Jump to content


Photo

What ISO to use with T3i

  • Please log in to reply
28 replies to this topic

#1 Rayh

Rayh

    Mariner 2

  • -----
  • Posts: 230
  • Joined: 22 Aug 2005
  • Loc: Gilbert, AZ, USA

Posted 04 April 2013 - 11:20 PM

I just purchased a Canon T3i to use for AP. I have doing AP for a number of years with a CCD camera. (QHY8) With that camera the conventional wisdom is to set the gain at 0 or 1 to keep from filling the charge wells so fast, thus maximizing dynamic range. What do I do with the ISO on a DSLR? Use a low number (like 100) to minimize noise and maximize dynamic range, or do I crank up the gain to 3200 or even 6400 to get as much brightness as I can?
Thanks,
Ray

#2 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4869
  • Joined: 11 Sep 2009
  • Loc: Gambier Island, BC, Canada

Posted 04 April 2013 - 11:28 PM

There have been measurements made to find the "Unit Gain" (the point at witch 1 unit in the RAW file = 1 electron recorded in the pixel). Shooting at unity-gain would be the way to maximise the dynamic range.

I believe that for most 14bit ADC cameras (such as the T3i) that point was at about ISO400.

(For many 12bit cameras such as my XT/350D and XS/1000D it is closer to ISO1600.)

#3 Toxic Coolaid

Toxic Coolaid

    Viking 1

  • -----
  • Posts: 582
  • Joined: 01 Oct 2011
  • Loc: NorthEast Tennessee

Posted 04 April 2013 - 11:29 PM

I've found it best to stick with 800 or 1600

#4 fmhill

fmhill

    Messenger

  • -----
  • Posts: 436
  • Joined: 17 Jul 2012
  • Loc: Cape Cod, Massachusetts, USA

Posted 04 April 2013 - 11:37 PM

Hi Ray,
I use a Canon 60Da and from what I understand after months of reading and studying about optimum ISO settings, cause and effect, the earlier Canon DSLR cameras produce lowest noise and greatest dynamic range at ISO-1000, the later models such as the 60Da produces lowest noise and greatest dynamic range at ISO-200... I'm not sure what the major reason for the difference, whether it is the newer sensor, the Digic 4 processor and later, benefiting from the lower ISO, or both together...

And there is a usable range of half to double the ideal ISO that is usable with relatively low noise and high dynamic range slightly less than ideal... With my EOS 60Da, I use 100 to 800 but try to stay on either 200 or 400 and adjust exposure time to compensate image brightness as needed... By keeping the ISO constant for a session makes adding dark frames that match ISO much easier at the end of a session, you are not having to make several sets which is time consuming when you want to get wrapped up and the gear put away...

#5 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4869
  • Joined: 11 Sep 2009
  • Loc: Gambier Island, BC, Canada

Posted 04 April 2013 - 11:38 PM

If you are interested in the detailes you can see data and analysis on this for *many* cameras at this web page here. Unfortunately the T3i is not in the list of tested cameras so some extrapolation is required but there are a few other relatively recent Canon cameras.

Just brace yourself for info-overload before clicking the link. :p

#6 Rayh

Rayh

    Mariner 2

  • -----
  • Posts: 230
  • Joined: 22 Aug 2005
  • Loc: Gilbert, AZ, USA

Posted 05 April 2013 - 12:22 AM

Thank you for the info everybody. Now I know that I want to start with the lower ISOs. I did not think of the 14 bit vs 12 bit but that does make a difference.
Ray

#7 SKYGZR

SKYGZR

    Viking 1

  • *****
  • Posts: 877
  • Joined: 13 Aug 2009
  • Loc: Planet Earth, Milky Way Galaxy, Speeding towards the Virgo Supercluster

Posted 05 April 2013 - 07:32 PM

Depending on the target, I'll bounce around..400, 800, 1600..never use higher..too noisy.

