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JimK

Diopters

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I have just returned from a week live aboard to Turks/Caicos - I used my D-70, 18-70 mm lens in Ikelite Housing - I left my #4 diopter at home & decided to use setup minus diopter - My photos for the most part were good to very good with some vignetting when lens was extended - my question is what is the purpose of the diopter other than eliminating the vignetting? - JimK

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Actually, diopters don't eliminate vignetting. If anything, they'll make it worse on extreme wide angles. A diopter simply allows the lens to focus closer than it normally would.

Most commonly they are used on wide angle lenses behind dome ports. The use of a dome port creates a virtual image in front of the dome that is roughly twice the dome's width. If an object is at infinity then it's virtual image would be formed 12" in front of the nodal point on a 6" dome and this might be as close as many lenses will focus. An object that was 3' away might have it's virtual image image formed 7" in front of the nodal point which would be too close for many lenses to focus.

A diopter is like a magnifying glass. A +4 diopter will change the focus point from infinity to 250mm and it's closes focusing point will be almost in front of the glass. So, with a diopter behind a dome, you'll be able to focus on objects that are at infinity as well as objects that are just in front of the dome port. You may or may not be able to get away without a diopter. It depends on you're lens/port combination.

Some people also use wet diopters which serve a different purpose (macro) and some claim that diopters will improve corner sharpness. I don't know if the second point is true or not, but I haven't been able to find a reasonable explanation for why it should. I imagine in some cases there are subtle nuances in the lens-diopter combination that make one combination sharper in the corners while other do not. Can someone else elaborate?

 

Cheers,

 

Mike

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A diopter also changes the focal length and the angle of view. In your case the focal length with dioptre becomes longer and the angle of view more narrow. If you remove the dioptre the angle of view becomes wider and the port or its sunshade might cause vignetting.

 

Cheers,

Juergen

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A diopter also changes the focal length and the angle of view.

 

I think you may have the functions of a diopter and a teleconverter confused. Diopters have no effect on the effective focal length or angle of view. They simply allow the lens to focus closer than it would otherwise.

 

Cheers,

 

Mike

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I am pretty aware of the difference between a diopter and a teleconverter. I am using both (seperate and in combination). Though this might not be the intention when using a diopter also a diopter changes the focal length . A formula like

New FL = 1 / (1 / FL of the lens + 1 / FL of the diopter)

is often quoted, but this formula is over simplified (it is indicating a diopter is always shortening the focal length). A more precise formula needs the knowledge about the position of the primary planes of the lens as well as of the diopter. Since these data usually are not available, just go for a practical test. My practical experience is, that the angle of view becomes smaler, if you combine a short focal length with a diopter.

 

Cheers,

Juergen

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my friend used a D70, 18-70mm combo for a while with no problems, and then added a +4 diopter in order to get closer focus, so he could get greater magnification for macro shots.

 

 

for very wide-angle rectilinear lenses like the 12-24mm, a +2 or +4 diopter may improve the corner/edge sharpness. for lenses like the 12-24mm, people will often test different dome sizes, extension lengths, and diopters in combination to find the optimum combination for their camera/lens/housing combination.

 

scott

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