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03-16-2010, 05:23 AM   #1
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Magcro Magnification Ratio Question

For those of you that like the short version, I'll state the question first. How would one calculate the magnification ratio of a non-macro zoom lens with a diopter attachment at any given focal length?

I have been experimenting with macro photography recently by using a Raynox DCR-150 on my DA 55-300 Zoom It is a diopter of 4.8 and seems to make some pretty good images.

I've been considering stepping up and buying one of the various prime macro lenses, either the Tamron AF90 or the Pentax D FA 100mm, both of which has a 1:1 ratio and a minimum focus distance of about 30cm.

All of this thinking brought a question to mind and I haven't been able to find any info about it, so I thought I'd throw it out here for some discussion.

I have read that you can calculate the minimum focus distance of a lens with a diopter adjustment by dividing 100mm by the diopter value. This seems to be the case for the Raynox because the diopter on that is 4.8 and everything wants to be at around 20cm to be in focus. So the question is how would one calculate the magnification ratio of the lens at each focal length of the zoom, say at 100mm, 200mm and 300mm? I suspect it is less than 1:1 especially at the long end, but I'd like to understand how to calculate it.

Someone here has to know, right?


Last edited by GregK8; 03-16-2010 at 12:21 PM. Reason: calrify language
03-16-2010, 07:41 AM   #2
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The thread "The Raynox Macro Club" has a lot of info on this subject:

Raynox Macro Club link

Newarts in post #91 provides the forumula for calculating closest focus distance:

formula -- post #91

There's more discussion in the posts that follow. IIRC, someone (can't remember who) says the working distance of a 100mm macro lens is less than that of the Raynox 150.

The formula for magnification when a close-up lens of strength D is put in front of a primary lens of focal length F is:

Magnification = F*D

Hope this helps.

Last edited by twokatmew; 03-16-2010 at 07:48 AM.
03-16-2010, 07:44 AM   #3
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I forgot to mention....

I've got a DA35.2.8 Macro and really like this lens. But ... to get close to a true 1:1 macro, working distance is practically nil, and I end up blocking light. I've got a DAL50-200 and a Raynox 150, so I'll be playing with that soon, as I definitely need more working distance at times. I believe you get 1:1 at ~140mm with a Raynox 150.

Last edited by twokatmew; 03-16-2010 at 07:51 AM. Reason: fix typo
03-16-2010, 07:54 AM   #4
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Minimum magnification = (Focal_Length_in_meters) x Diopters.

Example, 85mm lens with 4 diopter close-up filter: .085*4 = 0.34x

Further, Working distance in meters at minimum magnification = 1/diopters

For the Raynox 150 (4.8 diopters) this is about 8.3", 204mm - a nice working distance, a little longer than a 100mm macro lens at 1:1 (200mm).

Old Dave

PS The working distance calculation is for the distance from the optical center of the lens to the subject so is not precise in the real user's world.


Last edited by newarts; 03-16-2010 at 08:09 AM.
03-16-2010, 08:05 AM   #5
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QuoteOriginally posted by newarts Quote
Minimum magnification = (Focal_Length_in_meters) x Diopters.

Example, 85mm lens with 4 diopter close-up filter: .085*4 = 0.34x
For those of us using crop sensors, don't we have to take the crop factor into account?

That is, a +10 diopter yields 1:1 at 100mm in 35mm-equivalent format. Thus, on a 1.5x crop sensor, that's just under 70mm. Correct?
03-16-2010, 08:22 AM   #6
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QuoteOriginally posted by twokatmew Quote
For those of us using crop sensors, don't we have to take the crop factor into account?

That is, a +10 diopter yields 1:1 at 100mm in 35mm-equivalent format. Thus, on a 1.5x crop sensor, that's just under 70mm. Correct?
No. "Magnification" is with respect to the image projected by the lens, not the size of the sensor capturing that image. At a 1:1 magnification the image captured by a 36mm sensor is 36mm wide in object space & the image captured by a 24mm sensor is 24mm wide in object space.

The crop factor is taken into account when you enlarge the sensor image to a constant display size.


