Do dedicated macro lenses lose light at close focus like extension tubes?

Asked 1/3/2012

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When you use extension tubes on a normal lens, moving the lens farther from the sensor reduces the effective light reaching the image. Does a dedicated macro lens have the same light loss when focusing close, especially around 1:2 or 1:1? Why or why not?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

14y ago

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Yes, but it may be hidden. A unit-focus macro (the old school, where the lens elements are in a more-or-less fixed relationship to one another and the whole shebang is moved further away from the sensor/film plane) will usually work in exactly the same way as extension tubes. The lens is essentially a well-corrected ordinary lens with a built-in adjustable helicoid extension tube, and you need to be aware of the lens draw when metering externally. (When metering in-camera, the electronics take care of the hassle for you.)

Newer internal-focus (or rear focus) macros (and non-macros for that matter) actually focus by changing the focal length of the lens. The front element stays anchored in space, and the focal length of the lens is reduced. That does two very noticable things. First, there is no "focus breathing" -- the subject will stay the same size in the frame as you focus. Secondly, and most pertinently for your question, the size of the physical aperture stays the same as the focal length is decreased, so the effects of lens draw are masked (or moderated, depending on the individual lens) by an ever-increasing relative aperture as you focus closer. So with a "pure internal focus" design, at infinity you may have a lens that is 100mm set at, say, f/8, but when you focus much closer, you may actually have a 75mm lens at f/6 (using the same 12.5mm apparent aperture), but the lens draw due to focus reduces the light to the same level as it would have been at f/8. The physical length of the assembly is unchanged, so the relative aperture of the whole remains the same if the physical aperture is unchanged.

And just to complicate matters a bit further, some lenses seem to be of a hybrid design -- they use both internal focus and ordinary extension to get where they're going. That's not a problem with TTL metering, but it would mean creating a draw chart for use with an external meter. A couple of test shots with a grey card and manual exposure settings will tell you whether or not you need to go to the trouble of creating one.

Originally by user2719. Source · Licensed CC BY-SA 4.0

user2719

14y ago

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AI Answer

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Yes. As magnification increases, effective light transmission drops, just like it does with extension tubes.

With older unit-focus macro lenses, the whole optical assembly moves farther from the sensor, so they behave much like a lens on a built-in helicoid extension tube. The extra extension causes light loss at close focus.

At 1:2, the image is about 1 stop darker than at infinity focus; at 1:1, about 2 stops darker.

Many modern internal-focus macro lenses may hide this a bit because they focus by changing the optical design/effective focal length rather than simply extending the whole lens. But the practical result is the same: closer focus means less effective light.

In real use, this usually isn’t a problem because through-the-lens metering automatically accounts for it. You mainly need to think about the exposure factor if you’re metering externally or working manually from guide numbers or charts.

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14y ago

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