NikonGear'23

Gear Talk => Camera Talk => Topic started by: Øivind Tøien on February 27, 2020, 04:27:53

Title: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien on February 27, 2020, 04:27:53
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
(https://otoien.zenfolio.com/img/s/v-10/p3839256966.jpg)

1/4000 second
(https://otoien.zenfolio.com/img/s/v-10/p3839256968.jpg)

1/2000 second
(https://otoien.zenfolio.com/img/s/v-10/p3839256968.jpg)

1/500 second
(https://otoien.zenfolio.com/img/s/v-10/p3839256964.jpg)
(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:

(https://otoien.zenfolio.com/img/s/v-10/p3839310392.jpg)
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Hugh_3170 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.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien 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.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien 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:

(https://otoien.zenfolio.com/img/s/v-10/p3839455364.jpg)
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.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Bent Hjarbo 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
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Hugh_3170 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.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien 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:
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Frank Fremerey 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:

(https://otoien.zenfolio.com/img/s/v-10/p3839455364.jpg)
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
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Bent Hjarbo 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?
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Bent Hjarbo 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 ;)
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien 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/ (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 (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  ;)
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Bent Hjarbo 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.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien 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.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Seapy 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.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien 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.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Peter Connan on March 01, 2020, 17:59:20
I only saw this now, and it is already dark here. I will try and do a similar test over the weekend, maybe even before depending on how the week (and the weather) goes.

Will it be a problem if I use an ND filter instead of a T-shirt? Will be using the 24=120 at 120mm, is that OK? Alternatives are a 180mm Sigma macro or 500mm f4 Nikkor.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien on March 01, 2020, 22:57:46
Thanks for responding Peter.

The ND filter cannot replace the white T-shirt as the T-shirt is used as a diffusor and not particularly to attenuate light. Do not worry too much about getting the same colors as in my captures, it is not the point as long as no channels are blown or clipped.

I suspect that the Sigma 180mm would be the better choice, less chance of vignetting than a zoom when stopped down and still blurring out the T-shirt, but either choice would likely work.

If you have CNX-D try to exactly reproduce the editing steps above below so that we can compare results quantitatively. If other editing software is used it could be interesting to compare the raw files for that purpose, but we would have to figure out a way to transmit the files, as they would be too large to email (only 1/8000 sec would be needed for that).

Bent, the latter applies to your file too, if you still have the 1/8000 sec raw file.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Bent Hjarbo on March 02, 2020, 09:07:20
Øivind I still have files, weetransfer could be used to transfer the file.
PM me with an e-mail address.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien on March 02, 2020, 09:12:30
Thanks, PM sent.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien on March 02, 2020, 10:35:07
Thanks Bent, I received the 1/8000 sec files. I was going to wait, but I post the thumbnails now so that Peter can better align any processing of the result he gets to these adjustments. To better compare, I converted both files to to gray scale using the monochrome picture control with contrast +3, and pulled contrast slider all the way to the right, and then adjusted exposure so that right edge of the histogram is in the middle. So not quite as extreme contrast adjustment as I showed for the thumbnails earlier. [Edit: For better sensitivity. also increased picture control contrast to +3]

I think it looks very similar among the two D500 bodies, with slightly different patterns, but the pattern persists between exposures within each body (thanks for supplying multiple 1/8000 sec exposure, only showing one here):
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Peter Connan on March 02, 2020, 17:24:44
OK, will try with the 180 and a T-shirt.

I only have Lightroom, so will probably just WeTransfer the raw to you.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Roland Vink on March 02, 2020, 20:51:52
Forgive me if I got the wrong end of the stick, but isn't it common for very high shutter speed to result in slightly uneven exposure? Think about it - at very high shutter speeds the gap between the first curtain (open) and second curtain (close) becomes a narrow slit which passes across the sensor. To get a perfectly even exposure the first and second curtain must move at exactly the same speed and maintain an absolutely even gap all the way across. When the slit is very narrow, any slight variation in the width of the gap will result in uneven exposure - if the gap is wider at the start of the exposure and gets narrower, or is wider on one side of the image, then the exposure will be uneven. This would be true whether the first curtain is electronic or mechanical.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien on March 02, 2020, 21:50:57
Roland, your physical explanation is of course correct. It must be a very narrow slit at 1/8000 sec. Whether it is common that this is causing uneven exposure  in other camera models is an open question. My limited experience indicated that D7100 was still able to expose evenly at 1/8000 sec shutter speed. I wonder if mechanical shock from the very fast mechanisms in the D500 can be causing some wiggling of the curtains. The response in my D500 looks almost like a sine wave. So it is not only a change in speed, but also angle of the curtain. If it related to speed of the mirror/shutter mechanism, we would perhaps expect to see it in a D5 too, although tolerances might be tighter in those high cost bodies.

My first SLR was a Minolta SRT-101. Both my own and my brothers copy showed very uneven exposure at 1/500 sec to 1/1000 sec, which was the shortest shutter speed. It was unusable for high light intensity snow scenes during our skiing in the high mountains during winter break and Easter holidays, also because the shutter traveled horizontally so it became visible as vertical lines. We called it the rubber band shutter. We sent in for warranty repairs several times. In the end I gave it up and spent all my savings to exchange it for a Nikon F2. I wanted something reliable that could stand up to the hard use of our mountaineering ventures. I have stayed with Nikon ever since.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien on March 02, 2020, 22:04:13
OK, will try with the 180 and a T-shirt.

I only have Lightroom, so will probably just WeTransfer the raw to you.

Sounds good, getting the raw file will be the best alternative. I sent a PM to you with my email address.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien on March 03, 2020, 00:53:32
To answer the open question, I tested all of my available bodies. As for the D500 comparisons I used my 105mm @ f/8 though two layer of Tshirt, Monochome picture control with constrast +3, general contrast slider pulled all the way to 100, and normalized exposure so that right edge of the histogram is in the middle. That reveals that also the other bodies show some slight uneven exposure at the two shortest shutter speeds, although harder to detect as it is a left to right or top to bottom difference in exposure. So Roland's suggestion is confirmed, these short shutter speeds can be a challenge to accuracy due to the narrow slit. So astrophotographers beware of using those for exposing the flats! Otherwise it is too little to care about.

Top to bottom:
D500
D200 (I had to let sun shine directly a the T-shirt due to the limited ISO range)
D5100
D7100


Next, I also tested my AW1.  The fully electronic shutter exposes pretty evenly at 1/16000 sec compared to 1/500 sec  8).
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien on March 07, 2020, 11:00:47

Thanks Peter, I got the images and the note about dust bunny. The images basically confirm the observation in the other bodies, showing effects specific to the short shutter speeds (perhaps a little milder in Peter's copy). Together with the data from the other other bodies, I think it is safe to assume that high shutter speeds are not reliable for making flats in astrophotography or other instances where extremely small gradients across the frame matters. Here are the results of the three D500 bodies, Peter's, mine and Bent from left to right at 1/8000 sec, and for completeness below that the same four shutter speeds that is shown in the bodies above for Peter's D500:
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Peter Connan on March 07, 2020, 19:07:27
Thanks Øivind

The only thing that really concerns me (having already decided that the D500 is a huge step backwards from the D750 for the type of astro photos that I do) is the vertical smudge on the 1/4000th photo.
Title: Re: Call for test: Short shutter speed D500 - look for uneven exposure.
Post by: Øivind Tøien on March 07, 2020, 21:18:07
It looks like a loose hair straw - should be easy to blow away. The end of it might be in the next frame.