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

Started 7 months ago | Questions thread
Re: Differences in lens design
1

Tom Axford wrote:

There is a very simple and universal formula that relates the magnification (m) to the object distance (x) and the focal length (f) for any lens, which is:

x = f/m

However, in this formula the object distance is measured from the front focal point of the lens, which is typically slightly in front of the lens but varies considerably according to the design of the lens. Using this formula is also complicated by the fact that most modern lenses use internal focussing, which actually changes the focal length as the lens is focussed closer. Again, this varies very considerably with the design of the lens.

The lens manufacturer quotes the minimum focal distance as a measurement from the sensor plane (which is marked on the camera), because this measurement is much easier for the photographer.

To use the simple formula above you need to work out where the front focal point is (use the lens in reverse and see where it brings objects at infinity into focus), but you also need to know how the focal length varies with the focus distance. Lens manufacturers seem reluctant to publish any of this information with the lens specs (probably because they think it would just confuse many people).

Just to update these comments with the lens data very helpfully provided by Bill Claff.

At the MFD and the long end of the zoom, the Panasonic lens actually has a focal length of only 73mm instead of 105mm.  It's front focal point (F) is actually 29mm  inside the lens, and the working distance (object distance from the front of the lens, called O) is 117mm.  So the object distance from the front focal point is 117 + 29 = 146mm, which is equal to the focal length divided by the magnification.

The Sigma lens at its MFD has an actual focal length of 76mm.  Its front focal point is 69mm in front of the lens, and the working distance is 144mm.  So the object distance from the front focal point is 144 - 69 = 75mm, which is equal (within numerical error) to the focal length divided by the magnification.

The Sigma lens has been designed to give a much greater working distance for similar magnification.

I trust Bill will correct me if I have misinterpreted his data.

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