Seems like every third thread includes references to diopters!

Most really experienced uw photographers here seem to have their favorite combinations of lenses and diopters, yet the reasons for those choices are not always apparent to those of us with more limited technical expertise. I've used numerous diopters (because housing manufacturers told me I had to), but I've never felt like I really understood why I might choose, for example, a +2 vs. a +4 for a given lens and port.

I can't seem to find (despite archive searches) an overall "A to Z" discussion of diopter use. Can anyone point me to such a discussion? (Am I the only one who feels ignorant on this particular subject?)

From Photo Tips and Help: http://www.ikelite.com/web_pages/qdiopter.html

I wrote a piece about diopters in UWP magazine (Issue 1 I think) try www.uwpmag.com to download it.

The formula to calculate what diopter is needed is:

P (Dioptric Power) = 1000 / 4 x R (Radius of the Dome Port)

If your lens focuses to closer than 4 x R then you don't have to use a diopter BUT diopters may 'tend' to help reverse some of the curvature of the image produced by the dome port so can be beneficial in other ways than readjusting focus. Paradoxically this is supposed to be most pronounced with flat rear surface diopters (I believe) but I'm checking out a very high quality two element achromat at the moment to see how well this performs (some adjustment is needed at the moment as it affects the port's position and I'm shifting the port forward in 5mm increments to find the optimum.

If you think about it, the bigger the dome, the less likely you are to need a diopter.

The dome also has to be correctly aligned with the 'centre' of the lens which is why manufacturers offer various port and/or extenders.

The only really good book which I have found to cover this is a large and very expensive volume on scientific photography by S Ray, but it only contains a few (albeit very useful) pages about underwater photography.

There is a great article right here on wetpixel about this dioptre stuff .... http://wetpixel.com/i.php/full/dome-theory/

regards,
Julian

Sorry Julian, I'd forgotten about the piece right here!

Thanks to all for the great info re diopters and dome ports, but several people seem to be using diopters with macro lenses (i.e., flat ports) as well.

Are there general rules for flat port diopter use? For example, is the only purpose/function of a diopter with a fixed focal length lens and flat port to enable the lens to focus closer, and does a diopter keep the lens from being able to focus on distant objects (e.g., that occasional dolphin that swims by while you're shooting nudi's)?

Using diopters for macro is just to make your macro lens even more macro by letting you focus closer. They allow you to take a picture of something tiny, but cost you the ability to focus at infinity.

Diopters would only be used with telephoto macro lense because a 50/60 mm macro lens will allready focus to the point the object is pretty much touching the port.

The power used for macro is up to you. The more powerfull, the small an object you will be able to photograph, but a stonger diopter has a larger cost in terms of the ability to focus on large objects.

Your example is a bad one, because a dolphin will require a WA lens so no matter how you are set up to photograph nudi's you wont be able to photograph the dolphin that swims by.

A better example is you are shooting nudi's and cant take a picture of the angelfish that swims by....

To get around this, you can use a wet mount diopter that you can remove underwater.

Thanks, William - that helps (regarding using a diopter for macro).

As for the dolphin being a bad example, while I agree an angelfish is a more likely diversion, with clear enough water, I've taken pictures of some pretty big subjects with macro lenses. For example, this 4-ft Click to view attachmentClick to view attachmentbumphead parrot was taken with a 100mm macro lens and a 1D MkII with a 1.3 crop factor, which means it would be comparable to a 130mm lens on my full-frame 1Ds MkII. So I was being serious when I said dolphin! ;-)

Wow very impressive. When I used the canon-100 (on 1.6X) i learned not to even try anything bigger than a butterfly fish. I guess I've never seen clear water....

Good discussion. I like William's answer on macro dioptres. Spot on. There is always a lot of confusion on the dioptre vs teleconverter issue - but at least not in William's mind!

Alex

Well thanks for the compliment, but to be honest, I thought I being vague by leaving out by how much a diopter effects things.

Because I've never been able to figure that one out. I know how to determine min magnification. But I've never been able to figure out max magnification, or minimum focus distance which are what you really want to know.

Hmmm...should I start a new thread asking for a teleconverter primer?

I WOULD be interested in learning more about that. I've never used a TC uw...does it work the same as above water? And how does it affect macro shooting (e.g., min. focus distance, etc.)?

BTW, my bumphead parrot shot is definitely an exception - it was in Komodo from about 10 feet away (ambient light) in exceptionally clear water. I've tried shooting mid-to-large sized stuff with that 100mm lens in murkier water, and... let's just say I won't be posting any of those photos...

Just to add to the topics for discussion you can also use dome ports with macro lenses and by doing so retain their focal length (or field of view). Since they are used well stopped down, curvature of the image isn't a great problem. A friend of mine used a Subal DP84B Dome Port (as opposed to the FP84B Flat Port) with his 60mm micro Nikkor for a long time very happily!

Here are a few tips that helped me.

Photography stores use the word "diopter" for

(1) something you stick on the eyepiece of the camera to correct your vision

(2) something you screw onto the end of the lens to shorten the minimum focusing distance and increase the magnification

(3) something you push onto the outside of an underwater housing to shorten the minimum focusing distance and increase the magnification

and mostly if you ask for a diopter you will get (1).

The discussion on Wetpixel is about (2) and (3).

Woody's push-on diopter is type (3). It can be used to increase the macro capabilities (higher magnification, shorter focusing distance) of an existing underwater camera/lens combination.

The diopters mentioned on Ikelite's website are type (2). They look like filters and have similar screw threads to attach them to the front of a lens. The Ikelite website explains why you need to attach a +4 diopter to some zoom lenses, inside the housing, to achieve focus inside a dome port.

If you're looking to buy a type (2) diopter on the web, search for CLOSE-UP LENS. Sometimes they are listed under FILTERS - 'close-up filters'. The price is usually US $30-$100 depending on quality (you don't want a cheap one, which would cut out too much light and add optical defects to your wonderful pictures). The mega-expensive $600 ones are usually for video cameras.
Anything that looks square is probably an eyepiece corrector (1) rather than a close-up lens (2).

For the theory, see Imaginatorium 'What is a close-up filter?'

See Yusuf's discussion on image quality with diopters