The Canon MP-E 65mm is a very specialized lens for 5X magnification macro (Nikoners call it "micro"). It is one of the favorites amongst biologists, geologists, people into shooting rare gemstones, and such. This macro lens can fill the entire frame with a picture of a grain of rice! This amazing lens is also one of the most technically difficult lenses to use correctly. It is a completely manual focus lens where focus is achieve through moving the lens back and forth. Therefore, it is often necessary to use the focus rail mounted on the lens. Since sharpness is of high importance, let's "pixel-peep"; blowing up pictures to look at technical quality of the images.
It is important to understand that the depth of field on macro lenses is very shallow. Also, there is a difference between displayed f-stop and effective f-stop. Canon MP-E manual page 8 displays a chart of displayed f-stop and effective f-stop. For the remainder of this article, I will reference f-stops in the format of "displayed f-stop(effective f-stop)." With this lens, at 5X at f/2.8(f/16.8) the depth of field is only 0.048mm, and at f/16(f/96) it is only 0.269mm. That means if you're off by only 1/4 of a millimeter, your subject will be out of focus! Other technical difficulties make it a challenge to use this lens; there isn't enough light going into this lens even at 1600 ISO, thus one needs to use a macro flash (with near-full power) to get good results. I highly recommend shooting everything on complete manual mode to get a much more consistent result, minimum ISO for least grains, as well as a sturdy tripod, a focus rail, a remote release, and a good knowledge of the relationship between flash power and distance.
While this lens can go up to 5X and f/16(f/96), it doesn't mean all subjects should be shot at f/16(f/96). The reason is at such a high aperture, diffraction in the lens will render overall soft focus. Below is an original (a wedding invitation).
Cropped:
Every lens has an optimal aperture for sharpness, and from the above we see that the optimal sharpness at 1X magnification is attained between f/8(f/16) to f/11(f/22). Since every lens has an optimal aperture at different magnification, below is an illustration for 3X magnification:
It's very hard to see from this picture (Blogger keeps down-sizes this picture) but trust me, optimal sharpness at 3X is at around f/5.6(f/22.4). Now let's look at 5X:
Here we see that optimal sharpness (sweet spot of the lens) at 5X is around f/5.6(f/33.6). However, every picture at 5X is pretty soft. I believe a 12MP sensor on the Canon XSi has more resolution than the lens can resolve at 5X, thus I don't recommend shooting at 5X with modern high mega-pixel cameras.
In summary, for maximum sharpness at 1X, set your camera between f/8-f/11 displayed aperture. At 3X, set your camera to about f/5.6. God forbid you need to shoot at 5X with a modern camera, set it to about f/5.6. Anything less than optimal sharpness (the "sweetspot") will provide too little depth of field, and anything greater will result in lens diffraction-- softness in overall picture. As for flash, use the inverse square-root law. To double your flash power, you can either double the flash power, or just move the flash to-from the subject distance by 1/2.
Lastly, for 3-dimensional still-moving objects that require high depth of field, I still recommend setting the lens to the sweet spot despite its shallow depth of field. Instead, use focus stacking software to auto-stitch multiple partially focused pictures into one picture. Below is an example with 5 different partially focused pictures stitched into one picture:
Same picture as above just cropped further. This is a picture of one grain of salt:
Ok, now that we've found the optimal sharpness in this lens, go shoot bugs and exotic jewelry!
