equipment

Reassuring light

The worst nightmare for any underwater photographer is a flood. You know what they say: there are two types of underwater photographer- those that have flooded a housing, and those who haven’t…yet. Every time I get in the water with my camera, I always wonder if water will get into the housing. I think most underwater shooters have a ritual to follow when setting up the camera to avoid a leak, and I for one never used to deviate from mine. After all, some of the steps in my setting up process might be unnecessary, but my fear of a flood prevents me from changing the ritual.

Well, for a year now I have been using a leak detector, which lets me pressure test my housing before every dive, and it has made a massive difference to my confidence in how I have set up the housing (as well as allowing me to streamline my setting up ritual).

DSC_7365I use a device called Leak Sentinel, purchased from the very helpful Miso Milivojevic, who designed and manufactures them in Slovenia under the company name of Vivid Housings. The device fits into a spare strobe port on my housing; some of the air is evacuated from the housing with a small pump. If this under-pressure remains stable, then the housing has no leak and is safe to dive. The leak sentinel indicates this with a reassuring green flashing light.

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I have just upgraded mine with a new circuit board which adds some new features. The Leak Sentinel can now compensate for temperature changes (which are likely to affect the pressure and potentially cause a false positive); it now allows me to test the housing a day or so before the trip, and also warns me when the leak sensor battery is getting low.The device has already paid for itself because it detected a leak in the system, which I spotted and corrected, rather than finding out when I got in the water….

equipment, techniques

Macro magnification

Having tried my lovely new Nikon 105mm macro lens out on a recent trip to Scotland, I had borrowed a Nikon diopter and also used a teleconverter. Now I had discovered that I was able to get some pleasing images, I was interested in making some measurements.

By using different combinations of these three items (lens, teleconverter and dioptre) different magnifications can be obtained, but as usual there are trade offs to be made. On one hand, a teleconverter increases magnification without needing to get closer (ie same working distance), but the optics absorb some light, which means either brighter illumination or a larger aperture, which in turn reduces the (already very narrow) depth of field. On the other, a dioptre lens increases magnification without absorbing so much light, but reduces the working distance.

So which is the best combination to choose? The first step, I felt was to try all the combinations and measure the magnification, working distance and depth of field. This would provide me with objective data to make decisions.

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It seems to boil down to working distance- the dioptre is likely to give better results if the subject is suitable- shorter working distance (so hopefully less backscatter) coupled with higher magnification. However, the teleconverter will help with more skittish subjects because it gives high magnification at a longer working distance, but relies on not having too much suspended matter in the water. Using both is a possibility, will require some care to ensure that the appropriate part of the image is in focus, due to the wafer thin depth of field.

equipment, techniques

DIY fibre snoot

During my recent trip to Scotland, I was happy enough with my composition and the sharpness of the super macro images I attempted. However, on closer examination of the images, I found the lighting was not to my liking. The trouble is that when lighting small subjects (a few millimetres in size), I want the subject lit without lighting up the background (or the suspended stuff in the water).

The job of a snoot is to restrict and direct the light from the strobe. I have tried before with plant pots and plumbing parts, but have not been happy with the results. Either the light is not snooted enough, or there is too much
light loss.

There are some very good commercially available products, but these are quite expensive, so I was keen to pursue the DIY route. During my googling of the subject, I came across Rob White’s excellent photo site, where he described how he made a fibre-snoot from inexpensive parts. He was quite willing to offer me extra advice, and I had to give it a go!

20140318-223208.jpgThe fibre snoot in place. I first used a neoprene sleeve to hold it there, but this was not secure enough when I tried it in the garden.

20140318-223221.jpgthe snoot base is made from the middle of a DVD “cake box”, the arm is a loc-line kit I got cheaply from eBay and inside is a length of 6mm dia “optical fibre”. I use quote marks because at that large diameter it is not so much an optical fibre as a light guide. I works, though. You can see the arm is off centre and aligned to one of the flash tubes, to maximise light output.

20140318-223230.jpgThe fibre is slightly recessed in the end of the arm and the orange fitting is covered in black insulating tape to cut light loss.

20140318-223242.jpgI decided to use the screw threads built into the strobe head. These two short M3 screws fit nicely, though the black plastic is more flexible than I would like though. I’ll still use the neoprene sleeve I think.

20140318-223250.jpgThis shot gives an idea of the working distance at the near point, when using a +5 diopter on the Nikkor 105mm macro lens. A short working distance, but workable, i feel. I found it fairly easy to set up (in air!) on a static subject. Going to be challenging underwater, to not crash the end of the snoot into the subject.

20140318-225437.jpgThis shot gives an idea of the size of the pool of light from the snoot. The tip has an aperture about the same as the light guide (6mm).

20140318-225448.jpgAn early shot at closest focus of the 105mm lens.

20140318-225454.jpgSame subject using +5 diopter. Happy enough with the lighting.

20140318-225501.jpgAn photo taken outside- heather flowers. This is using the fibresnoot (only just out of shot), the 105mm lens and +5 diopter. Nikon D200, ISO 400, 1/160, f/32. strobe output about 3/4. Nice black background and fairly pleasing lighting without being too obviously “snooted”.

20140318-225507.jpg Exactly the same subject and optics, but not using snoot. Although I like the background in this shot, underwater this would be more “messy” and with more backscatter. I don’t like the hotspot in the foreground, either- needed to point the strobe “out” more.

The next step is to try a narrower tip…..