Using macro lenses & ports.

For underwater photography there are many and varied macro lenses available today. Of these there are two types which are the most commonly used.

The first is what has become regarded as the ‘standard’ macro lens and which has a focal length of 50~60 mm. Such lenses have a fairly short working distance (the distance between the front of the port and the subject) and so can be tricky 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

The second is of very approximately double the focal length of the first, typically 90~105mm. These lenses are useful in clearer water where they provide a longer working distance and allow for potentially better lighting. They are also useful for shy subjects and are often used for fish.

The latest designs of these shorter macro lenses now use internal focus mechanisms whereby internal optical adjustments are used to focus. This is very useful for underwater photography because it means that the physical length of the lens does not change as it is focused closer, as it did with older designs ,so a smaller port can be used which has benefits such as better use of the available working distance, less bulk, reduced buoyancy and so on. However there is a trade off with such designs because they actually shift their focal length as they focus and, because they are optimised to produce their best image quality in the sensor plane at a set distance from the sensor. They are less effective if they are used at increased magnifications with extension tubes as image quality quickly degrades. So whilst they are superb whilst working in the range for which they were designed, they are actually limited to this range.

There are a number of methods of increasing magnification with macro lenses and each has its advantages and disadvantages. Simplest of all is to use a diopter or close-up lens in front of of the macro lens. Such 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 underwater. Image quality is good with a well designed wet diopter but will fall off slightly towards the edges 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 ‘achromat’ design diopters which can be used like this as well as 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.

Alternatively extension tubes can be used. These are an effective solution for getting closer to a subject but work best with older type none internal focus macro lenses. They are even more limiting than internally fitted diopters and really should be used only when a very restricted range of focus 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.

A third 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.