#8 LeCarl

LeCarl

    Vostok 1

  • -----
  • Posts: 183
  • Joined: 16 Mar 2011

Posted 05 April 2013 - 09:03 PM

I tested all of the isos setting... so for me on t3i=
1600, 1600 and 1600, and if not good(burned) ---> HDR ;)

#9 mmalik

mmalik

    DSLR camera modifications

  • *****
  • Vendors
  • Posts: 5405
  • Joined: 13 Jan 2012
  • Loc: USA

Posted 05 April 2013 - 09:42 PM

What ISO to use with T3i


Not about T3i per se; here... is some relevant discussion applicable to any DSLR. Regards

#10 Rayh

Rayh

    Mariner 2

  • -----
  • Posts: 230
  • Joined: 22 Aug 2005
  • Loc: Gilbert, AZ, USA

Posted 07 April 2013 - 08:21 PM

Thanks, mmalik! Based on your link and the Clarkvision article, I can crank up the gain higher than the 400 I was using. At 400 my histograms and images seemed weak.
Ray

#11 mmalik

mmalik

    DSLR camera modifications

  • *****
  • Vendors
  • Posts: 5405
  • Joined: 13 Jan 2012
  • Loc: USA

Posted 07 April 2013 - 10:12 PM

I can crank up the gain higher than the 400 I was using. At 400 my histograms and images seemed weak.


Agree; you may start with 800 and go from there. Another important factor is whether you'll be modifying your T3i or not.


Last but not least, if you have budget and/or if you are able to return T3i, consider getting 60Da... instead for both astro and day imaging. Thx

#12 Rayh

Rayh

    Mariner 2

  • -----
  • Posts: 230
  • Joined: 22 Aug 2005
  • Loc: Gilbert, AZ, USA

Posted 08 April 2013 - 11:16 PM

At 2x the price and no lenses at all I ruled out the 60da. At least now I have a camera I can use for both AP and regular photography. My Nikon was useless for AP...just too hard to use and no software supported it.
Ray

#13 Suresh

Suresh

    Mariner 2

  • -----
  • Posts: 235
  • Joined: 05 Aug 2005
  • Loc: South India

Posted 09 April 2013 - 04:08 AM

The ISO is similar to gain in ccd , Jerry lodriguss re commends high ISO at high ambient temperature , stick to 200 ISO in cold climate and even 1600 ISO at temp above 75 f
Suresh

#14 Mike Phillips

Mike Phillips

    Soyuz

  • *****
  • Posts: 3522
  • Joined: 21 Feb 2006
  • Loc: Swift Creek, NC - 35.682 N, 78.743°W

Posted 09 April 2013 - 09:36 AM

If I am reading the unity gain charts right, my 7D would gain should be at ISO 1000 ??
I could have used this some time ago, I started shooting with my XTi at ISO 1600 and when I upgraded to a 7D last year, bit into the bigger is better and shot at ISO 6400. BIG MISTAKE! I shoot at ISO400 now and it seems better. I have questions that I'll put into another thread.

Mike

#15 Mike Phillips

Mike Phillips

    Soyuz

  • *****
  • Posts: 3522
  • Joined: 21 Feb 2006
  • Loc: Swift Creek, NC - 35.682 N, 78.743°W

Posted 09 April 2013 - 12:49 PM

This is fun!
http://www.dpreview....iews_data&ca...

Mike

#16 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4869
  • Joined: 11 Sep 2009
  • Loc: Gambier Island, BC, Canada

Posted 09 April 2013 - 01:07 PM

If I am reading the unity gain charts right, my 7D would gain should be at ISO 1000 ??


Actually that is the unity gain scaled to 12bit. The 7D has 14bit ADC however. Roger Clark *has* tested the 7D and the results are listed on a separate page (page found here). 14bit unity gain for the 7D is "ISO 246" according to his tests. The dynamic range numbers and read noise numbers would leave me to say ISO400 or ISO800 are still the best bets. The sample read noise/bias images certainly demonstrate how much higher the read noise is at ISO200 and below!

#17 mpgxsvcd

mpgxsvcd

    Surveyor 1

  • -----
  • Posts: 1951
  • Joined: 21 Dec 2011
  • Loc: Raleigh, North Carolina

Posted 09 April 2013 - 01:17 PM

Dynamic range always decreases with increased ISO as long it is a true ISO value and not a simulated or software created "Extended" ISO value.

Everything looks very linear for the Canon cameras that have been mentioned here. Therefore, I would think that your tracking/guiding time and light pollution would be the biggest factor.

Just take exposures for as long as your mount can accurately guide or track and then select an ISO that just begins to show the sky glow.


http://www.dxomark.c...ompare-camer...

#18 Tonk

Tonk

    Fly Me to the Moon

  • *****
  • Posts: 7129
  • Joined: 19 Aug 2004
  • Loc: Leeds, UK, 54N

Posted 09 April 2013 - 01:37 PM

Na - I'll turn that over. Pick the ISO nearest unity gain for your camera and then select an exposure time that doesn't wash out the background. Thats if your tracking is up to it.

