I plan to buy an Ikelite 200DL housing for my Sony A7iii + 28 mm f2 in order to upgrade from my current olympugs TG6 setup.
Ikelite offers two options, either the regular DL 8" dome port (recommended setup) or the compact 8" DL dome port (second choice). I wanted to know if anyone here have experienced both dome and if there's variation in optical quality or results. My main concern is principally linked to size and weight. All my photography is done by snorkeling rivers and stream, I wonder if the 8" dome port would end up too large or difficult to place in shallow or small rivers.
Cheers,
The compact dome port is a much shallower dome and invariably the centre of curvature of the dome will be placed well behind the entrance pupil of the lens, it's a compromise to get a more compact dome and also allows you to get in closer. 28mm is not that wide so the impact on optical quality is likely not as great as something like a 16mm lens. Google the ikelite 75344 and 75340 and you'll find a link with an image of the dome and you can see the difference - along with the dome dimensions
The 28mm f2 has a fairly long minimum focus distance of 290mm (AF) and I would guesstimate it focuses about 125mm from the port surface. Plugging that into a dome port calculator indicates the closest object it will focus on is about 320mm. I don't know how that fits in with what you are planning to shoot, max magnification is 0.13x, which is an object 270mm long filling the horizontal frame at that distance in air. I think it will be more like ~300mm object underwater due to the virtual image. The bigger dome places the virtual image further away so it should achieve more magnification and fill the frame with smaller subjects.
Thanks you for the calculations, this is great (do you have any ressources to understand the calculations?). I feel focal is correct for the use, this is not directly comparable, but I rarely zoom the tg6 and I quite satisfied with acheive composition.
This seems a reasonable minimum focus distance (we talk distance to sensor, right?) for the intended use, fish portrait of medium/large freshwater fish (crappy, pikes, trout) and "wide" shot of fish school or small critters with the environment (ex. crayfish under rocks). For narrow portraits of small fish (ex. darters/gobies) I plan to buy a 90mm macro.
Yes the min focus distance is from the sensor. The first guesstimate is for the compact dome port. The calculation starts with lens length as published and then add up all of the extension and port lengths. The only thing a little uncertain if the placement of the camera in the housing, i estimated the sensor is located between the power switch and flash bulkhead. You can use this calculator to calculate distance to the virtual image: https://oceanity.com.au/blog/view/dome-port-virtual-image-visualiser
The object distance and virtual image distance is distance from dome port.
On comparing with the TG-6 don't forget it focuses much closer than the lens we are discussing here and benefits from the the 1.3x magnification of flat port so effectively has equivalent field of view of about 32mm. I'm wondering if you might be better served with a 16-35mm f4 or perhaps a 24-70, though the 24-70 needs a diopter to achieve focus behind a dome. You know the size of the stuff you shoot of course, but the magnification achieved with the 28mm is quite small.
