What is a diopter in underwater photography

Close-up lenses: photograph tiny objects in full format

Close-up lenses are an indispensable part of modern underwater photography. DIVING photo expert Herbert Frei explains which additional glasses you can use to create spectacular close-ups of small sea creatures.

Close-up lenses do what their name suggests: you get closer because they shorten the distance to the subject and enlarge the object. These special glasses are usually defined by their refractive power (diopters) or focal length. Because they have to be mounted in front of a basic lens, they are also known as auxiliary lenses. Camera companies sometimes also speak of macro converters, which seems exaggerated, because such recordings can only be made if they are used together with a suitable lens. Then, however, with the appropriate number of diopters, you can almost playfully penetrate the natural reproduction scale of 1: 1.

With achromatic lenses, even sea urchin spines take on dramatic dimensions. Photo: H. Frei

Does the auto focus work?
Good to know: Even if a close-up lens is mounted, the autofocus works without any problems within the focus area. The reason is that close-up lenses practically do not swallow any light and the automatic focus adjustment therefore always has enough light available. However, the exposure changes if a close-up lens is connected in front of it. The cause is the change in focal length (macro optics and close-up lens), so that a different valid aperture results despite the identical aperture. The shutter speed also changes.

Close-up lenses on compact cameras
Do you even need them where compact cameras can usually be set between five and one centimeter close? You have to know that this extreme close-up setting of the compact zoom lenses can only be achieved in the wide-angle position. For example, if the compact model has a zoom range of 24–120 millimeters equivalent to a small image, it can only be focused on the shortest close-up setting with a focal length of 24 millimeters. If you zoom into the telephoto position, you have to lengthen the distance to the subject, otherwise the pictures will be blurred. But if they are recognized as sharp by the AF, then you are so far away that the images become dull and color-cast. The telephoto range cannot normally be used underwater. But if you put a close-up lens in front of the UW housing, this moves the subjects closer and you can also zoom into the telephoto range. The image scale becomes larger and the subjects can be better illuminated with an external flash due to the extended working distance. This is why close-up lenses really make sense on compact cameras. The only question is, how strong should they be? As a rule, close-up lenses with four or five diopters are recommended. In terms of refractive power, this is sufficient to expose the subjects not only large enough, but also from a reasonable distance. Stronger close-up lenses (up to eight dioptres) are not required for compact cameras and may be difficult to use for occasional photographers. Reason: The focus area cannot always be captured immediately with the zoom when the camera is brought up to the subject.

A case for close-up lenses - it can also be bigger: If you want to depict the little pygmy seahorses in full format, you couple the attachments. Photo: H. Frei

Attachment to the UW housing
The saltwater-proof close-up lenses are attached using 67 mm threads in the port of the UW housing. Sometimes an adapter is required if this part is slightly oval in shape rather than round. No matter how great the strength of the close-up lens, with compact cameras it is almost impossible to capture image scales of 1: 1 or larger. In principle, this would only work if the image sensor was at least 17.3 x 13 millimeters (Micro Four Thirds) and the close-up focus was five centimeters or closer.
Normally, close-up lenses are screwed onto compact camera housings. With special swivel adapters you can also fold them forward, which is very pleasant because you can quickly take a picture in the usual genre from time to time. Folding rings for compact camera housings are available from BS-Kinetics, Dyron and Nauticam, for example.

Close-up lenses on system cameras
In order to exceed the 1: 1 image scale, macro freaks mount close-up lenses on the outside of the port. They are either screwed on, plugged in or folded in front. The latter is the more elegant solution, as practiced at BS-Kinetics or Nauticam - but you have to be careful not to get caught on the reef with the handle open. Incidentally, close-up lenses are rarely screwed directly onto the camera lens. You can do that, but then you are no longer able to take anything other than close-up or macro shots.
Close-up lenses primarily make sense underwater as accessories to system cameras if a macro lens is used. If possible, a model with a long focal length, so that sufficient working distance (distance from the plane glass to the subject) is guaranteed. Because perfect illumination is more important than extensive subject enlargement.

Close-up lenses for swiveling forward
The housing manufacturers offer different fastening systems: BS-Kinetics and Nauticam swivel the waterproof close-up lenses in front of the flat glass. With their 67 mm thread, these can be exchanged for other diopter strengths. As a rule, the close-up lenses come from Inon. Swiveling forward has the advantage that you can work very flexibly and do not have to awkwardly stow the removed close-up lens. Screwed close-up lenses with M-67 thread are still used on a large scale.

