Canon MP-E 65 - How to determine magnification?

[ktuli]

Does anyone out there have the Canon MP-E 65mm 1-5x macro lens? If so (or if you have any suggestions for me), do you know how to determine what magnification level was used on a shot after the fact?

I was hoping that I would be able to find something in the EXIF info, but after looking this morning, I am not seeing anything.

Any help is appreciated!

- Bill

[Dizzy]

Good question Bill, as I've often wondered what the actual magnification of a given
shot was.

To measure the magnification of a given eyepiece in a telescope we divide the
focal length of the telescope by the focal length of the eyepiece (in mm).

Didn't read the entire text (yet) but this tutorial here on CiC might shed some
light on the issue:

https://www.cambridgeincolour.com/tu...era-lenses.htm

Mike

[ktuli]

Mike,

Thanks for the link, but I don't think it holds the answer. The MP-E 65 lens from Canon is a one-of-a-kind sort of lens - the only variable on that lens is the magnification. Everything is done at the minimum focusing distance (which I guess if I had that, I could determine the magnification) of the lens depending on what setting you have it at.

The lens is a prime, so there's no change in the focal length. It doesn't even have a conventional focusing mechanism either. When you turn the barrel of this lens, it changes both the minimum focusing distance and the magnification - you have to manually move the camera and lens physically closer or further away (we're talking in the range of centimeters here) to obtain focus.

It it a challenging lens, but as you can see, the results can be stunning... I just want to be able to figure out (after the fact) how much of that 1-to-5-times magnification I did get.

- Bill

[Dave Humphries]

Hi Bill,

To be honest, it is rather academic isn't it? - I mean; the picture is always going to be viewed at larger than your sensor size

If you really want to know, try this ...

It sounds like there is probably a direct relationship between the magnification and a figure that might be available as "focus distance" or "subject distance" in the detailed EXIF data - this figure isn't normally visible with most EXIF viewers.

If you go here; http://regex.info/exif.cgi
use the lower "local image" feature to find a picture on your HDD, then review the data, you might find it. I got a figure of 0.56m for one of my shots taken with a prime 105mm macro lens, I haven't done the next bit, so I don't know what the 'mag' is!

If you do find such a figure, you can then take some shots at each end of the range plus a few in the middle and know that one end is 1:1 and the other more than that (e.g. 5x), take say, 5 snaps (of a finely etched ruler) so you can see the distance from one edge of the frame to the other and relate that to your sensor size, thus giving you the magnification, then tabulate those figures against the corresponding focus distances recorded.

e.g. (fictional example)
f.d. = 0.150m = 1:1
f.d. = 0.100m = 2:1
f.d. = 0.080m = 3:1

Cheers,

[ktuli]

Dave,

A very good suggestion - and exactly what I use to figure out the magnification on my other macro lenses that go to 1:1 magnification. Unfortunately, though the EXIF data does provide that focus distance, the markings on the lens are "working distance".

Focus distance is the distance from the subject to the sensor plane; whereas the working distance is the distance from the subject to the front of the lens barrel.

This lens in particular is very tricky to work with in these terms because of the way it changes during zooming*.

For those folks who easily get headaches when trying to juggle numbers, I'd stop reading now....

So it seems that this lens, when at 1x magnification has a focus distance (FD) of 0.24 m or 9.4488189 in and working distance (WD) of 0.101 m or 4.0 in. Subsequently, it appears that at 5x magnification, the FD is 0.3 m or 11.8110236 in and a working distance of 0.041 mm or 1.6 in. However, when zooming* the lens through its magnification range, it grows a total length of about 133mm or 5.25 in - though the FD changes only a total of 0.06 m or 2.36220472 in. So what I think that means is that while the lens is changing is physical length (thus affecting both the FD and WD), it is doing so at a rate that means there will be FD values that will have multiple possible WD value mappings, so I don't think I could use the FD exif value to provide me a number that would correspond to the WD which would then allow me to know what my magnification was.

As a more generic way of putting this, I believe when I am working at 1x magnification, when I move to 2x magnification, my FD actually gets smaller, but then as I move to 3x, the FD starts to grow again, eventually passing through the value for 1x and continuing to grow larger through to 3x, 4x, and then 5x. The confusing bit here is that while you're doing this, the whole time lens barrel is growing in length, so it is getting closer and closer to your subject.

At least I think so... as long as I did all of my math right.

- Bill

* When I say 'zooming' in regards to this lens, I am referring to changing the magnification, not the focal distance.

[Snarkbyte]

As Dave suggested, there is in fact a relationship between distance and magnification. In fact, Canon includes a chart of this relationship for the MP-E 65 in the PDF document on "EF Lens" CD that should have been included with your lens. Just load the CD, open "Start.PDF" and navigate to the MP-E 65 section by clicking on that lens in the table. A shot of the table I snipped from the document is shown below. Of course, you still need to know the working distance, but perhaps the EXIF viewer Dave pointed to can give you an estimate of that. BTW, there's lots of good stuff on that CD, so keep it handy. Congrats on a great new lens!

[ktuli]

Al,

Thanks for the chart. I did have the CD, and I pulled it out last night, but didn't get around to looking at it till today. Unfortunately, like you said - that just gives me the magnification to WD ratio. WD is not recorded in the EXIF data. And unfortunately, the FD is not always saved in the EXIF.

Today I went the more methodical method and took 5 different photos - one at each magnification - and here are the FD values I got for them:

Mag - Focusing Distance
1x - 0.24m
2x - 0.24m
3x - N/A
4x - N/A
5x - 0.3m

I have no idea why 3x and 4x didn't report values, but even with 1x and 2x reporting the same value means I can't use that as a way to determine the magnification from the EXIF data alone.

I'm really starting to worry that the only what I'll be able to do this is to record it manually in a notebook or something.

- Bill