Im making my first case to try out split shots with. My question is: in practice how important is distance from lens to dome? Will I lose all underwater sharpness if my lens is 20mm too close? In your experience, how far off the theoretical dome extension length have you found still practically useful?
The long story is ........
I'm attempting to bodge an Ikelite 8" dome port (which I picked up cheapish) onto my Meikon case for Sony A6000.
I'm going to cut down and glue the extension port to the housing ... so I've pretty well got one chance to get the length of the extension right.
I'm using a 10-18mm lens (SEL1018f4) (15-27mm full frame equivalent) which worked pretty well with the 4" dome I had glued on there before. Hopefully the 8" dome will help me to get split shots.
I suppose the focal point (not necessarily the front element?) of the lens needs to be 102mm from the dome. I have marked the focal points at 10 mm and 18 mm on my lens for reference.
Now I come to choose how long to make the extension I discover that I can only get the lens back from the dome about 80mm before the dome housing starts to vignette my image with the lens at 10mm. At 14mm I can move it back to 90mm and at 18mm I can get the focal point about 100mm from the dome.
So my question is ... am I stuck with my lens at 18mm if I use this dome port?
Or will I experience only moderate edge softness if my lens is 20mm too close at it's widest 10mm?