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Paul Kay

Member Since 27 Nov 2003
Offline Last Active Oct 13 2018 11:15 AM

Posts I've Made

In Topic: The relationship between exposure and sharpness

26 September 2018 - 09:52 AM


Paul, underexposing does not in itself cause noise; only trying to correct that in postprocessing does (by amplifying shadow noise in underexposed areas). Maybe it's a semantic difference, but I feel it's an important one.

I am not sure what you mean about "shooting to the right incorrectly exposes an image". If you shoot to the right taking care to not blow out any highlights, you are maximally using (getting the highest possible signal to noise ratio from) your digital sensor. You can of course decrease the exposure in postprocessing a bit, without adding any noise.

In general, sharpness is not a function of exposure, within a wide range of reasonable parameters.




Since you have to correct 'underexposure' then its a cause of noise in an edited image. Its actual effect varies depending on how much an image has to be adjusted and which tones need the adjustment.


ETTR (Exposing to the right) introduces tonal discrepancies in my experience - its not a solution but is touted as 'data acquisition maximisation' (and supposedly produces a more malleable digital file). Its simply not that simple in my experience - I've tried innumerable methods of exposing and its best to exposure for the image you want rather than apparently maximising data capture.


'Sharpness' is a function of many variables, exposure is just one. It doesn't matter much, except when all the others are additive.


FWIW the camera I have found to produce surprisingly 'malleable' files and respond least badly to hauling up shadows has been the Leica M9 with its full frame CCD sensor. Not usable underwater and 18MPixel only but surprisingly effective. I find the Sony A7II can be good but then again it can be disappointing - I suspect this is down to tonality problems which I don't always find fixable in post processing. As always depends what you are doing - I have been printing to 30" x 20" for the last couple of years, which is a reasonable size, and discrepancies can start to show.

In Topic: The relationship between exposure and sharpness

10 August 2018 - 08:17 AM

Proper exposure affects other things but not sharpness. Shoot RAW, expose to the right, and all will be good.

Incorrect exposure increases noise and thus affects perceived 'sharpness'. Exposing to the right also, from my testing over many years, is, in effect, often incorrectly exposing the image (or effectively parts of it to be more accurate), and depending on specifics can have a similar effect to incorrect exposure in that noise levels can marginally increase in some areas. I am an advocate in shooting 'within latitude' and this 'latitude' needs to be assessed for each camera - some have greater 'latitude' in terms of exposure than others. All that said, choice of apertures, movement (even with flash/strobe illumination at times) and precise point of focus are usually more significant. 

In Topic: The relationship between exposure and sharpness

06 August 2018 - 03:05 AM

Can someone explain the physics to me?

If it was a simple, single reason then this wouldn't be a problem. 'Sharpness' is in itself a non-technical and rather wooly term, so firstly you would need to define what exactly you mean by it. Then there are numerous factors which affect our perceived degree of 'sharpness' in an image, some relatively easily defined (poor exposure leading to noise, use of too small an aperture resulting in diffraction limitation, etc.) but others are not (lighting, point of focus, etc.). So its not easy to explain the physics.

In Topic: Wetpixel's Nikon FX wide-angle lens review

07 May 2018 - 10:12 AM

The virtual image is only spherical for an infinite object distance. It is otherwise aspherical. The source for this fact is Chapter 8 in Optical Fundamentals of Underwater Photography by Gomer T. McNeil, 2nd Ed., 1972.

And for a subject at infinity the virtual images lies at a point which is 4 x the radius of the dome from the centre of the dome which is where the principal point of the lens should be positioned. This means that the centre of the mage is at 4R but because the virtual image is spherical, depending on the lens's angle of view, the edge of the image will be somewhere closer - exactly where will depend on its position and the dome's radius (radii) and thickness and refractive index - its complicated. But the camera lens is not much of a factor here because it can only image what it 'sees'. Its ability to do so will be dictated by the virtual image produced by the dome port and subject and any inconsistencies of the camera lens such as field curvature at closer focus. Its all a bit messy.


The 'less curved' dome idea is in effect the suggestion of positioning the camera lens closer towards the dome as opposed to ensuring it is aligned at the centre of the dome. You may want to try doing just this (should be easy enough). If I remember correctly, I think that you will find that it results in the trade off of reducing the field of view as opposed to doing what your diagram illustrates so is counter productive.


Sadly I suspect the 'filmdays' rule of thumb of 90 degrees being the maximum viably/easily correctable field of view still applies with dome ports though bigger does help .....


I still disagree about the unpredictability of lens performance underwater behind dome ports. Its lack of technical information which hampers prediction nothing else.

In Topic: Seacam S6 -> Nikonos bulkheads (WTB or any advice...)

03 May 2018 - 02:35 AM

Just one question (sorry, maybe its basic, but to confirm), if I connect Nikon D800 through S6 bulkhead with Seacam S6-S&S cable and Inon Z330 strobes, will I have TTL available? Or do I need to purchase separate TTL circutry eg turtle...?


Sorry Marco but I simply don't know. First you will need to check if you have any circuitry in your Seacam housing - it may be fitted already - it will be under the metal cover in the 'prism housing' part of the housing and is accessed by one screw. If so then the housing will support dual TTL with Seacam strobes, but I can't give you any information about compatibility with Inon Z330. You may need to remove the circuit board if its not compatible. I suggest that you post in the dSLR section here on wetpixel. If you don't get any answers then contact your Seacam distributor (http://www.seacam.co...roatia-slovenia) or Seacam direct. You will also need to check the Inon specifications - its possible that a fibre optic solution may actually be better and Seacam offer an Optical Slave Trigger at around 280 Euros too. Sorry not to be able to tell you but there are now so many combinations that I tend to deal with them on the basis of needing to when they arise.