70D question

Question

This is probably going to be a really dumb question, but I am going to ask anyway.  I just bought my first dslr camera, the 70d, and it's all a bit overwhelming for a newbie.  I intend on buying one of the DVD sets that explains the cameras features but in the meantime, the one thing I haven't been able to find in the manual or by clicking through the cameras menu is this, and it's my dumb question;
I will use this camera for wildlife photography and while watching the numerous camera review videos before I made my purchase I heard where they kept talking about how this camera has a crop sensor of 1.6x and by using this feature it would turn a 255mm lens into a 408mm lens?  Ok, so is this a feature you turn off and on or is this camera just always using this crop sensor?  And I think crop sensor was the correct term?
Dee
previous camera - Nikon CoolPix
new camera - Canon 70D
new to the dslr world, have a lot to learn

Answer 1

It's not a feature that you turn on and off... the sensor size is what it is.  It's one aspect of your camera that never changes.
I'll try to explain.
A "full frame" sensor simply means that the digital sensor is the same size as a single frame of 35mm film.  That size is roughly 36mm x 24mm.  A "full frame" Canon body would be either a 6D, 5D III or 1D X (not listing any of the cameras that Canon no longer markets).
Your camera has an "APS-C" size sensor.  That means the digital sensor is roughly the same size as a single frame of APS-C film.  APS-C is "Advanced Photographic System - Classic".   A frame of that film is slightly smaller... about 22.5mm x 15mm.  The APS-C crop-frame bodies from Canon include all the Rebel bodies (T3, T3i, T5i and SL1) as well as the mid-range 60D, 70D, and 7D.  Also, the EOS-M is an APS-C body (though that's a mirrorless camera or "MILC" and not a "DSLR").
The lenses, however are the same.  You can use the same Canon EF series lenses on your camera (and all Canon EOS cameras) as you can on a full-frame camera.  The EF lens projects the same physical size image circle into the camera body on your camera as it does on a full frame camera.
The APS-C cameras can also use Canon's EF-S lenses.  More on this later in the post. 
Since the sensor on your camera is physically a little smaller, it means a bit more of the image "spills off" the sides and is not recorded as part of the image.  
The APS-C format size is referred to as a "crop factor" because it's as though you took an image from a full-frame camera, cropped in to just use the center area... but then enlarged the print.  It makes it *seem* as though you enlarged the image ... or "zoomed in" a bit more.
The crop factor is 1.6.  This means that if you were to shoot a subject at the same focal length (suppose you use a 50mm lens) with a full-frame camera, and then again with the 70D, it will look as though the image from the 70D was zoomed in by 60% -- effectively the 50mm lens will look a bit as if we had used an 80mm lens on the full-frame camera.
So now that we've established that the "crop factor" on the sensor means you get a narrower angle of view... and that the narrower angle of view makes it seem as though you've zoomed in a bit, that seems to work in your favor at the "long" end (when you want a long telephoto focal length).  BUT... what happens when you want the opposite?  What happens when you want a wide-angle lens?  Now the crop-factor works AGAINST you.   It makes an otherwise wide angle lens (on a full-frame body) seem not so very wide at all.  Canon offers a different set of lenses that deal with this issue -- those are the EF-S lenses.
The EF-S are lenses specifically made for the crop-frame sensor cameras.  I have a Canon EF 100mm f/2.8L IS USM Macro lens for my 5D III -- it's a nice macro lens.  But for the crop-factor bodies, Canon makes the EF-S 60mm f/2.8 USM Macro.  It turns out if you multiply 60mm x 1.6 you end up with 96mm -- which is VERY close to the 100mm focal length Macro that they make in the EF series.  These two lenses provide roughly equivalent angles of view on their respective crop factor bodies (of course you can ALSO use the EF 100mm macro and get an even tighter angle of view.)
So back to your question...  you take the focal length of the lens, multiply it by 1.6, and that gives you a full-frame equivalent (technically it's not equivalent) -- 250mm x 1.6 = 400.  So it would be a bit like using a 400mm lens on a full-frame camera.
Here's an article at Cambridge in Colour that discusses it:  http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm
Here's a video that discuses it:  https://www.youtube.com/watch?v=yAdI5BBgFHQ
Tim Campbell
5D II, 5D III, 60Da