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Nikon D7500 ISO settings

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#1 Adhosler

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Posted 27 February 2020 - 09:51 AM

I’m curious to hear what you D7500 owners out there are using for your ISO settings? I have an explore scientific ED80 FCD 100 on an EQ6-R pro, but it’s unguided, so I’ll only be able to get 60-90 second exposures. I’ve only shot one target which was Orion at iso 800 for 60 seconds, the core seemed a bit blown out. My next target is Andromeda and I’m reading ISO 400 is ideal for the 7500’s sensor. I’m curious to know if there is one ISO setting that is used for most DSO’s (a one-stop-shop setting if you will) or does it vary depending on the target (obviously M42 is probably the exception being so bright)?

#2 bobzeq25

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Posted 27 February 2020 - 10:45 AM

M42 and Andromeda are both exceptional, not because they're bright (although they are), but because they have a very large range of brightness.

 

Generally you want to use the lowest ISO that doesn't have too much "read noise".  The lower the ISO the greater the dynamic range.  Bracken's excellent Primer explains in more detail.

 

https://www.amazon.c...d/dp/0999470906

 

Generally the ubiquitous early Canons use 800 or 1600.   Nikons 200 if the tracking is good enough.  Nikon does something strange at 100.  More here.

 

http://dslr-astropho...trophotography/


Edited by bobzeq25, 27 February 2020 - 10:54 AM.


#3 Kevin_A

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Posted 27 February 2020 - 01:46 PM

Unity gain for the D7500 is iso 292 which is slightly higher than the D5300 and furthermore the noise reduction algorithm on the D7500 kicks at iso 400 and drops dramatically... so with such a short exposure i personally would use iso 400 as the lowest noise setting for 60-90 second exposures. If you were doing 180 or 240 second exposures you could try iso 200 and see if that extra exposure time tames the higher noise for the tiny increase in dynamic range you get from the lower iso.  I think iso 400 will be best in your case as it is above unity gain and if you need to get the histogram higher by all means iso 800 still looks good for your given exposure times.


Edited by Kevin_A, 27 February 2020 - 01:51 PM.


#4 PhilipPeake

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Posted 27 February 2020 - 01:51 PM

Where do you get these numbers for the D7500?
It's practically "isoless".

3200 works fine for me.
If you are using RAW files, the histogram is pretty much meaningless.

#5 Michael Covington

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Posted 27 February 2020 - 02:24 PM

I generally go by PhotonsToPhotos, as in the chart below.  You want to set the ISO as low as you can as long as it is still giving you higher dynamic range.  Nikons with Sony sensors work well at low ISO settings (200 or in the case of the D7500, even 100).  Nothing is lost by using a low ISO.  You can brighten the picture up by processing without having overexposed the bright areas.

Earlier DSLRs performed best around ISO 800.

I don't see that unity gain particularly matters; what you want is dynamic range, which comes from good signal-to-noise ratio.

 http://photonstophot...htm#Nikon D7500

 

This chart, on the other hand, suggests that the D7500 switches modes somehow at 400, and ISO 400 might be best:

http://photonstophot...#Nikon D7500_14

 

So does this chart.  

http://photonstophot...,Nikon D7500_14

If I had a D7500, I'd use it at 400 unless some experiment showed that some other setting was better.

 

Should I get a D7500?  It looks like it's a step up even from the D5300, which is already a very good camera.



#6 Michael Covington

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Posted 27 February 2020 - 02:25 PM

Where do you get these numbers for the D7500?
It's practically "isoless".

3200 works fine for me.
If you are using RAW files, the histogram is pretty much meaningless.

If it's ISOless, that means the read noise is very low regardless of ISO setting.  So your dynamic range depends on the ISO setting itself, the lower the better.  


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#7 bobzeq25

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Posted 27 February 2020 - 02:55 PM

Where do you get these numbers for the D7500?
It's practically "isoless".

3200 works fine for me.
If you are using RAW files, the histogram is pretty much meaningless.

You're right, except...  (big except)

 

3200 will drastically reduce your dynamic range, and hurt your images.  It's just not the right thing to do.

 

Astrophotography targets have much greater dynamic range than terrestrial subjects.  What works for terrestrial often does not work for astrophotography.

