It's been raining here in Baltimore all day, so I figured this would be a good time to play around with my new Canon EF25 II macro extension tube. After a few close-up shots of my Xpan, I decided that the world didn't need any more camera porn today. I switched over to the broccoli plant that's been surviving living by the window for the past several months.
I don't have a true macro lens to compare it to, but I do notice quite a bit of softness in the full resolution image. This could be due to the excruciatingly narrow depth of field. I suspect that it's actually due to the fact that the lens is being pushed beyond its normal parameters by the extension tube. The image that's projected from the back of the lens is significantly larger when the extension tube is attached. As such, any softness in the lens quality is much larger as well.
This is similar to the rule of thumb when using teleconverters. A 2x teleconverter will double your lens's effective length, but will also halve its effective sharpness. This is fine if you've got a very sharp lens, but if you're using a zoom, you may be making some real sacrifices in image quality.
Hopefully I'll get my hands on a true macro 100/2.8 soon so I can make a real comparison. For now though, the EF25 should do for the amount of macro shooting I do.
In the meantime, maybe I'll try shooting in the rain...