Distance from dome port to lens? Can I get away with being 20mm out?

Hiya,

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?

Regards,

Peter

Getting the best fit behind a dome port is often a matter of trial and error. Maybe starting with the recommendations of the housing manufacturer and varying the amount of port extension (it helps to have several port extension tubes on hand). Most successful over-under shots have been done with fisheye lenses. I am not familiar with your gear so cannot provide more specific directions. Keep in mind that if the lens is squat very little if any port extension is needed. It is the longer zoom wide angles such as the more recent Nikon and Canon 16-35 lenses that are quite long and require a lot of port extension. The older primes used hardly any. The old Nikon 20D lens requires none (Seacam) for example.

Hi Peter,

Dome port set up is actually specific to the actual lens, so there is no generic information that you can use too work this out. Manufacturers typically physically test lens/port/extension combinations in order to figure it out. Lens positioning with a dome is critical however!

If it is not correctly positioned, your images will not be sharp.

I would watch this video:

https://player.vimeo.com/video/32130591

It is a pretty good primer on dome port theory

Split shots with a rectilinear wide-angle are difficult

Sorry to not be able to provide answers!

Adam

My friend Eiko Jones in BC made some great over unders with a... ready for it... macro lens. He shot them in a swamp and got some amazing shots of little frogs sitting on lily pads. I suspect that he has them posted on his website someplace...

Once I saw those, I realized that just because "they" say it can't be done, it doesn't mean it can't be done...

My friend Eiko Jones in BC made some great over unders with a... ready for it... macro lens. He shot them in a swamp and got some amazing shots of little frogs sitting on lily pads. I suspect that he has them posted on his website someplace...

 

Once I saw those, I realized that just because "they" say it can't be done, it doesn't mean it can't be done...

It would be nice to be able to actually see these pix. Getting both the air and underwater parts in focus at the same time (not using Photoshop!) is not that easy. To use longer focal length lenses one can use a split diopter lens - like a regular diopter but only covers one half of the lens so that one can focus at two distances. It is possible this is what your friend used. There is a bit of trial and error using split diopter lens with getting the subject to match the two distances. I failed the last time I tried.