Diopter required for Canon 17-85zoom

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.

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

http://wetpixel.com/forums/index.php?showt...=11936&hl=17-85

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

http://splash.smugmug.com/gallery/1204003

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

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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!

dhaas

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...

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.