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03-16-2010, 11:09 AM   #7
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BTW, while you can caculate the magnification in theory for a lens or combo you don't yet own, if you have the goods in your han, you can make it much simpler. Get out ruler, focus on it as closely as you can. However many inches fit across the viewfinder, that's the magnification. This works because an APS-C sensor is essentially 1 inch wide (slightly less, but close enough).
03-16-2010, 12:05 PM   #8
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QuoteOriginally posted by newarts Quote
For the Raynox 150 (4.8 diopters) this is about 8.3", 204mm - a nice working distance, a little longer than a 100mm macro lens at 1:1 (200mm).
Hmmmm, why would the specs for both the tamron @ 90mm and the Pentax D FA @100mm report a working distance of 30-31cm?

03-16-2010, 12:06 PM   #9
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QuoteOriginally posted by Marc Sabatella Quote
BTW, while you can caculate the magnification in theory for a lens or combo you don't yet own, if you have the goods in your han, you can make it much simpler. Get out ruler, focus on it as closely as you can. However many inches fit across the viewfinder, that's the magnification. This works because an APS-C sensor is essentially 1 inch wide (slightly less, but close enough).
Haha

So simple. It's amazing how one can completely over think things.
03-16-2010, 12:17 PM   #10
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QuoteOriginally posted by newarts Quote
Minimum magnification = (Focal_Length_in_meters) x Diopters.

Example, 85mm lens with 4 diopter close-up filter: .085*4 = 0.34x
So for the DCR-150 and the 55-300mm zoom, if I want a 1:1 ratio, I can divide 1 by the diopter to get 208mm, which is the exact same number as the working distance.

Interesting how that works. So if I set the focal length of the zoom to 208mm, in theory that should be the same 1:1 ratio as a 100mm D FA.
03-16-2010, 12:24 PM   #11
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QuoteOriginally posted by GregK8 Quote
Hmmmm, why would the specs for both the tamron @ 90mm and the Pentax D FA @100mm report a working distance of 30-31cm?
I'll answer my own question, because they are reporting the minimum focal distance, which I believe is the distance from the focal plane to the object, instead of the working distance, which is the distance between the front of the lens and the object.
03-16-2010, 12:34 PM   #12
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QuoteOriginally posted by twokatmew Quote
The thread "The Raynox Macro Club" has a lot of info on this subject:

...

Hope this helps.
Very helpful. I can't believe I missed that thread. I always try to make at least an honest effort at searching before I post a question, but I somehow missed that. It must have been in my results and I just past over it.

My wife does say that I'm not a very good finder. I can stare in the cupboard for an hour looking for a can of coffee that is right in front of my face, which she is only to happy to point out as she comes over and grabs it.
03-16-2010, 12:46 PM   #13
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QuoteOriginally posted by GregK8 Quote
I'll answer my own question, because they are reporting the minimum focal distance, which I believe is the distance from the focal plane to the object, instead of the working distance, which is the distance between the front of the lens and the object.
That's correct, and is quoted by lens makers because it is more precise, regardless of lens construction (Internal Focus, Recessed Lens, Hood, etc.)

The equation is:
Minimum_focal_distance=Focal_length(1+M)(1+1/M)

Dave

PS any discrepancy is likely because the effective focal length of a 90mm, internal focus lens (for example), is 90mm when focused at infinity, and can vary at other focusing distances.
03-16-2010, 03:13 PM   #14
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QuoteOriginally posted by newarts Quote
No. "Magnification" is with respect to the image projected by the lens, not the size of the sensor capturing that image. At a 1:1 magnification the image captured by a 36mm sensor is 36mm wide in object space & the image captured by a 24mm sensor is 24mm wide in object space.

The crop factor is taken into account when you enlarge the sensor image to a constant display size.
Thanks for the explanation. I think I'm starting to get it now.
03-16-2010, 03:15 PM   #15
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QuoteOriginally posted by GregK8 Quote
Very helpful. I can't believe I missed that thread. I always try to make at least an honest effort at searching before I post a question, but I somehow missed that. It must have been in my results and I just past over it.

My wife does say that I'm not a very good finder. I can stare in the cupboard for an hour looking for a can of coffee that is right in front of my face, which she is only to happy to point out as she comes over and grabs it.
I happened upon that thread by accident and have bookmarked it, as it has a wealth of info ... and lots of great photos! I'm glad you started this thread though, because I learned something new. Always a good thing!
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