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
Tom Axford Veteran Member • Posts: 9,823
Differences in lens design
2

Monicaa wrote:

My Panasonic 24-105 f4 has a MFD of 0.3m. It gives me a magnification of 0.5x at 105mm, which I have confirmed by taking photos of objects with a known size. This is 0.3m from the sensor plane to the object, with a working distance of about 10cm.

However, a 105mm macro lens with the same MFD (for example the Sigma 105mm 2.8 has a MFD of ~0.3m also), produces a 1.0x magnification.

How does this work? Two lenses at the same focal length and focal distance achieve different macro reproduction ratios?

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).

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