Call for test: Short shutter speed D500 - look for uneven exposure.

[Peter Connan]

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.

[Øivind Tøien]

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.

[Bent Hjarbo]

Øivind I still have files, weetransfer could be used to transfer the file.
PM me with an e-mail address.

[Øivind Tøien]

Thanks, PM sent.

[Øivind Tøien]

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):

[Peter Connan]

OK, will try with the 180 and a T-shirt.

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

[Roland Vink]

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.

[Øivind Tøien]

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.

[Øivind Tøien]

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.

[Øivind Tøien]

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  .

[Øivind Tøien]

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:

[Peter Connan]

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.

[Øivind Tøien]

It looks like a loose hair straw - should be easy to blow away. The end of it might be in the next frame.