General question on optics: what makes a macro lens so special in terms of optical quality?

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Dear Steve

Some manufacturers offer the same focal length in two versions: a 'normal' fast model and usually a one-stop slower macro model. Here's one for you: what makes a macro lens so special in terms of its optical quality? What's the DOF difference in terms of percentage for an F2.8 compared to an F4? Any guidance on literature on the subject of optics will be welcome. Thanks...Rene

-- rene z. quan (renequan@goplay.com), February 28, 1999

Answers

Rene, I had a response all typed up for you and my computer went down. This is a shortened version of it. In general, a normal lens will not be as well corrected for spherical aberration in close focus situations as a macro design. But, most symmetrical normal lenses used with extension will be accepable for macro work. Most macros are symmetrical because they automatically correct for coma, distortion, lateral color and field flatness. This gives the designer more freedom to concentrate on correcting spherical aberration. Closeup optics that can only use a few elements(loupes) are best when they use aspherics because they don't have the option of using many other spherical surfaces to correct spherical aberration. The P67 135mm macro is nearly symmetrical and is a Dynar design. To use macros without a floating element for infinity work, it is best to stop it down to reduce the spherical aberration. Floating element macros can work well at infinity and macro. You might want to look at Kingslake's "Lens Design" or his "History of the Photographic lens". SR

-- Steve Rasmussen (srasmuss@flash.net), February 28, 1999.

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