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I found some old filters (labeled +1, +2) which my dad used on his SLR back in the day. I'm not sure what they're good for or if they're useful on my DSLR. Can someone tell me more about them (i.e. intended use / effect)?

Hoya filter +1 Hoya filter +2

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Thanks for the edit @mattdm, it's a much better question now –  akid Jan 2 '13 at 13:19
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2 Answers

up vote 5 down vote accepted

These aren't really filters, they are called closeup lenses, at least around here. Close up lenses are one of the three ways to get macro capability. The other two are extension tubes and a deliberately built macro lens.

Apparently in some parts of the world (judging from Matt's answer), these lenses are referred to as "diopters". Technically, a diopter is a unit of measure of how much a lens bends light. Camera lenses are usually defined in terms of focal length, since that is more directly relevant in most cases. Diopters are useful when stacking multiple lenses. The units of diopters are defined such that the individual diopters from each lens add to make the combined diopter. For example, if you put a +1 and a +2 closeup lens on the front of your existing lens, this will be equivalent to putting a single +3 closeup lens there. Of course there are downsides to multiple filters, such as light loss and internal reflections, but we're talking about the diopter math here. It's also not exactly right because the two closeup lenses can't be exactly at the same place, but close enough for most purposes.

The diopter measure of a lens is the reciprocal of its focal length in meters. A +1 closeup lens is therefore basically a weak magnifying glass. It is a single-element lens with a focal length of 1 meter (1000mm). The purpose of a closeup lens is not to adjust the apparent focal length of the lens you put it on, but to effectively allow it to focus closer. This is the same effect of looking thru a magnifying glass with your eye. It allows you to focus on things closer than your bare eye can, which allows you to see small details you otherwise couldn't.

Closeup lenses have their place, but one downside is chromatic abberation. Since they are single-element lenses, their light bending strength accross the spectrum is dependent on how constant the material's index of refraction is accross that spectrum. In multi-element lenses, different materials can be used in the different elements to mitigate this, but with a single element you get what the single material gives you.

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I just tried it on my 35mm lens (the only one I have kicking around right now that fits). The decrease in minimum focus distance was not mind blowing (at +3), but it's enough to play around with. –  akid Jan 2 '13 at 23:20
    
@akid: That makes sense. +3 is not a very "strong" magnifying glass. Think about focusing a image with a ordinary magnifying glass. The focal length may be 10 cm or so, which would be +10 diopter. +3 diopter has a focal length of 333 mm. –  Olin Lathrop Jan 3 '13 at 0:33
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They look like diopters, filters which mount on the front of a lens and reduce the minimum focus distance. The +1 +2 etc. designates the "strength" of the filer, i.e. how much the minimum focus distance is reduced.

So you ought to be able to screw them onto any lens with a 52mm thread and take close up "macro" photos. Most single element close up filters have a significant impact on image quality, decreasing sharpness and increasing chromatic aberrations. The results are usually much better when using a dedicated macro lens, but you will be able to have a play around with macro photography using these filters. They can be stacked so by combining both you get a +3 dioptre effect.

See also:

Which suits better for hobby usage: extension tubes vs. close-up filter?

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