Macro Photography, Diopters and Math

I think I’m living in the movie Groundhog Day everytime I look out the window today. The raging snowstorm outside is reminding me WAAAAY too much of February’s Snowpocalypse. While the forecast is calling for only 12 inches of snow (ha!) I’ve decided to get productive today and research a subject I’ve wanted to know a little more about: Close-Up Diopters.

The world of macro photography is one of my favorites to visit. I personally like to get as close to I can to make my subject look alien. I have a couple of diopters, a +4 and a +10 (which is the diopter of choice). How do they assign a number to a diopter and what does it mean? I found a great page on Michigan Tech’s website, of all places, that easily explains some of the mathematical concepts I’m about to present so if you want even more in-depth information check it out (http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/A95/Close-Up/Close-Up-Lenses.html).

“Most close-up lenses are marked with a +d number in diopter unit, the power of the lens. The diopter (or power) of a lens is defined as 1000/f, where f is the focal length of the lens measured in mm. Thus, a lens of 50mm has a diopter of +20 = 1000/50, and a +4 diopter close-up lens has a focal length of 250mm = 1000/4. A close-up lens with larger diopter value has higher magnification.”

In the Holga, the native lens is f=60mm which means it’s focal length is 60mm (NOT to be confused with the MINIMUM focal length, which is something entirely different). The diopter of this lens = 1000/60 or 16.6 or, if you round-up, +17.

I often get asked how I use my diopters, especially in focus-free cameras. It is a tricky business, for sure and requires a bit of trial and error. The most important thing you need to know is how close you need to get to the subject, but how the hell do you figure that out? First off, you need the minimum focal distance which will tell you how close to your subject you can stand and still take a shot that is in focus. In a Holga, that would be 2.95 feet/89.95 cm. So the closest you can get to your subject with an unmodified Holga is about 3 feet. By the way, I found this out by looking it up on the web. Most cameras have this number listed in the Specs part of the camera’s manual.

My favorite tool, the +10 diopter, has a power that would equal 1000 divided by 10, or 100mm. This means your working focal distance or the distance between your lens and subject is 10 cm (100mm) or, for those of us who aren’t using the metric system, 3.937 inches.

This all means that when you use a +10 diopter on a Holga, you should be 10cm/3.97 inches from your subject in order to get it in focus. You’ll have to learn where your diopter’s sweet spot is and that will occur through trial and error. The stronger your diopter (the larger the “+number”) the smaller the area of focus and the greater the amount of bokeh you will get in your shots. At least, that’s the experience I’ve had with my plastic diopters.

Here’s a little chart that sums it up quite nicely. I can’t take credit for this. I got it from a Flickr group about 3 years ago. The person who posted it has been deleted but still, thank you to holga_pics for doing the calculations.

Don’t forget to set your Holga to infinity (the mountain symbol) when using a diopter:

Diopter Power              Working Focal Distance

+1                                      100 cm or  3′ 3″ (39.37″)

+2                                        50 cm   or  1′ 6 ” (19.5″)

+3                                     33.3 cm or  1′ 11″ (13″)

+4                                        25 cm or 9.84″

+5                                       20 cm or 7.87″

+10                                     10cm  0r 3.97″

I know what you’re (or at least, what those of you who are into geekery, like myself) are thinking. What formula is used to calculate these numbers? This statement from http://home.sprintmail.com/~awiner/diopters.html sums up the formula quite nicely.

“…the diopter power implies the focus distance. Take 100 cm and divide by the diopter. That’s the focus distance with the diopter. So, a +5 diopter wants to focus within 20 cm. A +2 diopter wants to focus within 50 cm. So, higher power diopter implies more magnification – if you can get that close.”

You’re really just dividing 100 into the diopter power number (+2, +3, etc). That’s it! Who knew it was that simple? Certainly not this math phobic girl.

For consistent results and quicker shooting in the field, make yourself a ruler or marker that will help you position yourself the correct distance from your subject. I use some old cardboard and carry it with me whenever I do macro work.

I hope this little exercise has been useful. If you’re still reading this, I think you get a medal.

Seriously.

Thanks for staying with me on my mathematical journey because knowing these concepts will allow you to use any diopter on any lens and get decent results.

Happy Macro-ing!

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About ipdegirl

analogue girl in a digital world View all posts by ipdegirl

4 responses to “Macro Photography, Diopters and Math

  • F.W. Redelius

    Thanks for the info it was very interesting
    FWR

  • Dilen

    Wow. I was wondering what it meant by the + number which were in the macro diopter filters. Thanks

    -Dilen

  • ipdegirl

    Dilen,

    Basically, the + in front of the number indicates the power of the lens. It’s another way of representing the fractional number of 1000/x.
    Check this quote out from the blog. I had to read it slowly a few times to re-acquaint myself with the terminology because I am mathematically stupid….

    “Most close-up lenses are marked with a +d number in diopter unit, the power of the lens. The diopter (or power) of a lens is defined as 1000/f, where f is the focal length of the lens measured in mm. Thus, a lens of 50mm has a diopter of +20 = 1000/50, and a +4 diopter close-up lens has a focal length of 250mm = 1000/4. A close-up lens with larger diopter value has higher magnification.”

    I think they use the + sign for people like me who would be scared off by all those large numbers and fractions 🙂

    Hope I helped clear it up for you!

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