D3X vs 5D2 - myth and reality

Started Jan 26, 2009 | Discussions thread
bobn2 Forum Pro • Posts: 61,163
Re: Still measurements

GaborSch wrote:

bobn2 wrote:
Honestly, I don't give a fig for their "measurements", nor for the
view of "most of you", for my measurements are good enough for me,
beside being consistent. As soon as I see a contradicting
measurement (which is not the same as a "statement"), I am ready to
revisit my ones.

Well, you might excuse us for not giving a fig for your measurements, then. Emil's measurements are posted on his site.
I am measuring raw channels .

Yes, but sensor DR is in th end to do with incident light not just the photoelectrons created in it. the CFA is part of the system, and needs to be taken into account.

A basic problem with measuring the DR on specific images under
specific illumination is, that that result is totally useless in
other setting or under other illumination.

A problem with any lab based measurement. What one seeks to do is make measurements which are based on commonly used situations or can be readily extrapolated to a range of them.

I am using the data gained from the sensor to measure the sensor's
chartacteristics. I don't understand your objection. Should I use
data from a specification sheet to support the specification sheet,
or what?

There is no triangulation against any calibrated instrument. Nonlinearities and messing around in the signal chain could be transparent to your measurements.

I don't understand this problem (but really). I measure the
saturation levels on any image created by the same model with the
same ISO. Although it does happen, that some copies have different
saturation levels, the deviation is always negligable from the
perspective of dynamic range. (The deviations are important when
determining if some patch really clipped ot not.)

The problem is simply that the saturation level is one end of the DR calculation. I've been trying to work out whether your 'measure noise at a fixed fraction of saturation' somehow gives you a measure of DR, I can't see that it does, but I could be missing something.

Thus it is enough for me to find some other shots to measure the
saturation levels; it is not relevant, that the actual images used in
these demo are clipping or not.

Besides, all the above becomes irrelevant, when you regard that my
demonstration used only the red channel, the saturation of which is
very firm.

I'll take your word for it.

But that is a very different statement and says little about the DR.
The bottom end of the DR is the read noise, and in your measurement
that is swamped by the shot noise, which is about 2.5 stops higher
than the read noise

1. The DR is determined by noise . Not by "this noise" or "that
noise" but simply be "noise".

We have a different understanding of DR then. Different levels of incident light have different noise in them. You should only be interested in sensor noise, you are measuring the noise in the light as well.

2. You would have to show me hard proof before I accept that the
shot noise is 2.5 times higher than the read noise in the range I
used for the demo

Well, you should also be giving hard proof that your method measures the DR. I gave you the calculations, that pretty much constitutes a proof, unless you want to dispute the measurements they are based on (oh, sorry I forgot, you don't give a fig for any measurements but yours). In fact, you could probably calculate the same figures from your measurements, I might do it if i get a little more time.

The bottom end of the DR ratio is set by what you think is the
minimum acceptable signal distinguishable from the read noise. If you
take it as at the read noise floor you get one figure. If you decide
you want it one stop above the read noise floor, your DR measurement
would be one stop less. So, if you had two cameras with one stop
difference measured to the noise floor, the difference would still be
one stop measured to one stop above the noise floor

I have not measured anything from the noise floor. I don't even have
to know where the noise floor is. (In fact, I don't know that.)

Than what you are measuring is not the DR. I think that's your basic misunderstanding. measure what you like, but don't call it DR, and don't make sweeping statements saying others are misrepresenting the DR based on your measurements, which are not the DR.

Consequence is that differences in read noise will be masked by shot

It is of no relevance whatsoever. Does the difference make one camera
better than the other? (Beside the max. 1/3 EV difference I noted at
the beginning?)

It is of clear relevance if you want to measure the DR. If you want to measure noise at some arbitrary fraction of FS, OK. In fact, the calculation I showed you is a pretty good indication of why in the majority of situations, most cameras produce pretty similar results. It's only in the deep so-called 'shadow noise' where the read noise floor becomes visible.

Sorry, Gabor, I just don't think your measurements support the
conclusions you make, they are quite consistent with the D3x have
somewhere around two stops more DR than the 5DII

I don't see this.


I think you are using a different definition of 'DR' to everyone else

I need to repeat it: I have not used the term DR in my demo . I used
the terms "noise" and "intensity".

You used the term 'DR' repeatedly. Just go back and read what you actually posted in your OP. If you want to use some other term, we have no dispute.

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