Shadowy Daisy

Shadowy Daisy
by damned-truths

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True macro lenses come in fixed focal length. Are there any macro lensens which have variable focal lengths? And please explain why yes or why not?

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up vote 4 down vote accepted

It depends on how you define fixed and variable. As you change the focus distance of many prime lenses, including some macro lenses, the actual focal length changes a little bit. Most fixed focal length lens' focal lengths are defined when the lens is focused on infinity and the light focused at the film/sensor plane is colimated when entering the lens. With Macro lenses that are also capable of focusing colimated light at the sensor plane the difference in focal length when focused at the minimum focus distance (MFD) will be greater than with a more typical lens that can't focus as close.

It also depends on how you define True macro lenses. If you are using a lens with a maximum magnification (MM) of 0.5x on a camera with a 2x crop factor sensor a resulting 8x10 print will show the subject the same size as if you had used a lens with a MM of 1.0x and a full frame camera.

The Canon MP-E 65mm 1-5x Macro lens is listed as a fixed focal length lens, but for all practical purposes the focal length is meaningless. At 1x the field of view (FoV) is about what one would expect for a 65mm lens, but at 5x the FoV is 1/5 that, or what one would expect from an approximately 325mm lens. The lens can only focus at a specific distance at any particular magnification setting. At 1x it has about 100mm working distance (the distance from the front of the lens to the point of focus). By 5x the working distance is only 41mm. Since the lens can't focus colimated light onto the sensor when connected to a camera with the registration distance for which it was designed at any setting, there is no real way to express focal length in the conventional sense.

And it also depends on how you define zoom lens. Another clue that the MP-E 65mm is a unique kind of (sort of) zoom lens is the chart included on page 8 of the MP-E 65mm 1-5x Macro User Manual. As the magnification is increased, the effective f-number for any given aperture setting also increases as one would expect when the same sized opening of the diaphragm is used for a longer focal length lens.

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I meant that e.g. a 50mm macro lens usually stays on 50mm and the photographer can't change that to for example 60mm just like a normal 18-55mm lens. Or are there any true macro lenses which have this ability ? – Julian Jun 14 '14 at 23:54
The one listed in the answer effectively does just that. The FoV is changed by a factor of 5 from 1x to 5x MM. Your 18-55mm only changes the FoV by a factor of 3. – Michael Clark Jun 14 '14 at 23:59
Because at 1x it provides roughly the same FoV as a 65mm lens focused on infinity would. – Michael Clark Jun 15 '14 at 0:14
@Julian Canon says it is a 65mm lens because it is a 65mm lens, it focuses collimated light at a distance of 65mm from the rear principal plane, which is the one and only definition of focal length. – Matt Grum Jun 15 '14 at 10:24
@MichaelClark that's not true, the lens can focus collimated light - just not at the distance where the sensor in a DSLR is. If you put a 65mm lens from a mirrorless camera on a DSLR it wont focus to infinity, but that doesn't mean that lens is somehow no longer a 65mm lens! Focal length is an intrinsic property of the lens (every lens) and has nothing to do with the purpose of the lens or what camera system it is mounted to, or field of view. – Matt Grum Jun 15 '14 at 10:28

Nikon makes a 70-180mm macro zoom lens. It focuses down to a 1:1.3 magnification ratio -- not quite what is considered "true macro" -- but with the 6T close-up lens it gets to 1:1. Supposedly quite good, though I have no first-hand experience.

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Macro lenses will usually stay at a fixed focal length because A) it is difficult to focus with zoom changing B) Zoom lenses can present stability issues when close to a subject There are a few macro lenses that the photographer can "lock" at the minimum focal length.

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