Do you want 1:1 aspect ratio or are you asking about 1:1 image scale?
For "scale", 1:1 is typical of a "true" macro lens (I say "true" because there are lots of zoom lenses that will work "macro" into their name or specs ... because they can focus on subjects at a fairly close distance. But that close distance usually isn't close enought to be 1:1 scale. It's usually 1:3 or 1:4 scale.
1:1 scale means the size of the object on the sensor is the same as the object in real life. I use a penny as an example. It has a diameter of about 19mm. The sensor is about 22 x 15mm (approximately and it will vary just fractionally by model for APS-C sensor size cameras like your 40D and 7D). This means a penny, when imaged at 1:1 scale, would be so large on the sensor that it wont entirely fit in the frame vertically and would just barely fit in the frame horizontally. You would easily see every tiny detail on the penny at that scale. That's what we often refer to as "true" macro.
To do that you need a true macro lens. Canon makes 4 true macro lenses (60mm, two 100mm choices, and a 180mm macro).
If you actually meant 1:1 aspect ratio... that's different. The 7D has a menu to change aspect ratio (I don't know about the 40D... it may have one also). The "catch" is that this is ONLY used as a framing guide when shooting in "live view" mode. If you look through the viewfinder, that's a physical view through the lens so it can't technically crop that to a square -- but in liveview mode it can.
The even bigger "catch" is that even if you pick a 1:1 aspect ratio, the camera technically records in 3:2 aspect ratio and you have to crop the image on the computer. The aspect ratio is really only providing you... the photographer... with an optical aid so that when you KNOW you plan to crop that image, you can see what will fit in frame when you take the shot. You still have to crop it afterwards.
Tim Campbell
5D III, 5D IV, 60Da
