help understanding diopters for lenses

[hammerhead man]

I am considering going to a digital SLR and I have heard about the need to have lens adapters for a camera (i.e. +1, 2, 3, or 4 diopters) that are needed for certain housings. Can anyone enlighten me on what these are and why they are needed or can anyone point me to an article that discusses this issue? I am really ignorant about this. If you would rather email me than post an answer, my email address is wjames3@cox.net.

Thanks,

Bill J.

 

[pete340]

I am considering going to a digital SLR and I have heard about the need to have lens adapters for a camera (i.e. +1, 2, 3, or 4 diopters) that are needed for certain housings. Can anyone enlighten me on what these are and why they are needed or can anyone point me to an article that discusses this issue? I am really ignorant about this.

"diopter" is a measure of the strength of a lens. When you get a prescription for glasses it starts out with +2 diopters or +4 diopters, etc. The bigger the number the more it magnfiies.

If you're using a Nikon D70 with the 18-70 mm lens in an Ikelite housing you need a +4 diopter adapter. It may also be called a "close-up" lens. It's a chunk of glass in a thin plastic housing, and it screws on to the end of the big lens. I think you need a 67mm one; that's the diameter of the thin plastic housing. It has to match the diameter of the big lens, so it will screw on.

The reason you need it is the spherical port that goes on the Ikelite housing. When you're in the water it acts like a diverging lens -- it spreads out the light rays that come through it. You have to overcome that, and so you need the close-up adapter. Other lenses are generally strong enough to do that on their own, so they don't need that adapter. The lens has to be able to focus to 12" to work with that port. The 18-70 doesn't focus that close, so you have to help it out.

I bought a set of +1, +2, and +4 diopters from Calumet Photo .com. But just about any photographic supply house ought to have it.

 

[fdog]

Dome ports create a "virtual image" that is very close...8 to 20 inches away, depending on the size of the dome.

This means that for an object that is actually 50' away, the camera will have to focus on the virtual image at, say, 16" away. (Numbers drawn from a particular equipment configuration, yours will vary).

This places a premium on close-focus ability of the lens. If the lens is an inexpensive zoom, or the dome is very small, the virtual image will be too close for the lens to focus it sharply.

The easy answer is to screw on a diopter, which allows the lens to focus closer, as Pete describes.

I am not a fan of diopters, as they are (relatively) cheap optics and degrade the image for my shooting, and I avoid equipment configurations that force their use.

A way to avoid the use of diopters is use of "primes", which generally focus much more closely than their zoom cousins, or, using large domes.

All the best, James

 

[fpoole]

A way to avoid the use of diopters is use of "primes", which generally focus much more closely than their zoom cousins, or, using larger domes

Heheh.. Whoops.. I was with you until the word "primes" popped up... Fixed length lens?

Whas'dat?? LOL...
inquiring minds wanna know...

You can get a Port (big enough? or???) something so you don't have to use diopters??

thanx..

(always learning something eh??)

 

[hammerhead man]

Thanks for the info. I'm also a little in the darks regarding "primes" what is the difference between primes and other lenses?

 

[fdog]

A "prime" is a fixed-focal length lens, as opposed to the zooms that are typically bundled with digital SLRs.

Here's a prime . This is a zoom that contains the focal length of the prime.

Although zooms are undeniabally handy, in the past, the concept of a "prime" was reinforced by being the choice of professionals. As nice as zooms are, in their early days their dismal optical performance relegated them to being an amateur toy. I would typically go out on assignment with three camera bodies, each with a different prime, to give me flexibality in perspective choice.

Modern zooms, however, are brutally sharp, and are a far cry from their early years. I would be perfectly happy going out on assignment with two bodies, one with a 12-24mm and the other with a 70-200mm. So, the glory days of the prime have passed.

Or has it?

Generally, when compared to equivalent zooms, primes:
>Are 1 to 2 stops faster.
>Are sharper, especially when compared to a zoom at its extremes of focal length.
>Have lots less flare and ghosts.
>Have way less distortion.
>Are lighter.
>Focus closer.

It's the first and last items that makes primes better suited for much of housed UW photography. And, the finest underwater lens ever made is a prime! The UW-Nikkor 15mm.

All the best, James

 

[hammerhead man]

If I get primes instead of a zoom does that eliminate the requirement for diopters? Also, if you only had one lens for close-up and wide angle which one would you select or does such a lens exist? I know that one of the options that can be selected from the canon rebel XT menu is for close-ups that I think allows the user to get real close to the object when taking a picture. Do you know whether this option works very well? I currently have a Sea&Sea DX-3000G that has a macro button that when I select it allows the camera to focus within 1cm of the object you are taking a picture of and I have found that this works quite well for close-up photos. Not sure if that also works well with SLRs.

Thanks for all the information you've provided. It has been very helpful.

 

[pete340]

If I get primes instead of a zoom does that eliminate the requirement for diopters?

It depends on the lens port and the lens. If you're using the Ikelite 6" dome you need to be able to focus to 12". If your lens can't do that by itself, you need a diopter.

 

[fpoole]

Ok, I think I'm getting it now...
If, as you say, the lens can't focus to the dome, then you need a diopter
Example (no relation to what's out there, just trying to understand it)

- If I had a lens that would focus to 3" and the dome/port was 2", I'd need a Diopter
if the dome/port was 4" then I would NOT need one..

- If I had a lens that would focus to 7" and the dome/port was 8", I would NOT need a Diopter.

- If I had a lens that would focus to 9" and the dome/port was 8" , I WOULD have to have a diopter

and if I didn't want to have for whatever reason an 8" dome/port but a 6" dome/port and the lens only focused at 8" I'd have to get a diopter inorder to use the 6" dome/port... (don't need to get into the size of diopter here, suspect that's a whole different subject eh?? )

Something along those lines??? so it is dependant upon the focusing and how big a "dome/port" you want to have...

Sorry, just takes a little repetitive thinking here to get it to sink in...
thanx for the help so far... ;-))

 

[pete340]

Ok, I think I'm getting it now...
If, as you say, the lens can't focus to the dome, then you need a diopter

Basically. You don't focus on the dome, though, but on the "virtual image" that it forms. With the 6" dome that's about 12" out. (That sounds wierd, I know; I learned basic optics in high school physics forty years ago, and virtual images didn't make sense. Now they do; sometimes you just have to give it time. <g>)

Ignore this if it isn't helpful. When you photograph an object that's a long distance away, the light rays that it reflects to your camera come in more-or-less parallel. Your lens bends them inward, which gives you an image on the film. The dome, with water on the outside and air on the inside, is a diverging lens. When the light from that object passes through the dome into the air the light rays get bent outward (they diverge). Your lens now has more work to do: in order to focus on the film it has to bend them back to parallel, to overcome that divergence, and bend them further, so that they'll produce an image on the film. So you need a stronger lens.