focal length (light bending ability) of the lens elements. The focusing mechanism on the MPE-65 works like a set of bellows, i.e. it just moves the whole lens back and forth. You can see this as the physical length of the lens barrel changes significantly when focusing. This has the effect of changing both the minimum focus distance and the angle of view, however the focal length of the lens elements remains unchanged.

It's common for prime lenses change field of view slightly on focusing, the effect is just exaggerated with the MPE-65. Lots of rules break down when you get into macro and super-macro photography, focal length, f-stop etc. cease to matter as the formulas for things like depth of field and exposure don't give accurate answers at macro distances. You have to ignore what it says on the lens and use your judgement or trial an error to begin with.

Note it's standard practice to state the focal length of a lens when focussed at infinity (to account for minor variations as mentioned above). As the MPE-65 doesn't focus to infinity I assume they measure it for the 1x setting, so the value is not directly comparable with other lenses.

Answered by Matt Grum

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