Using dioptres behind a dome port

Lenses used behind a dome port may require the addition of a +dioptre. The dome creates a curved image that sits 25 to 30cm out front and if the lens can't focus this close then a dioptre is needed.

My 18-55mm does work behind the 6" dome without a dioptre but performance improves with a +4. At the 18mm end there is less barrel distortion and the corners are sharper.

I never knew whether it also allowed you to get closer to the subject so some pool testing was required.

Adding the +4 brought the closest focus from 21cm to 13cm, and sharp corners only required f/16 rather than f/22. Of course you can use wider apertures if the corners are not critical.

Close focus wide angle CFWA

Here's another example of CFWA using the 18-55mm with a +4 dioptre.

St Leonards Pier again with better vis this time.

Short tailed nudibranch (Ceratosoma brevicaudatum). This is the most common nudi in Port Phillip growing to 150mm, which is big for a nudi.

Leaky shoulder valve this time! Got a cold wet left arm.

+4 dioptre investigation

Flickr contact Rikke Lind commented that it would be interesting to see a comparison of DOF, zoom level etc. with and without the +4 dioptre and I had to agree. I have always wondered whether dioptres actually magnify or just allow closer focus, so here's a series of images showing what happens.

The tests show that adding a +4 dioptre.......

Set focus of 1m reduces to 38cm and Infinity changes to 39cm.
Closest focus of 24cm reduces to 22cm, giving 1.44x mag.
So the focus range Infinity - 24cm changes to 39 - 22cm. Interesting, I thought the closest focus would be a lot closer not just by 2cm.

If the camera stays in one spot, the +4 gives a little bit of magnification, resulting in slightly reduced depth of field,
But if the camera is moved back to keep the image size constant then the depth of field is not reduced.
Note: Depth of field only depends on image magnification, (and aperture and sensor size) not which lens you use.

At 18mm zoom, aperture 3.5, the +4 changes barrel distortion into pincushion distortion and reduces corner blur and vignetting! Didn't expect that.



The reason we use dioptres underwater is mainly to enable your lens to focus on the virtual image created by the dome port, many lenses cant focus close enough on their own.
Or, as with my 18-55mm, to bring the closest focus to about 10cm from the dome for our rich but murky sub-temperate water.

18-55mm with +4 dioptre

I bought a Hoya +4 dioptre 52mm to try with the kit lens and the results are excellent. I can now focus 10cm from the dome. Perfect for our murky sub-temperate water.

Behind the 6" dome this lens gives reasonable macro magnification with useful wide as well.

Of course it's not as pin sharp or as fast focusing as the 60mm macro or Sigma 15mm Fisheye but it's definitely a useful middle range zoom lens.

It means I wont need to buy a Sigma 17-70 and 8" dome to fill the gap.

On this dive I photographed nudibranchs and sharks, however the nudis were big and the sharks small!

Cuttle colours 18-55mm

These shots were taken at The Springs near Pt Lonsdale. Vis was only 3m with heaps of sand stirred up by the strong current and swell. I was surprised at how little backscatter showed up with the strobes out wide and balanced background exposure.

This is the same giant cuttle over a fun 15 min interaction. Wonderful inquisitive critters.

I used the 18-55mm kit lens with a +2 dioptre behind a 6" dome. Will try a +4 dioptre for closer macro as soon as the wind subsides.

Coogee Wreck and Castle Rock

I was invited to dive on Freediver, Alan Beckhurst and Mary Malloy's excellent dive boat.

Surface conditions were not inviting at all with up to 20kn SW and rain on the forcast. There was a nasty short steep swell with wind chop making the heads quite exciting.

I used the 18 -55mm kit lens because I didn't know where we were diving. Should have used the 15mm FE in the wonderful blue 15 to 20m vis.

Many of my wide shots were underexposed using 1/60th and f/8 at ISO200 but salvagable. Could have increased the ISO and aperture maybe.

Camera issue of the day: At 32m I found one of the buttons was stuck down preventing any speed or aperture changes. I was probably a little narced so it took a while before I found the offender and freed it up. That was the main reason for underexposed images.

18-55mm kit lens with +2 dioptre at The Springs

St Leonards was flat but had zero vis so I toddled across the peninsula to The Springs, Pt Lonsdale. From shore the water looked calm and clear-ish but this is a shallow exposed site. The surge and stirred up sand made photography tricky. Vis was no more than 5m.

This is a great site for marine algae, or seaweed, having tons of different species. I am currently building up a collection of marine algae images for a proposed Port Phillip ID guide so seaweed it was for today. I also wanted to try the kit lens with +2 dioptre for middle range close ups.

The +2 brought focus plenty close enough, maybe 10cm from the dome. At f/8 there was corner softness in some shots but certainly not excessive. These are the keepers.

I had one issue to begin with. I couldn't turn the camera on. After a few panicked seconds I realized the button that activates the self timer was depressed. Once I lifted it and worked out how to cancel the timer all was well.