can anyone explain the why and how of using diopters on the front of a lens in a housing?

Appreciate it.

Larry Gatz

Hi Larry

Dioptres are commonly needed to resolve the problem of the camera lens not being able to focus close enough to achieve an image, and is particularly the case with dome ports.

A dome port forms what can be considered to be a concave lens in front of the camera’s lens.  The result of this concave lens is that a virtual image of the subject of the photo is formed at a point closer to the camera lens than the subject, e.g. with ikelite domes, the virtual image of a fish that is actually 1m away is formed at around 30cm from the dome.

This distance (30cm) is sometimes less than the closest focus ability of the lens you are using.  In this situation, a dioptre is needed.  In crude terms, a diopter acts like a magnifying glass, and can simply be thought of as a convex lens. 

Magnifying glasses work by placing objects inside their focal length, generating a virtual image that is larger than the original object, which appears to be positioned further away from the lens than the actual object. 

Thus the diopter on the camera lens causes the virtual image formed by the dome to be magnified and also (as far as the camera lens is concerned) be positioned further away from the lens.  So long as the dioptre’s virtual image is further away than the minimum focus distance of the camera lens, the camera lens will be able to focus, and you get to take your photo!

Note that this is all a major simplification of some pretty complicated optics - if it was simple, camera lenses would be alot cheaper than they are!

Hope that helps.  If it doesn’t make any sense, please let me know - I’ll try to do post some diagrams that might help.

Ok fine I understand that ...but I guess I have never seen the problem in the first place.

Does this occur with certain lens or all lens?

I shoot a 16-35 without any focus problems...or are you suggesting that I possibly don’t shoot macro with my WA so I don’t have the problem?

Any close focus WA images I have are sharp.

Thanks,

Larry Gatz

Hi Larry

The problem revolves around the combination of port and lens.  If the virtual image formed by the port is inside the minimum focus distance of the lens, a dioptre will be needed to enable focus.  If the virtual image is formed outside the minimum focus, then no dioptre will be necessary.  The latter situation is (all other things being equal) preferable!

If your 16-35 doesn’t give you any problems, and all your shots (even close focus WA) are sharp, then you don’t need a dioptre.  I have seen some people comment that a dioptre assists with edge sharpness, but again, if you don’t think you have a problem with edge sharpness, why add another piece of glass?

I don’t know what make of 16-35 you are using, and with what size and make of dome, but it may be that your lens has a close minimum focus, or that your dome forms virtual images further away than other domes, and so the combination you have works perfectly well without the need for a dioptre. 

Ikelite’s website indicates that the Canon 16-35mm lens works fine with their 6” and 8” ports without the need for a dioptre.  I’m not aware of a 16-35mm lens for Nikon, so I assume you are using the Canon lens, which, it would seem has a nice close minimum focus.

HTH