Kaegen,Barrelcactusaddict wrote: ↑Tue Dec 28, 2021 12:15 pmNow I'm a little confused, as to how shortening a focal length when focusing on a macro lens increases magnification. . . .
You seem to have paused posting, perhaps while contemplating Rik's question about optical formulae. If these are throwing you at all, I'll point out a simplified way of looking at this. Here goes:
Yes, most of the general use "macro" lenses for DSLRS do shorten focal length as they are focused closer. This is because the lens designers took a mechanical shortcut. Let me emphasize that point: This shortcut is more about mechanics, cost, and bulk than it is about optics.
Imagine you are using a simple lens on a view camera. The camera is a shallow, light-tight box with film at the back; at front, there is a bellows full of empty air between the camera and the lens. To focus on a subject at infinity, you adjust the length of your bellows so that the amount of space between the lens and the film (or later, sensor) is equal to the focal length of the lens. So if you have a 100mm lens, you have 100mm of space between lens and film when focused at infinity. (This is close to the definitions of "infinity focus" and "focal length.")
Now, imagine that you want to focus on a closer subject. To do so, you lengthen your bellows. For a subject at 1:1 magnification (1x), you need a bellows length that creates lens-to-film distance of twice the focal length of the lens. So for a 100mm lens, you have 200mm of space between lens and film at 1:1 magnification.
This is true for any simple lens shooting at 1:1: The distance between film and lens will be twice the focal length of the lens. So for a 200mm lens to focus at 1:1, it must be 400mm from the film.
In the days of view cameras, photographers adjusted focus by changing the length of their bellows ("bellows draw"). By the time of SLRs, most lenses included a focusing helicoid that replaced the bellows. A focusing helicoid is a tube that changes length as a portion of it is turned. Helicoid focus is not optically different from bellows focus--it's still a tube of empty air, simply in a more convenient and portable form.
For SLR photographers who were still willing to fuss with a bellows in the field, there were some good, inexpensive, and simple macro lenses made for bellows use. They lacked a helicoid. But these lenses--when combined with the required bellows--were bulky, a bit heavy, and quite fiddly to use in the field. (I can't begin to say how many shots I missed while wielding such things--while you are adjusting the bellows draw, the bug flies off.)
So camera companies came up with "macro lenses." (In quotes because most lenses of this type focus from infinity to 1:1, and many would argue that this is not truly macro.) These lenses are comparatively small, compact, and used helicoids, so no bellows to mess with. The problem is that helicoids are either limited in their range of length, or expensive and bulky. For a 100mm macro lens to focus the full range from infinity to 1:1--which most users expect--requires 100mm of length adjustment. This is difficult to do with a helicoid.
So here is where the design engineers accepted a tradeoff. The designed most macro lenses to shorten their focal length as they are focused down from infinity toward 1:1. As the focal length decreased, so decreased the need for a long helicoid. For example, if you decrease a nominally 100mm macro lens to 60mm as it approaches, 1:1, you only need a 60mm helicoid.
Does this make any sense?
--Chris S.