 

The histogram is NOT meaningless.   Space to the right of the skyfog peak holds all the good data.    Space to the left is useless, wasted.  The more you shift the peak to the right, the less room is available for your data, the more space is wasted.

 

This ain't terrestrial, things of little concern for terrestrial can have major implications for astrophotography.


Edited by bobzeq25, 27 February 2020 - 03:01 PM.

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#8 PhilipPeake

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Posted 27 February 2020 - 03:00 PM

If it's ISOless, that means the read noise is very low regardless of ISO setting.  So your dynamic range depends on the ISO setting itself, the lower the better.  

This is one of those things that I think is something of a myth, so long as you are using RAW files, and manual exposure.

 

The sensor has a fixed sensitivity. You can't adjust it in any way.

So expose a subject for 10 seconds at a fixed aperture and the same image is captured by the sensor irrespective of the ISO setting.

 

Where ISO starts to play a part in exposure is where the camera calculates the exposure time for a given aperture.

You are bypassing that.

 

The second place it comes into play is in determining how much amplification of the captured data is required to stretch the image before conversion to JPEG. That number also gets fed into the histogram display.

 

Older sensors required amplification of low level signal before stretching/JPEG conversion.

As sensors have become better, the point at which that is required has been pushed off further and further.

 

Try it.

 

Take an exposure @ 100, 400, 800, 1600 and 3200

 

Remember, each pixel is receiving EXACTLY the same data, so any difference in the images will only be due to differences in any amplification applied in copying that data to the RAW file. Since it is THE SAME data, there should be no difference. So one will not blow out data any more than any other.

 

Prove this to yourself by using the exact same stretch settings to examine each of the images.

I never saw any difference at all.

 

If you work with JPEG images, all bets are off.


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#9 PhilipPeake

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Posted 27 February 2020 - 03:02 PM

You're right, except...  (big except)

 

3200 will drastically reduce your dynamic range, and hurt your images.  It's just not the right thing to do.

 

Astrophotography targets have much greater dynamic range than terrestrial subjects.  What works for terrestrial often does not work for astrophotography.

 

The histogram is NOT meaningless.   Space to the right of the skyfog peak holds all the good data.    Space to the left is useless, wasted.  The more you shift the peak to the right, the less room is available for your data, the more space is wasted.

 

This ain't terrestrial, things of little concern for terrestrial can have major implications for astrophotography.

 

But ... as I said, same exposure, at different ISO loads the SAME data into the sensor, but will display differently on the histogram.

Changing the ISO setting does not change the sensitivity of the sensor.


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#10 bobzeq25

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Posted 27 February 2020 - 03:03 PM

This is one of those things that I think is something of a myth, so long as you are using RAW files, and manual exposure.

 

The sensor has a fixed sensitivity. You can't adjust it in any way.

So expose a subject for 10 seconds at a fixed aperture and the same image is captured by the sensor irrespective of the ISO setting.

 

Where ISO starts to play a part in exposure is where the camera calculates the exposure time for a given aperture.

You are bypassing that.

 

The second place it comes into play is in determining how much amplification of the captured data is required to stretch the image before conversion to JPEG. That number also gets fed into the histogram display.

 

Older sensors required amplification of low level signal before stretching/JPEG conversion.

As sensors have become better, the point at which that is required has been pushed off further and further.

 

Try it.

 

Take an exposure @ 100, 400, 800, 1600 and 3200

 

Remember, each pixel is receiving EXACTLY the same data, so any difference in the images will only be due to differences in any amplification applied in copying that data to the RAW file. Since it is THE SAME data, there should be no difference. So one will not blow out data any more than any other.

 

Prove this to yourself by using the exact same stretch settings to examine each of the images.

I never saw any difference at all.

 

If you work with JPEG images, all bets are off.

Answered above.  Your ability to resolve areas of different brightness levels is damaged, with old sensors or new, there is little difference.  8 bit jpgs are worse, but 14 bit RAWs still have a problem. 

 

It may be the same data, but you're squeezing it into a smaller space, with fewer levels.  Your full well capacity is drastically reduced, while the read noise floor stays about the same.  That read noise dependency is what defines "ISOless".  

 

Both bright areas and dark areas will be clipped more at higher ISO.  See this book for considerable detail, illustrative figures.  This is well understood by experienced astrophotographers, there is no controversy here.  Terrestrial thinking does not apply.

