more newbie ?? - possible reasons for soft photos

I took my Sigma 18-200 out the other day just because I wanted to see what would happen. I used the Subal housing with dome port and my Inon 220s strobe.

Some photos turned out pretty well but I noticed many were a little (or a lot) soft instead of sharp as I have come to expect. I can't get the lens/camera to give me the same results on land so I was wondering:

- is it because of the dome port?

- is it because of the water and floaties?

- is it because I should have used a dioptre or something?

- will it happen in a flat port?

- is it an aperture problem...I shot a lot between f8 - f11, though I can't see a pattern?

- is it a photographer error ?

I did tests in the housing and out of the housing on land...both produce nice crisp images at all focal lengths.

Curious as to possible causes and solutions. And, no, there was no real reason to stick this lens in the housing - there are certainly better options for long range and for wide angle. I just have the luxury to play and why not!

If you got soft and sharp photos in the same dive under the same conditions you can eliminate all of the above except photographer error... Just kidding, but I do think you will get more consistent results with a close-up diopter. Try testing the lens/port combination in a pool under controlled conditions. If you use the lens behind a flat port you won't need a diopter.

Luiz

Yes, it's because of the dome port. If the lens is too close or too far from the dome then the center of the photo may be sharp, but the corners soft. At some zoom ranges, it will probably work fine - that's why some are sharp and some are not.

Cheers

James

OK, thanks. What confused me was the photos are sharp out of water, but all the underwater ones are at least a little soft.

So, will adding a dioptre improve things and if so, what strength to start with (or do I just jump in and guess again)?

Problem could also be incorrect lens position for the dome port. For technical details on using dome ports check out these links:

http://www.marinethemes.com/domespacer.html

http://wetpixel.com/i.php/full/proper-dome...-and-placement/

http://wetpixel.com/i.php/full/dome-theory/

I think the correct dioptre for the Subal DP-FE2 dome port you are using is a +2.

In addition to the correct dioptre you're going to need the correct spacer, specially when using the lens at the wide end of the zoom. Check out the last paragraph of the first link above for a quick and dirty method for selecting the correct spacer.