D3X vs 5D2 - myth and reality

Started Jan 26, 2009 | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 61,123
Re: Measurements

GaborSch wrote:

bobn2 wrote:

The difference has been observed and demostrated by Emil and others,
so at the moment it would seem that your measurement is the outlier

As I am measuring this instead of speculating about it, I have no
problem whatsoever.

I think Emil and the others were measuring rather than speculating , also. When one measurement disagrees with others, it's not speculation to say its an outlier. It's possible it's right and the others are wrong, but unless you can show the error in their methods, most of us are likely to take the majority view.

Read noise, sure. As for DR (more on that later) this depends on full
scale count (not necessarily the same as saturation) which can be
different from channel to channel. What this means is that a camera's
DR does indeed depend on the actual illuminant - hence Iliah Borg's
magenta filter trick

Sorry, you are mixing up two different issues.

1. Calculating the actual intensity, relative to the saturation
point: you need to know, when saturation occurs. I do know that
(though sometimes some camera copies contradict that). Anyway, don't
mix up the binary numerical range with teh saturation levels.

Good to know that.

2. The composition of the illuminant. I have a magenta filter, which
I use (extremely seldom) to increase the dynamic range of the
camera , NOT of the sensor .

Not exactly clear, actually. The top end of the DR of the sensor will depend on which of the colour channels tops out first. This in turn will depend on the illumination coming through the CFA, which is usually thought of as part of the sensor . Thus different sensors with different CFA characteristics will display their best DR with different illuminants. A basic problem with your method and all the others is that in the end you are using the sensor to measure itself. Kind of unavoidable without a lot of expensive lab kit (although I seem to remember John Sheehy came up with a proposal for a simple piece of kit which would do it), but you always need to be aware of the limitations caused by that.

Let's put it another way, how do you discover the 'saturation points'?

I am looking for raw files with saturated areas.

Good. Now I find this hard to square with your statement "It is not necessary to have any high exposure when measuring the DR; only the dark end is interesting."

I have not calculated the DR at all.

I find this difficult to square with your statement 'The result
proves, that the claim "two stops higher DR than the 5D2" is
ridiculous , like some other claims. In fact, the DR of the D3X is
max. 0.5 EV greater than that of the 5D2 at ISO 100, and at ISO 400
the 5D2 is already better.'

Back to my statement: the DR depends on the ISO, on the subject and
on the actual illumination, AND on your acceptance of noise.

I am not saying how high the DR of this or that camera is. I am
saying, that this or that camera creates a certain level of noise at
a certail level of illumination (measured on the raw channel)
. The
difference between the D3X and the 5D2 is FAR from the two stops,
even from one stop.

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.

That's an SNR figure, not a DR. I've never been able to convince
myself that DxO's 18% grey noise value is a very useful metric

Not have I. I don't give a fig for how DxO measures the noise.

Good, we agree on that, then.

True, but it's always proportional to the ratio between full-scale
and read noise. All that happens is you might set your 'acceptable'
lower limit some amount above the noise floor

???

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.

On many cameras 7EV from saturation is 6EV or more above the read
noise floor.
Shot noise will be about 3 stops greater than read noise at that level.
(Edit: I should have said 5 stops above read noise floor and shot
noise 2.5 stops above read noise)

Consequence?

Consequence is that differences in read noise will be masked by shot noise. As a numerical value, assume we have two cameras, one with a read noise of 6e-, the other with a read noise of 24 e-, if the FWC is the same, the former will have 2 stops better DR. On your measurement, you're measuring with a photon shot noise of say 136 e-. For the first camera, the observed noise will be sqrt(136^2 + 6^2) = 136, for the second the observed noise will be sqrt(136^2 + 24^2) = 138.

The effect is slight, but at the same exposure the shot noise in 900
pixels of 5DII will be lower than 900 pixels of D3x

Sorry, this is far out of the range we are dealing with (the sample
of 900 pixels is at -8.28 EV, barely over black clipping).

Yes, I said it was small.

Anyway, create more suitable raw images and we can talk about it.
Until then it is not worth of discussion.

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

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Bob

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