How do you decide if you need to add a dioptre to a particular lens before you hit the water? And how do you decide which one?

Have a look at:

http://wetpixel.com/forums/index.php?showtopic=10372

For macro shooting it depends on the subject matter you want to shoot. The smaller your expected subject matter, the more the need for stronger dioptres.

I not going to comment on wide angle.

Alex

I like using diopters on my 105mm, i am not a big believer in the idea of cropping so to fill the frame on certain subjects then a diopter is very handy.

I know you love nudis, if you want to get really tight with the 100mm then i think you would like using something like a +1 or +2 which allows you to focus closer and therefore fill the frame better.

like this with a +1.


However, if you want to take more fish portraits and stuff that are a little shy then the diopter will limit how far you can get close to a subject and still achieve focus, the stronger the diopter the less room you have to work with. Also, when you add a diopter, your depth of field shrinks and sometimes doesn't let you get everything in focus, but that limited DOF is a good creative tool as well, this is a +1 i think




As for wide angles, i think Paul has steered you in the right direction as this has come up quite a bit recently

for wideangle, the 2x diameter rule is kind of popular, but dome diameter doesn't work when the curvature is small, or in other words, if the radius isn't half of the diameter. Lens length, etc. is left out too with this rule. Therefore, I personally don't like it.

To obtain lens' topside closeup capability underwater behind a dome:

Diopter Strength = 100cm / 3.03 x dome curvature radius.

full story: http://wetpixel.com/i.php/full/dome-theory/

Julian

Interesting! Using your forumula Julian, you will never obtain a result of "no diopter is needed!"

Even using a 27cm dome (huge) your formula still yields a result that a +1.2 power diopter is needed.

Cheers
James

Damn Mike!! You're not half bad at this photography thing.

Following is simple explanation. +4 recommendation is stronger than needed for some lenses, but did not differentiate to keep explanation simple:

http://www.ikelite.com/web_pages/qdiopter.html

James,

that’s right. Every dome, no matter how big, has its own negative focal length. The formula indicates what dioptre strength would equalize this effect. Then one can decide between zero and the result (based on individual criteria like corner sharpness, split shot possibility, etc.).

Even if a lens does focus underwater without a dioptre to infinity (thumb rule 2 x diameter), it is not assured that the close-up distance is always sufficient.

Without dioptre, close-up distance is decreased with any dome (compared to topside). That’s not dramatic, but it depends on dome model / size, lens close focus ability and individual needs. For example I wouldn’t recommend the Ikelite charts for the Seacam Superdome as dioptre strength is mainly given (or better limited) by dome’s radius.

Having that said, I don't use dioptres with my wideangle zooms.

Julian

Thanks Todd, i knew i liked Canadians for some reason...

You know, looking at these photos i bet if i used sRGB they would look better on the website eh? Oh well, too lazy to change em now...

So much for a simple answer!

Thanks for the links and information - it's far far more than I was looking for.

Guess I was looking for a quick rule of thumb I'm a KISS girl!

Simple solution is to try it - given that generally there are 4 different 'close-up' lenses available (+1, +2, +3 & +4) I'd suggest trying a +2 or +3 to start with and see how this works FOR YOU! If the effect is not sufficient (ie you want to focus closer) try a stronger one, if its too much try a less powerful one. Depending on your lenses' filter thread, 'close-up' lenses can be reasonably priced as sets which might be worth the investment.