Following on from a discussion on wetpixel I tried a (somewhat subjective) experiment by combining my short flat port and extender tubes so that I got the 'tightest' fit between macro lens and port glass (using a 60EFS lens) to see whether the result differed in any way from my 'standard' set-up consisting of a single longer port which left a small gap between lens and port glass.
Although I haven't conducted any scientific testing, my initial impression after half a dozen dives is that the results are as sharp and seem to exhibit slightly less chroma in the corners, virtually all of which can be eliminated in Photoshop. The problem is assessing the differential of the colour fringing around high contrast boundaries in the frame corners - which is where it is noticable - but the telling difference is that the slider settings in Photoshops raw converter are slightly lower so I believe that it may be correct to say that lens to front port distance should be minimised for optimum results.
I'd be interested to hear from anyone else who has tried doing this and see what other opinions exist.
As an aside, I also shot the set-up with and without a wet diopter and whilst the diopter did its job, it certainly did reduce performance to an extent and the image curvature it introduced was evident - although not to a high degree. In my mind, whilst wet diopters have their place, they are not suited to really critical applications where a prime macro will still win out.
