### Why are macro lenses "better"?

greenspun.com : LUSENET : Large format photography : One Thread

I love macro photography but don't have the money to buy a dedicated macro lens. I've been using a 210/5.6 Symmar-S with what seem to be fine results. I've always heard that macro lenses are optimized for higher reproduction ratios (RR), but I don't understand why. Does it matter if your subject is planar or not? I've read that process/copy lenses are better for flat subjects but wonder whether they'd be any better than a normal lens for flower (3D) macro work. How can a lens be "optimized" for infinity to 1:10, or 1:10 to 2:1?

See if my simple logic about optics is correct. When you photograph with a lens that produces an image circle that just barely covers the film, you're using all the glass in the lens (ie. the edges as well as the center). Since the edges are usually not as sharp, resolution suffers a bit. With higher magnification (say 1:1 since this is what I often shoot at), you're using much less of the periphery of the lens elements and, therefore, more of the "sweet spot" (ie. the center of the glass) which should result an image on the film that is sharper corner-to-corner and with less distortion than you would have made at infinity. If my logic is correct, ANY lens should make a great macro lens since, at high RR, you're using more of the lens' sweet spot.

If someone could comment on my logic and clarify why and when macro lenses are better, I'd greatly appreciate it. (I have a rudimentary understanding of optics and college physics, but I guess the simpler the better.)

-- Tony Karnezis (karnezis@aecom.yu.edu), January 20, 2002

Tony: I can't begin to give you the formulas and physics behind lens designs, but I do know that lenses can be optimized for a particular focus range through element configuration and spacing between the elements. The lenses designed for close work can give you better defination and, as you stated, a flatter field of view. For example, a camera lens can be used on an enlarger. It will give you an image that can be focused on the baseboard. However, it will not give you an image that is as sharp corner to corner as a well designed enlarging lens. Both lenses are built to work best at a particular focus distance. Someone smarter than I, which is not difficult to find, can give you the math behind the optics.

Regards,

-- Doug Paramore (Dougmary@alaweb.com), January 20, 2002.

They're better because they're better.

More seriously, a lens can give best performance at only one magnification ratio. A lens designed to give best image quality focused at infinity (magnification ~ 0) gives less than best at high magnification, e.g., 1:1. Similarly, a lens designed to work best at high magnification will much less good at low magnification.

That said, lenses that are nearly symmetrical work quite well at a wide range of magnifications. That's why your Symmar-S gives you good results. Tessars aren't symmetrical. The 100/6.3 Luminar, a macro lens, is a Tessar type. Zeiss advises using it only at magnifications > 0.8:1. Mine is useless at infinity, much much better close up.

If you are serious about working at higher magnifications, consider getting a shorter lens. You didn't tell us what camera you're using, but if its a double extension (~ 300 mm max) field camera, with a 210 you can't make 1:1. By the way, there are quite ok macro lenses available at very low prices. Tominons as made for use in a variety of Polaroid cameras, including the MP-4. You may be able to afford an ok lens that will do what you need.

Cheers,

Dan

-- Dan Fromm (76266.333@compuserve.com), January 20, 2002.

True macro lenses are generally well corrected for coma at high magnifications, which can be a very noticable problem. If you try different lenses side-by-side, you will really see the difference.

-- David Goldfarb (dgoldfarb@barnard.edu), January 20, 2002.

Tony:

Aberrations, particularly spherical and coma, are corrected for particular reproduction ratios. As an example, we used to run a physics lab for college students where they measured the aberrations of a lens. Normally that would be hard, since in most lenses these are small. We used an aerial lens, optimized for infinity... but we used it backwards (with a collimated light source). The aberrations were large and easily observed.

-- Glenn C. Kroeger (gkroeger@trinity.edu), January 20, 2002.

The last part of your post is a bit in error. If the lens is wide open, you're using the entire area of the elements, regardless of the format or distance. Light rays from the subject will strike the entire surface of the lens, and be refracted to the appropriate spot on the film to make the image. As you get closer, the angle of the rays from different points on the lens, to the subject, changes. The correction is different for that condition, than further away where the rays are closer to parallel. (way oversimplified!) Most lenses work pretty well outside the range claimed by the manufacturer. If you do extreme closeups, where the distance from the lens to the film is greater than the lens to the subject, it may help to reverse the lens. This gets the angle of the rays (coming in and going out) closer to what the lens was designed for. The worst condition is probably 1:1 or close to it, and that's where a good macro will shine when compared to a normal lens. Of course, stopping down helps immensely, until you get into diffraction.

-- Conrad Hoffman (choffman@rpa.net), January 20, 2002.

A process lens is normally optimized for 1:1, and if you look at the construction, it it more or less totally symmetrical. At 1:1 the distance in front of and behind the lens is the same, and the "same" lens construction in front of and behind the center of the lens makes this particular construction perform extremly well. The different abberations that are produced in the front are reversed as the light rays "takes exactly the same path out", but on the other side. I'm not saying that macro lenses and process lenses are the same, but they are similar in the aspects of symmetricality. (or whatever it is called?)

While a Symmar is fairly symmmetrical, i.e. similar construction of the front and the back of the lens, the construction is corrected and optimized for the fact that the distance between the lens and the film is a bit more than 210 mm (in your case) while the distance between the lens and the subject might be infinity. It is obvious that the front and the back of the lens are different, as they have different sizes etc. The same goes for most normal and wideangle constructions that are corrected for normal ranges.

-- Björn Nilsson (b.w.nilsson@telia.com), January 21, 2002.