#19 Campos

Campos

    Vendor-MonoMods

  • -----
  • Vendors
  • Posts: 532
  • Joined: 26 Dec 2012
  • Loc: Portugal

Posted 09 April 2013 - 04:11 PM

Im my oppionion, crancking the ISO higher then 400 (on my 350D)appart from increasing the noise level to much for my taste, it washes out the colors on the stars. Using lower ISO values like 400 or even 200 and increasing the exposure accordingly is the way to go, the drawback is that for increased sub lenghts you will need to have a very good polar alignment and goos tracking or field rotation starts to creep in your subs...

Cheers,

Luís

#20 Patrick

Patrick

    Voyager 1

  • *****
  • Posts: 11412
  • Joined: 15 May 2003
  • Loc: Franklin, Ohio

Posted 09 April 2013 - 10:06 PM

This is fun!
http://www.dpreview....iews_data&ca...



That's an excellent website for seeing the effects of different ISO settings. Thanks for the link!

Patrick

#21 Alfredo Beltran

Alfredo Beltran

    Viking 1

  • -----
  • Posts: 501
  • Joined: 01 Aug 2010
  • Loc: Bogota, Colombia

Posted 10 April 2013 - 09:41 PM

I own an unmodified Canon T1i (14 bit) and last year I measured the gain and read noise of each ISO from 100 to 3200 following Craig Stark's methodology as stated here and this is what I found:

ISO 100
Gain: 2.209
Read noise: 15.98
Full well: 36184
Dynamic range: 2265

ISO 200
Gain: 1.149
Read noise: 8.77
Full well: 18830
Dynamic range: 2148

ISO 400
Gain: 0.574
Read noise: 5.11
Full well: 9406
Dynamic range: 1840

ISO 800
Gain: 0.282
Read noise: 3.59
Full well: 4612
Dynamic range: 1283

ISO 1600
Gain: 0.142
Read noise: 2.69
Full well: 2320
Dynamic range: 863

ISO 3200
Gain: 0.067
Read noise: 2.54
Full well: 1098
Dynamic range: 433

This shows that at ISO 400 gain is around 0.57, and at ISO 200 1.15 with very low read noise values, which are similar to more expensive dedicated astro ccd cameras, but with much lower full well capacity because of the 14-bit converter. So, those are the values where the dynamic range is maximum is around ISO 200 and 400. Craig has an interesting article on that here .

Nevertheless, it's my understanting that the fact of having lower read noise values at higher ISOs mean that dimmer details can be gathered in less time. But there's a trade off and is the much lower dynamic range. To me, it looks like 3200 isn't worth because the read noise is almost the same as ISO 1600, but the dynamic range is cut by half. On the opposite, ISO 100 has too high read noise and the fact that has 2.2 of gain means that 2.2 photons are required to have an electron on the sensor... Not worth it. It seems that from 400 to 1600 would be the best choice, according to what each one considers best for the object, setup, location, etc.

What are your thoughts on this?

Best regards,

Alfredo

#22 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4869
  • Joined: 11 Sep 2009
  • Loc: Gambier Island, BC, Canada

Posted 10 April 2013 - 10:24 PM

Alfredo: Excellent information! Sounds to me like for the T4i ISO1600 is ideal for *faint* objects without too much dynamic range and ISO400 is the best balance point of read noise and dynamic range for objects with high variances in brightness.

Interesting. I would have guessed sticking with ISO400 would be best with that camera in all situations but that low read noies down at 1600 is should good for faint targets!

Given the close similarity in sensor to the T4i I wonder how closely the T3i would track these results?

#23 Samir Kharusi

Samir Kharusi

    Vanguard

  • *****
  • Posts: 2001
  • Joined: 14 Jun 2005
  • Loc: Oman

Posted 11 April 2013 - 12:28 AM

Some comments. The very wide range of people's preferences expressed here demonstrates that ISO choice does not matter much :question: Having said that let us be concerned with what actually matters:

