Crop factor/Lenses - I'm a little confused

[cortisolman]

I understand that some lenses are designed for non-full-frame-DSLRs, but I wold have assumed this means you don't have to multiply the focal length by the crop factor.
Now I read that you do still have to multiply the focal length by the crop factor, so what's the point of the special lenses. (question arising from wanting a wide angle for landscape shots)
Clarification all around would be appreciated, thanks.

[tomberkley]

Yea it is wierd.
I give you a example a common "general perpose lens" which the focal lenth is 24-70mm in the full frame format
for an aps-c camera (1.6x crop factor camera) the equivilent focal lenth is 17-55mm

So for wida angle 14mm on full frame is uxtra wide and evan anything up to 35mm can be considered wida angle.
For a 1.5x/1.6x crop factor camera body to get the same wide shot you will need to go down to 10mm or about there sigma make a 4.5mm fisheye,which i can assume is prity wide, not to sure how good it is tho image/sharpness wise.

What brand do you shoot with?
I know canon make a good 10-22mm ef-s wide angle, as do nikon 10-24mm these are the extra wida angle lenses made for 1.6x factor cameras. and all the third party lens makers make extra wide lenses.

I have a canon 40d (1.6x) and am saving for the canon 24-70mm L which will not be as wide as if i had a full frame camera i will still use it for landscapes. So waht i mean is i am not going to buy any canon ef-s lenses because i hope to upgrade to fullframe in the future and when I do I dont want to have to sell all my es-f lenses because they wont work on my fullframe camera
but if you cant see yourself changing camera then the lenses made for crop cameras are much cheaper and can give some really good quality images (providing you dont buy the crap ones...)
hope this helps
tom

[Rick0r]

They lenses designed for cropped sensors are cheaper, seeing as they don't have to be the high quality required for professional bodies.

[cortisolman]

Thanks guys. Makes good sense.
Tom, I haven't actually got a DSLR yet, I'm trying to figure out which lenses will best suit me, short and long term, so I can make the best possible decision, in order to minimise future upgrade costs - same thing with you getting the lens suitable for full frame.
So I just have to save up to get a really nice, really wide lens.

[fokker]

All lenses are specified in their 35mm-equivalent format to save confusion.

The thing to remember is that 50mm or whatever is still 50mm perspective no matter what camera/sensor it is on. All the sensor size changes is how big the captured square of an image will be out of the full image circle given by the lens. You get the exact same result if you take a photo on a full frame camera and then crop it down to 1.6x smaller in photoshop then if you had taken the photo on a 1.6 fovcf camera in the first place.