Author Topic: Call for test: Short shutter speed D500 - look for uneven exposure.  (Read 704 times)

Øivind Tøien

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Background:
I have been having problems with gradients in the background of my astro photography lately. Just this week I realized that what I thought was light pollution gradients was a pattern, repeating during imaging at different locations and of different areas of the sky. I narrowed that down to problems with the flats (that are used to compensate for vignetting). After much experimenting I have realized it was due to a combination of using ISO 1600 (vs ISO 800 with mode light pollution) and use of a light panel for making the flats that is quite intense and has to be used a maximum intensity to avoid flickering. This resulted in a shutter speed of 1/4000 sec for the flat. At that speed and 1/8000 sec the shutter exposes unevenly across the frame in a fixed pattern. (Any gradients across the panel was compensated by averaging at least flats rotating 45° between each; this can be done in camera in the D500 by 8 multiple exposures, but I also did the averaging of up to 40 frames in DSS).

The final test today was much simpler using single frames and I ask other D500 users here to repeat it to see if their sample of D500 responds the same way:

1. Set D500 to daylight white balance, Auto ISO and f/8 (allowing ISO to possibly go as high as 6400 or beyond depending on light conditions), use medium long to long lens; my final test was with the 105mm f/2.5 set at infinity, but it should not matter as long as it is not much wider.

2. Pull two layers of a white T-shirt tightly over the lens (can also use a milky Plexiglas plate), go outside,  out of the sun in the shade if clear, or in overcast weather.

3. With 0 exposure compensation expose frames from 1/8000 second down to 1/500 sec in 1 EV steps letting auto ISO adjust correct exposure, while holding camera overhead free of nearby objects, pointing towards the sky.

4. To process, in CNX2 CNX-D pull contrast slider all the way up (and if under a clear sky saturation all the way down to avoid over saturation of the blue channel). Similar processing in other software is of course also OK. Export files to a relatively small size JPG like 800 pixels wide that makes it easier to detect patterns.

On my D500 sample I find that it exposes unevenly across the frame at 1/8000 sec and 1/4000 sec, but only slightly at 1/2000 second:


1/8000 second


1/4000 second


1/2000 second


1/500 second

(The dust mote that dropped down from the lens at frame #2 easily went away with the sensor cleaning mechanism applied once - nice to see that it works!).

The effect would not very noticeable in normal photography as contrast is seldom boosted this much, and particularly not with BIF where it would most likely show up, but astro photography is very sensitive to this due to the need for light pollution subtraction during processing and application of the flats (which is a division).

My D7100 does not show this and one could worry that it could be an early sign of a compromised shutter in my D500. (The body is just out of warranty but I have evidence in the flats that it occurred long before that.)

Thus I am interested if this is just a characteristic to the very fast shutter/mirror mechanism in the D500 or if it is limited to my copy of the D500. In the former case I will just forget about it, and in any case find a weaker light panel for my flats that does not require such short shutter speed.


Added: here is night sky capture from mid January of the Flaming Star Nebula region demonstrating the same (inverse) pattern:

Øivind Tøien

Hugh_3170

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #1 on: February 27, 2020, 06:34:08 »
Any chance of repeating your tests with a different D500 body?

Maybe you have a friend with one or your employer might have one in another department or you could hire one?  Just my 0.02c worth.  Good luck.
Hugh Gunn

Øivind Tøien

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #2 on: February 27, 2020, 06:48:48 »
Any chance of repeating your tests with a different D500 body?

Maybe you have a friend with one or your employer might have one in another department or you could hire one?  Just my 0.02c worth.  Good luck.

Unfortunately I do not know anyone else around here with a D500. That is why I call for NG members with a D500 to perform this simple test.
Øivind Tøien

Øivind Tøien

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #3 on: February 27, 2020, 08:25:17 »
I notice the same pattern in my first master flat with the light panel spring last year (2019-03-30) based on 24 captures at 1/2500 second (light output of the panel was likely lower as the flats were done outside on site at -5°C). So it is unlikely a phenomenon that has developed over time, but has been there all from the start:


Master flat averaged by DSS, based on 24 captures, rotating light panel 45° between each capture at 1/2500 sec, ISO 1600, with 300mm f/4 PF at f/4.5 (front aperture + internal aperture). Contrast and levels globally adjusted to make the asymmetric pattern more visible.
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Bent Hjarbo

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #4 on: February 27, 2020, 10:43:42 »
Hi Øivind
Did my best, but I get grayish very light blue pictures.
Two layers of a white T-Shirt WB on Sunlight.
The processing is done in Lightroom, I have the Nikon software, but did not find the handles, never used is and it is very slow ;)
To get nearly the same color as you the saturation is actually turnes up!
The 1/8000 shot is ISO 12800
The two middle shot is adjusted +0.33 in PP to get the same light level.
Hope this helps.
Br Bent

Hugh_3170

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #5 on: February 27, 2020, 11:04:12 »
I have a D500, but it is not with me and I will be away from home until early April. 

