>I was after something between a 150 to 200 mm lens as I want the >working distance. Can any one make any reccommendations on a >particular lens.
Scott,
It depends to some degree on what camera you have. If you have a film camera or DSLR and what brand/model. If you have a DSLR, the crop factor (assuming it isn't full frame) will give your lens a little more working distance by default.
Also, I have a 50, 90, and 180 mm macro. I rarely, if ever, use the 180 because it provides too much working distance (39" @ 1:2). At that distance, I have trouble manipulating/controlling specimens. Of course, it does allow you the distance needed for frame filling shots of rattlesnake heads, but frankly, I found it is often simply too long for the job at hand and so I leave it home. My 90mm macros have been my standard herp macro lenses for years. Now that I have a DSLR with a 1.5x mag factor, the 135mm field of view I get from the same distance is even better!
For example, the Konica Minolta 200mm macro G lens (a lens I will own one day!) has a minimum focus distance of 1.7 feet at 1:1 magnification (1.33:1 on my DSLR). So the front of the lens will be 20 inches from the subject. However, the lens itself is 8" long and the camera is another 2" thick. So now the viewfinder is 30" inches from the subject. Your arms are a few inches behind that, and now you are a yard from you subject at its closest focal distance which will fill the frame with something the size of a nickel. This may sound cool, but generally, you want more in your picture than an area the size of a nickel and so you have to back up. Now you are 5-6 or more feet away from a flighty critter, trying to both control it, focus, compose and take a photo. For the same photo with a 90mm macro, I can be around 2 feet from the subject. A little close to an angry prairie rattler maybe, but much better to deal with a non-stop salamander.
Here's a pic with a 90mm macro from about 2 feet back (a little close maybe, but there was some glass between us!) -

There are lots of choices available, depending on your camera system (I only know these as I was shopping within the last few weeks) -
Nikon offer a 60 and 105 mm macro.
Canon offer 50mm, 60mm, 100mm, and a 180mm macro lenses.
Konica Minolta offer 50mm, 100mm, and 200mm macros.
Pentax offer a 50 and 100mm macro as well.
So if you have a Canon or a Minolta SLR, you can stick in system and get a 180-200mm macro. The "problem" is that these two lenses are from the respective companies' best series of lenses (the L series for Canon and the G series for Konica Minolta). Both therefore are extraordinary quality lenses which will set you back about $900 for the KM and $1200 for the Canon.
If you have the cash, you won't be disappointed with either of these lenses (assuming they will fit your camera
).
So assuming you don't want to shell out $1000 for the lens, you are going to have to investigate other brands.
Sigma make a 105 ($399), 150 ($629), and 180 ($689) mm lens. Optically these lenses are very good and if you get the newer EX series lenses from Sigma they are very well built and really are outstanding quality lenses. I used an older Sigma 90 Macro as my main macro lens for years and it was a great, well built, very sharp macro lens and the new EX are supposed to be even better.
Tamron make a 90 ($439) and 180 ($679) mm macro lens currently. I recently bought the 90 mm (SP AF 90mm f/2.8 Di) and I really love the thing. With my 7D it gives me an effective field of view of a 135 mm lens and plenty of working distance. It is tack sharp, focuses quickly and is a pleasure to use.
Here's an uncropped pick with mine. This is a TX Brownsnake whose head is less was probably 5-8 millimeters long -

Vivitar, Phoenix etc, make generally inferior lenses. The only exception is the old Vivitar Series 1 100mm Macro. This lens was a real bargain for around $100 new and it frequently produced better test results than the Canon, Nikon, Minolta, Pentax lenses of the same focal length. Unfortunately, they don't make this lens anymore and photographers who knew snatched these gems up!
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Chris Harrison