Wetpixel Live: Dome Port Essentials

One of the most frequently discussed topics here on the Wetpixel forums is the selection of appropriate dome ports. Many topside photographers assume that the dome's function is simply to provide a waterproof port and tend to be unaware of the fact that it also acts as a critical component in an underwater optical system. @Alex_Mustard and @adamhanlon discuss how ports can make or break the quality of imagery that can be produced underwater.
 

Hi. Thanks for another usefull episode. 

Does using f stop 12 , 16 and up won't bring us to Diffraction ? Maybe I won't have soft corners but I risk the sharpness of the entire image. Don't I ? 

Any thoughts about this ?

f12 or f16 shouldn't be in the area where diffraction is really a problem for most systems (it scales with sensor size). On MFT diffraction will start to become noticable when pixel peeping on image tests at about f11, but in real world application I would say f16 is where the limit is. However you also need to adjust the apertures mentioned in the video to sensor size, so if Adam says f16 (on FF) on mft you would be using around f8. 

The diffraction limit is where effects start to be noticed and resolution drops, some of this can be recovered at least partially by judicious sharpening and requires pixel peeping at 100% magnification to even see.  It certainly does not mean the image is suddenly unusable, it gradually becomes worse at smaller apertures.  To be accurate it varies with pixel size, that is to say it noticed at wider f stops with smaller pixels. 

As pixels get smaller they will eventually be smaller than the resolution limit of the optical system, so having smaller pixels does not increase resolution.  The net effect is that as MP go up the aperture at which a loss of resolution can be detected gets larger.   Conversely you should expect similar performance to a lower MP when you stop your high MP camera down into diffraction.  Exactly how much varies with the lens you are considering and also the sensor - whether it for example has an anti-alias filter or not. 

You need to consider this in relation to resolution loss when shooting through a dome.  The premise behind developing wet lenses like the WWL and WACP is that the  loss of resolution across the frame when using a dome is such that kit lenses behind a WWL/WACP can out-perform a top line pro wide angle lens behind a dome.  The net effect for a lens behind a dome port is that the diffraction limit is noticed at smaller apertures than it is on land, due to the resolution loss of the dome/lens optical system swamping the small loss due to diffraction when it first starts to impact.

If you look at the lens test results such as what DXO mark publish you will also notice resolution of wide angle lenses is not that great.  For example the Nikon 16-35 f4 lens is rated at 19 MP that is their data indicates it resolves no better than 19 MP so resolution will start to be a problem at smaller apertures than you might expect.

Thanks. My setup is:

Nikon Z72 (FF 45 MP) no AA filter

lens: fisheye 8-15 (I use only the 15 mm

So - what is my "risk zone"?

The f stop requirement really is for rectilinear,  you are stopping down to bring the edges of the curved virtual image into focus.

Fisheye is more forgiving, the optics are fundamentally different and I would guess you probably won't need to stop down past f13 and likely get away with less.   f11-13 Try some reef scenes at varying f stops and see how like the results.  Theoretically you start to go into diffraction at f9 or so, but f11-13 should only give slight softening and that could well be overwhelmed by the lens/dome/shooting through water system aberrations and really only noticeable when peeping at 100% or printing very big.

Thanks !

With full frame (Z7II) and fisheye, I would try and keep apertures at f/8 or above however, this is somewhat dependent on dome size too. The barrel distortion of the fisheye does definitely hide some of treh corner issues, but they are still there...

As a rule of thumb, I will use ISO and/or shutter speed to try and maintain as closed an aperture as possible. Ultimately, you are always better off getting the shot but the take away is to always try and prioritize aperture over our other "exposure" settings when using a dome. 

Likewise, the bigger the dome, the better the corners! Of course, practically it is not always practical or desirable to be shooting with a 9" dome, but port choice should take into account the fact that optical compromises are made when using smaller ones! Of course, this is also dependent on sensor size...