I have written a number of articles on underwater photography and this one tries to explain the differences in types of macro lens and how best to use them underwater.
Using macro lenses & ports.
For underwater photography there is a large variety of macro lenses available today. Of these there are two types which are the most commonly used.
The first has a focal length of 50~60 mm. Lenses of these focal lengths have a fairly short working distance (the distance between the front of the port and the subject) and so the subjects they are used for can be awkward to light when used at their closest focus (usually 1:1). However for many subjects they are very useful lenses indeed and especially in murkier waters, and there are now conical shaped ports available which allow for better placement of lighting.
The second is of very approximately double the focal length of the first, typically 90~105mm. These lenses allow for potentially better lighting and are also useful in clearer water where they provide a longer working distance.. They are also useful for shy subjects and are often used for fish.
There are others including yet longer focal length macro lenses and special purpose macro lenses – both are for specialised applications and they are used underwater, but infrequently.
The latest designs of many macro lenses now use internal focus mechanisms in which shifting internal optical components are used to focus. This is very useful for underwater photography because it means that the physical size of such a lens does not change as it is focused closer, which it did with older designs, so a smaller port can be used, and even a conical shaped port. This has benefits; better use of the available working distance, less bulk, reduced buoyancy and so on. However there is a trade off with such designs because as they focus they actually change their focal length. So their working distance can vary and not be quite as predicted. Because they are optimised to produce their best image quality in the sensor plane at a set distance from the sensor, they are often less effective if they are used at increased magnifications with extension tubes, as image quality quickly degrades. So whilst they are superb within the working range for which they were designed, they are actually limited to this range for optimum results.
However there are a number of methods of increasing magnification with many macro lenses and each has its advantage and disadvantage. Simplest of all is to use a diopter or close-up lens in front of of the macro lens. Diopter lenses are available in a variety of types, strengths and qualities. For underwater use there are two types, wet and dry.
Wet diopters (such as those produced by Seacam) fit into the front of the lenses’ port and are very versatile because they can be attached and detached at will whilst underwater. Image quality is good with a well designed wet diopter but will often fall off slightly towards the edges and corners of the image
The alternative type of diopter is one which is screwed into the macro lenses’ filter thread and used inside the port (which may require a slightly longer port). There are some high quality diopters of ‘achromat’ design, which can be used like this as well as much simpler, cheaper designs. Whilst effective these are not versatile and once fitted they are on for a whole dive, which, as they increase the closest focus distance but also decrease the furthest focus distance of the lens, can be limiting.
Yet another alternative is for extension tubes to be used. These are an effective solution for getting closer to a subject but do work best with older type, none internal focus, macro lenses. The downside is that they are even more limiting than internally fitted diopters and really should be used only when a very restricted range of focusing is acceptable. They also result in a darker viewfinder images are substantially increased exposure (more flash power is required) so are not an ideal solution. There are some specialist lenses (such as the Canon 65mm MPE Macro lens) specifically designed for higher magnification macro photography but these are very limiting indeed and only really usable for very specialist shots.
Yet another solution is to use teleconverters. There has been much discussion on www.wetpixel.com regarding appropriate lens/teleconverter/diopter/port combinations, and for further information its well worth visiting and searching through this excellent website.
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