Canon 17-40mm Dipoter with Dome?

Anyone out there using the 17-40? Does it require a diopter behind a dome port?

Mike

I'm using it with a S&S compact dome port and no, it doesn't require one. The minimum focus difference seems to allow it to work out.

It's a good question though.

I bought a B&W NL4 Diopter recently for my 17-40L for underwater use. I'll try it once I get a dome port to see if there's a difference.

The 17-40L will focus close enough to work diopter-less under a dome port. What I haven't been able to find out so far is whether having to focus on a very near image would affect AF performance, and whether the optical performance would be better/worse with a diopter.

With the +4 diopter on above water, infinity focus on the lens focuses on a subject 30cm away.

It seems to me that diopter selction for wide-angle zooms is a bit of a black art, and no-one really knows the definite answers (the simple answer to your question, as Tom pointed out above, is 'No'. But other sources say you need a +4 diopter. Does this mean a +1 or +2 diopter would work best...?) A set of diopters and a pool would seem to be called for.

Hi,

yes sometimes it looks like treated as black art.

but it is no hidden secret. to focus (at least) to the underwater infinity image is 3,03 * dome radius.

when a dipotre should compensate the negative refraction of the dome you can quickly work it out by numbers to determine the dioptre value.

regards,

Julian

but it is no hidden secret. to focus (at least) to the underwater infinity image is 3,03 * dome radius.

when a dipotre should compensate the negative refraction of the dome you can quickly work it out by numbers to determine the dioptre value.

I'm glad that I can quickly work it out - hey, wait a moment, no I can't, I've got no idea what you're talking about! Can you explain how this works (the second bit about determining the diopter value).

BTW, I'm currently in the process of re-learning primary school mathematics, so once this is explained I should be able to do it, especially if it involves equilateral triangles or long division.

Hi Rob,

let's boost your "primary school mathematics" to master divisions

(it's nothing more, really)

the dioptre number is just a different value to express the refraction.

for example your brand new ewa marine dome with nasa coating has a radius of 10cm.

100 cm

----------- = D

3.03 * r

r = 10 cm -> dioptre value of 3.3

theoretically this would compensate the dome negative refraction effect.

The 100cm value is fixed. But glass thickness is thrown out here. so there still will be differences when your ewa marine "TÜV" proven dome has 5cm glass

:wink:

Julian