RAW, mRAW, sRAW?

Spiritflier

Spiritflier

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Simon (Prefer 'Si')
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Hi all,

This is one for the techies on the forum... I'm currently using the 7D having upgraded from the 40D (which will stay with me until it packs up!).

I've never bothered paying much attention to the three RAW settings but I'm now a bit curious as to how the camera creates images based on these formats.

If I shoot at on full RAW, my camera creates a file using all 18mp. On mRAW, the camera uses 10mp and so on... I'd like to know what criteria the camera uses to decide which pixels will form part of the progressively smaller image. Since the sensor size is the same on the 40D and the 7D, an image will relatively cover the same amount of space regardless of the pixel count (although it will obviously cover more actual pixels on the 7D)...

In other words, does it use all 18mp and then downsize the file to a 10mp equivalent or does it selectively discard (or switch off) certain pixels?

The reason I ask is that at on the 7D, images taken at ISO100 in RAW and viewed at 100%, show slightly more noise than I can see if I shoot the 40D at maximum resolution and I wondered whether it was just because of the increased resolution.

Thanks in advance,
Si
 
Chaz Photos said:
I asked Canon last year about this at Focus on Imaging and the main guy who did the lecture on the 50D said he had not been told how it is done.
Click to expand...

Hi Chaz,

Thanks for the reply... I had a mooch about on Google but couldn't find anything of any use either. I did a little experiment last night and created a 3888x2592 (10mp) TIFF from an 18mp .CR2 RAW and found that the noise level appeared to be lower which I'm guessing was because although the image was proportionately the same size on both counts, the smaller image resulted in less noise being visible. It wasn't exactly a scientific test but it did start me thinking. :)
 
Some image processing must take place to reduce the pixel count, however I suspect that this done by reducing the pixels sampled in the original image. ( Probably a bit like PhotoCD gave you various resolutions of the same image.)

I suspect the noise is a result of magnification, which your tests seem to confirm.

I've not found much use for sRAW on my 1Ds as the 5Mp file is almost twice as large as the image file. If I need a smaller file I use my 5D or trusty ( but ageing) 20D
 
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