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Why is my image grainy at only 100 ISO?

dlesandrini
Apprentice

Last night I tested out my new iOptron SkyTracker loaded with my used Canon Rebel T2i.

 

I pointed towards the Orion constellation, through a palm tree and the results were really, really grainy.  ISO was set to 100 and the kit lens was at f/5,6 and 55mm (51mm according to Lightroom ... see attached screen shot.)

 

Can I assume there's a problem with this camera?  I bought it used.  Should I take it to the camera doctor?  Oddly enough, later shots I took at 400 ISO looked better, but still grainy.  Any ideas?

 

This is my first post.  If you want me to upload the actual photos, I can do that.  

 

LR grainy.png

3 REPLIES 3

Waddizzle
Legend

I think your histogram illustrates most of it.  You're taking photos that have very little dynamic range, which are not much above the noise floor of the image sensor. 

 

I think that says 10 second exposure.  Your tracker should allow for much longer shots, which means more light gathering and more dynamic range.  Astrophotography is complicated. 

 

[EDIT] I would also advise using the widest zoom setting on the kit lens, 18mm.  You'll get less star trails, and will able to take longer exposres without tracking.

--------------------------------------------------------
"The right mouse button is your friend."

Thanks ... I think you're spot on.

 

My fear was that there was something wrong with the camera because there seems to be an unbreakable rule out there that High ISO gives more noise than Low ISO.  I'm learning that this isn't true.

 

One blog I found informs that one should watch the histogram.  Yes, night photography will push the histogram to the left but he advises that it not slam all the way to the left wall.  One needs to collect more light and the 10 seconds with ISO 100 wasn't doing that.  400 ISO at 60 seconds was strangely less noisy.  He had an explanation for it but I haven't finished reading the blog yet so I can't pass that on.

 

Yes, I am dissappointed with the tracking.  With 55mm focal length, the polar aligned tracker should have done 60 seconds without star trails.  It's probably user error.  I'll try again tonight.  Thanks for your input.

You used Lightroom?

What the ISO setting on the camera does is amplify the analogue signal before readout and digitization. When amplifying the signal the shot noise gets amplified too, making the signal to noise ratio is the same. However the read noise does not get amplified as it happens after amplification. When you shoot at low ISO and brighten the image in software, the shot noise, and the read noise both get amplified, giving a higher level of noise. The biggest influence on noise is the total amount of light falling on the sensor. If you have a very dim scene with little light falling on the sensor then you will have noise regardless of the ISO setting. Likewise if you have tons of light falling on the sensor you will have very little noise regardless of the ISO setting.

EB
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