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Diopter required for Canon 17-85zoom

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#1 ccorogenes



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Posted 21 February 2006 - 11:44 AM

I recently purchased an Ikelite housing for a Canon 20D DSLR and a port for the Canon 17-85mm Zoom. Ikelite reccomends using a +4 Dopter for this lens port combination. I still don't understand why and if it is absolutely necessary. Can any one explain or does anyone have experience with the lens and port combo.

#2 kriptap


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Posted 21 February 2006 - 12:06 PM

Read this and do a search you'll find quite a bit about this lens etc:


#3 mever



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Posted 21 February 2006 - 08:57 PM

Simple answer is that the system will focus with far less accuracy without the +4-
dome point alters " virtual image". ie where the target appears to sensor as to where it really is.......it will work sometimes but less often........I know cause I have forgotten to put it on several times :)
Then you can't get a focus lock ie red flash in viewfinder, won't shoot exposures without focus and that is when you remember the diopter............
GREAT combo though..........examples here with diopter


it just works better with it get the B + H version it allows good macro topsite as well.............

Marty Everhard
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Canon 20D Ike Housing Dual DS 125 Strobes -50mm , 17-85 mm , 18-55mm , 10-22mm , 100mm macro, 70-300 mm All Canon
Also Sealife Reefmaster DC310 W/Dual Strobe & Attachment lenses which I started with several years ago

#4 dhaas


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Posted 22 February 2006 - 06:32 AM

Additional explanation is the +4 isn't "nice to have" with the Canon 17-85mm lens behind the special Ikelite dome port.... IT'S REQUIRED TO ACHIEVE full range focus UNDERWATER.

Underwater a dome port creates an "apparent image" the lens thinks is maybe 10" - 12" in front of the lens. Since the Canon 17-85mm lens can only focus to 1.2 feet you NEED a diopter to increase close focus capability.

Above water where any port simply acts as a window (IN AIR) forgetting to put your +4 on will not become noticeable. UNDERWATER as many state without the Ikelite recommended +4 diopter you will not be able to get a focus lock except maybe 8-10 feet away (!!!)

When you install your +4 and want to take a test shot ABOVE WATER you need to get VERY CLOSE to get a focus lock. Underwater you'll then have full range focus capbility behind a DOME port.

Hope this explanation helps!


P.S. - I have always used B&W diopters in the last couple years. They are very good quality for the money. Available at B&H in NYC with no hassles...
David Haas

#5 ccorogenes



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Posted 23 February 2006 - 06:15 AM

To all who replied to my inquiry many thanks. I am relieved to know I made a good purchase and now understand the +4 Diopter requirement. I will have an opportunity to try it out in Bonaire next month.