Do all macro lenses loose light at close focusing distances?

Ernie Misner wrote:

Sasquatchian wrote:

The short answer, of course, is YES. But here's the reason. As you focus closer you have effectively changed the focal length of your lens (it's only its rated focal length at infinity) but the actual physical size of the aperture remains the same, and since the aperture number is only a ratio between the focal length and the actual diameter of the aperture opening, that f/number changes. f= focal length divided by the diameter of the opening.

So to further elaborate, on a 100mm lens f=100. f/4 equals 100 divided by 4 or a 25mm opening. Extend that lens closer to focus closer, say to 200mm, then f equals 200mm divided by that same aperture opening of 25mm and you now have f/8. So even though the numbers on the outside of the lens have stayed the same, the actual numerical aperture is not two stops less than the markings on the lens.

To give you a rough rule of thumb, a 1:1 macro reproduction ratio loses two f/stops from the marked aperture number. A marked f/5.6 is in fact f/11 even though it still says f/5.6.

When you move even closer you lose more light. At a 2:1 ratio or double life size, you lose another stop of light so that your marked f/5.6 is really now f/16.

Now you can start to see the implications of stopping the lens down too far when doing macro photography, as you'll start to see significant image degradation due to diffraction when you go much past an effective f/11.

Your friend who claims that this doesn't happen on his lens is wrong but you might not want to tell him that in order to avoid a pointless argument. Good enough to know it for yourself and to know it's just simple physics that no one can change or alter.

Thank you for the technical reasons for the loss of light at high magnification. Definitely not to be confused with creating more of a shadow on the subject as you move in closer to the subject. Do normal (non macro lenses) actually not get any darker at their minimum focusing distance?

Technically they do but usually it's not enough to notice or have a visible effect. And to add to what I wrote, if you happen to be using aperture preferred auto exposure, you would get an automatic compensation for any kind of lens. I remember shooting with the Nikkor 105 Micro on my (now) old Nikon F4s with the Nikon Databack on it and that databack would read out the actual compensated aperture as you focuses the macro down to 1:1, which was kinda cool. And if you're shooting digital, you're going to see the image on the back of the camera or on a tethered computer anyway and be able to compensate. I can see a slight change in exposure when doing massive focus stacking from front to back.