Fisheye Dome Port Focusing

Ok I don't really need answers infact as there is nothing i can do about this it doesn't really matter but whilst diving and thinking today and taking photos my mind started up its own conversation ..

I had always heard that the fisheye lens focuses on the dome port not the subject ... and i noticed my lens kept on adjusting its focus .. so I wondered .. WHY ?

why does the lense insist on re focusing all the time if it is staying focused on the same thing.

photo from today

I think (and I'm happy to be corrected) that when you are using a dome port, the virtual image that everyone talks about being "about a foot away", (which obviously depends upon the dome curvature) is actually the virtual image for infinity focus.

So I suspect that in a shot like the one you just posted, where the primary image (ie the diver) is closer than infinity focus, the virtual image is closer to the lens than the infinity focus virtual image, and the camera actually need to focus closer than "about a foot away".

The boat, though, would be at effective infinity focus, so switching from the boat to the diver as the focal point of the image would involve the camera refocusing.

Nice shot, BTW.

As Craig said, the focus range is "just" shifted underwater. But there is still a range within the virtual image moves related to the real distance. The lens still has to focus, but on a different range, like in this example chart.

Julian

Interesting chart Jolly. Can you explain how these values are reached, I.E. AOV v. dome radius, lens mm v. chip size v. radius, etc. Thanks

so in actuality .. the lens is not focusing on the dome itself .. it is focusing in a range beyond the dome in order to make the primary subject in focus rather than any background.

so i either heard wrong or people were simpifying things for me ... which often needs to be done.

Interesting chart Jolly. Can you explain how these values are reached, I.E. AOV v. dome radius, lens mm v. chip size v. radius, etc. Thanks

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The chart is from an article I have written some time ago about this stuff:

http://wetpixel.com/i.php/full/dome-theory/

AOV (= Angle of view?) has no influence on the focus distance itself. It's the dome curvature predicting the distance of the virtual image underwater. And the distance from the lens' centre of perspective to the film / sensor if you want values relating to the lens' distance adjustment.

Julian

Thanks Julian, Interseting article and comments. How if I can only calculate the Olympus 8 mm fisheye with the 170 mm Athena dome.

Phil

Hi Phil,

do you want to create a similar chart for the Athena Dome? All the data, like described in the article, would be necessary. The problem is that most manufactures do not provide them. Usually they just state the dome diameter which is quiet useless for this. Furthermore, I don’t know the flange focal length of the Olympus E System (distance camera bayonet <-> sensor) and how they put the dome in relation to the lens. Is the Athena dome the regular Olympus dome?

Julian

Hi Julian,

The Olympus dome for the PT-E01 and now E-02 was obtained from Reef photo and video. As far as Ryan and I can tell it is the same Athena glass dome (including coatings) that is called OPD-F 170 (fisheye) on the Athena web site. The diffrence is that it is mated with a metal base which threads into the metal ring on the front of the housing. Even the removable dome shade and dome port cover appear the same, except for the Oly logo. I will attempt to obtain the flange focal length of the E system when I go to PMA next month.

Phil