When is 50mm NOT 50mm?

What is the "effective focal length" ?

Crop factors are used to give people who were already familiar with FLs in 35mm film (or "full frame") terms an idea of what to expect from a given lens on another platform (like APS-C). I suppose they're particularly handy on tiny sensor digicams where it's really difficult to understand how long 19mm is (is it a 1/2.5" sensor, 1/2.3", 1.1.6" ?) or how wide 5mm is. Personally, I think people would better understand if they were labeled .5X-5X for 1/2 "normal" to 5 times "normal" where normal is defined as the diagonal of the sensor ... the 10X terminology for zoom range is already confusing to a lot of people who assume that, like a pair of 10X binoculars, the 10X means you have "10 times magnification". But anyway ...

A 50 is a 50, and it's both actually and effectively a 50. The fact that it's equivalent to 75 on full frame shouldn't matter to you, unless maybe you're coming from an old SLR and buying your first DSLR and need to know what an 18-55 gives you. (Or if you're coming from a point & shoot and were accustomed to its zoom range being expressed in 35mm terms). Once you use APS-C for a while, you stop using the crop factor, because just as you used to "know" that a 50 is normal, you get used to the idea that between 28mm & 35mm is normal ... no multiplication necessary.

Think of it another way. If a 66mm lens on APS-C is equivalent to a 100mm lens on FF, then by definition, a 100mm lens on a FF body is equivalent to a 66mm lens on APS-C. Why don't the manufacturers label that 100mm lens as a 66mm effective lens ? After all, with the advent of mirrorless, there are stories of people selling off their FF kits and moving to lighter APS-C ... now they need to apply crop factors in reverse. Wouldn't it have been easier if their FF lenses were labelled in APS-C equivalents ? I'm only trying to point out that this equivalence is an arbitrary thing used to compare two systems; if you're not comparing, there's no need to use it.

One more point to consider ... Sony makes a 50mm Alpha mount lens for its SLTs that works on both APS-C and full frame models.  And Sony makes a 50mm Alpha mount lens for NEX.  If you were to label the 50mm NEX lens as 75mm because that's the full frame equivalent, what would you expect if you put the 50mm Alpha lens on the camera with an adapter ?  You'd get the same thing, even though one is labelled 50 and the other 75.  It's really simpler when actual focal lengths are used.

- Dennis

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