Canon 6D won't focus lock underwater. Wazzup?

[JohnN]

So I can't get my new-to-me 6D to focus lock reliably. The configuration:
Ikelite with 4-Lock port and dome
2x Ikelite strobes
1 Focus light
50 mm lens

The camera has no problem out of the housing with achieving a focus lock (green LED is solid), but underwater, I can get maybe 1 out of 4-5 shots.

I'm in Raja Ampat, with very good visibility, so there should be sufficient ambient light. With and without the focus light turned on, I have the problem

Could this be a firmware problem on the camera? There are < 8K clicks on the body so I believe it is "like new"

Any suggestions are most welcome

 

[John C. Ratliff]

Are you using a dome port? A dome port has an imaginary image with a focus distance very close.

SeaRat

 

[David Haas]

Searat is correct.

If your 50mm lens can't focus too 12" or closer and your housing has a DOME port the "apparent image" a dome creates UNDERWATER is too close t focus to what the lens sees......

Even when a subject is actually 3' away the lens SEES a subject maybe 12" or closer image on the dome port.

David Haas

 

[JohnN]

Thanks. Since the post I’ve learned more than i care to about the virtual image. The solution for the 50mm lens turns out to be a +4 diopter

 

[John C. Ratliff]

Thanks. Since the post I’ve learned more than i care to about the virtual image. The solution for the 50mm lens turns out to be a +4 diopter

…or a flat port.

The photo below is of a crawdad (crayfish) in the Tualatin River, taken with a Canon F-1N film camera, FD 50mm Macro Lens, behind the dome port on an Ikelite housing. Your +4 diopter will in effect convert your lens into a macro lens, but you won’t have distant images (I think).

SeaRat

 

[John C. Ratliff]

Here's what Ikelite Underwater Systems said in their S.L.R. Case Instruction Booklet concerning the dome port:

****DOME PORT****

The dome port #5503 (photo 17) is definitely recommended for use with 28mm to 17mm wide angle lenses. Dome ports create an unusual focusing phenomenon as it creates an apparent image underwater that is a few inches in front of the dome. The camera lens must focus on this apparent image rather than the actual image which may be several feet away. Viewfinder or pre-set focusing is required since the distance mackings on the lens may have no relationship to the actual image distance.

****PRE-SET FOCUSING****

You will find when using the dome port that by presetting the lens focus distance between 12 & 12 1/4 inches, the camera will be focused from about 18" to infinity under water, eliminating the need for focusing. Lenses that do not focus as close as 12" will require the addition of a +3 close-up lens. With the +3 close-up lens in place read from the information sheet enclosed with the close-up lens, for the setting for focus at 12 to 12 1/4 inches. Set your camera lens at that distance. This should put everything from about 18" to infinity in focus underwater eliminating the need for future focusing.

I have used my Canon FD 24mm F-2.8 mm lens with great success behind a dome port (see the photo below of red-sided shiners and the Umpqua Northern Pike Minnow.

SeaRat