understanding the crop factor

stumac

stumac

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I see alot of posts trying to explain the crop factor and how it affects the focal lengths of a lens and i think ive worked out an easy to understand way to explain it.I am no expert so please correct me if im wrong.
so......You have a white wall, now draw a rectangle on it,this rectangle is your full frame sensor,now draw a rectangle slightly smaller inside the bigger one,this is your crop sensor.now get a projector,this is your lens, and project an image on the the wall so that it fits to frame with the large rectagle,so this is what your full frame camera can see.the image in the smaller rectangle is what your crop camera can see,so even though nothing has moved position(weather that be the photographer,the zoom ring or the subject)the crop rectangle shows an image that appears to be zoomed in or cropped but in fact the image is still exactly the same size.
so a 50mm lens is a 50mm lens its just the rectangle/sensor that sees it differently

hope I make sense and hope this helps
 
Good link, I think it backs up what um saying just alot mire detailed
 
stumac said:
I see alot of posts trying to explain the crop factor and how it affects the focal lengths of a lens and i think ive worked out an easy to understand way to explain it.I am no expert so please correct me if im wrong.
so......You have a white wall, now draw a rectangle on it,this rectangle is your full frame sensor,now draw a rectangle slightly smaller inside the bigger one,this is your crop sensor.now get a projector,this is your lens, and project an image on the the wall so that it fits to frame with the large rectagle,so this is what your full frame camera can see.the image in the smaller rectangle is what your crop camera can see,so even though nothing has moved position(weather that be the photographer,the zoom ring or the subject)the crop rectangle shows an image that appears to be zoomed in or cropped but in fact the image is still exactly the same size.
so a 50mm lens is a 50mm lens its just the rectangle/sensor that sees it differently

hope I make sense and hope this helps
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I think the problem with this explanation, for a beginner, is that a) it says "now get a projector" which very few people actually have these days and some folks won't even know what one is or does, and b) it then says "this is your lens".

This is confusing since the whole point of the explanation is to understand that actually the crop factor has nothing to do with the lens (and focal length doesn't change) and it's pureply down to the size of the sensor.

The best description I've heard is to imagine a full frame sensor, and then take some masking tape and cover over a bit from the top, bottom and sides. You have then 'cropped' the sensor, made it smaller and so reduced the field of view, but the lens hasn't changed.

Therefore, in order to restore the orginal field of view, you must use a shorter focal length lens which naturally forms a smaller image that will fit into the reduced cropped area.
 
OK, but this is pretty straightforward, and there's no reason to make it more complicated than it needs to be. I think the confusion for a lot of newcomers is the idea - sometimes touted by salesmen - that crop cameras actually increase the 'power' (focal length) of a lens.
 
It may be nice to understand crop factor but for most do you need to, look through the view finder and what you see is what you get.
 
paulminus273 said:
It may be nice to understand crop factor but for most do you need to, look through the view finder and what you see is what you get.
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Not so.

I know going through the viewfinder of a 550d (cropped) that i was taking a photo of my car, now the lens was a 50mm prime and i could stand no further back, going by the viewfinder view, i was going to miss out on a bit of the front and rear bumpers. Then when i put the pic on to my pc it was all there, bumpers and more, but i think this is more to do with the percentage of view from the viewfinder rather than crop factor.
 
Dexy101 said:
Not so.

I know going through the viewfinder of a 550d (cropped) that i was taking a photo of my car, now the lens was a 50mm prime and i could stand no further back, going by the viewfinder view, i was going to miss out on a bit of the front and rear bumpers. Then when i put the pic on to my pc it was all there, bumpers and more, but i think this is more to do with the percentage of view from the viewfinder rather than crop factor.
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Exactly that, nothing to do with the crop factor. If you were on live view, you would have seen the whole car.
 
so your view finder crops the cropped image again
what sort of idiot designed that lol
Surely it would make life much easier if what you saw through the VF was what you got ,certainly for setting up the shot
 
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To do so increases cost and size.

It does make like much better to have a 100% viewfinder and that's one of the reasons why people spend loads of money on pro cameras.
 
Nawty said:
To do so increases cost and size.

It does make like much better to have a 100% viewfinder and that's one of the reasons why people spend loads of money on pro cameras.
Click to expand...

Agreed, but you can usually make the necessary allowances, if you're used to the camera.
 
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