Happy to see if my sample shows the same issues when I get back.

Unfortunately I do not know anyone else around here with a D500. That is why I call for NG members with a D500 to perform this simple test.
Hugh Gunn

Øivind Tøien

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #6 on: February 27, 2020, 12:36:46 »
Thanks so much Bent. It is very subtle, but I do think I see a slight tendency for the same pattern (same locations dark upper left and lover right darker) when I examine the 1/8000 exposure as in my capture, but it is hardly visible at 1/4000 second even with further contrast enhancement, so perhaps not as much as with my copy. Will be interesting if we could get one more response. It is anyway too little to worry about from an imaging perspective unless one do flats for astro photo at these shutter speeds, then almost anything matters as everything is pressed to their limits.

Hugh, how can you stand being away from your D500 for that long?  :o :'(  ( ;) )

Added: Here are thumbnails for a further extreme global edit of Bent's captures to releveal patterns:
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Frank Fremerey

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #7 on: February 27, 2020, 14:08:40 »
I notice the same pattern in my first master flat with the light panel spring last year (2019-03-30) based on 24 captures at 1/2500 second (light output of the panel was likely lower as the flats were done outside on site at -5°C). So it is unlikely a phenomenon that has developed over time, but has been there all from the start:


Master flat averaged by DSS, based on 24 captures, rotating light panel 45° between each capture at 1/2500 sec, ISO 1600, with 300mm f/4 PF at f/4.5 (front aperture + internal aperture). Contrast and levels globally adjusted to make the asymmetric pattern more visible.


I see something in your posts, Øivind Tøien, but I do see nothing in the other posts
You are out there. You and your camera. You can shoot or not shoot as you please. Discover the world, Your world. Show it to us. Or we might never see it.

Me: https://youpic.com/photographer/frankfremerey/

Bent Hjarbo

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #8 on: February 27, 2020, 14:20:42 »
Glad to be any help.
I can see that this is a problem that I never will run into  ;)
Can it be an issue with consistency of the imaging sensor?

Bent Hjarbo

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #9 on: February 27, 2020, 14:37:01 »
I did a test with my Z6 with silent shutter.
Maybe you can twist these images as well ;)

Øivind Tøien

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #10 on: February 28, 2020, 08:29:28 »
I see something in your posts, Øivind Tøien, but I do see nothing in the other posts

I am not sure, but Bent did not state anything about boosting contrast that I did in my captures and later performed rather aggressively in Bents captures for the thumbnails? The purpose is to reveal patterns rather than a natural response of the  shutter/sensor combination.

Bent, can you clarify that?

Glad to be any help.
I can see that this is a problem that I never will run into  ;)
Can it be an issue with consistency of the imaging sensor?
I do not think so since it only occurs at high shutter speed. If the pattern occurs more or less similarly at all shutter speeds then I would say it is quite open. Bill Claff has many examples of fixed pattern noise in light exposures on http://www.photonstophotos.net/. It does not look anything like what we see here at high shutter speeds in the D500: http://www.photonstophotos.net/Charts/Sensor_Heatmaps.htm

I did a test with my Z6 with silent shutter.
Maybe you can twist these images as well ;)


Hmm, these look more like a cloudy sky without T-shirt  ;)   ::)
But since you ask; they look rather artistic, and pretty similar at all shutter speeds  ;)
Øivind Tøien

Bent Hjarbo

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #11 on: February 28, 2020, 08:56:28 »
I did boost the contrast in LR, on all images, also from the Z6.
Regarding the Z6 images I had the T-shirt on, but it was more cloudy  ; I don’t know why they are so different.

Øivind Tøien

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #12 on: February 28, 2020, 10:24:06 »
I did boost the contrast in LR, on all images, also from the Z6.
Regarding the Z6 images I had the T-shirt on, but it was more cloudy  ; I don’t know why they are so different.

Thanks for confirming, Bent. I suspected there was already some contrast boost as I only applied half as much in CNX-D with your images to get the extreme results shown in the thumbnails as I applied to my own image shown above. A T-shirt is far from a perfect diffusor, although it is commonly used as a simple method to obtain flats in astro photography. But then it is practice to average a number of shots, rotating it between each exposure. I have a milky Plexiglas panel on the way, but it might take a while to get here.

For reference here are thumbnails of my own images. I had to go to another 100% contrast boost applied to the JPG's in CNX-D to get anything looking close to your thumbnails, so I think Lightroom initially applied more contrast to your images than what I had in mine.
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Seapy

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #13 on: February 28, 2020, 13:14:57 »
Øivind, if you dismantle an old flat screen monitor, there are usually sheets of diffusion plastic which I retain for suchlike tasks, singly or multiple layers for greater effect.
Robert C. P.
South Cumbria, UK

Øivind Tøien

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Re: Call for test: Short shutter speed D500 - look for uneven exposure.
« Reply #14 on: February 28, 2020, 20:34:03 »

Thanks for the tip, Robert. I will keep that in mind if I come across a suitable candidate.
Øivind Tøien