sharkmelley

Having looked at this in more detail, it's certainly true that the D7500 is using less aggressive spatial filtering than the D5300.  However this does not explain the observed difference in thermal fixed pattern noise (FPN).   The D7500 really does appear to have a higher FPN than the D5300.

The practical consequence of this is that you are more likely to need dark frame calibration with the D7500 than with the D5300.

Mark

Thanks for reviving this thread.  I've been playing with a Nikon D5300 recently and noticed that the spatial filtering (a.k.a. hot pixel suppression or HPS)was causing all kinds of problems with star colours - mainly turning them green, just like the latest variant of the Sony star eater algorithm.

It's all being discussed over on Cloudy Nights:  https://www.cloudynights.com/topic/635441-aa-filter-spatial-filter-and-star-colours/

It's early days and it's not yet fully understood.  However, it appears that the Nikon D810A (the one built for astrophotography) doesn't have the same issue because the spatial filtering is much less aggressive.

So it's just possible that the D7500 is also using less aggressive spatial filtering than the D5300 and that's why it appears noisier.  I'll take a new look at Mike's D7500 darks and see what I can determine.  It'll definitely provide another interesting data point for my analysis.

Mark

15 hours ago, MikeODay said:

Hi Mark

please see below links to single dark frames:

5300, 240sec, ISO250 : https://www.dropbox.com/s/fu27b1tx5wbu00c/D5300_dark_240s_250iso__030.nef?dl=0

7500, 240sec, ISO400: https://www.dropbox.com/s/cymscuakzpcu9mp/D7500_240s_400iso__0030.nef?dl=0

Cheers

Mike

 

Thanks for those.  There's nothing obvious that strikes me about them immediately, by means of explanation.  But the D7500 certainly seems to have more hot/warm pixels.  I will take a closer look later.

Mark

There is something I find odd about the results from the D7500.  Nikon cameras use a spatial filtering algorithm in longer exposures in order to remove hot/warm pixels.  It's not a severe algorithm like the "star eater" spatial algorithm used on Sony cameras but it should still remove isolated hot/warm pixels.  So I'm surprised to see such a long right-hand-side histogram tail. 

I would certainly be interested in taking a look at a representative raw dark frame from both the D5300 and D7500.

Mark

From your histograms it certainly does look like the D7500 sensor has a greater number of outlying values.  On the other hand those histograms also support your earlier blog where you stated that the dark noise in the D7500 is lower.  Your bias subtracted darks are an anomaly because of the large number of clipped values.

I think the overall message is that the D7500 is an improvement over the D5300 as long as you use well matched darks.  But that tentative conclusion needs to be tested further.

Mark

A very interesting read!  There are very few people doing proper testing of thermal noise in DSLR cameras but it is the one thing that can make or break an image, especially on the warm nights that we rarely have in the UK.

A couple of comments on the histograms.  Both of them seem to be clipped at the low end i.e. the left hand tail of the histogram is missing.  Why is this - I thought Nikon had fixed the black clipping issue?   The multi-peak nature of D7500 histogram is very strange indeed.  It appears to indicate different populations of pixels behaving differently.  I don't recall ever seeing this before.

If you still have both cameras you could potentially perform a side by side test to measure how fast the noise rises from frame to frame.  This might provide a lot of useful information.  It's the approach I took on the testing in the graph below:

Successive 5 minute exposures were taken from a camera switched on after acclimatising to the 20C ambient temperature.  The noise in a difference frame (2 successive darks frames subtracted) was measured and converted to a dark current estimate for each point in the graph.

The reason I subtract the dark frames is so the graph represents the thermal noise levels expected from calibrated frames.

If you have a good set of darks taken under controlled conditions I'd love to see how they would compare on that graph.  Generally speaking it's the Canons that become very noisy.

[Later Edit]  I've realised that the black clipping in the histogram is because you've bias subtracted the master dark.  The shape of the D7500 histogram is very puzzling though.   Unfortunately both your histograms go off the top of the scale, so it's difficult to make a meaningful comparison.

Mark