I need a wide angle lens that won't distort for taking some architectural pictures. I am told a "rectilinear" lens is what I need. Does the sigma 12-24mm zoom lens qualify? It says "asphereical" is this the same thing? any info would be helpful.
Rectilinear means straight line. A rectilinear lens projects undistorted straight lines onto the film plane so that they are rendered straight in the image. Aspherical designates a lens element formula that results in an image forming piece of glass that is a portion of a hyperbolic or parabolic (I forget which) rather than spherical geometric solid. Neither implies the other, although it is theoretically possible for a rectilinear lens to incorporate aspherical elements. Your best bet, other than to use a view camera, is to get a Canon tilt/shift lens for your Canon camera or a Nikon perspective control lens for your Nikon camera. Minolta's tilt/shift lenses were made for Minolta MD mount only, not Minolta AF and are hard to find.
You will not get the fish-eye effect, but you will get some degree of barrel (lines bowing out) and/or pincushion (lines bowing in) distortion. You will also get keystoning (parallel lines converging) and leaning (the building appears to be leaning back) when the lens is tilted up or down. You could get an expensive PC (perspective control) lens or you could try to correct with Photoshop.
It's a terrific lens. As Mark says, there is a bit of barrel distortion at the edges of the frame, especially at the widest angles. It's barely perceptible for normal shots, but you'd see it if you're worried about straight lines for architectural use. One of the good things about the lens is that it is "full frame" for 35mm film cameras, unlike the Canon 10-22mm zoom which only works on their APS-sized DSLRs. But if you do use a DSLR with a smaller sensor, there's virtually no distortion visible -- what little there is, is at the edge of the 35mm frame, and my 20D's chip doesn't see that part of the image
True rectilinear wide-angle lenses are very expensive, this one is a good value that gives an extremely wide field of view along with very minimal distortion at a reasonable price.
Most people are so used to seeing some linear distortion that they accept it as normal and do not object to it. It is usually a problem only in cases where architectural accuracy is important for some specific reason. Looking back at your post, I see that is the case, so stick with the advise to get the PC lens. They are also called tilt-and-swing lenses.
The photographer who told you that you need a rectilinear lens gave you good advice. You need a purpose built lens and your 12-24 is not built for the purpose for which you want to use it. It would be like using a slotted screwdriver to drive a Phillips head screw. You can do it, but it is not the most efficient way to do the job. Follow the good advice that you have already gotten and get the results that you need.
Thanks for all the feedback. The purpose I am using it for taking pictures of very large luxury estates for a Real Estate Broker. I need something that will have a wide enough angle of view, but not look distorted like a fisheye. A large portion of the pictures will be of the grounds and landscaping, so I guess perfectly straight lines are not necessary. From what you have said, I am thinking the the sigma 12-24mm will do the job. Any more opinions are appreciated.
Any camera with a mirror in the light path will not give distortion free images. You may get lenses with lower deistortion than others, but you always end up significant distortion. Rectilinear in your case means that radial symmetric lens distorion was corrected to some extent. When putting a lens with a focal length of less than the distance between mirror and film plane infront of the camera, you produce extra distortion (basically any lens with less than 35 mm focal length). So, in the digital world you are more or less doomed, unless you spend a lot of money - really a lot. You may correct for lens distortion, photoshop has some plug-ins to reduce the distortion effect. This is your best bet.
Actually I have a camera that distorts only 4 µm at the most up to 81 mm away from the image center that is 0.0049% anything you can buy today for an SLR will be at least at the 1-2% level, for rangefinder cameras you may get a 0.4% - and the typical film size (half the diagonal) will never be anywhere close to 81 mm! Lovely camera...!