Not a huge fan of my GoPro

[Barmaglot]

What did you buy again? APS-C Sony and what lens and dome are we talking about?

He posted a couple pages ago that he got a Sony A6300 in a Nauticam housing with Zeiss 12mm f/2.8 and 180mm glass dome.

 

[Chris Ross]

He posted a couple pages ago that he got a Sony A6300 in a Nauticam housing with Zeiss 12mm f/2.8 and 180mm glass dome.

Thanks, I went looking and gave up after a few pages - we went through so many iterations and back and forth, I just lost track.
12mm f2 you could probably shoot at f8 I would guess if you needed to, but wider lenses would probably need stopping down further.

 

[bvbellomo]

Thanks - I was just about to reply.

I still do not understand having a virtual image at the 4 x the radius of the dome - meaning my 180 has an image 36 cm in front of the lens. So if I am photographing a shark 10 meters away, I don't know if I want to focus 36cm or 10 meters. As long as I can autofocus, it really only matters for curiosity. I just worried it would autofocus to 10 meters when I'd want 36 cm (or vice versa). This is not something I can test in my fish tank, as if something 36cm away is in focus, the farthest objects in the tank are also in focus.

 

[Barmaglot]

It will focus just fine, don't worry, although a shark 10 meters away will be barely a speck with a 12mm lens in a dome. Here is a substantial sized manta at something like that distance, shooting a 10-18mm lens at 18mm:

Obviously not an image to share for any reason except as an example of what not to do.

 

[Chris Ross]

The lens does not know or "care"about the actual distance - when it's underwater all it can see is the virtual image and that is what it is focused on - it is focused at very close distance not at the distance of the physical subject - you can check in EXIF - though not all readers will extract that data. If you used a MF lens you would even be able to see this on the lens itself. This is a good write up of why it happens - Understanding Flat Port and Dome Port Theory - Blog | Oceanity

 

[bvbellomo]

The lens does not know or "care"about the actual distance - when it's underwater all it can see is the virtual image and that is what it is focused on - it is focused at very close distance not at the distance of the physical subject - you can check in EXIF - though not all readers will extract that data. If you used a MF lens you would even be able to see this on the lens itself. This is a good write up of why it happens - Understanding Flat Port and Dome Port Theory - Blog | Oceanity

I appreciated this. It explains the importance of port extensions. If your lens is too far forward, it should behave somewhat like a flat port, losing some advantage of a dome. But if your lens is too far back, you can get worse refraction than if you used a flat port.

I still don't understand what a "virtual image" is. Every change in medium (water, glass, air) refracts light, which is how lenses work. In some cases, it is useful to think about what the image would look like with only some of these medium changes - is that a "virtual image"?

Part of why I don't understand autofocusing underwater is I don't understand autofocusing in general. A quick wikipedia search leads me to believe my Sony uses phase-detection, which should work regardless of how the light is refracted - really any "Passive" system should work.

 

[Chris Ross]

AF will work regardless of what type it is, the virtual image is not there it just appears to be there because of the optics, it comes from the dome acting as a negative lens. There is really no complete explanation about what goes on without getting into optical formula. If you want to try to digest it, this link goes through the formulae, Optics of Dome Ports
but like I said the explanations are a little difficult to follow unless you have some grounding in optics.

 

[bvbellomo]

Not sure if I should keep posting on this thread, as this is manufacture/model specific. But I got everything working nicely with the dome, but not the kit lens and flat port.

I got the zoom gear to attach nicely to the lens, and it zooms nicely when I turn the gear by hand. But it does not seem to work inside the housing. The port has a knob labeled "MF" which I assume is manual focus, but it doesn't do anything, is about a half centimeter too far to turn the zoom gear, and I don't see anything to zoom.

I do have 2 plastic gears left over that looks similar to eachother that I don't understand and haven't fit on anything. I believe 1 came with the diopter holder and the other with the zoom gear. They are somewhat similar to the "locking gear" on the diopter holder, which I was able to attach but I also don't understand. Are you supposed to turn the "locking gear" underwater? Or is it just how the holder attaches?

I haven't had time to try the diopter underwater, but it is blurry in air. Is it supposed to be? Or do I have it mounted wrong?

Do I need to mount the manual focus gear? I don't plan to use manual focus.

Any idea what my other gear is?

My trip is less than a week away.

 

[Barmaglot]

Is this the gear that you have for the 16-50mm? I don't have personal experience with Nauticam housings, but from the way it looks, it's highly likely that it's supposed to mesh with the gear on the housing (the one driven by the big knob on the left side), not the port.

Regarding diopter - it's going to be blurry if you're outside of its working distance; you've mentioned that you have a CMC-2 - by Nauticam's port chart, its working distance is 69mm to 121mm, although I don't know if that distance is counted from the sensor plane (probably not), lens front element, port glass, or diopter front element. Port glass is the most sensible probability, but again, I'm not sure. Regardless, when your subject is either closer or further away than that range, the lens will not be able to focus. The maximum magnification with CMC-2 is quoted as 1:1.4, so a subject 33mm (1.3 inches) wide will be able to fill your entire frame at the closest focus distance. You can zoom your lens to 50mm, switch to MF, focus it to minimum distance, then just keep moving the camera closer to the subject until things come into focus on the screen/EVF.

 

[bvbellomo]

Thanks! That is the "zoom gear" that attaches nicely to the lens. It does not seem to mesh with either the big knob nor the port knob. Most likely one of my "left over" gears is the MF knob, which I guess I do need for the diopter.

I am able to focus the diopter out of water, from 69mm to 121mm in front of the port seems about right. I was expecting more range.

Any idea why the diopter holder has a gear that turns?