Crop factor cameras and zoom.

A bird!, originally uploaded by norbography.

This BLOG entry has nothing to do with birds. Its all above zoom and cropped sensor cameras. I use this photo because it is perhaps one of my better shots with my 70-200mm zoom lens.

I must admit, I have had a bit of a bug up my arse for months regarding this topic. The thing that has burred me up is the statement you hear from some quarters re. cropped factor sensors giving you more reach! They dont. I am going to quote a few different sources here, but I am going to explain it as best I can.

A cropped sensor has no way of making the camera lense magnify the object you are taking a photo of. Full stop. Now, to quote some people far more eloquent than myself.

Dale Allyn says in a Flickr thread here Picture your film camera (or Canon 5D) capturing an image of the front of a school, from building edge to building edge. It doesn't matter what lens we use for the shot or the focal length.

Now picture a smaller rectangle drawn on that image that is about 38% smaller than the field of view captured by the film camera shot. It would exclude the the ends of the school building, some of the sky and some of the foreground, just like we were using a longer lens – just like a lens that was 1.6 times longer in focal length. That's the view that we would record with an XT, 20D or 30D.

This is how a crop factor sensor "sees" the scene. It's a smaller sensor than the film (or full-frame sensor like the 5D) and so the rest of the view is just sort of splashed off of the sensor edges. (Not really splashed off, but for our purposes here). It's sort of like having a projection screen set up in your house, but the projector is set so that 38% of the projected image is cast outside of the screen boundaries. We don't see or enjoy the details outside of the dimensions of the projection screen.

There is an excellent explanation here.

When it is all said and done, a cropped sensor gives you the field of vision of a longer lense, but not the reach/zoom of one.

Phew.