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To diopter or not diopter


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#1 chewie70

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Posted 06 September 2006 - 01:54 PM

I know this question has been asked many times and I have even asked it myself. I leave for Guadalupe in 2 weeks to photograph the GW's. I will be using the Aquatica D200 housing w/ a Nikon 17-35mm f/2.8 and 8" dome with proper extension. Aquatica's reference states that no diopter is needed with this lens. I was planning buying a B & W +2 diopter to have just in case, but no one seems to have them in stock. My next option is to get either a +1 or +3 diopter. Unfortunately, I've been working so much, that I haven't had a chance to borrow any diopters and test it out for myself. ( I know, don't go to battle with an unproven rifle)...... Anyway, I would greatly appreciate any advice. I know James and Tim gave me some advice on this matter a while back, but that was regarding a different lens. My instinct is to go with the +3???

Matt

#2 Rocha

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Posted 06 September 2006 - 02:04 PM

Yep, I would take the +3 just in case. But I think that in this particular case you won't need it because you will take photos of great whites right? The diopter will give you better corners, but it really doesn't matter if the corners are just blue water.

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#3 chewie70

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Posted 06 September 2006 - 04:01 PM

Thanks Luiz, I'll probably just do that. I'd rather spend the $60 and know that it's there if I need it.

#4 Alex_Mustard

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Posted 06 September 2006 - 10:11 PM

I'd strongly recommend using a dioptre.

Alex

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#5 maelstrom

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Posted 07 September 2006 - 05:56 AM

I was just at Guadalupe, and used the 17-55 without diopter, usually inthe 35-45 range. With the new rules and wardens on the boats, very few sharks will get close, and I don't think diopter is necessary when they are 10-30 feet away.

Hal

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#6 sharky1961

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Posted 07 September 2006 - 06:57 AM

Hi Hal,

what new rules??


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#7 UWphotoNewbie

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Posted 07 September 2006 - 07:31 AM

I was just at Guadalupe, and used the 17-55 without diopter, usually inthe 35-45 range. With the new rules and wardens on the boats, very few sharks will get close, and I don't think diopter is necessary when they are 10-30 feet away.

Hal



Everything will be unsharp anyway if its 30-40 ft away underwater......

Scary shot Hal! :rolleyes: :unsure:

Edited by UWphotoNewbie, 07 September 2006 - 07:33 AM.

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#8 chewie70

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Posted 07 September 2006 - 09:56 AM

Thanks for the advice everyone. I ordered the +3 last night to have just in case.

BTW, what new rules???

#9 maelstrom

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Posted 07 September 2006 - 04:42 PM

Thanks for the advice everyone. I ordered the +3 last night to have just in case.

BTW, what new rules???

We had the administrator for the whole biosphere on the Solmar V 2 weeks ago. She was writing rules and policy the whole trip: 1. Only 6 boats at a time, 2. No sharks baited up to the cages, 3. No lures, 4. No mammal blood for chum, 5. There will be a ranger on every diveboat to enforce the above, 6. No flights to the island, as the Nautilus had already advertised they were doing .

Having never done this before, I can't say how much impact this was having, but I got the impression that previous trips had much more up close and personal shark encounters. Apparently, there had been some abuse in the past, and it was publicized for advertising purposes. So, we all pay the price.

Hal

Everything will be unsharp anyway if its 30-40 ft away underwater......

Scary shot Hal! :rolleyes: :unsure:

It was 35-40 mm focal length, not feet. Probably averaged 20 feet.

Hal

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#10 chewie70

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Posted 07 September 2006 - 07:02 PM

That really sucks. I hate it when rules are pretty laxed and people have to push the envelope, which in turn spoils it for everyone. Hopefully it will chill out a little by the time that I get there.

Thanks for the info and nice shots.

Matt

#11 photovan

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Posted 07 September 2006 - 07:11 PM

funny how governmants act on impacts to individual species, but do jack s#!% about the impacts climate change will have on species and system viabilty...
darren

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#12 V_kids

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Posted 09 September 2006 - 01:36 AM

I'd strongly recommend using a dioptre.

Alex



Hi to all Wetpixel member!

i'm really interested with this "diopter" discustion.
where can i find more information about this diopter??
I really dont understand why do we need a diopter for UW shooting.

Currently im using
Nikon D50 (with sea & sea housing)
Nikon 10,5mm
Nikon 12-24mm
Nikon 18-55mm
Nikon 60mm micro
Sigma 105mm macro

Which of the lens above that i need to put diopter on, for underwater shooting.
Im really confused
i never had any problems shotting UW using these lens

please advise me ;)
im really confused :P

Thx u in advanced


Andree
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#13 mattdiver

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Posted 09 September 2006 - 01:48 AM

There have been a number of discussions on this topic on Wetpixel. I suggest you do a search to find the relevant threads.

