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<blockquote data-quote="WayneF" data-source="post: 576357" data-attributes="member: 12496">Extension tubes DO CHANGE THE EXPOSURE.&nbsp; Loss of about 2 stops at 1:1 (because they extend the lens).&nbsp;&nbsp; It is the extension, which is ALSO TRUE of macro lenses at the same 1:1.&nbsp; The camera meter can meter it, but a handheld meter must be adjusted manually.&nbsp; Your f/2.8 macro lens can't do more than about f/5 at 1:1 because of this (technically it is f/5.6 and 2 stops, but they can do some internal focusing tricks). &nbsp;And marking of f/10 becomes f/20 (at 1:1).&nbsp; The macro lens is designed to show the new increased fstop number with&nbsp; extension (shows new f/20).&nbsp; Extension tubes are also affected but probably can't show&nbsp; the numbers.This is because extension of the lens DOES CHANGE The focal length.&nbsp; Focal length is distance from film or sensor plane to the second design node in the lens. So any extension increases focal length, even lens extension to focus close at say 4 feet does somewhat. The Marked focal length number is only true if focused at infinity.fstop number = focal length / lens aperture diameter.&nbsp;&nbsp; Increase the focal length 2x, and the fstop number goes up 2x. This is a two stop loss of light.FWIW, if at 1:1 magnification, then (by definition)the focal length (distance from film or sensor plane to second node in lens)is equal tothe working distance in front of lens (to first node in lens).Which is why at a 40mm or 60 mm macro lens only has one or two inches of working distance at 1:1,but a 105 mm or 200mm has a few times more working distance at 1:1.</blockquote>

[QUOTE="WayneF, post: 576357, member: 12496"] Extension tubes DO CHANGE THE EXPOSURE. Loss of about 2 stops at 1:1 (because they extend the lens). It is the extension, which is ALSO TRUE of macro lenses at the same 1:1. The camera meter can meter it, but a handheld meter must be adjusted manually. Your f/2.8 macro lens can't do more than about f/5 at 1:1 because of this (technically it is f/5.6 and 2 stops, but they can do some internal focusing tricks). And marking of f/10 becomes f/20 (at 1:1). The macro lens is designed to show the new increased fstop number with extension (shows new f/20). Extension tubes are also affected but probably can't show the numbers. This is because extension of the lens DOES CHANGE The focal length. Focal length is distance from film or sensor plane to the second design node in the lens. So any extension increases focal length, even lens extension to focus close at say 4 feet does somewhat. The Marked focal length number is only true if focused at infinity. fstop number = focal length / lens aperture diameter. Increase the focal length 2x, and the fstop number goes up 2x. This is a two stop loss of light. FWIW, if at 1:1 magnification, then (by definition) the focal length (distance from film or sensor plane to second node in lens) is equal to the working distance in front of lens (to first node in lens). Which is why at a 40mm or 60 mm macro lens only has one or two inches of working distance at 1:1, but a 105 mm or 200mm has a few times more working distance at 1:1. [/QUOTE]

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