lens - Variable focal length for macro lenses?

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?

Answer

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 collimated when entering the lens. With Macro lenses that are also capable of focusing collimated 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. A simple single element lens must be twice as far from the camera's imaging plane as its focal length to provide 1:1 magnification of an object in focus at the same distance as the camera's imaging plane but on the other side of the lens.

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 angle of view (AoV) is about what one would expect for a 65mm simple single element lens focused at unity (or 1:1 magnification with the subject 260mm from the camera's imaging plane) which is about the same angle of view as a 130mm lens focused at infinity. At 5x the AoV is 1/5 that, or what one would expect from an approximately 325mm lens focused at unity, which gives an AoV about equal to a 650mm lens focused at infinity. The lens can only focus at a single, fixed 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 collimated 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.

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.