I'm learning photography and all its elements. Probably this question is really stupid, but I have to ask.
I'm not dealing well when the question is equipment and one of the reasons I know this is because of the negative comments in my answer:
How do I choose a telephoto zoom lens?
When I asked that question, I didn't know these lenses are for different purposes. In fact, I still don't know. To me, they all look like telephoto and I can take the same kind of pictures with them: birds, surfing, etc.
All I know about lens categories I learned here: http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup
But I don't understand several things, like:
How can a EF 17-40mm f/4L USM be an "Ultra-Wide Zoom" and the EF-S 18-55mm f/3.5-5.6 IS be a "Standard Zoom"? One starts with 17mm and the other one with 18mm, they look the same! In my mind, a wide angle lens should start, at least, with 10mm.
How can a EF 50mm f/1.8 II be "Standard & Medium Telephoto" and EF 50mm f/2.5 Compact Macro be "Macro"? They have the same focal length!
Here's what I think is true at the moment: A lens has two important aspects: focal length and aperture. That's all I have to care about. Of course it's not correct, that's why I want some advice to get in the right way of thinking when I buy my next lens. So, what can I read to make the right differentiation between them? How can I know which lens is better for my purposes?
OBS: I'm a fan of the chosen answer in this question: How to choose a lens for my first DSLR? I love to keep things simple and I don't care too much about technical details. It can seem like paradoxical stuff because of this question, but it's not. I'm asking this because I want to buy the right lenses without wasting money, but in the simplest way.
Answer
How can (EF 17-40mm f/4L USM) be "Ultra-Wide Zoom" and (EF-S 18-55mm f/3.5-5.6 IS) be "Standard Zoom" ?
The EF 17-40mm lens fits a full-frame (35mm size sensors) camera. The EF-S 18-55mm fits only a cropped-sensor camera (APS-C size, like any Canon DSLR other than 5D and 1D). On a crop sensor camera, your lens' Field of View (FoV) at the 18mm end is similar to a (18 x 1.6 =) 28.8mm lens on a full-frame camera (which is considered the "standard" sensor size for lens classification).
This is why the 17-40 is "wide" and the 18-55 is "standard".
How can a (EF 50mm f/1.8 II) be "Standard & Medium Telephoto" and (EF 50mm f/2.5 Compact Macro) be "Macro"? They have the same focal length!
Macro is not about focal length, but rather maximum magnification - which is proportional to the minimum focusing distance (MFD). Just how close can you be to your subject and still focus.
A lens has two important aspects: focal length and aperture
These are usually the most important features, but certainly not the only ones. The focal length will determine your FoV (and thus affect your perspective). The max Aperture will determine your minimum Depth of Field (DoF) and your ability to take photos in low light. Other than that, you should consider lens quality - optics, but also construction. Image Stabilization may be important to you. More specific needs will determine whether you need a Macro lens, weather resistant lens (like Canon L series), Diffractive Optics (DO) lens (for compactness) or catadioptric lens (relatively low weight for big apertures and long FL, with extremely low chromatic aberrations.
For starters, though, you should really concentrate in the first couple of items. You should know from previous experience what range of parameters you need for your photography style.
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