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

I believe some zoom lenses change their focal length when focusing close in order to achieve closer focus.

We measure subject distance from a point called the front nodal. Should the lens be a single element symmetrical, this point would fall in the middle of the glass lens. We measure focal length from a point called the rear nodal. Again, if we are dealing with a simple lens this point would be at the same location. Now camera lenses are quite complex. The focal length is measured from the rear nodal to the focused image when imaging a subject at infinity (like a star).

Now a complex lens is constructed using many glass lens elements. Some are cemented to their neighbor, some air spaced. Some are convex and some are concave. The net power of the combination places these two cardinal points in bizarre locations, they are often inverted to make the lens barrel shorter and or to accommodate the mirror swing and or focusing mechanism. To this end, these front nodal might fall in air, forward of the lens barrel. The rear nodal might be shifted rearward to allow for mirror swing and the like.

To achieve unity (magnification 1) the image plane falls two focal lengths behind the rear nodal and the subject distance falls two focal lengths forward of the front nodal. The focal length at 1:1 is the distance subject to image divided by 4.

In other words, these measurements are somewhat complex.

Focus breathing.

https://www.dpreview.com/videos/8010417625/dpreview-tv-what-is-focus-breathing-and-why-should-you-care

I noticed that when demonstrating the first 28-200mm that we had for sale.

At 200mm focused to the closest focusing distance it had a similar field of view of a fixed 135mm .

Past about 4-5 metres (I can't remember now) it started to look like a 200mm.

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).

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?

I was just going to say that one's a macro lens design and the other is not.

(Maybe I should run and hide... )

atom14.

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?

For more detailed information on Distance I suggest the Optics Primer - Distance article in the PhotonsToPhotos Optics Primer.

You'll find that something called the internodal distance (or hiatus) is important especially as close distances.

You can also see both lenses at the PhotonsToPhotos Optical Bench.

The Panasonic Lumix S 24-105mm F4 Macro OIS

Best viewed "original size"

And the Sigma Macro 105mm F2.8 EX DG OS HSM

Best viewed "original size"

The internodal distance is H'-H in the positions.
For the Panasonic it's -31.65mm and for the Sigma it's 9.68mm
Optically this is the chief reason for what you are observing.

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.

Tom Axford wrote:
...

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.

This is correct but the underlying reason is the relative positions of the principal planes as I noted (internodal distance also known as hiatus).

The PhotonsToPhotos Optical Bench is interactive and also has numerous display options. The following screenshots may make the point better.

Panasonic, best viewed "original size"

Sigma, best viewed "original size"

The F dots are the focal points, the P dots/lines are the pupils and the H dots/lines are the principal points/"planes".

Blue is from the object side and red is from the image side.

Between the two lenses the chief difference is the distance between the front and rear H positions.

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?

Stated focal lengths are for infinity focus, and depending on the lens, the focal length may drop quite a bit at close focus. That's why max magnification needs to be stated as a lens spec.

John Sheehy wrote:

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?

Stated focal lengths are for infinity focus, and depending on the lens, the focal length may drop quite a bit at close focus. That's why max magnification needs to be stated as a lens spec.

How is this relevant? The focal length at minimum focus distance for both lenses is nearly identical at about 75mm.

bclaff wrote:

John Sheehy wrote:

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?

Stated focal lengths are for infinity focus, and depending on the lens, the focal length may drop quite a bit at close focus. That's why max magnification needs to be stated as a lens spec.

How is this relevant? The focal length at minimum focus distance for both lenses is nearly identical at about 75mm.

How is the max magnification less relevant than stated focal length and MFD to the person looking for magnification?

John Sheehy wrote:

bclaff wrote:

John Sheehy wrote:

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?

Stated focal lengths are for infinity focus, and depending on the lens, the focal length may drop quite a bit at close focus. That's why max magnification needs to be stated as a lens spec.

How is this relevant? The focal length at minimum focus distance for both lenses is nearly identical at about 75mm.

How is the max magnification less relevant than stated focal length and MFD to the person looking for magnification?

The OP isn't "looking for magnification"; he has observed that at at the same MFD two different lenses achieve different magnification. Focal length at MFD does not explain that observation.

If I may take the numbers Bill already provided and give a "physicist" explanation with a couple of formulas.

For a thin lens, the distance between the object and its image is given by:

(2+m+1/m) * f = object-image distance ala MFD

where m is the magnification and f is the actual focal distance measured at MFD. This formula works exactly only when the two principal planes of the lens coincide (the blue H and the red H' in Bill's diagrams). For neither of these two lenses the two planes coincide. In fact, for one lens the blue H goes in front of the red H' and for the other lens it is the other way round. This mismatch, the distance between these two planes, is called hiatus and it can be included in the above formula, so it works again:

(2+m+1/m) * f = MFD - hiatus

Let's plug the numbers in:

Panasonic: (2+0.5+2) * 73.3 = 298.2 + 31.7 = 329.9 mm

Sigma Macro: (2+1+1) * 75.7 = 312.4 - 9.7 = 302.7 mm

All checks out.

So, yes, there is focus breathing for both lenses and neither lens is 105 mm at close focus, but the difference between the two lenses is very small: 73.3 mm vs 75.7 mm. What makes the difference for the magnification is the hiatus: the negative hiatus for the Panasonic zoom makes the effective path the light rays travel 31.7 mm longer but the effect is opposite for the Sigma.

The bottom line is that the MFD alone does not define magnification. You need to know both the actual focal length at the MFD and lens hiatus.

I bet you are glad you did not ask in the technical forum section....

Monicaa wrote:

FrancoD wrote:

I bet you are glad you did not ask in the technical forum section....

This is very fascinating, I've spent the last couple of days reading more and more into it, but I can't say I'm any clearer on it haha

Single elements? I can do that. Double elements? I can even figure that out! But when you get to the complexity of these formulas intuition goes out the window.

I'm going to re-read everything another few times, because I think all the info is here to explain this, I just need to make sense of it.

I actually ordered the Sigma DG DN 105 macro to experiment and see if having each lens in hand will help me intuit an understanding. Any excuse for GAS

Unfortunately, having both lenses probably will not help you much.  If you remove the lens, there is probably no way for you to set it to its minimum focus distance and do experiments with it in that setting.

However, you probably can experiment with the lens at its infinity focus setting, which will be interesting even if not totally relevant.

ThrillaMozilla wrote:

John Sheehy wrote:

That's why max magnification needs to be stated as a lens spec.

Yes, I hardly ever need to know the specification for the minimum focusing distance. The maximum magnification, however, is crucial, and cannot be derived from the minimum focal distance without considerable additional information.

Arguably working distance (to the front of the lens) is more useful information than subject distance (to sensor/film).

ThrillaMozilla wrote:

John Sheehy wrote:

That's why max magnification needs to be stated as a lens spec.

Yes, I hardly ever need to know the specification for the minimum focusing distance. The maximum magnification, however, is crucial, and cannot be derived from the minimum focal distance without considerable additional information.

Thank you for appreciating my main point, which someone else wrote off completely because my reason was not complete.