Hi guys. Another beginner photographer recently made a comment in another thread
about focal lengths of lenses changing depending on what body (camera) the lens is on. Since there hasn't been anything on this topic for a while, I thought I'd write a post on the issue.
Please note that I am far from an expert on this issue, but I do feel like I can understand it and explain it in person. If you find a mistake in my post or disagree with me, please write a post by all means.
Instead of writing my own post on the issue, I decided to do some searching to find out how other people had explained it (and to solidify my understanding of the topic). I managed to find some really good articles on it, which explain probably better than I could.
While reading through the links, don't be put off by the maths and the geometry, you will be able to work it out eventually
(though doing geometric optics in a university physics course does help).
So here goes:Probably the best explanation I've seen, including geometryWritten for Canon lensesWritten from a Pentax point of view
The bottom line is that one should think in terms of field of view
(or angle of view), and not in focal length
. Focal length is a physical property of the lens, whereas field of view is a function of sensor size (and image circle size). I.e. a 50mm lens has a focal length of 50mm, regardless of which body you put it on.
I don't know what the terminology for other makes is, but in Nikon parlance, DX
bodies and lenses have an APS-C
sized sensor, which is a crop of a 35mm frame.
Nikon FX bodies have 35mm-sized sensors, i.e. the same as a 'regular' film camera. If you put a DX lens on an FX body, you will see dark edges on your frame (known as vignetting). This is because the image circle produced by a DX lens is smaller than an FX sensors. Note: an 'image circle' is the picture produced by the lens that falls onto the image sensor.
Because a DX lens needs to produce a smaller image circle than an FX lens, because the sensor is smaller, DX lenses can be physically smaller than an FX lens, for the same focal length. Thats why a 300mm DX lens is physically smaller (narrower and shorter) than a 300mm FX lens.
You will also notice that a lot of compact and bridge (i.e. non DSLR) cameras have an 'equivalent' focal length marking. This does not mean that the focal length is different, it means the field of view is different. The lenses of compacts and bridges are much much smaller than SLR lenses for the same focal length.
This is again because the sensors in such cameras are much smaller than even the APS-C format. (Smaller lenses generally mean lower quality, more noise, more grain etc, because of the electronics.) (Note I am not talking about megapixel count here, only physical size).
For example my Panasonic Lumix DMC-TZ1
has a 10x optical zoom (35-350mm equivalent) lens which is physically much smaller than the equivalent DX or FX lens would be (taking into account the fact that the Lumix has a a 'folded lens').
So if you really want to sound smart while talking to your photography buddies, rather talk about field of view instead of focal length
The 'mite formerly known as joshilewis
FGASA Level 1 GuideGlen Reenen TR
15-18 June: Berg-en-dal