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01-15-2012, 10:48 PM   #1
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Need help calculating macro...

I have a Sigma 90mm f2.8 macro lens. It goes to 1:2 on its own. (Minimum focus about 27cm = 11") When I attach the matched "life-size" apochromatic adapter, it goes to to 1:1. (Minimum focus about 18cm = 7") I have confirmed this with the ruler test. At 1:2, it's 48mm wide. At 1:1, it's 24mm wide. Quality of images is excellent.

(BTW, the minimum focus is a rough estimate, but I'm trying to calculate from object to camera sensor.)

So, I also have an old Sigma Zoom K II 1:4.5 70-210mm that is listed as going to 1:3.5 as a macro. Using the ruler test, it goes about 83mm wide which would be 1:3.46, so that seems to be right. (Minimum focus about 90cm = 36") It's front filter size is the same 52mm as the Sigma 90mm, so I screwed on the "life-size" adapter expecting that it would halve (or double depending on how you look at it) the macro size, i.e., make it a 1:1.75 or so. BUT when I did the ruler test, it now is going only 18mm wide. That is, it has become a 1.33:1 (note, larger than 1:1) macro. (Minimum focus is now only 30cm = 12") The quality of the image was actually quite good. That adapter appears to be of rather high quality.

I'm not quite sure how the lens physics work to make that happen, but what then would happen if I got a 58 to 52 step-down ring and attached that life-size adapter to my DAL 55-300mm? It is listed as having a .28 magnification (= about 1:3.57 macro) with minimum focus of 140cm (about 55").

It would be nice to think I could 1:1 or better macro and not have to be so close as my 90mm lens requires.

Thanks for any calculations and help!

01-16-2012, 01:31 AM   #2
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What is "...the matched "life-size" apochromatic adapter" exactly? Normally, I would imagine this to be a teleconverter attached to the rear of your lens.

However, you further speak of "...a 58 to 52 step-down ring and attached that life-size adapter to my DAL 55-300mm!" This indicates that your "adapter" mounts on the front of your lens.

So, it might be helpful if you could identify your "adapter" in more detail.
01-16-2012, 04:48 AM   #3
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It sounds as if the matched adaptor is a close up diopter in the range of 7-10 diopters. I would need to do a little math to work out the exact focal length.

As to what happens to the zoom, the reason you get more magnificationn is that the zoom is doing macro at the minimum focal length, and ad a sult the close up adaptor allows this to give higher magnification.

To work out exactly what is happening, there is a simple test. Put your 90mm at infinity, and measure the focus distance om the front of the lens to the subject, this distance will be the focal length of the adaptor on its own.

The diopter value is 1/focal length.

When you add this to any other lens, the resulting focal length will be 1/(1/focal length+diopter of add on lens)
01-16-2012, 09:33 AM   #4
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@StoneG: Yes, the adaptor screws on to the front of the lens. See HERE. It's a 52mm.
@LowellGoudge: I'm guessing that what I'm calling an adaptor (and what it calls itself as a "life-size attachment") is what you more properly call a diopter.
I do have some of those +1, +2, +3 diopters, but this attachment is clearly doing much more, so your guess of 7-10 may well be right. I really don't seem to lose any quality with this diopter.
I'll have to work on measurements and math, but your explanation makes sense why its magnifying value is higher with the longer focal length.
Thanks!

01-16-2012, 09:37 AM   #5
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Here's my estimate: The sigma is an IF lens so its focal length at 1:2 is about 60mm with an image distance of 90mm. Adding the close-up lens changes the mag to 1:1 but does not change the image distance. Therefore the effective focal length of the combined lens+close-up lens is 45mm. Therefore the close-up lens focal length is about 180mm (5.6 diopter).

The nominal mag of the close-up lens on the 300mm lens is 300/180 = 1.67. The total mag of the extended 300mm lens plus the close-up lens is:
m ~ (1+0.28)(1+1.67) - 1 = 2.4
01-16-2012, 01:32 PM   #6
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@newarts: Wow, thanks! That's impressive magnification.
I'm guessing that for a macro like that, vignetting (going from the 58 to 52mm) shouldn't really be much of an issue.
Whether it's possible to get a stable shot at that kind of magnification is another matter....
Thanks again!
01-16-2012, 03:01 PM   #7
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QuoteOriginally posted by mgvh Quote
@newarts: Wow, thanks! That's impressive magnification.
I'm guessing that for a macro like that, vignetting (going from the 58 to 52mm) shouldn't really be much of an issue.
Whether it's possible to get a stable shot at that kind of magnification is another matter....
Thanks again!
Vignetting will not be a problem; stability will be more of a problem
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