Close-up lenses to stick
Another very common type of close-up lens attachment is sticking. This is how Seacam works, for example, with its wet diopters and the "MIP80" achromat. Lens holders also make it easier to remove and store. The image quality, especially of the "MIP80" achromatic, is hard to beat. The Seacam wet diopters can also be coupled to increase the image scale. The Austrians supply a special fastening ring for this. According to the manufacturer, there should be macro freaks who put up to four wet diopters on top of each other. If the edges of the picture have no visual relevance, you can do that. For example, when taking pictures of tiny pygmy seahorses. Otherwise, for optical reasons, such combinations are not advisable because the chromatic aberration increases extremely: This is a disturbing aberration because the RGB colors intersect the optical axis differently.


Small really big with close-up lenses. Photo: H. Frei

Which magnifications can be achieved with close-up lenses?
On a 100 mm macro lens, focused at 1: 1, the image size increases to 1.25: 1 with a 4 diopter lens. At eight diopters, the refractive power creates an image ratio of 1.8: 1, and 12.5 diopters increases the image ratio to 2.25: 1. At this magnification, not every subject fits into the viewfinder. The illumination is not a problem. With two flash units - one on the right, the other on the left - macro motifs can be perfectly illuminated.

Close-up lenses and dome glasses
If wide-angle zooms are used behind dome glasses, a strange optical discrepancy occurs every now and then. If the zoom cannot be focused closer than 30 centimeters and the dome glass is small and strongly curved, you can no longer set it to infinity under water. The cause is the configuration of the dome glass, which creates a negative refractive power due to the water-glass-air layers. To avoid this dilemma, you either have to enlarge the dome glass, which is not always possible, or mount a close-up lens on the zoom. Mostly with a power of two or four diopters. With this optical trick, which, however, somewhat restricts the angle of view, the zoom behind the curved windshield can then be used again as on land.
However, assembly problems occur if the zoom has a very short focal length and a strongly curved front lens. Then you may not be able to attach a close-up lens. Lens mount diameters beyond 90 millimeters - not uncommon with high-speed zooms - also encounter obstacles: such close-up lenses are rarely available. It is also due to the fact that these glasses are only used to a limited extent in overwater photography. There they are increasingly being displaced by real macro lenses.

Diopter and achromatic lenses
A diopter is the refractive power of a lens with a focal length of one meter. The larger the diopters, the greater the refractive power of the lens. This shortens the focal length of the system. Incidentally, two close-up lenses that are cemented together and whose optical errors cancel each other out are called achromatic lenses. They are more expensive than normal close-up lenses and are always used when the refractive power is very high, for example with more than eight diopters. This improves the imaging performance. The limit of the refractive power of close-up lenses and achromatic lenses should not exceed 15 dioptres.

Couple close-up lenses
You can use two close-up lenses together, but you should mount the stronger one first. When coupling three or four close-up lenses, the imaging performance drops sharply at the edges of the image. We can therefore only recommend this procedure if the edge and corner sharpness are not important - ergo the subject is very small and sits in the center of the image. Basically, a strong close-up lens is always better than the coupling of two or three close-up lenses with the same refractive power.

Use under water
Lenses that have already been screwed in or folded down on land should be removed again underwater in order to be able to wipe off the air bubbles stuck in the space. There is always the risk that they slip out of your hand. Therefore, you should not perform this maneuver on a steep reef or over a coral field. The glass surfaces of the close-up lens must be free of finger stains and grease smudges. Coarse scratches also degrade the imaging performance. After the dive, you should definitely wipe off the adhering water droplets, otherwise water stains can form over time, which can cause glass burns. Especially when the UW housing is still being dried in the sun. This surface damage can no longer be removed with normal means. The thread of screwable close-up lenses should occasionally be smeared with a little grease. The threads are prevented from seizing up when salt crystals are deposited. Therefore, always rinse well with fresh water after diving in the sea! It is best not to tighten screw-on close-up lenses all the way. After a while, they can no longer be removed without any problems. Then only a hair dryer (carefully heat the thread) or a screw solvent from the car accessories will help. People with golden hands can also try carefully with water pump pliers. Don't press too hard or the glass will break.

