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Close up filters – how to calculate magnification factor?

David Lim , Feb 13, 2001; 02:34 p.m.

Last week, I bought a set of Hoya close up filters (+1, +2, +4) to go with my Canon 22-55 lens. At 55, the lens has a maximum magnification of 0.2x according to Canon. If I put a +4 close up filter, what will be the magnification? Is there a formula to calculate the magnification factors of +1, +2, etc given that we know what the max magnification of lens Y? If not, are there rough rules, say at a +1 filter will equate to Z times?

Thanks.

Responses

David Goldfarb , Feb 13, 2001; 06:58 p.m.

I know there's a formula for this, but I find it easier just to put the diopter on the lens and look through the viewfinder at a ruler. If you have a 100% viewfinder, it is 36mm wide, so if the field of view is 36mm, the magnification is 1:1, etc.

C.G. Ouimet , Feb 13, 2001; 07:45 p.m.

According to the excellent John Shaw's "Closeups in Nature" ...

Magnification = focal Length of lens / focal length of Diopter

where the focal length of the diopter = 1000 / diopter strength.

For example, a 3 diopter has a focal length of 333mm (1000/3). So the magnification of a 3 diopter on a 55mm lens would be 55/333 = .165.

By the way, using a 1 and 2 together yields a 3, a 2 and 3 together yields a 5.

C.G. Ouimet , Feb 13, 2001; 08:05 p.m.

Actually, if you look at the math closely, it's doable in your head ...

Magnification = (Diopter x lens focal length) / 1000

Example: (3 x 105) / 1000 = .315

Example: (5 x 105) / 1000 = .525

Jim Strutz - Anchorage, AK , Feb 13, 2001; 08:43 p.m.

"Magnification = (Diopter x lens focal length) / 1000"

Wouldn't this be with the lens focused at infinity? How does the close focusing of the lens come into play here? I admit I'm confused on this subject. I've always used the focus-on-the-ruler method.

Carl Vogt , Feb 13, 2001; 09:03 p.m.

After a little searching I found a great web site that addresses the issue of Diopters at length.

http://www.smu.edu/~rmonagha/mf/diopter.html

Basically, it looks like +1 is a starting point changes in magnification are relative to values with the 1+.

Based on the tables at the above link, it would seem if the field size is 50% smaller with the 2+ the image size is 2x ; if the field size is 33% smaller with the 3+ the image size is 3x , the field size is 25% smaller with the 4+ the image size is 4x etc.

The same table is also in the "Kodak Pocket Photoguide" pg 37

Hope this helps Carl

Alexandre Vaz , Feb 14, 2001; 06:07 a.m.

I’m not a close up expert, actually I only bought this week a close up lens from sigma to use with my 75-300. But I think those diopters don’t make a difference with 22-55. Close up lens, allow you to focus closer than you normally could. As it as been previously said, a +4 close up lens with 22-55 allow you to focus at about 25 cm, doing a 1/5 relation, but 22-55 already focus at that distance... So if you have a 75-300, you can in fact make a magnification bigger than 1/1. With 55mm you would need +10 to make 1/2(meaning that you have a field of view twice as big as 24x36=48x72)

Brian Chandler , Feb 14, 2001; 06:52 a.m.

Response to Close up filters Ehow to calculate magnification factor?

1/u + 1/v = 1/f

That's really all you need to know, assuming you can imagine a diagram of a lens in your head, and it means that everything is done by adding and subtracting reciprocals. That's why the unit used is the "diopter/dioptre", which is the reciprocal of the focal length in metres. (It very greatly helps if you don't have to jumble up inconsistent sets of units as in at least one of the websites mentioned above.)

So "+4" means 25mm lens. Alternatively your 55mm is a 1000/55 = 18 dioptre, so adding, the combination is 22 dioptres, or f = 45mm. And you say the greatest magnification is 0.2, so (object:subject distances) u:v =5:1, so:
1/55 = 1/5v + 1/v = 6/5v
So v = 55 * 6/5 = 66 (i.e. lens is 66mm from film)
Now add closeup lens, so f=45, and
1/u + 1/66 = 1/45
1/u = 1/45 - 1/66 = 0.02222 - 0.01515 = 0.00707
u = 140
And the ratio u:v gives the magnification (as is obvious when you imagine the diagram), thus 140:66, so just over 0.5

Several people have pointed out that the best way to find the magnification for an actual (purchased) combination is to measure it with a ruler. True, but it doesn't hurt being able to work it out in advance.

You might find this page (of mine) helpful - at least it has a diagram: imaginatorium.org/stuff/cufilter.htm
The title, as is supposed to be obvious, is a joke.

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