Dome Ports – Why they Work (or don't)!

Paul Kay · Issue 1, Aug 2001 · pp. 24–25

Products mentioned: Subal fisheye dome port, Nikon 17-35mm AFS lens

Have you ever wondered why it is considered best to use a dome port with a wide-angle lens? After all, flat ports are much cheaper, and can be bought for many lenses, so why bother with an expensive and sometimes vulnerable dome port? Well, the maximum theoretical angle of view that it is possible using a flat port is 96 degrees, and in practice it is much less (and no, I’m not going into the complex optical theory of why - see details of the book at the end if you must know). In fact, the accepted cut-off focal length of the lens usable with a flat port is reckoned to be 35mm (on the 35mm format). Any shorter focal length generally requires a dome port, and to get the best out of a dome port, it is essential to choose the right one and use it correctly. So for those of you with technical longings here follows the reasoning. For everyone else, there’s the information on the effects of us…