Author Topic: Exposure offset correction [3rd party focusing screens, adjust default expo]  (Read 860 times)

chris88

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Hello everyone,
I had an idea this morning which is about "exposure offset correction".

Introduction:
For my taste the Canon 6D which I own often produces slightly underexposed images. So I often adjust my exposure to +1/3EV by standard. As I do so nearly every time, I am not using my light meter in the viewfinder like intended. Instead of setting it to "zero" to get the right exposure I need to set it to +1/3 EV every time. This is not the worst thing ever, but it feels slightly irritating doing so. Therefore the best thing which I could think of is to be able to offset adjust the exposure measurement to my taste.
Now, since some weeks I have bought a third party focusing screen and noticed that the pictures are looking even darker with the new focusing screen. In the Canon menu I can adjust my cameras exposure perception by  the customer function C.Fn III where I can choose between three presets for the following Canon focusing screens: Eg-AII, Eg-D, Eg-S. But none of those three presets seem to really fit to my third party focusing screen. So again, the best thing which I could think of is to be able to offset adjust the exposure measurement to my taste.

The Idea:
Can't we use the same way as the customer function for focusing screen selection uses to set exposure offset corrections to set our own "exposure offset"?
So, is it possible to identify which registers are modified by the customer function for focusing screen selection and then write down our own values in there?
This way we might be able to tune our cameras to an exposure perception which also fits our own, very individual taste.

What do you think?
Would this be possible?
And if yes, who is willing to help developing such a functionality?

Many thanks is advance and best regards,
Chris

a1ex

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If you run the camera firmware in QEMU and change that option from CFn III, you get the following:
Code: [Select]
[MPU] Received: 22 21 02 08 06 00 00 00 03 00 14 05 02 00 00 00 01 00 00 00 00 00 09 00 01 00 01 00 00 00 03 03 02 00  (unknown - PROP_CFN_4)
[MPU] Received: 22 21 02 08 06 00 01 00 03 00 14 05 02 00 00 00 01 00 00 00 00 00 09 00 01 00 01 00 00 00 03 03 02 00  (unknown - PROP_CFN_4)
[MPU] Received: 22 21 02 08 06 00 02 00 03 00 14 05 02 00 00 00 01 00 00 00 00 00 09 00 01 00 01 00 00 00 03 03 02 00  (unknown - PROP_CFN_4)

In other words, the main CPU is forwarding this option to the MPU (which handles metering in photo mode, among others). What exactly the MPU does with this preset, I'm afraid I'm unable to tell, since I'm not familiar with its CPU architecture.

Sorry, unable to help with this one...

chris88

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Hmm, that's really too bad.
But hey, thanks a lot for looking into it. Thought we might have a chance - but I didn't know that the MPU will disrupt my plan.

Chris