Originally posted by stevbike Consider the fact that you may be getting a little bigger then life anyways on a D-slr considering the fact that the image is cropped...
The crop-sensor doesn't affect magnification. A 1:1 setup at close-focus distance will deliver 1:1 images whatever the frame size.
Originally posted by lars.o Are there any lenses that offer magnification greater then 1:1 without additional equipment such as extension tubes?
No.
Quote: Is using extension tubes to go beyond 1:1 even a good idea? Are there other options?
There are two ways to increase magnification: extension, or supplementary optics.
First, some definitions. A camera lens is one that can be directly mounted on a camera. A camera macro lens has extension built into its body, such as my Macro-Takumar 50/4 or Vivitar-Komine 90/2.8 macro. An enlarger or bellows-macro lens has no focusing mechanism, is meant to be used on extension. Extension means bellows and/or tubes; a reversed camera lens with a deep front inset also provides some extension.
Increased magnification with a macro or non-macro camera lens, or to an enlarger or bellows-macro lens, can require adding LOTS of extension, especially with longer focal-length lenses. So if you have a 1:1 100mm camera macro lens, you'll need another 100mm of extension to reach 2:1. I have used longer such setups but only with a shoulder-stock mount for stability -- and the gear looks like a firearm!
Adding extension is easier with shorter lenses. A 1:1 50mm camera macro lens only needs another 50mm of extension to reach 2:1. But working just two inches away, you will probably scare the bugs! Non-macro camera lenses can be mount-reversed, with extension added, for fine macro results. But a reversed prime still has a working distance under two inches.
One trick I recently discovered: I put a lousy A35-80/4-5.6 zoom, Pentax's worst, on a mount-reversal ring, and got good macro results! At 35mm it reaches about 2:1; at 80mm it reaches around 1:2 at 6in / 15cm working distance, and can also focus past infinity. But high magnification still means working *very* close.
So, what about supplementary optics? A corrected close-up adapter like the Raynox DCR-150 or -250 allows for pretty close work with decent results. But the classic tool for clean optics is with reverse-stacking of lenses. Magnification is a simple ratio. The lens mounted on the camera is the PRIMARY; the reversed lens is the SECONDARY; and magnification is the ratio of focal lengths, PRIMARY : SECONDARY. So a 50mm lens reverse-stacked on a 100mm gives 100:50 or 2:1. Stacking a 35mm lens onto a 105mm gives 105:35 or 3:1. Extreme magnification can be reached this way! BUT... working distance will be under two inches. Reversed primes always have a working distance equal to their register, which for Pentax lenses is around 4.5cm.
A teleconverter is another form of supplementary optics. This will increase your working distance by its TC X-factor, but will also degrade image quality to some extent. This IQ loss probably won't be too great with a TC that is well-matched to the camera lens. I have even use a variable-magnification macro TC. It worked for me; but then, my standards are low.
So, the awful truth: For great magnification you must either work very close, or use LOTS of extension on longer lenses, or degrade image quality a bit. A magic wand does not exist. The laws of optics are merciless.