In one of the conversations that I had with Mark Plonsky
he mentioned that he had used a 500D close-up filter with his 100mm macro and liked the bokeh
that the close-up filter produced. Since I had been having problems with isolating the subject in the areas where I shoot, and I hate spending time in Photoshop doing things with the computer that I could be doing with the gear, I decide to give the 500D a try.
Mark, as usual, was right -the bokeh is excellent!
The 500D does take a little getting use to. I don't think that it gives me less depth of field, but it seems to compress the area of focus in the sense that a scene goes from being in sharp focus to being completely out of focus over a shorter distance. The area of sharp focus is the same as not using it, but the area between sharp and blurry is reduced -if that makes sense. Take a look at the background in the image with this post. I shot that female Violet Darter in a peach orchard, but all you see is a diffused sea of green due the way the 500D smoothes out the background.
There is no loss in image quality -period.
The 500D is designed to correct for chromatic aberration and it helps to keep the image circle sharp and, unlike an extension tube, there is no loss of light. The magnification that I get with the close-up filter is almost the same as a 25mm extension tube (1.375x for the 500D verses 1.39x for the 25mm tube). Given the choice between using the 500D and a teleconverter I'll take the close-up filter. But there is one problem...
Don't use a teleconverter with the 500D.
Stacking tubes between the camera and the lens with the 500D is no problem (I've taken a lot of images above 2x with the 500D and tubes), but I wouldn't recommend using a teleconverter with the close-up filter -way too much glass and there is no increase in working distance.
Since I was using the 500D on a prime lens the maximum focusing distance is reduced to 1/2 of a meter from the front of the lens (about 18"). The minimum focusing distance varies with the prime lens that the 500D is attached to, and for Canon's 100mm macro it's about 13cm (5 1/4").
With a telephoto lens the way a 500D affects the working distance and focus is completely different. If you're finding it difficult to get sharp images of critters close to the lens then it might not be your technique, it may just be the glass.
Most telephoto lenses are soft at the minimum focusing distance of the lens -by design. The primary use for a telephoto is to take images of distant subjects, so they are built to be sharp when focused toward infinity.
One way around the limitation is to use a close-up filter (like the 500D) since it will fix the focus at about 1/2 of a meter (approximately 18 inches) from the front of the lens. Since the focus is fixed (the focus ring will only change the area of sharp focus by about a centimeter or 1/2 of an inch) you can set the focus ring toward infinity and the resulting images are pretty sharp. The focal length of the lens changes the magnification, so to get "closer" to the subject all you have to do is zoom in. It sounds like it would be a pain to use, but once you get comfortable with it it's pretty easy to guess just how close you need to be to get to the subject in focus. One added benefit that I've found is when shooting butterflies -often I can get sloe to them, but once I stop moving I can't get closer to them without scaring them off. With a 500D on a telephotos lens I don't have to get close to get more magnification -all I have to do is turn the zoom ring...