I recently purchase a Samyang 8mm f3.5 Fisheye after reading a review by Ken Rockwell.
I found these lens readily available on Ebay and picked one up for a good price, less than $200.
I dont have access to a swimming pool as such for proper lens testing but I has able to take this lens out for a dive yesterday to see how it performs and make a rough comparison to the Tokina 10-17mm which I normally use. To start off the lens is a manual focus lens which i set the lens to its minium focus length of approximately 30cm or about a foot.. As you expect you need to be close to the subject as this lens is really wide but surprisingly it didnt have the distortion I was expecting and it feels to have less distortion than the Tokina 10-17mm @ 10mm.
First impressions are that the lens doesnt focus as close as the Tokina but I was expecting that and it isnt as sharp as the Tokina. I had to set the aperture at a setting between f8 and f11 on the lens....which I expected to be f9 (wasnt marked on the aperture ring).
Here are a few shots from yesterdays dive taken on a Nikon D7000 with Aquatica housing and 8 inch dome with 2x Ikelite DS161 strobes. Quick mention I also upgraded to the new Ikelite Li-ion battery packs made for the DS125/160/161 strobes. I must admit I like these new battery packs as they are lighter than the older packs abd have a quicker recycle time and give twice as amount of flashes than the standard battery packs. More bang for your buck.
The shots are taken a setting between f8-f11 and ISO100. The images are not cropped or edited only resized for web. Only adjustment was shutter speed. This collection highlights what one can see on a single dive are Julian Rocks, Byron Bay, Australia, which is one of the best dive sites in Australia.
Only early days with this lens but I am enjoying shooting with it and hopefully fine tune its performance. Hopefully getting a Manta photo during the week to add to todays collection. This lens has potential to make a great video lens for those like myself who are using the DSLR's more and more in making video's.