Lens length confusion

Started Jan 14, 2013 | Questions thread
AltLens Regular Member • Posts: 107
Re: Lens length confusion

malteser01 wrote:


30 years ago photography made sense to me. I used to have a Canon EOS 650 and enjoyed it very much although it burned through expensive batteries pretty quick - still works. Anyway, I'm getting intersted again in photography and the main issue I have is with lens lengths.

I just read the review of the Sigma 35mm 1.4 DG HSM (what a mouthful) and saw that it has a total length of 94mm. Now, in my simple pea brain, I seem to recollect that my 50mm Canon EF lens was 50mm from film to front of lens so the lens was just under 50mm total length or thereabouts - that's why it was 50mm (and of course it's a full frame camera since there was nothing else in those days). Furthermore, the 135mm lens was around 135mm long and longer lenses were similarly obvious lengths - i.e. what it said on the barrel was what it was.

So why is it that this 35mm lens is 94mm long when it's meant to be 35mm at full frame? How does that work?!

Sorry if this seems such a basic question, but it just doesn't make sense to me.


The 50mm, or 35mm, or whatever the focal length is, is still basically the distance from the focal plane to the focusing lens.  Internal focusing is one reason a lens barrel is longer.  The lens still moves in an out to achieve focus, but you don't see it because the overall length is longer than the longest focusing length, and the whole operation is contained within the barrel.  I'm sure there's more to it.

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