What diopter for Tokina 12-24

Hi guys,

I had a couple of trips with the Tokina 12-24 with the Ikelite 8" dome, and have been having some problems.

First, the corners are a little mushy (but still much better than with the 6" dome!)

Second, I have trouble focusing if the subject is more than 2 metres away. This is in good light and clear visibility, and the AF is having trouble locking focus.

Is this due to my lack of using a diopter? If so, which one should I be using?

And just to clarify, is diopter = close-up lens?

Thanks in advance! I'm hoping to get this squared away before my trip to Pulau Weh next week

I use a B&W +3 dioptre (close-up lens) with the Nikon 12-24 and a 9" dome, and I prefer it to a +2. You should see much improved corners.

Wait a minute -- you're having trouble focusing if the subject is MORE than 2m away? But you can focus on subjects that are closer? That's a bit odd. Normally I would expect the diopter to help with the closer stuff. That being said, I used the Tokina 12-24 behind the Sea and Sea fisheye dome port with no diopter and the corners were a tad bit soft but not so much that I obsessed over it. Perhaps a +2 would have helped that. Diopter definitely required with a smaller dome, however.

Wait a minute -- you're having trouble focusing if the subject is MORE than 2m away? But you can focus on subjects that are closer? That's a bit odd. Normally I would expect the diopter to help with the closer stuff. That being said, I used the Tokina 12-24 behind the Sea and Sea fisheye dome port with no diopter and the corners were a tad bit soft but not so much that I obsessed over it. Perhaps a +2 would have helped that. Diopter definitely required with a smaller dome, however.

hi guys, thanks for the replies.

Mike, yes I found it odd that it's the subjects that are further away that give me AF problems. I have no problems focusing close-up. Still, it could be the primitive AF in my 350D that's giving me problems. However I still think it's got a more advance AF system than a 1D3

What's the trade-off between using a stronger diopter? Say between a +2 and a +4. Will I lose field of view? How much do I lose?

Sorry I've got zero experience with diopters and dome ports, so just need something to put things into perspective (literally!)

thanks again!

Per Ryan of Reefphoto recommendation, I use a B&W +3 behind the Ikelite 8" dome port. Other times I don't use it at all like shooting those hammerhead in Galapagos a couple weeks ago, it's so far away & didn't really matter anyway.