Dome ports and depth of field

[Nickbell]

I have recently taken purchase of the Zen dome port for my Epl-2 &epo3 housing &9-18 lens.To get a better understanding of the depth of field characteristics of that lens (on land /in air) I downloaded a depth of foeld calculator for my tablet (Indie Film Gears DOF Calculator).Very interesting. My question is firstly, does a dome port give you the same angle of view underwater as the lens would have topside ? Secondly, if it does ( as claimed) can you assume that the lens would now also have the same Depth of field underwater as topside, or does the effect of the dome port play with actual distance relationships and totally destroy this relationship.

Cheers.

 

[bvanant]

In general I think that a lot of field of view issues depend on the particular dome. As for DOF, since the virtual image is now much closer (infinity is at 2x the dome radius) the image behind the dome will have a larger DOF than the real world image.
Bill

 

[derway]

I believe the angle of view, for a spherical dome, will be exactly what the lens sees in air, yes.

But the big problem here, is that virtual image is curved.

This makes it much much tougher to get the corners of the virtual image in focus, than it would be if the image were flat, or further away.

So you will need to shoot with a higher fstop, and use much greater DOF, to have a sharp image.

 

[deeper thoughts]

interesting

 

[unclebob]

does a dome port give you the same angle of view underwater as the lens would have topside ? Secondly, if it does ( as claimed) can you assume that the lens would now also have the same Depth of field underwater as topside,
Cheers.

This article covers the details underwater:
Dome Theory :: Wetpixel.com

But basically, domes form a virtual image when underwater and it's the virtual image you are focusing on. The virtual image extends from just outside the dome itself (for very close objects), to 3.03 times the radius of the dome (for objects at infinity). So there is a massive difference between using the lens above water and below water when shooting through a dome. With a dome underwater you will always be focusing very close.

derway brings up the other issue which is that the virtual image you are focusing on is curved. Rectilinear lenses like the 9-18mm are designed to focus in a plane (which is great if you are shooting photos of a brick wall above-water from straight on), so the curved virtual image created by the dome requires very large depth-of-field to keep the corners in focus. More likely, rectilinear lenses behind a dome are more likely to always have out of focus corners.

Note that a fisheye lens behind a dome works great because the fisheye lenses don't have a flat focus plane, it's curved like the virtual image that one is focusing on. This allows for sharp corners underwater (assuming they are sharp above water

So underwater, there is a massive difference between DoF (and focus distance) and even focus plane than there is above water.

The Dome Theory link above covers a lot of other details...

 

[DazedAndConfuzed]

Maybe it would be better off to get the absolute cheapest wide angle lens there is, the ones that focuses the edge so bad since it does not have enough elements to focus a flat plane image. Something like a 1 element lens would probably do.

 

[PHIL RUDIN]

Trying to understand dome theory is a bit like trying to understand the opposite sex, to many variables are involved like zoom V. prime, glass V. plastic and using one port for more than one lens. In my mind it is better to just look at the results and decide if you can live with them. The Olympus 9-18mm zoom and the Panasonic 7-14mm zoom have been well used and many images have been posted on this site and others. I will include links to my Olympus 9-18mm portfolio using the ZEN Underwater four inch dome and a link to the Panasonic 7-14 thread that has several of my images taken with that lens, most with the E-M5 and Nauticam six inch dome port.

In the 7-14 thread if would be best to read all of the pages because you will see that domes even advertised for the same lens do not all work the same. The point is that the final images are very exceptable from both of these lenses. I did not use F/stops above F/8 all that much and the corners are well within the exceptable range for ANY rectilinear lens. You can argue all you want that fisheye lenses have sharper corners but many images just don't look as good using a fisheye lens as they do using a rectilinear lens.

Depth of field is more a result of F/stop and sensor size than of dome design. Keeping in mind that water is 800 times more dense than air you would expect a bit more softness as you move away from the lens towards more distant subjects.

While dome theory is an interesting subject the end result is what everyone is going to be looking at.

Welcome to zenunderwater.com!

http://www.scubaboard.com/forums/panasonic-planet/413982-panasonic-7-14-mm.html

Phil Rudin

 

[glee719]

I have a hypothetical question. What happens when you use a wider lens in a smaller dome port? For example, if I were to just use the 4" Zen dome for the Panasonic 7-14mm lens at 7mm?

 

[PHIL RUDIN]

The results will be the same as if you use the Olympus 12-50 in a port designed for the 14-42 zoom, it will vignette in the corners because the port glass/acrylic is not as wide as the angle of view of the lens.

Phil Rudin

 

[glee719]

Thanks Phil!