T3i sensor size vs. full frame and how does that effect lens measurements?

Question

I am new to both photography and the T3i camera. I have been reading Jeff Revell's book "Canon EOS Rebel T3i / 600D: From Snapshots to Great Shots". In chapter two he talks about "Lenses and Focal Lengths". In discussing wide-angle lenses he says
"As for which lenses would be considered wide angle, anything 35mm or smaller could be considered as wide."
Latter he discusses normal lenses and says
"The normal lens for full-frame and 35mm cameras is the 50mm lens, but for the T3i it is more in the neighborhood of a 35mm lens."
I understand that there is a 1 / 1.6 ratio between the T3i and a Full Frame camera. In the first quote does the 35mm refer to the T3i measurement or a full frame measurement?
Is there a standard nomenclature to indicate if you are specifying a "full frame" value verses the T3i value? If so, what is it?
When I look at one of my lenses, say the 18-55mm lens, is this the T3i measurement or the standard full frame?
Thanks in advance for your assistance.
--Jesse
P.S.  I could not find a publisher’s forum for this book or I would have asked the question there.

Answer 1

Jesse-T3i wrote:
"As for which lenses would be considered wide angle, anything 35mm or smaller could be considered as wide."
In the first quote does the 35mm refer to the T3i measurement or a full frame measurement?
Is there a standard nomenclature to indicate if you are specifying a "full frame" value verses the T3i value? If so, what is it?
When I look at one of my lenses, say the 18-55mm lens, is this the T3i measurement or the standard full frame?
The fast answers are:
When he mentions the 35mm as the basis for determining what's "wide" he is referring to the APS-C crop-frame size sensor.
There is no nomenclature to indicate if you are "full frame" vs. "crop frame" on a lens because all lenses are reported in true focal lengths.  There is a nomenclature to indicate if the lens was designed to only project an image circle onto the sensor which is large enough for "crop frame" ... which is what the "EF-S" lens is.  If the lens is for "full frame" (which also works on all crop-frame) then it will have the designation "EF" for most lenses, but Canon also has a few specialty lenses such as the "TS-E" (tilt shift) and "MP-E" (macro photo) lenses which also provide full-frame sensor coverage.  Incidentally... this is just Canon's way of tagging a lens... Nikon uses the terms "FX" and "DX" (for full frame and crop frame respectively).  Everyone has their own tag.  Just remember that focal lengths reported are true (well... there's a TINY bit of round-off allowed) and ignore the sensor size.
Here's the background which may help you understand why this is.
The "normal" focal length is based on the human eye, and to understand it, it's easier to think in terms of "angle of view" rather than "focal length."
If you sit and stare straight ahead without moving your eyes or head to "look around", there is an area of vision that the average person can see (and it does vary), but the "angle of view" for that area is about 40 degrees (horizontally). Of course our vision doesn't present our brain with a nice little rectangular image... it fades and is irregular, but it's generally accepted that the angle of view covers about 40 degrees.
The LENS that provides 40 degree angle of view coverage on a "full frame" camera turns out to be a 50mm lens.  Images taken with such a lens will seem neither stretched out nor compressed... they'll seem "normal" to your brain.  Also, if you can see an object without having to look around to take in the view, then your camera can "see" it with that lens.  
A "full frame" camera is one which has a sensor size which is approximately the same as a single frame of 35mm film.  The dimensions are roughly 36mm x 24mm.  
There are many different crop frame sizes, but for DSLR cameras, the most popular (by far) is the APS-C size.  This means the sensor is roughly the same size as a single frame for the "Advanced Photo System - Classic" film.   You can think of this as being roughly 21mm x 14mm (although it varies by a just a few millimeters -- on your camera it's really 22.3 x 14.9mm).
If you were to use a movie projector and movie screen and you projected an image so that it fit perfectly on a 12' x 8' screen and called that "full frame", then a "crop frame" would be derived by shriking the movie screen down to about 8' wide by about 5-1/3' tall but (and here's the key) without doing ANYTHING to the movie projector.  Essentially that means you're projecting an image intended to fit ona 12x8' screen... but the real screen is smaller.  What happens to the part of the image that doesn't fit on the screen?    It just spills off the sides and is lost.  That is EXACTLY what happens inside a crop-frame camera.
Canon EOS "EF" lenses are designed to project an image large enough to fill a full-frame sensor ... so when used on a crop-frame camera some of that image just spills off the sides.   This means that if your lens was providing a 40 degree angle of view (measured horizontally) then the crop-frame is only capturing the 26 degrees in the middle of it.  If you want a 40 degree angle of view again... you'll need to change lenses.
And this is where your author suggests a 35mm lens is "normal" for a camera with an APS-C body.  The math actually works out in the neighborhood of a 31mm lens, but nobody actually makes a 31mm lens... and a 35mm focal length (commonly available) is pretty close.  Also 28mm lenses are commonly available and are ALSO pretty close.  It's my personal opinion that 28mm is probably a bit more normal than 35mm, but that's because as an avid amateur astronomer, I am fairly used to looking through Plossl design eyepieces which provide a 50 degree apparent angle of view and my eye actually can see the edges of the frame, but if I go a bit wider... I can no longer see the edges of the field without "looking around" (regardless... 40 degrees seems to be the established norm and THAT is the value that everything is based on.)
For purposes of buying lenses, the focal lengths are NOT converted or adjusted in any way.  For example, Canon makes "EF-S" lenses designed specifically to work with their crop-frame bodies.  So when the kit lens that comes with the camera says it is a zoom with an 18-55mm focal length range... it really is 18-55mm regardless of sensor size.   (Canon isn't multiplying or dividing focal lengths and putting a different value on the box just because it's intended for a crop-sensor camera.)
Hope this helps.
Regards,
Tim
Tim Campbell
5D II, 5D III, 60Da