View Full Version : Converting focal lenghts from film to DSLR
I know that the smaller DX sensor means that what I used to know as a "standard" 50mm prime on film SLR's would require a different focal length on a DSLR, but is there a formula for working out equivalents, or is it a sliding scale ? i.e if it's two thirds, does what used to be 100mm (@ 35mm film) now equate to 66mm, and 200mm in old money equate to 132mm on a DX lens ?
depends on the crop factor of the body ? not good with nikon stuff but I think the crop factor is 1.5, therefore 50mm is 35mm equiv of 75mm..
The focal length is the same, doesn't matter what camera you put a 50mm lens on the lens will always be 50mm. The image circle will be 'cropped' to give the effect of a longer focal length lens. Depending on the camera the 'crop' factor will be from 1.6 to 1.3, to find out the new cropped focal length of your 50mm lens just multiply it by the crop factor of your camera. The D300 is a 1.5 crop so 50 x 1.5 = 75. Just remember it's still a 50mm lens. Hope that helps.
to be honest im not sure why people worry about this so much, your lens either zooms in/out as much as you want it to or it doesnt.
The focal length of the lens does not change.
But the APPARENT focal length does.
So your 50mm lens on film would be 50mm on a fullframe DSLR, or 50 x 1.6 (the crop factor) on a Canon crop body = 80 mm.
On a 1.5 crop body , 50 mm -> 75mm
On a 1.3 crop body, 50mm -> 65mm.
That is the most important thing to remember.
Thanks folks - I should have said apparent focal length, I realise that 50mm is 50mm :-)
Neil_G I'm not worried, just interested, but thanks for your insight.
Patrick, on your D300 simply multiply the actual focal length of the lens by 1.5 to find out what focal length would give you the same angle of view on your old 35mm film SLR. IIRC, somewhere in the manual there is a list of common focal lengths and their equivalents.
(The actual crop factor is apparently closer to 1.52 but 1.5 is close enough for this purpose!)
Ignoring the fact that the FL doesn't change at all, a 50mm lens is a 50mm lens etc., and it's only the field of view that changes anyway. Probably the most important thing is that it's of absolutely no relevance at all to anyone unless they're used to thinking in 35mm terms anyway. For anyone starting out with digital it won't mean a thing.
However, as you clearly do tend to think this way (as I do) all you need is the "crop factor" and even that's a pretty loose term, but for a Nikon it's around 1.5x ...
So... if a shot you're looking at calls for a 50mm lens (to you) take 50 and divide it by the crop factor ie., 50/1.5 = 33 so pick anything between a 28mm and 35mm lens and you'll be fine.
Conversely, if you have a lens of a particular FL and want to know what the equivalent field of view will be on you crop body then take the FL and multiply it by the crop factor. Say you have a 135mm lens... 135 x 1.5 = 200 (ish)
But it's a lot easier just to look throught the viewfinder and see what you get :)
Ah the old argument about lens size
There are some who tell you it is not different and a 50mm is a 50mm no matter what it is on.
Well one thing you need to look at is the DOF on a full frame it will be different to a ‘crop’ sensor so now tell me it’s the same.
Yes it is a 50mm lens but it is on a camera that has a smaller sensor so will not work like a 50mm lens the field of view will change as your DOF will so read into all this what you want. You never get everyone agreeing with the same thing.
If you are looking for a lens that gives you a normal view then you take the diagonal measurement of the sensor
unless they're used to thinking in 35mm terms anyway.
I am, hence the question.
Thanks for all the replies, I now have something I can work with when looking for s/h lenses.
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