Basically, dome ports project a virtual image that is closer to the port than the actual subject. A (much simplified) rule of thumb is that this virtual image is located at about 3 times the radius of your dome port. If your lens can focus that close, you should be alright, if not you'll need a diopter to allow your lens to focus on the virtual image. Check this out for more details:
http://wetpixel.com/...ll/dome-theory/

In addition to the above, there is some evidence suggesting that even if your lens can focus close enough, using even a weak diopter will significantly improve corner sharpness.

#14 John Bantin

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Posted 09 September 2006 - 02:10 AM

A dioptre shortens the focal length of the lens thus allowing it to focus closer for the same rear-node/image plane distance.
A dome port produces a virtual image a short distance in front of it.
Depending on the radius of the port, this distance varies.
If your lens can focus using the range of its normal mechanism that is good. I always made a rule that adding a crude dioptre to a prime lens that had been computed to give as few aberrations as possible was not the best way to go. For working without a dioptre, you need to use the widest radius (and therefore the widest diameter port available). I never had much success with compact ports anyway because I hated the stretched blurry edges of the image.

Then I got the 12-24 Nikkor Zoom and found it would not focus close enough for all purposes (it was good with big animals at more than a metre away) - so I ate my words and fitted it with a plus-2 dioptre supplementary lens.

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#15 davephdv

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Posted 09 September 2006 - 09:22 AM

I like a +2 diopter with wide angle lenses. It does seem to improve edge thickness.

If they can't bait the sharks up to the cage that will have an effect on how close the sharks get. The sharks are only interested in the bait. Not you or the cage. I would use the 17-55 over the 12-24. I was in Australia with a top UW Photographer. He stated that he thought the best lens for getting full frame shark shots was the 28 mm (on a 35 mm camera)

They don't use mammal blood for the sharks in Guadeloupe. They use tuna blood as that is what the sharks are believed to be there for.
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#16 humuhumufish

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Posted 20 November 2006 - 12:15 PM

I just acquired the Nikon 17-35mm lens, and I'll be using a +2 diopter for it. Which company should I look at for diopters and what size, is it bigger than 62s?

Thanks,
Graham

#17 mrbubbles

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Posted 20 November 2006 - 12:28 PM

I was recomended to use a 4+ diopter with an aquatica/17 to 35 nikon. Is that to strong?

#18 echeng

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Posted 20 November 2006 - 06:27 PM

They don't use mammal blood for the sharks in Guadeloupe. They use tuna blood as that is what the sharks are believed to be there for.

Actually, they used beef blood mixed with fish chum until the new regulations took effect.

If your dome is large enough, you may not need a diopter if you don't need focus up to the port. I took my lenses into the water without diopters to see what kind of image I'd get and how close I could focus. There is no replacement for in-water testing!

The sharks still come close to the cages. Hal was on our Wetpixel trip, which was unfortunately a slow one for sharks. A large male hung out under the boat for much of the trip, and it was theorized that he kept the others away. Who knows...

We did watch as sharks swam all around the other boat there. :)

Lawrence, great white adventures, etc will be featured on Leno tomorrow night. I may even end up in the footage if I made the cut. I'm sure they will be making fun of us... :)
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#19 Paul Kay

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Posted 21 November 2006 - 12:36 AM

I don't know why diopters are still an apparent mystery!

The necessary dioptric power is given by a formula

P (dioptric power required) = 1000/4 R (radius of the dome)

so for a 20cm dome the diopter required would be 1000/ (4 x 200) = 1.25 and so on.

The virtual image is at a distance of 4 x R from the CENTRE of the dome (which should also be the site of the lens's entrance pupil if it is correctly set up.

The only problem with all this is that it means that with an appropriate diopter is fitted the lens is set to infinity for an underwater subject at infinity (NOW at 4 x R from the centre of the dome) and so the lens cannot be used above water - which may or may not be helpful!

It would though, be very useful for manufacturers to actually state their dome's radius in their specs but this is rare in my experience.

Diopters with a flat rear face will tend to induce some reverse (opposing) curvature into the sensor/film plane image, but probably not a lot. As you can see from other discussions image cruvature remains a fundamental problem with simple concentric dome ports.

A listing of dome diameters, appropriate diopters and the position of the entrance pupil on wetpixel would be very useful - I'll see what I can dig up tostart things rolling.
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#20 mrbubbles

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Posted 22 November 2006 - 02:21 PM

pgk: Is there any harm in using a diopter stronger then recomended by that formula?