DebAnn Loc: Toronto

Can you elaborate on this? If you buy a 50mm lens specifically designed to work only on a crop sensor camera, does it still perform like an 80mm lens?

blackest Loc: Ireland

Yes but it depends on the sensor size my crop sensor has a multiple of 1.5 so a 50mm has an angle of view the same as a 75mm lens on full frame.
If you took that lens and put it on a m43 camera (the ratio of height to width is different but we can ignore that) the crop factor is 2 because the sensor is smaller so on that camera the 50mm lens has a field of view the same as a 100mm lens on full frame. The light going into the lens is the same but because the sensor is smaller, light is spilt on to the back of the camera and only the central part is caught.

It's the smaller diameter front element that restricts the field of view.
http://www.photographymad.com/files/images/crop-factor-sensor-size.jpg

When you print a 6 by 4 from a full frame camera you are enlarging it less than when you print a 6 by 4 from a crop sensor camera.
If you enlarged by the same amount the crop sensor photo would have big white borders, if you cut those off and placed the crop image on the full frame image you would see everything in the picture is at the same size.

If you crop the jpeg from your camera you see a narrower view the same as you would as if you had used a longer lens.
As lenses get longer they bring to your camera a smaller and smaller slice of the world.

Hope that helps some.

DebAnn Loc: Toronto

Thank you. I wish I didn't find it all so confusing.

blackest Loc: Ireland

For practical purposes the view you get is all that matters, a wider lens has a wider field of view than a longer focal length lens. The Ef-s lens has a smaller image circle than an EF lens, but canon designed the EF-S lens so it will not fit a full frame camera. You can use either on the crop sensor and they will perform similarly. A fast 50mm lens is extremely versatile and anything f2 or faster is mostly fast enough. Most interesting photo's are taken with relatively short lenses, if you can smell the coffee the baking bread you are probably around the right distance. You want to be a photographer not a sniper and most of the interesting stories are told from up close.

The rest of the technobabble is exactly that. I really should post less and take more photographs

DebAnn Loc: Toronto

You are very helpful. It's the math stuff that stumps me. What I don't understand is this. If a manufacturer designs a lens for a specific camera (in this case, a crop sensor camera), and he labels it 50mm, shouldn't it behave like a 50mm and not something higher? I understand it would be different if the lens is usable on both a full frame and a crop sensor. But I think a 50mm crop sensor-dedicated lens should be 50mm. Am I being unreasonable?

I have another question. I just purchased a 10-20mm lens which I'm assuming is actually something like 18-whatever on my crop sensor. If I actually wanted a lens to work at 10-20mm, is there a lens made that comes lower than that.?

blackest Loc: Ireland

A 50mm does behave like a 50mm on any camera body the difference is only really due to the increased enlargement used in order to fill the paper.

http://www.bobatkins.com/photography/technical/field_of_view.html

10mm is very short for aps-c about the same as 15-16mm on full frame. It's about the limit really. If you want a wider field of view you need to stitch shots together (the opposite of cropping).

DebAnn Loc: Toronto

Now I get it. Thanks so much for your help.

Bloke Loc: Waynesboro, Pennsylvania

It can be a bit confusing, I admit. By convention, focal lengths for all DSLR cameras, and I guess pretty much all other smaller-than-medium-format cameras, are given as 35mm equivalent. This is now what we call full-frame. It avoids confusion for lenses like Canon's EF range, which are usable on full-frame *and* crop sensor cameras. Really, it isn't anything worth worrying about, though. A 50mm is a 50mm... On a crop sensor, it will *act* like an 80mm, but it is still a 50mm. I know, that still seems weird to me, too!

As far as your other question, 10mm is really pretty wide, even on a crop sensor. I use the EF-S 10-18mm on my T4i, and I have never felt that was not wide enough for pretty much anything. I can squeeze a little extra width with the Rokinon 14mm on the 5DII, but I have never noticed the lack of those extra 2mm...