 

https://www.amazon.c...d/dp/0999470906

 

Part of the unintuitive nature of AP.  Real answers only come from detailed mathematical analyses, your instincts will mislead you badly.

 

3200 gets you _nothing_ you can't get _better_ at 200.  You just need to track better for longer subs, and stretch the data more.  You can do that, because you have far more levels to populate.  16 times more levels.

 

That's how it's done.

 

This is why you had to use two different exposures on M42.  That's always a better idea for that target, at that ISO, it's mandatory.


Edited by bobzeq25, 27 February 2020 - 03:29 PM.

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#11 sharkmelley

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Posted 27 February 2020 - 06:01 PM

 

Take an exposure @ 100, 400, 800, 1600 and 3200

 

Remember, each pixel is receiving EXACTLY the same data, so any difference in the images will only be due to differences in any amplification applied in copying that data to the RAW file. Since it is THE SAME data, there should be no difference. So one will not blow out data any more than any other.

 

Prove this to yourself by using the exact same stretch settings to examine each of the images.

I never saw any difference at all.

 

If you work with JPEG images, all bets are off.

Maybe I've misunderstood what you are trying to say.

 

For the same exposure, it's true that the sensor itself receives the same number of photons whatever the ISO.  But the information written to the raw file is different in each case.  Each time you double the ISO, the values written to the raw file are doubled, more or less.  So it's definitely true that the highlights in an ISO 3200 exposure will be blown out more than at ISO 200.

 

Try it - you'll soon see it's true.  So if the read noise is the same at each ISO, the lower ISOs have much greater dynamic range.

 

Mark


Edited by sharkmelley, 27 February 2020 - 06:09 PM.

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#12 17.5Dob

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Posted 27 February 2020 - 08:09 PM

This is one of those things that I think is something of a myth, so long as you are using RAW files, and manual exposure.

 

The sensor has a fixed sensitivity. You can't adjust it in any way.

 

So expose a subject for 10 seconds at a fixed aperture and the same image is captured by the sensor irrespective of the ISO setting.

 

Absolutely false....

The gain is applied before the file is written. You can't recover later in RAW.


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#13 17.5Dob

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Posted 27 February 2020 - 08:21 PM

I see a HUGE difference in star "burnout", just between ISO 200 and ISO 400 with my D5300....I can't even imagine shooting at ISO 3200....

OP, try ISO 400 to start.


Edited by 17.5Dob, 27 February 2020 - 08:22 PM.


#14 Michael Covington

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Posted 27 February 2020 - 11:29 PM

Here is how we know the ISO setting is not simply a multiplication performed by a computer when decoding the raw file:

If it were, then the noise in the image would always be proportional to the ISO setting.  After all, under that theory, it is the same raw image, and then all pixel values are multiplied by a different quantity when the image is decoded.

 

But in fact that is only (approximately) true for the latest "ISOless" sensors.  Earlier sensors have a quite nonlinear effect where there is more noise at low ISO settings than that theory would predict.  That is why we use our classic Canons at ISO 800 rather than 200 despite the loss of dynamic range, but we use newer cameras at lower ISO settings.

 

Here is a comparison of older and newer DSLRs.  Neither one has a perfectly straight curve.  But the newer one is a lot straighter.

http://www.photonsto...,Nikon D5300_14

 

By the way, while you're there, look at the curve for the D7500.  It contains a surprise and reveals why I suggested ISO 400 for the D7500.

 

By the way, I also did the experiment.  I took flats with the same exposure at ISO 200, 400, and 800.  Opening them as "raw" (not deBayered) in both PixInsight and MaxIm DL, I got approximately factor-of-2 differences in average pixel value.  That, however, does not totally rule out the possibility that the software is reading the ISO setting and applying it while decoding the raw file as an image.  (I'm 99% sure it's not, but I can't prove it.)  What proves it is the curves in PhotonsToPhotos.  Camera performance changes nonlinearly with ISO setting, differently for different cameras.

And that is why the original question made sense!  To find out the best ISO setting for the D7500 we have to look at its performance.
 


Edited by Michael Covington, 27 February 2020 - 11:46 PM.

