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DSLR Crop Factor myths ( APS-C sensor ) (1 Viewer)

Musoman

PETE - Nikon/Sony Shooter
United Kingdom
I suspect many people know this already, but for those that don't, its worth noting B :)

First off, a lens's ability to magnify an image does NOT magically change ( increase ) just because it's on a different ( 1.5 crop ) camera. What does change is the field of view - which is how wide the scene is.

So a 500mm lens on a Full Frame DSLR , or an old 35mm SLR body ( the 35mm SLR is the comparison that 'marketeers' love to quote ) is 500mm. It will still be 500mm when its fitted to an APS-C sensor / crop DSLR body. There is no magical FL increase to 750mm on a 1.5 Crop DSLR.

Take out a £10 note or $10 bill. The Queens face ( or whoever, depending on your currency ) takes up a bit of space on the note but there's still plenty of room. Now fold a bit of each edge back, about 1cm per side will do.

The result is the queens face APPEARS to take up more space and looks a bit bigger compared to the note, right? What's really happened is the size is the same but the area you view is smaller, just like using a lens on a smaller sized crop sensor.

So the numbers that say a 500mm lens is "Equivalent to..." are just comparisons of the field of view. Plus the advertisers like it because it's misleading and makes you think you're getting more magnification so will shell out more money and be happy about it.

Another way of describing it..

If you mount a given lens (say a 300mm) on a 35mm camera, and take a picture, you could stand at the point you took that picture, and see what part of the scene your eye can see, and compare it to what got recorded on the film. If you draw a line from the point the camera was located to the point on the extreme left of the scene in the photo, and do the same for the extreme right, the angle between those lines (from the camera) is the field of view.

With a 300mm, that field of view would be about 6.5 degrees. Now take the 300mm off and put a 500mm on, and you'll get about a 4 degree field of view from the centre point... anything more than 2% either side will be out of the photo.

So if you go back to the frame of 35mm film shot with the 300mm and crop about (roughly) half the celluloid off, you can end up with the same field of view on the film that remains as you'd have got with the 500mm.

When you use a digital SLR with a "cropped" sensor, that's effectively what you're doing. The sensor will be a lot smaller than that 35mm piece of film, so you've effectively chopped the 'out' bits of the film off, compared to either a 35mm film, or a "full frame" sensor.

So ..... if you put a 300mm lens on a 1.5x crop sensor camera and a 450mm lens on a full frame camera, the scene you get in your picture will be about the same.

If I put a full-frame camera on a tripod, mount a 450mm lens and take a picture, and I then put a 1.5x camera with a 300mm lens on the same tripod and take a picture, we'll end up with a picture of more or less the same scene. i.e. an "equivalent" field of view.
 
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Agreed: a clear explanation.


Although I think I did understand this: it's implied in the phrase crop factor rather than magnification factor so I don't feel short changed.

But what i'm not clear about is the effect on image quality:

presumably a full frame sensor with more pixels (larger pixels? 'better quality' pixels???) gives a better image but if you want the same field of view you have to throw away 1/1.6 of the pixels anyway. So everything else being equal if you want that larger view of the Queens head are you better to have a full frame sensor and throw away pixels in the crop or to use the 'cropped' image from the smaller sensor?

Does the crop factor have any effect on perspective?

Does image taken with a 500mm lens on a full frame sensor and then cropped in the printing process to 750mm equivalent show different perspective effects to the same 500mm lens on a smaller sensor?
 
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