Why does my 24-105 have a different magnification (macro ratio) from a 105mm macro lens at same MFD?

Started 6 months ago | Questions thread
ThrillaMozilla Veteran Member • Posts: 7,468
Re: Why does my 24-105 have a different magnification (macro ratio)

I'm going to try to give you a simple answer. Just read on. (It's not substantially different from the answers you have received, but you may find it simpler. Do read to the end. You don't need to understand all of it.) By the way, Tom Axford's reply was spot-on. Bill Claff's reply also was, albeit a little more complicated.

The magnification is the height of the image on the sensor, divided by the height of the subject. One lens gives you a maximum magnification of 1, while the other is limited to 0.5, or half the magnification. You knew that already.

You asked why two lenses have different magnfications at approximately the same minimum focal distance. A simplified answer is that the magnification is determined by the distance from the front focal point, not from the sensor. The front focal point can be almost anywhere, but the minimum focal distance is measured from the sensor. Because some lenses are long, with lots and lots of lens elements inside, but others are short, the two numbers bear no relation to each other. So the minimum focal distance is not much help.

(Incidentally, cameras used to have a mark at the film plane, and that made sense when the distance from film to subject was marked on the lens barrel. But most modern cameras don't have that mark, and most modern lenses lack the scale, so the minimum focal distance is not as useful as it used to be.)

Now here's the hard part. It sounds like you want to know the working distance, which is simply unknown at this point. That depends not only on how thick the lens is (which Bill Claff can help with), but also on how much metal and plastic is on the front side of the lens at maximum magnification. Even something as simple as a lens hood or a filter would change the working distance.

If that's what you need, there may be a slight possibility of calculating that information. Maybe the manufacturer gives specifications for the length of the lens from flange to the end of the lens, and maybe someone can guess the state of the lens corresponding to that measurement. If the length is specified from end to end, maybe someone knows or can guess the last few millimeters, the distance from flange to the back of the lens. The distance from flange to sensor is known. From all that information maybe Bill Claff can estimate the distance from the front nodal point to the subject position at maximum magnification, and from that, estimate the working distance.

You see, it's not so simple to calculate the minimum working distance from the available information. Maybe you can ask the manufacturers.  They should actually specify that, even if they don't.

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