Very soft corners -- anything I can do?

OK, the third of three questions regarding testing my new Canon 30D/Ikelite/DS-160 kit.

In this case it's relevant to point out that I am shooting with the Canon 17-85IS lens in the dedicated Ikelite dome port for this lens, with a B&W 4-diopter filter as suggested in the Ikelite docs.

I am seeing very severe corner blurring and chromatic aberration. I am assuming that this is a wicked combination of:

1) This is not the world's greatest lens in the first place (CA-wise in particular)

2) The dome will introduce some distortion

3) The 4-diopter filter isn't helping much here, either.

Is there anything I can do "relatively easily" -- read as, I don't want to buy a whole new rig, or new lens; but a different filter, or a different port, maybe? -- that can be done here?

Thanks,

Dan

Perhaps an 8" port and a +2 diopter.

Try taking some test photos (of a newspaper) with the camera out of the housing and the +4 diopter fitted. I think you'll be surprised at the poor corners and CA.

Cheers

James

Perhaps an 8" port and a +2 diopter.

Yah, I looked at this. The 8" port doesn't allow zooming with this lens, at least not according to the Ikelite web site. Not an option. If I want fixed focal length, I'll use one!

Interestingly, upon reviewing the specifications (again on the Ikelite site), I noticed that for the Canon 18-55IS, they claim that "no diopter filter is required", as it can focus close enough. This lens has a MFD of about 10"; the lens I'm using has an MFD of about 13". If I use a +2 filter I should be fine.

I might just get one of the new 18-55IS lenses (the new kit lens) and put it in an 8" dome port. I can probably make do with a +1 diopter.

Thanks,

Dan

I just answered my own question here -- I called Ikelite and found out that the 6" dome port creates a virtual image that is at approximately 12". This lens focuses down to about 13", so I can probably get away with as little as a +1, and definitely a +2, filter. I will try this out, and report back.

Regards,

Dan

.....at what aperture?

May have to use this combo at f/16 (for example) so get reasonable corners.

Oh, just saw your related post. Seems like you were using f/8.