Prices and the diopter madness
If the close-up lenses are of good quality and are manufactured as achromatic lenses, they cost almost as much as a good macro lens, depending on the diopter strength. Because an adapter or a special macro port is often required. So you have to consider how far you want to expand the object. It makes little sense to install an expensive close-up lens for 300 euros on a compact camera if the entire camera is worth less than 400 euros. In the segment of a professional system camera that is operated with an expensive 100 mm macro, on the other hand, an additional few hundred euros for an adapter and ancillary lens are hardly significant. Visually it is clear, but emotionally it is not. Macro lenses can be exposed to ten to 15 diopters with a clear conscience, so that object magnifications of 2: 1 to 3: 1 with acceptable imaging properties are realistic. Close-up lenses with diopters above 15 are attachments whose optical quality deteriorates significantly in connection with macro lenses. But that doesn't mean you shouldn't buy them. In this magnification range, it is no longer important that the edges of the image must be explicitly sharp. This is impossible, if only for reasons of the shallow depth of field, for subjects that are not flat. The aim is the exposed and extroverted enlargement of tiny motifs at image ratios between 3: 1 and 5: 1. One should not forget: Such magnifications can only be achieved with excellent buoyancy control, because the tiny ones have disappeared from the viewfinder faster than you can focus.


Seacam system
If you want to exceed the maximum image ratio of 1: 1 specified by macro lenses, you need additional lenses. Seacam has developed a high-quality macro system that avoids the known optical disadvantages and ensures sharp-edged images right into the corners. The achromatic lens consists of two close-up lenses, the optical errors of which are mutually canceled out. The chromatic aberration has been eliminated except for small, invisible residues. In combination with a 100 mm macro lens, the magnification effect is at an image ratio of 2.4: 1. The autofocus works quickly and trouble-free with the "MPI80". The attached attachment sits parallel to the glass of the macro port. Decentering errors, such as those that can arise from mechanical folding forward, have been eliminated. The Seacam achromat is fixed by an O-ring friction, which is designed so tight that the attachment cannot accidentally come loose. A lens holder holds the "MPI80" when it is not needed. The adapter can be used with 90 to 105 mm macro lenses.
Focal length: 80 millimeters, 12.5 diopters, magnification up to 2.4: 1 when the lens is focused at 1: 1. Fastening: plugged into the macro port.
Prices: 365 euros (macro port), 395 euros (achromat), 125 euros (lens holder).

There are two special close-up lenses (5 and 10 diopters) for Aquatica housings that are plugged into an adapter on the ports for D-SLRs. They are fixed with an O-ring - a black one for five diopters, a red one for ten diopters. They also have an M-67 thread. They cannot be coupled with one another.

BS-Kinetics close-up lenses.




BS Kinetics
The lenses are swiveled forward with a special carbon folding mechanism. These come mainly from Inon (4, 8 diopters) or Subsee (5, 10 diopters). By the way, BS-Kinetics also manufactures swivel mechanisms for close-up lenses on small compact cameras.



Subsee close-up lenses are mounted on Ikelite macro ports. The achromatic lenses with 5 and 10 diopters can be attached using a folding adapter. www.digital-dive.de

Close-up lenses are swiveled forward with a precise folding attachment. The achromat has four lenses in three groups. Magnifications up to 4: 1. www.nauticam.com

The "M-77" macro lens can be mounted with an adapter ring. The manufacturer is currently developing a new folding adapter.

A screwable close-up lens with M-67 thread is available for compact and system cameras.

Has four of its own close-up lenses in its range which, in combination with macro lenses, enable image sizes between 1.3: 1 and 3: 1.

The top forge from Steyr offers the largest range of external collapsible close-up lenses: five models up to 25 diopters with image scales up to 4: 1.

If you have a UK Germany housing, you can screw close-up lenses with an M-67 thread to the macro ports.




The UW-Camerastore is the largest UW-Camerastore in Europe with more than 16 years of experience.

The UW camera gate is the largest underwater camera business in Europe with more than 16 years of experience. The showroom in Oud Gastel in the Netherlands is open Monday to Saturday and even waits, repairs and tests the housings in a pressure vessel. The UW-Camerastore is an authorized service center for Nauticam, Inon, Fantasea, and a distributor for Canon, Nikon, Olympus, Panasonic, Hugyfot, Ikelite, Fantasea, Gates, Keldan, Sea & Sea, Sealife, Sony, GoPro, Subal and many other brands . www.uwcamerastore.com