Shooting wide angles without diopters - can it be done?

I am trying to decide on the last things before taking my first rig under.

I will shoot wide angle and do not need to focus very close at all - think reef scenes or schooling sharks at a distance (not what I will be shooting - I will shoot a "sea gypsy" community in Asia, so think more like pearl divers). I may want to do some half in/half out shots, so I guess a diopter is not a possibility then? And I was wondering if it is needed at all if I don't get too close to the subject. Not ultra close up but perhaps 1.5-3 meters mostly. I don't know if the virtual image created by the dome actually moves in and out according to focusing distance, but if it does, then perhaps I can go without diopters and still get OK corner sharpness?

I will shoot a 5D in a Subal housing with the FE2 dome.

Also, if anyone has used either a Sigma or Canon fixed 20mm and have any pointers on it, I am all ears.

All best,

David

sure you can shoot without a diopter, as long as the lens doesn't have terrible close focus. i don't really bother anymore with my tokina 11-16mm as it looks just fine when you stop down, i prefer the wider FOV and the option for over/unders. granted, i'm shooting it in a 9.25" dome, think the FE2 is an 8", not sure what the quality difference will be, but i'm sure its doable.

To quote myself from a previous thread:

The dome-shaped interface maps objects at infinity (in real space) to a point approximately 3 dome radii away from its front edge (in virtual space). Objects in real space which are closer than infinity are in turn closer than 3 dome radii in virtual space... so the real field (zero -> infinity) is compressed into a virtual field (zero -> 3 x dome radii).

 

Setting your camera lens to focus at any point outside of this virtual field will result in an unfocused image.

So, while your subject is 1.5-3 meters away in real space, your lens will be focusing on the virtual image which is significantly closer - so much closer that your 20mm lens may not be able to focus on it. Adding a diopter may be necessary to ensure that focus is possible throughout the entire virtual field, but you'll need to do some tests to figure that out, since the math to determine this is quite involved.

Diopters do not increase depth of field or "corner sharpness". However, using a diopter with some amount of edge distortion can 'cancel out' some of the distortion that is inherent in the lens/dome combination.

Keri

In general you will not see a huge benefit when using a WA lens with a big dome. Some of the zoom lenses benefit from the use of a +2 single element diopter (see test shots by Alex M) as it helps correct for the curved virtual image.

I shoot the Canon 16-35II in a 9" dome and don't use a diopter.

Cheers

James