Difference in focal length between zoom and prime

aerobandit

aerobandit

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Piotr
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I don't know if anyone else noticed it, but once I compared canon 500mm f/4 with sigma 50-500mm. I was expecting the same focal length, but Sigma was noticeably shorter, I would say around 420mm on the long end compared to canon. I tried to google this, but didn't find anything. Is this the case with all zooms that their not really what it says on the case in terms of focal length?
 
rob-nikon said:
Could the explanation be focus breathing? I've heard of a few lens do this and give a shorter point of view or focal length.

Edit: looks like you are not the only one to notice it, try focusing closer to infinity to see if it changes.

http://www.fredmiranda.com/forum/topic/1083130
Click to expand...

That's exactly what I've noticed. I was going to buy new Tamron 150-600mm, but now I wonder how badly it would be affected by focus breathing??
 
I had a Canon fit 50-500 many years ago, and from what I recall of the reviews at the time the actual focal length was some way short of 500mm. Still a cracking lens though..
 
I saw a test a few years ago between a Canon 400/5.6 prime and a Sigma 50-500 at the long end and shooting from around 10 metres or maybe a bit less I can't remember exacktly, there was hardly any difference in them with the Siggy being just a bit longer - probably about 420mm assuming the Canon was 400mm. The person who owned the Siggy could not believe it . Of course when you shoot something at infinity the zoom lenses will come nearer to their advertised length but even then there can be a tolerance of around 5% or even a bit more.
 
There are quite a few "reviews" on the subject,
just Google "lens breathing"
Here's just one
 
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