Thanks.
Exposure Time and Blow-out
- hatflyer
- Mars Ambassador
- Articles: 0
- Posts: 131
- Joined: Sat Jul 30, 2022 2:27 pm
- 1
- Location: US
- Status:
Offline
Exposure Time and Blow-out
I have an 85mm f/ 1.4 lens on my Nikon D5500. I set it to ISO 100, f/ 4. But long exposure come out way too bright. Does that limit the time of exposure, or can I change a setting to be able to take long exposures?
Thanks.
Thanks.
- KathyNS
- Co-Administrator
- Articles: 0
- Posts: 2616
- Joined: Thu Apr 25, 2019 11:47 am
- 5
- Location: Nova Scotia
- Status:
Offline
-
TSS Photo of the Day
TSS Awards Badges
Re: Exposure Time and Blow-out
The only reason to shoot long exposures is to collect enough photons. If you are getting too many, cut back on the exposure times.
Terrible light pollution might cause over-exposed images. Misjudging the brightness of the Moon could also do it. (Hint: the Moon is a rock in bright sunlight. The correct exposure is a daytime setting no matter what the camera's meter says.)
Some targets are hard not to over-expose. M42, the Great Orion Nebula, is notorious for this. An exposure long enough to capture the dim outer fringes of the nebula will be too long for the very bright inner core. Conversely, the correct exposure for the core will under-expose the outer regions. Special high dynamic range (HDR) techniques are necessary to capture it all. (That is beyond the scope of Beginner Astrophotography.)
Terrible light pollution might cause over-exposed images. Misjudging the brightness of the Moon could also do it. (Hint: the Moon is a rock in bright sunlight. The correct exposure is a daytime setting no matter what the camera's meter says.)
Some targets are hard not to over-expose. M42, the Great Orion Nebula, is notorious for this. An exposure long enough to capture the dim outer fringes of the nebula will be too long for the very bright inner core. Conversely, the correct exposure for the core will under-expose the outer regions. Special high dynamic range (HDR) techniques are necessary to capture it all. (That is beyond the scope of Beginner Astrophotography.)
DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
- hatflyer
- Mars Ambassador
- Articles: 0
- Posts: 131
- Joined: Sat Jul 30, 2022 2:27 pm
- 1
- Location: US
- Status:
Offline
Re: Exposure Time and Blow-out
So does stacking 50 images add photons? Or does it only allow to reduce noise to signal?KathyNS wrote: ↑Wed Aug 31, 2022 7:03 pm The only reason to shoot long exposures is to collect enough photons. If you are getting too many, cut back on the exposure times.
Terrible light pollution might cause over-exposed images. Misjudging the brightness of the Moon could also do it. (Hint: the Moon is a rock in bright sunlight. The correct exposure is a daytime setting no matter what the camera's meter says.)
Some targets are hard not to over-expose. M42, the Great Orion Nebula, is notorious for this. An exposure long enough to capture the dim outer fringes of the nebula will be too long for the very bright inner core. Conversely, the correct exposure for the core will under-expose the outer regions. Special high dynamic range (HDR) techniques are necessary to capture it all. (That is beyond the scope of Beginner Astrophotography.)
Btw, my camera has an HDR feature. Is it as simple as using that?
- KathyNS
- Co-Administrator
- Articles: 0
- Posts: 2616
- Joined: Thu Apr 25, 2019 11:47 am
- 5
- Location: Nova Scotia
- Status:
Offline
-
TSS Photo of the Day
TSS Awards Badges
Re: Exposure Time and Blow-out
What matters in this case is the number of photons per frame. 50x 10-sec frames and 10x 50-sec frames will capture the same number of total photons. But the 10-sec frames are less likely to be over-exposed than the 50-sec frames. The two combinations will give you the same total exposure time (500s) and pretty much the same SNR. The difference is that, if the longer frames have saturated pixels, data has been lost.
So, if you are finding saturated pixels, you should reduce the exposure times of individual frames, and increase the number of frames to compensate.
I have no idea what you camera's HDR feature does. You should consult the manual for specifics. It might help, or it might not.
DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
- hatflyer
- Mars Ambassador
- Articles: 0
- Posts: 131
- Joined: Sat Jul 30, 2022 2:27 pm
- 1
- Location: US
- Status:
Offline
Re: Exposure Time and Blow-out
So what is the benefit of tracking if 50 frames of 10 sec is the same as 10 frames at 50 secs, if there are no star trails at 10 second exposures? You would probably have to move the camera a bit if no tracking tho?KathyNS wrote: ↑Wed Aug 31, 2022 9:41 pmWhat matters in this case is the number of photons per frame. 50x 10-sec frames and 10x 50-sec frames will capture the same number of total photons. But the 10-sec frames are less likely to be over-exposed than the 50-sec frames. The two combinations will give you the same total exposure time (500s) and pretty much the same SNR. The difference is that, if the longer frames have saturated pixels, data has been lost.
So, if you are finding saturated pixels, you should reduce the exposure times of individual frames, and increase the number of frames to compensate.
I have no idea what you camera's HDR feature does. You should consult the manual for specifics. It might help, or it might not.
- JayTee
- Universal Ambassador
- Articles: 2
- Posts: 5642
- Joined: Thu Apr 25, 2019 3:23 am
- 5
- Location: Idaho, USA
- Status:
Offline
-
TSS Awards Badges
TSS Photo of the Day
Re: Exposure Time and Blow-out
Before you start your imaging, it is advisable to take several different images of a single object using increasingly longer and longer exposure times or higher and higher ISO values. What you are going for is a histogram that looks similar to the ones described in this article.
https://www.astropix.com/html/astrophot ... raphy.html
Cheers
https://www.astropix.com/html/astrophot ... raphy.html
Cheers
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
- KathyNS
- Co-Administrator
- Articles: 0
- Posts: 2616
- Joined: Thu Apr 25, 2019 11:47 am
- 5
- Location: Nova Scotia
- Status:
Offline
-
TSS Photo of the Day
TSS Awards Badges
Re: Exposure Time and Blow-out
I wasn't suggesting 10 seconds as an actual exposure time. It was just an example to put some numbers on it. You will need to determine the proper exposure time for yourself. Start with what you have and adjust it.
Even if the exposure time were only 10 seconds, you wold see movement with any reasonable focal length.
DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
- Baurice
- Vendor
- Articles: 0
- Posts: 1337
- Joined: Sun Aug 18, 2019 10:42 pm
- 4
- Location: England
- Status:
Offline
-
TSS Photo of the Day
TSS Awards Badges
Re: Exposure Time and Blow-out
It would depend on what you are trying to photograph.
I would also say that an undriven exposure of about 5 seconds is the maximum at the celestial equator but you can use longer exposures as you get nearer the pole.
For the moon and planets, you need a much shorter exposure but also a longer focal length. I attach myDSLR to my telescope to get focal lengths of 1.5 metres and 4.5 metres. I do a lot of shots at 300mm, though.
I would also say that an undriven exposure of about 5 seconds is the maximum at the celestial equator but you can use longer exposures as you get nearer the pole.
For the moon and planets, you need a much shorter exposure but also a longer focal length. I attach my
Create an account or sign in to join the discussion
You need to be a member in order to post a reply
Create an account
Not a member? register to join our community
Members can start their own topics & subscribe to topics
It’s free and only takes a minute