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

Started 11 months ago | Discussions thread
bobn2 Forum Pro • Posts: 59,032
Re: Olympus OM-D EM 5 Mark II - RAW in Manual/ Aperture / Program mode

hcjghr wrote:

bobn2 wrote:

hcjghr wrote:


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.

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