We need high stacking efficiency. Low stacking efficiency implies that we have to shoot MUCH more integration time in order to match an integration time that used longer subs. As explained on other threads, to get high stacking efficiency the skyfog mountain on the histogram has to be clearly separated from the Read Noise. We can assure ourselves of high stacking efficiency (Skyfog Statistics Limited Regime) by using subs that are long enough to place the Skyfog Mountain at well over 30x the Read Noise. Let us say we aim for the Skyfog peak to be at 100x the Read Noise. It is of course unfortunate that the Read Noise increases as you decrease ISO. From the numbers reported above, 15.98 at ISO 100 and 2.69 at ISO 1600, it is obvious that to place the Skyfog peak at 100x, the sub at ISO 100 has to be long enough to place that peak at 1598 units and at ISO 1600 at 269 units. I.e. for similarly high Stacking Efficiency the subexposure at ISO 100 has to be 5.9x as long as the sub at ISO 1600. This may not matter much in bright suburbia and you are shooting at f2, but seriously, do you really want to use 6x longer subs when you are using a slow focal ratio OTA at a dark site? 30-minute subs anyone, when you can get away with 5-minute subs simply by changing ISO? IMHO loss of dynamic range is often over-valued, since a single exposure at low ISO is enough to bring back color on bright stars and stuff like M42 always require multi sub lengths anyway. Finally, the dimness of the target has little to do with how long a sub should be. All the signal you record is Skyfog+that super dim nebula wisp. It's never the nebula wisp on its own. Take care about where you place that Skyfog mountain (100x Read Noise is good, at your chosen ISO) and then your integration time determines your SNR on the dim stuff. Use the BoC histogram. High skyfog + fast focal ratio implies low ISO, otherwise you will have to shoot several hundred subs. Little light pollution and slow focal ratio and ISO 1600 works well. All the discussion above "proves" that you can get great images at almost any ISO, PROVIDED the subs are long enough commensurate with the Read Noise at that ISO.

#24 Patrick

Patrick

    Voyager 1

  • *****
  • Posts: 11412
  • Joined: 15 May 2003
  • Loc: Franklin, Ohio

Posted 11 April 2013 - 11:54 AM

We can assure ourselves of high stacking efficiency (Skyfog Statistics Limited Regime) by using subs that are long enough to place the Skyfog Mountain at well over 30x the Read Noise. Let us say we aim for the Skyfog peak to be at 100x the Read Noise.



Samir, as always, your comments are very much appreciated!

I may have missed some of your comments in other threads, so I wonder if you could elaborate a little on how one would measure "30x" or "100x" of the read noise vs the skyfog peak. Are you just talking about the gap between the left side of the histogram to the skyfog mountain, or something else? I've read your minimal exposure webpage several times but I don't recall you mentioning read noise as a percentage of the skyfog peak.

Thanks,

Patrick

#25 Samir Kharusi

Samir Kharusi

    Vanguard

  • *****
  • Posts: 2001
  • Joined: 14 Jun 2005
  • Loc: Oman

Posted 12 April 2013 - 01:05 AM

This thread pursues the concepts in depth. People who use astroCCDs are fully aware that to get high stacking efficiency you need your skyfog mountain to be well separated from the Read Noise. Steve Cannistra has a nifty spreadsheet that works well with commonly measured astroCCD parameters. The problem with DSLRs has always been that those parameters are either not known or rather obscure, requiring measurement by the user (or Roger Clark). My referenced thread tries to merge the astroCCD concepts with commonly known properties of DSLRs; specifically BoC Histogram, length of subs and stacking efficiency. The Read Noise in DSLRs is between 0.3% and 1% of the BoC Histogram. It's basically the histogram of a Bias Frame. But DSLRs then confuse the whole issue with varying ISO, hence my earlier comments on this current thread. If the Read Noise did not vary when you vary ISO, then we could use the same short subs at ISO 1600 at any ISO. We would then start a worry on quantization resolution in the A to D converter, which is also better at ISO 1600 than at lower ISO. But this resolution comes with a compromise on Dynamic Range, hence the search for Unity Gain. But all these are trivial compared to the enormous need for INTEGRATION TIME in order to overcome light pollution. Our images are always going to be limited by noise, so we look for "reasons" like ISO. No! Just double your integration time and you WILL notice improved SNR. Not enough improvement? Double it again, and again. All assuming that your skyfog mountain on each sub has been well separated from the X-origin on the BoC Histogram. Sorry, but there is no easy answer. Always recall that the HST Deep Fields used more than a hundred hours integration time...

Patrick, to know how your Read Noise appears on a histogram, you have to convert a Bias Frame to a Linear tiff or fits file and display it in an image editor that does not skew histograms. 30x to 100x Read Noise is then easy to figure out as to where your skyfog should fall, but things do get complicated by the fact that spreadsheets (like Cannistra's) require data on a Linear scale, while the BoC histogram is nonLinear (it involves a gamma stretch). Or spend hours translating everything into astroCCD terminology, like eRMS, skyflux in ADU/minute, etc and end up frustrated :tonofbricks: My referenced thread tries to walk the reader through those translations.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics