Question about flat vs dome ports

I have an Ikelite housing and both the small and big dome port. I understand that in order for a lens to work in either port it needs to be able to focus to a distance of less than 12 inches approximately. The question I have is; does the flat port have the same limitation?

No the lens does not need to be able to focus closer than 12 inches to work behind a flat port, however you can not use wide angle lenses behind a flat port.

Also, 12 inches is for the 6" dome, for the 8" dome the lens needs only needs to be able to focus at 16 inches.

Also, adding a diopter (+2 or +3) to a lens that can not focus close enough, will make it work fine.

Also remember that focusing measurment are always done from the "film plane" which in today's camera is were the sensor is positioned. some camera still have a symbol in the top back made of a circle with a line accross its center, that is to indicate where to help you wit your measurement.

A dome project an image at 4X its radius from its optical center

A quick way to predict where your virtual image will fall in front of a dome is to take the radius of a specific dome X3 and you have a rough idea where your subject will appear. In the case of a 6" dome, the radius is 3", one of the radius is inside the dome, the other three are outside, making for an approximate focusing of 9 inches, as you can see, not too many wide angle lenses can focus that close. that is one of the reason why some people choose a bigger dome, this way they can bypass the diopter if they wish too.

Regards