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.
