blurry edges

Hello,

First post here so please take it easy. 

My setup is a Sony a5100 with the kit lens (16-50mm), with a Meikon housing (which has a fixed flat port). I have a single S&S YS-02 (manual) strobe and most of the time I use a Inon UCL165M67 +6 Diopter as I mostly do macro photography.

As you can see in the picture below while the central part of the image is sharp (at least kind of ), the edges are very blurry.

Picture details: Sony a5100 with the kit lens (16-50mm)@16mm, f/10, 1/125, ISO 200 in the Meikon housing (flat port), S&S YS-02 and Inon UCL165M67 +6 Diopter. The subject is a Charonia variegata with a length of about 20cm/8inch fount in a recent dive here in Crete/Greece.

I know that for such "large" subjects the diopter is not actually needed, however this particular image is a good example of the problem I have. I get similarly blurred edges for most of my macro shots - although for macro it is less important because anyway I usually heavily crop them hence the blurry edges are usually cut. I think I also get it but to less extend even in Wide Angle photos.

What is the cause of these blurry edges  and hence how can I avoid them?

Is it just the shallow DoF due to the diopter? Is it due to the optics/quality of the diopter itself? Or the kit lens? Or the flat port of the housing? Or maybe I am doing something fundamentally wrong? Or maybe a combination of all these?

I am asking because I have some money (few 100euros/dollars) for equipment upgrade. I was thinking to get a second strobe, but if this problem is due to let's say the diopter, I'd prefer to get a better one (subsee +10 maybe) for now to overcome this and the strobe later.

Thanks a lot in advance for any suggestions and nice to meet you all

This shot does not need a diopter zoom in and go further away a diopter is not meant to be used at wide end

Thanks a lot for the reply.

11 hours ago, Interceptor121 said:

This shot does not need a diopter zoom in

As I said, I know that for this subject the diopter is not needed. This was just an example I fount in my archives illustrating this problem. I usually discard such shots and hence I couldn't find anything better.

11 hours ago, Interceptor121 said:

a diopter is not meant to be used at wide end 

The problem is that towards the tele end (40-50mm or so) my system (with the diopter) gives me super tiny DoF (less than a mm maybe) even at very small apertures. As a side note I have actually noticed that the aperture size under these conditions has very small effect - DoF is similarly tiny for say f/10 and f/22 - I don't usually go that small anyway.

Hence I am usually trying to shoot towards the wide end to gain as much DoF as I can and then crop the subject out.

Anything else I could try? Other than change my system of course

Thanks a lot!

16mm (wide angle) and flat port is the problem. This is the borderline and a dome port is recommended to avoid blured edges. You may try the 30mm macro lens. It's relatively cheap and will fit to your port (most probably)

16mm (wide angle) and flat port is the problem. This is the borderline and a dome port is recommended to avoid blured edges. You may try the 30mm macro lens. It's relatively cheap and will fit to your port (most probably)

The edges of the frame get progressively blurrier due to distortion with a flat port, this is why your wide shots are blurry at the edges.  this link explains the optics:  https://oceanity.com.au/learn/understanding-flat-port-and-dome-port-theory/

Thanks for all the replies - great info.

My conclusion is that all the factors mentioned (flat port, wet lens, camera lens etc) contribute to this.

Eg even on dry land my lens has a lot of distortion on the sides. Add to this the flat port, the diopter etc and that's the result. At least now I know it is not only due to my diopter.

I did some experiments yesterday during a dive. The distortion even at 30mm is noticeable, although much less than the one at 16mm.

So another trade off to live with: low focal length with wide field, larger DoF but distortion vs long focal lengths with narrower view but shallower DoF and less distortion.

Thanks a lot!