Olympus OM-D EM 5 Mark II - RAW in Manual/ Aperture / Program mode

Started 9 months ago | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 58,131
Re: Olympus OM-D EM 5 Mark II - RAW in Manual/ Aperture / Program mode

hcjghr wrote:

bobn2 wrote:

hcjghr wrote:

bobn2 wrote:

hcjghr wrote:

Hi,

This question is regarding RAW files and how the values are affected when different camera modes (P/Av/Tv/M) are used. I'm working with the Olympus EM5 Mark II camera.

Well, first, 'raw' is an adjective, not an acronym, so it is a 'raw file, not a 'RAW file'.

Through some experiments I have noticed that raw values (after black level subtraction) are not consistent when taken with different modes.

The clearest example is if I take a photo 3 photos one after another in a controlled environment (no light except a LED light) using

- P mode (where camera sets the Aperture and Shutter speed)
- Av mode (selecting same aperture and camera sets the same shutter speed)
- M mode (selecting same Av and Tv setting)

Looking at the mean values of patches I noticed that images from P and Av mode are consistent while M is different (images taken from same position in same conditions):

Manual (M) mode:
Mean: 0.14235, 0.15051, 0.01841

Aperture priority (Av) mode:
Mean: 0.15901, 0.16807, 0.02075

Program (P) mode:
Mean: 0.1605, 0.16968, 0.02098

( I have more than just this example)

Those don't look like raw file values to me. A raw file value would be an integer value, in the range 0-4095. This looks to me like the values after processing.

You are correct. The values I've posted are linear RGB values after demosaicking although I have looked at the CFA as well and the tendency is the same. The values are actually mean values over an area to also remove a bit a plausible noise.

The differences aren't so great an tend to indicate that you haven't precisely centred the meter in M mode.

Can you explain what you mean by that? I expected that if the parameters (Av, Tv and ISO) are the same when taking an image with different modes (P, Av, M) the values of the acquired raw image should also be the same? Or P and Av have intermediate values for the parameters which are not reported in the exif. (Sorry if I'm just repeating the question but I really want to get to the bottom of that and it seems you might have an answer for me:))

Setting the ISO sets the target exposure that the metering tries to match. In manual mode you can set to the closest 1/3 stop (or whatever you have set up as the step size). In P and Av mode, the shutter is continuously variable, and will try to match to a small fraction of a stop. In Tv mode, the aperture mechanism can similarly be set to a much higher precision than the steps available in M. How these things are reported in the EXIF varies from camera to camera.

That is really useful to know (maybe its common knowledge but I was always under the impression that P and Tv/Av modes just set the parameters (as you would set them in M to the closes 1/3 stop).

It's actually a matter of modern digital cameras following the behaviour of the old analog ones. In those, the shutter and aperture control were analog and stepless, so they would just be organised to time the shutter or open the diaphragm until the meter was exactly centred.

Also just a follow up..then in M mode do the settings remain exactly as the user sets them or the camera changes them to fit better given the metering info it collects?

In M mode, the metering and exposure meter should be completely decoupled.

Do you know where I could find where/if the camera saves that in the EXIF? I'm currently mostly interested in the Olympus EM5 Mark II ...but would be great to know if there is a common place with this information as maybe somebody else with a different camera will have the same questions.
Thanks again for all the explanations!!

There's a tool called exiftool,

which allows you to read the EXIF in detail. You'll see it's very complex with much duplication. Some manufacturers might put the exact shutter speed and f-number in there somewhere. But, you should be aware, that your camera and lens probably isn't well enough fettled to get them exactly in any case.

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Tinkety tonk old fruit, & down with the Nazis!
Bob

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