Canon EF lenses on a croped sensor body

hashcake

hashcake

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I'm trying to get my head around the true focal lengths when using lenses such as a Canon 70-200L f/4 IS and a 24-105L f/4 IS on a 40D.
I know there is a formula for calculating this but I have no idea what it is.
I'd appreciate it if someone could explain this.
 
hashcake said:
I'm trying to get my head around the true focal lengths when using lenses such as a Canon 70-200L f/4 IS and a 24-105L f/4 IS on a 40D.
I know there is a formula for calculating this but I have no idea what it is.
I'd appreciate it if someone could explain this.
Click to expand...

Multiply everything by 1.6 :thumbs:
 
That is, multiply focal lengths by 1.6 - everything else, f/ number etc. stay the same
 
As above, x1.6.

But...

Remember that the true focal length of a 70-200 is 70-200, that doesn't change. It is what it is, 70-200mm. With an APS-C what you see is effectively a crop cut from the centre of the frame which creates the effect of multiplying your zoom range by 1.6.

So, 70-200mm on a Canon APS-C camera looks like 112-320mm.
 
woof woof said:
As above, x1.6.

But...

Remember that the true focal length of a 70-200 is 70-200, that doesn't change. It is what it is, 70-200mm. With an APS-C what you see is effectively a crop cut from the centre of the frame which creates the effect of multiplying your zoom range by 1.6.

So, 70-200mm on a Canon APS-C camera looks like 112-320mm.
Click to expand...

You beat me too it, thanks :)
 
Thanks for the replies chaps.
 
The light-gathering power (i.e. the aperture) stays the same, but the effective aperture for depth of field increases by approximately 1.6x too.

So, a 100mm f2 lens on an APS-C body has the field of view of 160mm on a full-frame, f2 light gathering power and depth of field equivalent to about 160mm f3.2 on full-frame.

Andy
 
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