Shallow depth of field

Painted Goby (Pomatoschistus pictus), Loch Goil, Scotland. Nikon D200, Nikon 60mm macro lens with +5 diopter. 1/200s, f/18, ISO200

I was very pleased to be placed in the top four images in a recent BSoUP competition. The theme was “shallow depth of field”. My image was of a Goby taken a few years ago in Loch Goil, Scotland. This shot has been lurking in my catalog without being shown and it has been nice for it to see the light of day.

These tiny fish are found in large numbers in the very shallows of many of the Scottish sea lochs, flitting about on the sand.

I often spend time shooting these guys at the end of a dive. Although they look a little plain from a distance, when you see them close up, they have wonderful markings on their scales.

They tend to keep very still and then, without warning, dart a short distance. This means they are quite tricky subjects and you don’t get long to work an individual. They home in on any disturbed sediment, hoping for a tasty snack. The best technique is just to lie very still and let them move into range.

This image was taken with a 60mm macro lens with a Marumi dry diopter on the front. This provides greater magnification at the cost of depth of field. With lens alone, a wide aperture of around f/4 seems to produce pleasing shallow depth of field, but this image was shot at f/18 and still has a very shallow depth of field.

I don’t usually crop my images very much, but in this case I felt the image had more impact in portrait format than the original landscape orientation. I was very pleased with the pose, giving a nice curve to the fish with its mouth open, but unfortunately its pectoral fin on image left was not extended. This was why the shot has not been shown before, but for this competition, I wondered if it could be improved. I copied the splayed pectoral fin, gave it a mirror flip and then moved it into position on the image left. I was very pleased with the result and so, it seems, was the competition judge.


Dome port repairs

I had a bit of a shock as I stripped my housing down after my previous dive, when the “dome” of my dome port fell off! I discovered that my vacuum leak detection system had been all that stopped my housing filling with water in a catastrophic way. Looking up the replacement cost of the dome gave me a great encouragement to make a DIY repair…

The dome, which is a Sea&Sea mini dome, is constructed from an aluminium base, with an acrylic dome held in place by a bead of sealant. I have been using this dome since 2004 and has done hundreds of dives. I was able to gently pull the whole bead of sealant off, which left not a trace on the acrylic but some residue on the aluminium base. I carefully removed the latter using a flat plastic blade I made by applying a scalpel to a “bic” pen lid. I then carefully cleaned both parts with alcohol.

I did notice that I’d managed to make a hairline scratch on the *inside* of the dome (presumably through careless cleaning or handling), so I took the opportunity to polish the inside using my kit… Whilst I had easy access to the inside surface.

A question on the BSoUP Facebook page directed me to a product called TSE399 from a company called Techsil. Whilst some have used standard bathroom sealant to make a similar repair, this seems to have mostly been “in the field”. The difference between the Techsil product and standard bathroom sealant is that the former flows and the latter does not, as well as being quicker setting.

The acrylic has a groove around the edge, whilst the aluminium has a rebate, into which the dome sits. It seems that this is what provides the seal, as well a large creating a bead to hold the dome in place. The TSE399 is said to have excellent adhesion to both plastic and metal; I hope so, as it’s all that’s holding the dome in place!

To fit the dome, I decided to run a thin bead of sealant into the rebate on the port base. I let this cure for a few minutes, with the idea of creating a bed for the dome to sit on. This didn’t work as well as I hoped, as it did not set enough, so when I placed the dome in place, it sank into the sealant and a small amount overflowed. The final step was then to run a thin bead into the groove in the acrylic. It’s important to keep the dome flat until the sealant has solidified, as it will flow, which creates a smoother finish than I think I could have achieved with something similar to bathroom sealant.

Having left the dome alone for 36 hours for the sealant to cure, the final step before pressure testing was to give the exterior of the dome a quick polish to remove the one or two scratches it had picked up in the last couple of years; I’m sure these did t show in the images; tiny scratches fill with water and disappear from view underwater, but I wanted everything to be as shiny as possible before returning to use.

dives, techniques

Long exposures

I finally broke my 2021 duck with a dive at Capernwray quarry in Lancashire; the weather was unkind, so a sea dive was not an attractive option. Capernwray is blessed with good visibility and the trout patrolling the shallows are an enticing photographic subject.

I chose to try various long exposure techniques to give an impression of movement. Using an exposure time of 1/4s whilst moving the camera briskly to “overtake” the subject gives a pleasant motion blur (and it tends to hide the backscatter too). A burst of flash at the beginning of the exposure freezes some detail on the subject and adds some colour. In order to maintain a decent depth of field, an aperture of at least f/8 or f/11 is needed and that necessitates as low an ISO setting as possible. The small aperture also forces a high strobe output.

I also tried a zoom blur. It is quite tricky to get the zoom action quick enough to show in the exposure, but it’s probably easier with the camera housed. Underwater, the zoom is accomplished with a flick of the finger on the zoom wheel (rather than gripping and rotating the zoom collar when holding the camera by hand).

I was most pleased with the results when the camera was close to the subject (for these large trout, less than 30cm) and when there is some object in the background. This generates significant motion blur on the background and allows the strobe to illuminate the subject.

These trout are quite large, so too big for any of my macro lenses. I chose my trusty Tokina 10-17 lens, but struggled to get enough opportunities with the fish close enough to fill the frame. A longer lens would have perhaps been better; next time I might try a teleconverter to bring the subject a little closer.

This technique requires a lot of trial and error, and the “hit” rate is pretty low. Nevertheless, I managed to capture half a dozen “keepers”.