Goughy
Posts: 4
Joined: Sun May 26, 2013 5:32 pm

Auto exposure is off?

Sun May 26, 2013 5:42 pm

I've been playing around with the camera module for a few days now, and mine *seems* to be over-exposing all the time. I've tried a number of the settings (-ex snow, -ex beach, -ex auto, -ex back, -ex spot) etc. but they are all coming out with the same sort of image. Any ideas what might be causing this? I've also tried matrix and spot metering with the same results

Out of about 40 shots of the sky, 38 were over exposed, with 2 looking about right to me.

Normal (over exposed):
over.jpg
the normal exposure (looks over to me)
over.jpg (3.55 KiB) Viewed 2765 times
What I want (happens less than 5%):
correct.jpg
What i want :)
correct.jpg (4.33 KiB) Viewed 2765 times
Is there a calibration process that I've missed? I've updated and upgraded the firmware and OS, but new to Linux and messing around with the pi, so any thoughts appreciated.

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Grid
Posts: 22
Joined: Fri Jan 04, 2013 5:02 pm
Location: Lodz, Poland

Re: Auto exposure is off?

Mon May 27, 2013 6:42 am

Can you share the command line you are using to take the shots?
Fix instead of throwing away. Save the planet one gadget at a time.

Goughy
Posts: 4
Joined: Sun May 26, 2013 5:32 pm

Re: Auto exposure is off?

Mon May 27, 2013 9:22 am

I was using

raspistill -t 0 -o somefilename.jpg -w 1920 -h 1080

but then to reduce the chance of mistakes i've changed it to

raspistill -t 0 -o somefilename.jpg

for the testing with the same results.

I've also tried:

raspistill -t 1 -o somefilename.jpg

However, having just tried a (small) sample, it appears that removing the -t switch gives the correct exposure - at least on the three frames I tried. I'll try it on a broader sample set and report back!

In the meantime - any idea why the -t switch would cause it to overexpose?

jamesh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 27075
Joined: Sat Jul 30, 2011 7:41 pm

Re: Auto exposure is off?

Mon May 27, 2013 9:26 am

It's possible that with -t 0 (basically 0 ms before you take the picture), that the picture is being taken before the exposure cycle is complete.

Try -t 2000 to see if its gets more consistent.

I'd never intended the -t option to be set so low!
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed.
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Maxion
Posts: 138
Joined: Mon Dec 03, 2012 2:22 pm

Re: Auto exposure is off?

Mon May 27, 2013 9:28 am

Is there only sky in your shot? Or is there something else as well?

Goughy
Posts: 4
Joined: Sun May 26, 2013 5:32 pm

Re: Auto exposure is off?

Mon May 27, 2013 9:33 am

Cool - I'll try with the longer -t :)

As for the sky picture, no - there's a washing line in it too :)

pi-man-uk
Posts: 62
Joined: Tue May 08, 2012 1:41 pm

Re: Auto exposure is off?

Mon May 27, 2013 10:04 am

Just wanted to post my results - had been a little underwhelmed with the camera images to start with - using "-t 0"... i had just picked this up somewhere on the forum. Images didn't look sharp - a little grainy perhaps... so I read this and tried -t 2000. A lot better :-) .. tried posting results but only 64k allowed per photo so a little on the small side.
timeout-0.jpg
timeout 0
timeout-0.jpg (51.8 KiB) Viewed 2659 times
timeout-2000.jpg
timeout 2000
timeout-2000.jpg (56.39 KiB) Viewed 2659 times
thanks..

kaos
Posts: 108
Joined: Mon Mar 26, 2012 8:14 pm

Re: Auto exposure is off?

Mon May 27, 2013 10:13 am

@JamesH: Has a minimum worst-case time needed for the auto exposure to "lock" been figured out? Is there some feedback available from the MMAL interface to indicate such "lock"? (Will obviously need modified raspistill command to be useful).

Oh, and btw, thanks for all your work on this little gem!

--
Regards, Kári.

Goughy
Posts: 4
Joined: Sun May 26, 2013 5:32 pm

Re: Auto exposure is off?

Mon May 27, 2013 10:54 am

Just ran a set of 60 frames against the sky (beautiful and blue for the first time in ages in the UK)

As the -t switch isn't really important to me for this application I've removed it, and the images are much, much better - thanks for the advice :o)

jamesh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 27075
Joined: Sat Jul 30, 2011 7:41 pm

Re: Auto exposure is off?

Mon May 27, 2013 11:01 am

kaos wrote:@JamesH: Has a minimum worst-case time needed for the auto exposure to "lock" been figured out? Is there some feedback available from the MMAL interface to indicate such "lock"? (Will obviously need modified raspistill command to be useful).

Oh, and btw, thanks for all your work on this little gem!

--
Regards, Kári.
It depends on the scene. If the scene matches matches what ever current exposure is set to its doesn't have to use too many frames to decide its right, whereas if its a long way off it needs more frames to get its exposure right. (IIRC - i'm no expert on the grim details)

I *think* there is a callback the the code could use to find out whether its ready, or perhaps I can poll something - I'll need to investigate. Can you post a specific request for that in the features request thread so I don;t forget it.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed.
I've been saying "Mucho" to my Spanish friend a lot more lately. It means a lot to him.

kaos
Posts: 108
Joined: Mon Mar 26, 2012 8:14 pm

Re: Auto exposure is off?

Mon May 27, 2013 9:33 pm

jamesh wrote:It depends on the scene. If the scene matches matches what ever current exposure is set to its doesn't have to use too many frames to decide its right, whereas if its a long way off it needs more frames to get its exposure right. (IIRC - i'm no expert on the grim details)
Yes, I expected the actual time to be variable, hence my interest in worst-case.
jamesh wrote:I *think* there is a callback the the code could use to find out whether its ready, or perhaps I can poll something - I'll need to investigate. Can you post a specific request for that in the features request thread so I don;t forget it.
Thanks, I've just done so.

--
Kári.

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