diopters

[matt215 3]

i'm a little embarrassed to ask this but, what the heck does a diopter do? i have one. i think it's a +4. when and why do you use a diopter?

[frisco 0]

Matt,

 

you would use a diopter mainly on macro lens to achieve an higher magnification .... Beware that if you use underwater a diopter that screws on your lens like a filter you will loose the abilty to focus at infinity; you focus range will be limited from say 2" to 10" from your port, according to the lens used (shorter focus distance with a 60mm macro and a little bit longer with a 100macro).

 

Hope this helps ..... Francesco

 

BTW which brand of diopter do you have ??? I have a Canon 250-D that I use with my Canon 60mm macro lens for super-macro and I am quite happy with it.

[Bent C 18]

Diopters can also be used to increase corner sharpness om wide angle lenses. I use a diopter for my Canon EF 17-40, and the corners are somewhat better with a diopter than without.

 

regards

 

Bent C

[Glasseye Snapper 47]

Diopters are basically magnifying glasses. The shorter the focal length the stronger the diopter. Canon diopter names indicate the focal length. Francesco's D-250 has a focal length of 250mm and my D-500 has a focal length of 500mm. The diopter strength is also often given as 1000/focal_length, so the D250 is a +4 diopter and the D500 is +2.

 

A magnifying glass focuses parallel rays into the focal point and, inversely, it defocuses rays coming from the focal plane into parallel rays. Light coming from a very long distance (infinity) also has parallel rays. Therefore, with a diopter on your lens, objects at the focal distance of the diopter appear to come from infinity and your lens will no longer be able to focus on objects farther away. The advantage of a diopter is that you get greater magnification.

 

Things are a bit more complex when diving with a dome port. Underwater, a dome port acts like a lens making infinity appear to be right in front of the dome, the actual distance depending somewhat on the radius of the dome. If your lens cannot focus that close then is becomes impossible to focus on anything. I forgot this on my last holiday and put my 50mm canon lens behind a dome and couldn't for the life of me figure out why I could not get any focus. Problem was, the lens cannot focus closer than 45 cm, which was to far for my 6" dome. You can prevent this by adding a diopter that helps your lens focus closer. It can also improve sharpness in the corners of the image as Bent pointed out.

 

Bart

[matt215 3]

Thanks for the replies. That all makes sense. I just bought one a few years ago, and the guy at the store told me why I needed it at the time, but I didn't remember what he said. Now I know...

[fish 0]

Hi all

 

I have a canon 17-40 as well and I just noticed some lack of corner sharpness - therefore considering diopters. Do you have any experience with diopters with this particular lens and do I need to consider something else (dome and so forth)?

 

Also, does the diopter decrease my wide angle?

 

cheers

[Tjsnapper 0]

Hi Fish I have a 17-40 mm f4 L and use a B+W +4 dipoter in my 8"dome, it has increased corner sharpness a fir bit, especially if stopped down a bit.

Hope this helps.

[fish 0]

Thanks Tristan

 

But why should I go for a +4 instead of +2? Do these diopters have any other side effects?

[apurbasg 1]

i'm a little embarrassed to ask this but, what the heck does a diopter do? i have one. i think it's a +4. when and why do you use a diopter?

 

Hi Matt,

 

On which lens are you planning to use the +4 diopter?

 

I use a +4 Hoya diopter for my 18-70mm Nikon lens in a 8" dome port. Its minimum focus distance is about 12". As Bart pointed out, with the 8" dome port more often than not it becomes a necessity to use a diopter as the virtual image falls within the min focus distance of this lens.

I would suggest that first see if your minimum focal distance is short enough, say about 6" or less? If so, then avoid using a diopter as you will not be able focus at infinity any more. This hinders focusing ability at slighter longer distances a lot. Unless your main criteria is to increase the magnification its not worth the trade off.

 

You can use this on a macro for greater mags, but personally I would prefer a TC instead.

 

 

Cheers!

Apu

Edited August 29, 2010 by apurbasg

[fish 0]

So what is the equation behind the increase in magnification caused by the diopter?

[Panda 3]

The main use for a dioptre is to bring the focus range of the lens closer.

Dioptres don't magnify much by themselves, they allow you to move closer to the subject.

Useful for allowing a lens to focus close enough to work behind a dome port, or to allow a normal lens to act as a macro lens.

The down side is dioptres are simple single element lenses and so can increase colour fringing and peripheral distortions and your focus range is restricted.

 

Here ares some test images I made using a Nikon 18-55 and +4 dioptre

http://www.flickr.com/photos/pandasimages/...57604406703377/

[echeng 0]

So what is the equation behind the increase in magnification caused by the diopter?

 

If you focus a lens at infinity, you will be focused at 1 / [DIOPTER NUMBER] in meters. So using a +1 diopter means that you can't focus closer than 1 meter, a +2 = 1/2 meter, etc. It shifts the possible focus distance closer.

 

I'm not sure about the magnification part. We need a proper optics person to answer.

[reefnet 0]

I'm not sure about the magnification part. We need a proper optics person to answer.

 

You can find all the equations you need at http://scubageek.com/

 

Les