Re: need advice on lenses

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Posted by chrish on April 07, 2003 at 23:27:05:

In Reply to: need advice on lenses posted by Tenaka on April 07, 2003 at 19:18:12:

Unfortunately, the answer is a little bit technical, and if you just jump in without understanding the technical aspects, you will probably buy a lens you are unhappy with. This question comes up here quite often, and the answers are quite often overly simplistic, so I will try and be thorough, without too much "jargon".

Two issues are important when trying to get "close" for macro shots, the magnification ratio produced by the lens and the focal length of the lens.

First the ratio - This number is a ratio of the size of the subject to the size of the film frame. A frame of 35mm film is 35mm wide (~1.4 inches). So if you fill the frame with an object that is 1.4 inches across, you are shooting at a 1:1 magnification. For practical purposes, a quarter would take up the whole frame. Some specialized lenses will let you magnify the image this much.

For herp shots, I think a 1:2 ratio is fine. This will allow the frame to be filled up with something that is 2 x 35 mm (or 2 x 1.4 inches) across. This would be something the size of a 35 mm film box, for example.

Many "macro" zoom lenses on the market only allow magnifications of 1:4 or even 1:5. At those magnifications, you are filling the frame with an object approximately 4 x 1.4 = 5.6 inches across. Still fairly good for whole body shots of medium sized herps, but certainly not "macro" shots.

I suggest you figure out how small the smallest objects you want to fill the frame with would be. Then divide that by 35mm (1.4 inches) and that number will give you the max ratio you need. So if you want to fill the frame with something the size of a toad (lets say 3 inches long), you would need a lens that can magnify something to a 1 to 3/1.4 = 2.1, or a 1:2 ratio. So you need a lens capable of at least 1:2 magnification (a 1:4 lens wouldn't be able to do this for you, a 1:2 or 1:1 lens would).

And remember, just because an object is only 1 inch long (a cricket frog, for example), at 1:2 magnification it won't necessarily "fill" the frame, but it will occupy enough of it that you will be able to get a good "macro" shot. You wouldn't necessarily want it's nose at one end and its cloaca at the other!

The other variable is the focal length of the lens. The longer the focal length of the lens, the further the subject will be from the lens. So a 50mm macro lens may produce a 1:1 magnification when the subject is 1 inch from the lens, while a 100mm macro can achieve the same magnification (1:1) from a distance of 6 inches.

This lens to subject distance matters because if the lens is too close to the subject, you flash may not work properly, you may startle the subject, or you may even endanger yourself (being 1 inch from a rattlesnake wouldn't be a good idea!).

Be aware that when you read most lens specs, they tell you the minimum focusing distance. This is useful to know, but irrelevant in regard to the magnification. A 50mm lens that focuses to 6 inches isn't going to provide much magnification, a 200 mm lens that focuses that close will. Make sure you find out the maximum magnification ratio.

Many 70-210 "macro" zooms will be in their macro mode at the longer end of their focal length range (i.e. at the 210 end), which is good. Then you can focus "close" to an object and still stay a distance away.

There are a lot of options available. I would suggest that a macro lens that can focus down to 1:2 would be adequate for most of your needs. You will also want the lens to do this at focal lengths of at least 90 mm or more so you can maintain adequate lens to subject distance.

Look at the specs for zoom lenses online. Don't get hung up on getting a Minolta brand lens either. Tokina, Tamron, Sigma and even Vivitar makes some decent inexpensive macro zooms (avoid Phoenix lenses like the plague!).
Decide what sorts of magnifications you will require, decide what focal lengths you want (remember - most of your shots will not be in macro mode, so consider the advantages of the other focal lengths as well) and then do some research online.

www.bhphotovideo.com has good prices and has good lens charts showing things like maximum magnifications.

Hope this helps, without burying you with information!

Follow Ups:

Re: need advice on lenses Tenaka 04/08/03 (0)
Very good explaination, Chris.....np Bill Moss 04/08/03 (0)