Is it possible to see the magnification ratio when using RF 100mm F2.8L Macro IS USM?

[p4pictures]

I recently worked with a photographer who was using the EF 100mm f/2.8L Macro IS USM lens for technical photography where the magnification ratio was set on the lens using the markings in the focus distance window, and then the camera moved to achieve focus. The EF macro lenses have markings for 1:1, 1:1.5, 1:2, 1:3, 1:5. The photographer wanted to know how or if this could be possible with the RF 100mm Macro lens, and I'm - for once - stumped. The lens / camera can display the focus distance scale, but what I think is needed is the option for a magnification ratio scale.

[AtticusLake]

Knowing the distance to the subject, the lens could display the magnification -- if it knew the sensor size.  However, RF lenses can be used on both RF and RF-S cameras, so the lens can't know the sensor size.  When they brought out the EF lenses, I guess they weren't thinking about APS-C.

Theoretically the camera could do it, since the camera knows the focus distance (roughly) and of course its own sensor size.  My guess, though, is that it would be rough, and so not consistent; when the camera is displaying an actual number, the user might expect it to be exact, and that might not be possible.  And of course it would depend on you having the subject in focus.

Does the camera know the exact focus distance?  I kind of don't think so.  Of course it can get exact focus, but it does that by looking at the image, not by measuring the distance.  The lens can report the distance it's focussed at, by inferring it from the positrions of the elements inside the lens; but I think that's one of those situations where a microscopic movement of the elements would make a big difference in the answer.

All in all if I was the engineer asked to implement this, I would be a bit uneasy about it.  But still, it seems like it should be possible, if they could convey that the displayed number is approximate.

[p4pictures]

I'm sure that the magnification ratio is independent of the sensor size. For a 1:1 ratio - life-size, then a 36mm x 24mm subject would fill the frame on a camera with a full-frame sensor, while for a camera with an APS-C sensor then a 22mm x 15mm subject would completely fill the APS-C sensor. The ratio is the same, but the size of the sensor and therefore how much is captured changes.

The EF and EF-S macro lenses have an additional magnification ratio on the lens distance scale, it's not super accurate but is used by a range of photographers in technical applications such as forensics, dentistry, medical etc. They need repeatability and one option is to use the magnification ratio to ensure a subject is captured at consistent size. They usually use manual focus, set the ratio on the lens, and then move the camera to achieve focus to ensure the magnification is as desired. 

Most recent EF/EF-S and all RF/RF-S lenses report distance information to the camera. Probably some encoder on the lens focus mechanism returns the distance it is focussed at. This makes sense to me so that the distance display can be shown on the LCD screen when using manual focus with RF / RF-S lenses. 

Looking at the old EF 100mm f/2.8L Macro IS USM lens, the 1:1 magnification mark lines up with the 30cm / 0.99ft mark. As the RF lenses can show the distance then for the RF 100mm macro there should be an option to show magnification ratio instead of distance, it is unlikely to need to be any more accurate than the old method of lining up the figures in the focus window on the lens.

[AtticusLake]

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@p4pictures wrote:

I'm sure that the magnification ratio is independent of the sensor size.

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Ah, I beg your pardon -- of course that's true.  I'm thinking of "relative to filling the frame", which is something else, as you rightly point out.

So there would be no reason for the RF lens not to have a magnification scale on the lens -- if it had a focus distance window.  But the RF lenses don't have this, or any way to indicate the physical focus.  So the RF version would be for the camera ti display it.

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Most recent EF/EF-S and all RF/RF-S lenses report distance information to the camera. Probably some encoder on the lens focus mechanism returns the distance it is focussed at. This makes sense to me so that the distance display can be shown on the LCD screen when using manual focus with RF / RF-S lenses. 

Yes, that's right.  The question is how exact is it.  When you're looking at a magnification as a scale on the lens, your expectation of accuracy may be different to seeing a number on an LCD screen.  So I'm speculating that might be the reason not to do it.  Seems like a missed opportinuty, though, if they could come up with some way to indicate how approximate it is.

[tomnguyen1]

I hope this can help you,

The RF 100mm f/2.8L Macro IS USM lens doesn't have a built-in magnification ratio scale. To determine the magnification ratio, you can use the focus distance scale on the lens to calculate it or use a reference object to measure the size of the object in the image. Additionally, some macro lenses come with a dedicated hood that has a built-in magnification scale.

This is everything I know, hope this can help you
-Tom-