It has been a while, hasn't it?!?! Got to start writing again.
Right now I'm going to go back to macro and close-up photography. Something I started back at http://cornicello.blogspot.com/2007/05/which-macro-lens-should-i-get.html
Here I want to talk about the various ways to do close-up photography, especially concentrating on supplementary lenses (sometimes called Closeup Filters).
They typically come in powers or diopters, such as +1, +2, +3, +4
These affect the working distance (distance from the lens to the subject), as opposed to focusing distance (distance from film/sensor to the subject). They shorten the focal length of the lens they are attached to. The shorter the original lens, the less effect. And because they shorten the lens focal length you run into issues like short working distance (no room for lights, too close to some live subjects) and too much background coverage (you see the house next door, the sky, etc., etc.). My suggestion is to use the longest practical lens you can for the situation. Minimum 100mm for full-frame Macro (or 60mm for a crop factor digital camera). But preferably 150 to 200mm. This is so you can isolate your subject from the background. 50mm macro lenses are great for flat copy type work (postage stamps, etc.). But in the field they are generally too short for most situations.
The formula for figuring out the new focal length is 1000/((1000/focal length)+diopter)
50mm lens with a +3 diopter
1000 / ((1000/50)+3)
1000 / (20+3)1000 / 23 = 43.5mm
24mm lens with +2 diopter
1000 / ((1000/24)+2)
1000 / (41.7+2)
1000 / 43.7 = 22.8
100mm lens with +1 diopter
1000 / ((1000/100)+1
1000 / (10+1)
1000 / 11 = 91mm
100mm lens with +3 diopter
1000 / 13 = 77mm
No matter what the original focal length of the lens, it will focus at the same distance. At infinity that distance will be approximately 1 meter divided by the diopter power. A +1 lens focuses at 1 meter (~39 inches) in front of the lens. A +2 focuses at 1/2 meter (~19.5 inches). A +3 at 1/3 meter (~13 inches). A +4 focuses at 1/4 meter (~10 inches), etc. You can combine them. For example a +1 and a +3 would equal a +4 and focus at a maximum of .25 meters. You lose distant/infinity focus when using closeup lenses. If you combine closeup filters put the stronger one closer to the camera and don't stack more than two.
As all lenses with the same closeup lens attached focus at the same distances, the longer lenses will show more magnification. But with the increased magnification you run into sharpness issues, especially around the edges of the frame.
Canon and Nikon make dual-element close up filters that are better quality with longer lenses. Canon offers two. The 500D is a +2 diopter and the 250D is a +4 diopter. The names come from the working distance. A +2 diopter lens focuses at a maximum 1/2 meter (500 mm) and a +4 focuses at a maximum 1/4 meter (250mm). They recommend the 250D for lenses from 28mm to 135mm and the 500D for lenses from 75mm to 300mm. The 250D is available in 52mm and 58mm sizes. The 500D is available in 52mm, 58mm, 72mm, and 77mm sizes.
The Nikon closeup filters have names like 0, 1, 2, 3T, 4T, 5T, and 6T. The 3T and 4T are for medium telephoto lenses (like the Canon 250D) and the 5T and 6T work with longer lenses. The 0 through 4T filters are 52mm in diameter, the 5T and 6T are 62mm.
Focusing and composing can be quite different in macro photography than in general photography. If you are interested in reproduction ratios, for example you want to get an image that has a 1:1 size relationship between the subject and the image projected onto the film/sensor) you would set the lens focus for the 1:1 ratio and then move the entire camera back/forward to achieve focus. If you turn the lens's focusing ring you change the reproduction ratio. If you are not as concerned with an exact scientific number, you can combine lens focus control with camera distances. But it may be a little confusing at first. And you have to remember that you will have a maximum focusing distance and infinity focus will not be available.
Other alternatives for close-up photography are teleconverters and extension tubes.
Teleconverters magnify the image while maintaining the same closest focus distance of the lens. So a 100mm lens with a 1.4x converter becomes a 140mm lens, but still focuses as close as it did without the converter allowing you some extra magnification.
Extension tubes move the lens further away from the camera body, allowing the lens to focus on objects closer to it.
Both converters and extension tubes involve light loss, but through-the-lens metering should compensate. Still consider that you will need more light or longer exposures with these. And you can combine them, too.
Macro lenses are the best option if you will be doing a LOT of macro work. Their downside is extra cost. Again, my preference is for something at least 100mm focal length with a full frame camera. With a cropped sensor camera, I would start at around 60mm or longer.