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What does underwater close-up lens do?

April 24, 2018

What does underwater close-up lens do?

If you are reading this, you probably have already seen underwater pictures of tiniest sea critters and want to be able to produce something similar. Most of these pictures have been taken with macro or close-up lenses of some sort. If you want to understand better what these lenses do, then read on.

The first question is; what is the difference between a macro lens and a close-up lens? It seems like these terms are often conflated and mixed up. In general, a macro lens is something you attach onto your camera body and a close-up lens or more accurately close-up conversion lens is a secondary element you attach on your camera lens to alter the primary lenses range. Both lens types can, of course, be used to take underwater photos and often are combined to produce even better images.

For this article, I’ll be concentrating on the external close-up conversion lenses as they are extremely handy and often inexpensive way to get started with UW macro photography. These lenses can be taken off and exchanged during the dive as they have been designed as “wet-interchangeable” and are waterproof on their own. In a typical dive, I keep my close-up lens in my bc pocket until needed and then simply screw it on my housing port to take the photo.

tiny cuttlefish

A basic way to describe an underwater close-up lens would be to call it an underwater magnifying glass. It will allow the primary camera lens to focus closer and thus include more of your small target into the frame. Close-up lenses are usually specified by their optical power and some manufacturers call them diopters after the unit of measurement. This number is often advertised with the lens. The typical underwater close-up lenses range from +6 to +15 diopter. However, some manufacturers omit the dioptre values and choose to advertise their lenses with the magnification factor; 2X, 3X, 4X etc… for more accurate description. If you want to roughly compare these lenses you can use the following formula: M = D × 0.25 + 1, where the D is dioptre power and M is the magnification. Just remember that due to the optical qualities of water the topside readings are often not accurate when using the lens in water.

One question I always get asked about is - why do I need the lens at all, as my camera focuses super close on its own? It is true that many today’s compact cameras focus extremely close and often have built-in macro photography modes. 95% of these cameras, however, have one limitation. The macro mode only works when your lens is in its shortest focal length (i.e. the widest). As soon as you zoom the lens in the closest possible focusing distance escapes to 25-40cm. This is the key issue underwater. If you want to take good macro shots underwater you want to use a lens with longer focal length as this will narrow the angle of view. But as the primary lens can’t anymore focus very close it needs the help of the secondary diopter lens. The killer combo of the narrow-angle of view and the ability to move away from the target is what you want to aim to. By using the cameras own close-up modes, you are usually 1-2cm away from the target, which is unpractical underwater and often casts a massive shadow over the target. With a close-up lens added you can ignore the built-in macro modes, zoom in and move away from the target (5-10cm typically) but still fill the frame or even more. Moving away from your target also allows you to light the scene better with external lights as there is simply more space to position the units. Here’s a little illustration that shows you what I am talking about:

  using zoom with close-up diopter

A great thing about UW close-up lenses is that you get started without breaking the bank. The lens construction is often quite simple, and most lenses will work with all cameras. Today most close-up conversion lenses have M67 threads and match 95% of housings and ports on the market. A basic Inon UCL-165 close-up lens with +6 dioptre costs only £139.00. A Weefine WFL-03 +12 lens is £185.00. There are some more expensive lenses also for the true affinicados like the Nauticam CMC-1 or the Super Macro converter SMC-1, which is specially designed to be used with larger DSLR lenses.

Whichever close-up diopter lens you decide to go for it will open the world of underwater macro photography and put you on the right track of becoming the master of this art.

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