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DSLR: Exposure time vs ISO

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#1 AlexL

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Posted 26 January 2022 - 03:04 AM

Last night I took 45 minutes integration time of Orion nebula (M42).

Camera: Canon EOS 700D

Lens: Canon EF-S 18-135mm f/3.5-5.6 IS STM

ISO: 800

Tracking: SW star adventurer 2i

Lights: 40x90 seconds (but only used 30)

Darks, flats & bias: 20 each

Sky brightness: 18.8 mag./arc sec2 (using iOS Dark Sky Meter app)

 

Since during the weekend I used 90 seconds on M31, and the histogram was at about 30%, I used the same exposure time for M42.

Now, the histogram after stacking with DSS looks to be at about 50%.

 

My question is: to bring the histogram back to the left should I decrease exposure time or to decrease the ISO?

If one decreases the ISO from 800 to 400, would expect also an increase of dynamic range...

 

 

Thanks

Attached Thumbnails

  • gimp_m42.jpg


#2 17.5Dob

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Posted 26 January 2022 - 03:16 AM

Forget everything you know about daylight photography. Every camera has an ideal ISO to use which minimizes you cameras read noise and maximizes your dynamic range. In most Canon cameras, that's ISO 800 to 3200. I'd say your 700D needs to be shot near ISO 1600. Leave your ISO there and adjust your histogram by changing your shutter speed. Shooting lower ISO will actually increase your noise.

This excellent article discusses ISO in astrophotography and uses your camera as a test example.....https://petapixel.co...6400 and higher.

Edited by 17.5Dob, 26 January 2022 - 03:39 AM.

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#3 PhilHoyle

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Posted 26 January 2022 - 12:52 PM

You should learn processing with some type of AP software. Having a histogram at 50 is not a bad thing. You just need to learn how to stretch.

Phil

#4 unimatrix0

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Posted 26 January 2022 - 02:31 PM

That image is still workable. I rescued worse /more over exposed stacks of mine. 

 

First try to color balance it by using levels and histogram to have all R/G/B covering each other (individually adjust on the levels per channel) , and then use the levels again to bring up the black level to the bottom of the peak. Later on you still have to do more adjustments, but should work 

 

With ISO, this really depends on the Camera. There is no written in stone rules, but according to measurements, Canons work the best from ISO 800 and up. So leave it at ISO800 all the time and just adjust the exposure time . 

 I shot this using 10 second exposures at ISO400  with my Nikon.  It is said to be an ISO independent camera , but below ISO400, the way the camera saves the raw files , it introduces rings in the image. So I stick with ISO 400 all the time. 

And Orion is the brightest I ever have to shoot, so 10 seconds is the shortest I will ever use anyway, and it still works. 

 


Edited by unimatrix0, 26 January 2022 - 02:35 PM.

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#5 AlexL

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Posted 27 January 2022 - 01:56 AM

Many thanks for your comments :)
I will try again this weekend with a shorter shutter speed.

I will keep you informed of the outcome.



#6 Spaceman 56

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Posted 04 February 2022 - 03:55 AM

 

With ISO, this really depends on the Camera. There is no written in stone rules, but according to measurements, Canons work the best from ISO 800 and up. So leave it at ISO800 all the time and just adjust the exposure time . 

 I shot this using 10 second exposures at ISO400  with my Nikon.  It is said to be an ISO independent camera , but below ISO400, the way the camera saves the raw files , it introduces rings in the image. So I stick with ISO 400 all the time. 

 

I have a Nikon D5600. what ISO would you suggest please?

 

thanks Spaceman 56



#7 17.5Dob

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Posted 04 February 2022 - 04:38 AM

I have a Nikon D5600. what ISO would you suggest please?

thanks Spaceman 56

If you're shooting high dynamic range targets, M42, Andromeda, Pleiades,etc. or trying to preserve as much star color as possible, use ISO 200. If you are shooting fainter fuzzies in an empty field, or shooting narrowband filters, then ISO 400.

Edited by 17.5Dob, 04 February 2022 - 04:40 AM.

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#8 Spaceman 56

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Posted 04 February 2022 - 04:55 AM

thanks Dob. so in simple terms the brighter the target the lower the ISO, and the duller the target the higher the ISO?



#9 17.5Dob

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Posted 04 February 2022 - 05:00 AM

thanks Dob. so in simple terms the brighter the target the lower the ISO, and the duller the target the higher the ISO?


Not particularly...you could just shoot everything at ISO 200, as long as your mount is capable of the longer exposures needed. ISO 400 has a fair bit lower read noise while still retaining much of the dynamic range which helps when shooting through OSC narrowband filters to keep the exposures under 10 minutes.

#10 Spaceman 56

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Posted 04 February 2022 - 04:31 PM

you could just shoot everything at ISO 200, as long as your mount is capable of the longer exposures needed. ISO 400 has a fair bit lower read noise while still retaining much of the dynamic range which helps when shooting through OSC narrowband filters to keep the exposures under 10 minutes.

I have just bought a Skywatcher AZ/EQ5 Pro Mount, and I will shoot with a Nikon D5600. and One Shot Color.

 

up to now I have been doing manual tracking (by Hand) with subs of 25 seconds and stacking in Siril.

 

I did some shots on a Pentax at ISO 200 and others on the Nikon at ISO 800.

 

when the clouds go away I will try with shorter subs and maybe drop the Nikon to 200 or 400 ISO and see whats best.

 

I am just trying to get a ballpark starting point and move forward from there. I think the tracking mount will add some extra dimension.

 

thanks for the assistance. spaceman 56




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