XAPBob
Posts: 91
Joined: Tue Jan 03, 2012 2:40 pm

Metering modes

Thu Oct 03, 2013 9:24 am

I'm struggling to find any description of the metering modes beyond the documentation of what they are called.

I have a PiCal which is acting as a security camera.
When it's daytime, and the garage door opens, the outside of the image is very dark, and the person (usually me) in centre frame is basically a silhouette. When the lights are on in the evening the whole image is basically uniform.

I suspect that a more centre weighted (or even just "defined area" metering) would be beneficial, but the modes available aren't obviously described (nor is the default mentioned anywhere I can see)
Metering Mode options :
average,spot,backlit,matrix

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

Re: Metering modes

Thu Oct 03, 2013 10:00 am

I haven't seen any detailed definition, but going by the terminology I would think that "spot", or possibly "matrix", is what you are after. "Spot" should calculate exposure based on the center of the image only. "Matrix" should calculate the exposure from several spot readings, and if it is cleverly done it might help. Mind you, matrix metering seems to be a badly defined beast in general, see http://en.wikipedia.org/wiki/Metering_m ... e_metering. You might also want to try "backlit". It certainly helped in my case, which admittedly is probably less severe than yours: My camera is an outdoor surveillance camera (http://kari.wifi.is) and shooting what are essentially landscapes. I found that with the default setting (I think it is probably "average") it has a tendency to give beautifully detailed cloudscapes, but leave the foreground rather dark. Using -mm backlit I generally get a well exposed foreground (which is what I am mostly interested in) but may get blown highlights in the sky. You will probably be best off experimenting with all the modes and deciding what fits your purpose best.

--
Regards, Kári.

poing
Posts: 1131
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Re: Metering modes

Thu Oct 03, 2013 10:35 am

I'd use the -ev option in daytime that goes from 25 to -25. When you use -ev 25 the capture of a normal scene becomes completely washed out. If you do not use the -ex option the shutter speed will never be longer than 1/8, with -ex night it will never be longer than 1/4 while with -ex verylong it will go to 1/2 but hangs the camera (bug).

That said, the lens is prone to flare (loss of contrast when a lot of light enters it) so I guess in your usage scenario the person entering the garage will get very 'flat' on the capture but maybe recognizable.

jamesh
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Re: Metering modes

Thu Oct 03, 2013 10:45 am

Dave Akermans first HA balloon flight with the Pi used matrix I think and the pictures at high altitude were a bit pants for exactly this reason. Switching to average worked a treat.
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unwohlpol
Posts: 13
Joined: Tue May 28, 2013 5:03 pm

Re: Metering modes

Thu Oct 03, 2013 3:13 pm

Matrix metering gives me the best results... talking about shooting landscapes and prevent highlights (sky/clouds) from clipping.
http://www.ungutknut.at/webkem/out1.jpg

BTW: It says "average" in the EXIF even though I chose matrix.

"Average" gives too bright pictures with white sky (without using EV - because EV is not possible if you want to fix the ISO to 100).
"Spot" makes the sky too bright when aimed to the ground and the ground too dark when aimed at the sky.

XAPBob
Posts: 91
Joined: Tue Jan 03, 2012 2:40 pm

Re: Metering modes

Thu Oct 03, 2013 3:18 pm

Probably a question for jamesh:

Based on the EXIF data is average the default?

jamesh
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Re: Metering modes

Thu Oct 03, 2013 3:22 pm

unwohlpol wrote:Matrix metering gives me the best results... talking about shooting landscapes and prevent highlights (sky/clouds) from clipping.
http://www.ungutknut.at/webkem/out1.jpg

BTW: It says "average" in the EXIF even though I chose matrix.

"Average" gives too bright pictures with white sky (without using EV - because EV is not possible if you want to fix the ISO to 100).
"Spot" makes the sky too bright when aimed to the ground and the ground too dark when aimed at the sky.
The EXIF is a reported problem, but unfortunately the path the setting takes though the GPU means its translated to its nearest equivalent via a number of different places (internal setting, mmal, OpenMAX, exif). Each conversion is the best fit but the sum total is that the final EXIF doesn't exactly match what you might expect. Not sure what to do about it.

The default is mmal_param_exposuremeteringmode_average, which is OMX_MeteringModeAverage which is ISP_TUNER_AGC_METER_NORMAL which in the exif is Centre Weighted average.

Spot is spot->spot->spot
Backlit is Backlit->backlit->other
Matrix is Matrix->matrix->average->average

So it's all a bit of a dogs dinner terminologywise.
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poing
Posts: 1131
Joined: Thu Mar 08, 2012 3:32 pm

Re: Metering modes

Thu Oct 03, 2013 6:09 pm

unwohlpol wrote:...EV is not possible if you want to fix the ISO to 100
It is when you use -ex night or -ex verylong (but the latter may hang the camera). Use the latest build from here: http://www.raspberrypi.org/phpBB3/viewt ... 65#p426025

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daveake
Posts: 184
Joined: Thu Jul 12, 2012 12:07 am

Re: Metering modes

Thu Oct 03, 2013 8:35 pm

jamesh wrote:Dave Akermans first HA balloon flight with the Pi used matrix I think and the pictures at high altitude were a bit pants for exactly this reason. Switching to average worked a treat.
Actually I switched *to* matrix from [whatever the default is - spot I was told].

"space" images are a bit tricky because you don't want the black sky to affect the exposure. Manual exposure is probably the best bet.

Dave

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