Pixel density revisited

Started Oct 22, 2008 | Discussions thread
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bobn2
bobn2 Forum Pro • Posts: 49,851
Re: Pixel density revisited

DMillier wrote:

It's difficult isn't it.

For me, printed output is the goal, so the quality of the prints at
different sizes is important. DPReview sometimes comment on whether
differences are visible in prints but it tends to be vague like "only
visible in very large prints".

I would find an assessment of the test images printed to some
arbitrary size (eg A3, A2 etc) useful. But to evidence this would
require scanning sections of the print and there would inevitably be
complaints about the methods/equipment used.

There will be complaints about anything, but the issue really is comparability. What you want to now is not the precise output with your printer but to be able to make a useful comparison.

Likewise, if the noise patches were all scaled to a common size,
there would be arguments about the re-sampling method etc.

That's one of the problems with doing everything with proprietary software. It is asserted that using ACR makes everything 'fair', but I a strong suspicion ACR uses different algorithms for different cameras. Using something pen like dcraw means whatever it is it does, at least you know it. Likewise, using open source software to do the resampling means that you know the characteristics of the resampling.

I also suspect, that one of the effects of such tests would be to
show that the output from most modern cameras is very similar given
appropriate settings. And a review site whose message was
predominately "most cameras of similar spec give siilmilar results"
would get boring fast....

It would be nice to know if that is the case, rather than this constant denigration of the efforts of manufacturers who are trying to increase the amount of information that their cameras capture.

bobn2 wrote:

Phil Askey wrote:

I think a 30+ page review correctly qualifies any statement about
pixel density and noise. Where is this reverse evidence?

It could be any length, but if it didn't contain any statements
relating noise content to final output size, it wouldn't be
qualified, would it? I've read through it quite carefully, and I
can't find any pointer there that would help me understand the
relative noise between the cameras for usual output sizes. There are
a lot of statements about how noisy the A900 is, but all related to
per pixel noise.
As for the evidence, Emil has pointed you to a post of his, there's a
long series of posts which you didn't follow, but did discuss and
present the evidence in some detail, there's John Sheehy's
demonstration under the title 'the joy of pixel density' , and
finally there have been extensive discussions of the physics behind
it, which back up the position that in theory there is no causal link
between pixel density and final image noise content at any given
image size (with the caveat that there are noise effects such as
random telegraph noise, which come into play at very small
geometries). These discussions included a number of people who are
research physicists (not me, I hasten to add), and included Eric
Fossum.
I'm not really asking you to go back on your testing methodology
completely, it just seems to me that including a set of equal sized
crops from a given proportion of the frame (resized using a sensible
resampling method, there are plenty here who could advise) would be
much more helpful for people to assess the likely image quality from
any camera, and to make comparisons between cameras of different
pixel densities and sensor sizes. Do it in addition to the 100% crops
if you like (and please, in RAW), but it would improve the reviews
still further. Similarly, the crops from the 'bottle' scene are
difficult to make judgments from when they are presented at vastly
different output sizes.

bobn2 wrote:

These statements on noise and pixel density need to be qualified. The
message that people are getting from statements of this sort is that
increasing pixel density reduces image quality in an absolute way,
and there is no evidence to support that, rather the reverse, in fact.

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Bob

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Phil Askey
Editor, dpreview.com

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Bob

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Bob

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