Tokina 12-24 behind 8" dome - do i need diopter???

Hi

Relatively new to U/W photography, but have invested in a Subal ND80 with 8" dome and have been using a Tokina 10-17 F/E very nicely.

Need to do away with F/E effect so have bought a Tokina (yeah, I like 'em!) 12-24. Subal and others say I need a -2 diopter behind the dome BUT when it's fitted my lens wont focus on anything more than about 6" away. Take off the diopter, and it focuses fine from 6" to 60'...

Am I missing something here?? Do I actually need the diopter??

Advice / experiences welcome....

Subal and others say I need a -2 diopter behind the dome BUT when it's fitted my lens wont focus on anything more than about 6" away. Take off the diopter, and it focuses fine from 6" to 60'...

Are you testing this above or below water? The dome port won't function as a dome port above water as it works due to the change in optical density between water (outside the port) and air (inside the port). A dome port brings the apparent image much closer to the lens.

Are you testing this above or below water? The dome port won't function as a dome port above water as it works due to the change in optical density between water (outside the port) and air (inside the port). A dome port brings the apparent image much closer to the lens.

Well Ive been testing above water, due to go in the pool tomorrow. So that will be it? Doh! Like I said, kinda new.....

Thanks!

=)

Try to shoot something u/w with fine detail in the corners of the frame. Maybe put a plastic sign or similar with small text or bar code. If edges & corners are not as sharp as you like, then try a +4 diopter. My reference is the Nikon 12-24 with Nexus FP170 dome (about 6.75" dia). +4 works well for my setup but YMMV.

Hi

 

Relatively new to U/W photography, but have invested in a Subal ND80 with 8" dome and have been using a Tokina 10-17 F/E very nicely.

 

Need to do away with F/E effect so have bought a Tokina (yeah, I like 'em!) 12-24. Subal and others say I need a -2 diopter behind the dome BUT when it's fitted my lens wont focus on anything more than about 6" away. Take off the diopter, and it focuses fine from 6" to 60'...

 

Am I missing something here?? Do I actually need the diopter??

 

Advice / experiences welcome....

I used the Tokina 12-24 behind a Sea and Sea fisheye (9") dome for years. Never used a diopter. Occasionally there were issues with the corners; in particular, when the stuff in the corner of the image was close to the camera (think close-focus wide angle). However, since I am MUCH less fanatical about corner sharpness than many here (I, for one, have never spent an hour in a pool shooting the wall tiles ) I never worried much about it. If I had, I might have tried a +2 . . .

I have a similar issue with the same Tokina 12-24 but a Hugyfot housing with a 174 mm dome (about 6.8 ").

According to the Hugy specs, you can use this dome + lens by putting a 40mm extension ring between the port and the housing and no diopter. I guess this is to put the virtual image a little further. I'm going to try it this weekend, so I'll tell you how it was.

I have a similar issue with the same Tokina 12-24 but a Hugyfot housing with a 174 mm dome (about 6.8 ").

 

According to the Hugy specs, you can use this dome + lens by putting a 40mm extension ring between the port and the housing and no diopter. I guess this is to put the virtual image a little further. I'm going to try it this weekend, so I'll tell you how it was.

Yeah, sorta makes sense: Im using a 50mm extension with the Subal and, yes, it works perfectly underwater! Yay!

Zoom gear slippage, however, meant I had to stop my pool test short. Im told this sometimes happens when tolerances on the lens rubber grip are not as tight as they should be. Ive been advised to use duck tape (gaffa tape in the UK) on the inside of the zoom gear to pad it out a bit.

Anyone else come across zoom gear slippage? Gonna go out and bu some tape tomorrow if people think it's a good solution. Should I really be getting slippage??

=)

What do you mean by zoom gear slippage. Is the zoom ring slightly too big for the lens? Is it too tight causing the housing gears to slip? All lenses have some variation in manufacturing tolerance so this is not an uncommon problem. One solution to the first thing is to place a layer of tape inside the ring to make its inside diameter slightly smaller.

According to the Hugy specs, you can use this dome + lens by putting a 40mm extension ring between the port and the housing and no diopter. I guess this is to put the virtual image a little further. I'm going to try it this weekend, so I'll tell you how it was.

An extension tube has a similar effect to a close-up lens on the front of the lens. Both allow the lens to focus closer than they would normally - which required for dome ports unless the lens already has a very close focusing distance. The tube does it by simply moving the focal point further from the focal plane (the same thing the lens does when you focus more closely). The close-up lens does it by modifying the optics before the lens. One problem I see with using a extension tube is that the effect of the tube is dependent on the focal length of the lens. If you use it on a 12-24mm lens, the effect will be far greater at 12mm than at 24mm and may even be too great - i.e. you may not be able to focus on distant objects underwater.