Do modern IF Macro lenses have true constant apertures?

Question

From my basic understanding f/stop is the ratio between the aperture size and length of the lens.

A majority of macro lenses feature an extending barrel. So naturally, a fully-extended f2.8 lens, gives you a smaller f-number. eg. Sigma EX DG 70mm Macro

However many of the current macro lenses feature internal focusing, where-in there is no barrel extension, just a moving element. Eg: Sigma 105mm EX DG OS Macro, Tamron 90mm Di VC USD

Since I own an older lens I would like to know if the newer lenses keep the base aperture all the way?

Answer 1

When a lens is imaging an object that is that is an infinite distance away, we measure the distance, lens to film/sensor and pronounce this measurement distance to be the focal length. Thus if we mount a 50mm lens and focus on a star, the back focus distance is 50mm. When we close focus using this same lens, the back focus distance increases. This is because all lenses have limited ability to refract (bend inward) the image forming rays. As an example, to obtain a life-size (1:1) image of an object with a 50mm lens, the back focus distance increases to 100mm. The now increased back focus is technically not the focal length, but nevertheless the f-numbers associated with this lens are no longer valid. This induces a substantial loss in image brilliance when we close focus.

The formula to figure out how much compensation to apply is (M+1) X (M+1). M = magnification. Thus for the life-size set-up M = 1. To solve with M = 1. (1+1) X (1+1) = 2 X 2 =4. In other words the light loss is 4X. Since each f-stop equals a 2X change, a life-size setup requires that we open up 2 f-stops.

Macro lens to the rescue: The macro lens has two key characteristics. 1. The macro is optimized for close focusing. Ordinary camera lenses are optimized to image a world wherein the subjects are spread out over different distances. The macro is optimized to image flat objects like stamps and/or objects that display slight contour. The macro lens then projects an image onto the flat surface of film or digital sensor. 2. The macro is designed to maintain constant image brilliance as you close focus.

How is the constant image brilliance accomplished? The front lens group of the macro magnifies the size of the entrance pupil (aperture). This action is a variable, as you focus closer and closer, the outside world sees a larger and larger entrance pupil. It is this action that maintains a constant exposure as you close focus.

Bottom line: Most all macro lenses are optimized for close focus work and slightly compromised when tasked to image distant objects however image brightness is upheld as you close focus.

Answer 2

The basic principle is that 1:1 magnification requires 2x focal length extension which will therefore lose 2 f/stops (certainly NOT constant aperture). f/2.8 maximum will become f/5.6 at 1:1. True of internal focus too, the front of the lens may not move, but focal length is extended (like internal zooms), and 1:1 requires 2x focal length extension, regardless if internal or frontal extension. 2x focal length is 2 stops.

My Nikon 60 and 105mm macro lenses are internal, and report this as f/5 instead of f/5.6. I don't know why not f/5.6, but that's close, and I suspect that's a reporting issue due to internal shifts, and not a theory issue. They do 1:1.