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#15 LauraMS

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Posted 28 February 2020 - 01:24 AM

I shoot most of my 7500 images at ISO1600 or ISO3200. For timelapse I have used ISO3200, e.g. here. I have tried only once with ISO400 but had very bad results.

 

Here is a Heart and Soul and h and chi Pers with 43 90sec eposures with an old 105mm lens at f/d=4. I used a STC Duo filter to suppress light pollution at my Bortle 5 site. Acquired raw (nef) files, stacked in DSS, some cosmetic correction with Lightroom:

2019-09-30 Heart and Soul 43x90sec 105-40 ISO3200 STCDuo crop3 Kopie.jpg

 

I would like to see others using successfully ISO400 with exactly this camera. As mentioned, I had bad experience. I have to admit though that I only recently found out that there are two raw setting, one with compression of NEF files. I cannot exclude that this has influenced by results.

 

Laura



#16 LauraMS

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Posted 28 February 2020 - 02:27 AM

and here 90 20sec  ISO1600 stacked image of the eta Carinae region under hazy conditions and low elevation:

 

2019-02-24_eta_Carinae_Region_30min_90x20sec_ISO1600_105mm_Dark_oldFlat_PI_integration1_ABE_crop3.jpg



#17 sharkmelley

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Posted 28 February 2020 - 04:44 AM

By the way, I also did the experiment.  I took flats with the same exposure at ISO 200, 400, and 800.  Opening them as "raw" (not deBayered) in both PixInsight and MaxIm DL, I got approximately factor-of-2 differences in average pixel value.  That, however, does not totally rule out the possibility that the software is reading the ISO setting and applying it while decoding the raw file as an image.  (I'm 99% sure it's not, but I can't prove it.)  

I can give you the other 1%.  Until recently PixInsight was using DCRaw for its decoding.  I have looked in the source code of DCRaw and it is definitely reading the values straight from the raw file.

 

Mark


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#18 Kevin_A

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Posted 28 February 2020 - 08:37 AM

What you want is a setting that gives you the most dynamic range with the least noise that gives you separation on the histogram. All this will be achieved based on your own personal criteria such as lens speed, sensor quality, light pollution and tracking quality.

Dynamic range is king to me (to a point) and all else is just a compromise to get the most possible based on tracking stability, lens speed, exposure time, sensor characteristics and light pollution. Sometimes i have to settle for less DR when using slower lenses, depending on which tracker i use ...as my portable rig just isnt great above 60 seconds.... so i bump up ISO to compensate.

In the future we will probably all have to reduce our image sub exposure times, raise the iso and increase lens speeds due to the thousands of new satellites they plan to launch that will ruin many of our very long exposures.

In the end all we want is better signal and best dynamic range... everything else is just variables used to achieve this based on our geographic location and equipment we use.

I am not sure their was a mention of your light pollution level as that makes a difference to our comments too.

Cheers.....

 

fyi.... based on your equipment,sub lengths and your comments regarding blowing out the core(but not knowing your sky quality), then ISO400. 

 

I moved from a Bortle 8 to a Bortle 4 sky recently and now all my settings have changed to move my histogram to 1/3rd (dark site is way lower than 1/3rd), so i now either increase my sub lengths or raise my ISO with my same lenses to get to the same level. Now it depends more on how many planes and satelites are out and about and how many subs i am willing to throw away. ISO and light pollution are not issues to me, and neither is tracking... but % of keepers are.

 

Basically you have to tailor your ISO and variables to your own situation and see what works best for you.... as there is no real 1 stop shopping for all or any of us for that matter... as some people even live near airports... so short subs are all they can ever do... then they up the ISO a bit to get the most signal, do more subs and get less throw-away subs for their short 30 second max. exposure times. Ya gotta do what ya gotta do....


Edited by Kevin_A, 28 February 2020 - 12:51 PM.

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#19 Michael Covington

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Posted 28 February 2020 - 09:24 AM

I can give you the other 1%.  Until recently PixInsight was using DCRaw for its decoding.  I have looked in the source code of DCRaw and it is definitely reading the values straight from the raw file.

 

Mark

Excellent!  There's no reason to suppose libraw is different, and people have certainly not reported a change in PixInsight's behavior with the switch to libraw.

 

So... case closed, I think.




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