I've used film in a number of SLR cameras but Im becoming interested in the new digital side of the house. I understand the sensor's size causes any lens length to be multiplied by 1.6, so that the camera takes pictures through a 100mm lens as if it were a 160. That being said, how do you purchase a wide angle lens for this? If you buy a 20-something lens it at least becomes a 32mm lens. Also, is it true that the lenses intended for this sensor size will not work with future, "full" size sensors? What you are asking about is the FLM or Focal Length Multiplier or FOVC (field of view crop) effect. With a 1.6x FOVC you are not really using a 100 mm lens as 160mm, but you are viewing what you would have viewed as if the lens were 160mm. So its not really a multiplier but really a crop factor. You can check it out for yourself in the link provided.
Since we are talking about Canon lets use Canon terminologies. The Canon EOS family of cameras use the EF series of lens. All EF lenses will fit on a EOS body but they will have different FOVC based on the body its attached to. So the EF 28-135mm will be a real 28-135mm lens on a full frame body (like 5D or 1Ds Mk.II) but will "appear" like a 45-216mm lens on an XTi which has a 1.6x crop factor. See the link provided and judge for yourself if you are really getting 216mm or its merely an appearance of 216mm.
Canon created another line of EF lens called the EF-S lens for its 1.6x cropped body cameras. These will ONLY work on 1.6x bodies like 30D and XTi etc but NOT on the 5D or 1Ds Mk.II. These include the EF-S 18-55mm lens. It is a REAL 18 to 55mm lens which projects an image that matches the sensor size, with an EF lens part of the image fall outside the sensor. It should be noted that at the wide end there will be vignetting on a full frame body but on a 1.6x that part falls outside the sensor so there is none.
Now for true wide angle Canon has the EF-S 10-22mm lens that is specifically meant for the cropped bodies and so using the 1.6x factor you will get an image on a cropped body that equal to what a real 16mm lens would project at the wide end.
What does all this mumbo jumbo mean in the real world? Cropped bodies are not going to go away. Its cheaper for them to produce an APS-C size sensor than to make a full frame sensor (and hence the price difference). By making cheaper cameras they are able to sell more units so it makes perfect business sense. And the last I checked only Canon make the Full Frame sensors and other have to follow suit (Nikon D2X is a 1.5x FOVC body), when that happens the price of a full frame camera body will fall but it will not affect the cropped 1.6x bodies.
The other thing to note is that Canon also has the 1.3x bodies in the 1D Mk.IIN and the 1D Mk.III. These APS-H sensors have the added advantage of no vignetting and little crop since its only1.3 vs 1.6. You're absolutely correct in saying that lenses made for the current digital sensor size won't work properly on cameras with full-frame 35mm sensors. That said, cameras today are pretty darn good! And you'll probably keep your camera for years to come!
There are digital only zoom lenses made that begin around 17 or 18 mm. These would be similar to 28mm lenses on full-frame cameras. Not super-wide angle, but useful in most circumstances.
If you currently shoot with the EOS system, your present lenses will work on a Rebel Xti, as I'm sure you know... Greg is absolutely correct. The 12-24 length is becoming well-known as the "digital wide angle lens of choice." I don't own one, but I do own some that go to 17-18 mm as Greg mentioned.
Here is a mini-tutorial I made myself to compare focal lengths. This is NOT a lens test or a camera test! It is merely intended to show the difference between various focal lengths. The camera was a Nikon D200 and the lens was the Nikon 18-200 VR lens. There is further explanation on the image itself. It would help if you click on "All Sizes" above the image.
http://www.flickr.com/photos/7189769@N04...
Here are a few "wide" shots to consider.
This was taken from about 8-10 feet away at 18 mm:
http://www.flickr.com/photos/samfeinstei...
My wife's first picture with her D50 at 18 mm:
http://www.flickr.com/photos/samfeinstei...
This is about 50 yards away at 19 mm:
http://www.flickr.com/photos/samfeinstei...
A DX or Di (etc) lens will vignette on a full-frame sensor, because it is idealized for the smaller sensor of most digital cameras. | 