RAW to PSD = massive file size difference, why?

[Chris22753034tl7n]

Why are my ARW files at 59MB turning into 172MB size files when I simply export from camera raw at 8 bit Adobe RGB at same dimensions and resolution? Thanks for any help. 

[Stephen Marsh · Accepted solution]

Raw = 1 channel

Rendered = 3 channels

https://www.adobe.com/digitalimag/pdfs/understanding_digitalrawcapture.pdf

[Mariam Hovhannesyan]

Here is some answer that I found on web.
Hope this can help

"First of all, size in MB doesn't directly mean anything for print production. Resolution means everything.

When you open raw image in Photoshop, it saves in memory not only raw image itself, but also a copy of the image that you made while import with Camera Raw, so, basically you have original and Camera Raw version of the image in memory. That's why the size is approximately double.

When you save psd file, if you save it with support for previous Photoshop versions, file size can grow up dramatically. So if you don't need support for previous versions of Photoshop, always remember to uncheck that option upon saving.

If you want to keep your files in raw, I suggest you to use Digital Negative (DNG) file format. It's native for Adobe and in the same time uses losless compression."

[D Fosse · Accepted solution]

The main explanation is that a raw file is a single channel, while the RGB file has been demosaiced and encoded into three channels. So that triples the size right there.

Then there are other minor factors, but mainly it's the number of channels.

[D Fosse]

<duplicate post deleted>

[Chris22753034tl7n]

OK, thanks. Sounds kind of like the raw file is compressed. Is there meta data in the file that instructs camera raw? 

[D Fosse]

The raw file is not compressed. 59 MB x 3 = 177 MB. That's the explanation. A raw file can be compressed, but normally isn't.

There will always be small variations from the x3 formula, some of it from variations in bit depth or layers or other properties of the RGB file, all of which affect file size.

But that's getting sidetracked by irrelevant detail. The fact remains, and will remain, that an RGB file will always be considerably larger than a raw file, due to how the file is constructed. Just accept that and stop worrying about it. There is nothing wrong here. This is normal.

[Chris22753034tl7n]

OK, tough guy, thanks. 

[Conrad_C]

OK, thanks. Sounds kind of like the raw file is compressed.

By @Chris22753034tl7n

Not necessarily compressed, but as was explained, mostly about raw being only one not-yet-demosaiced monochrome channel, instead of three RGB channels. Convert to CMYK and it might be 25% bigger than RGB, because a fourth color channel was added. Add some layers, masks, and channels and it goes up some more.

Although TIFF/PSD file sizes sound “unusually large” to those used to seeing the smaller file sizes of highly compressed JPEGs and single channel raw files, the size of an uncompressed RGB TIFF/PSD file is actually the natural file size of that image. The math proves it. For example, if you have a 6000 × 4000 pixel image, then its natural file size at 8 bits per channel should be:

6000 * 4000 pixels = 24,000,000 (24 megapixels)

* bit depth of 8 bits per pixel (24,000,000 * 8 = 192,000,000 bits)

* number of channels (192,000,000 * 3 = 576,000,000 bits)

and convert from bits to megabytes (576,000,000 bits / 8 bits to a byte / 1,000,000 to get from bytes to megabytes = 72MB)

That’s what I get from my 24-megapixel ARW files. Raw, they are around 49MB. When expanded to RGB at 8 bits per channel, uncompressed TIFF is 72MB (just as the math said) and PSD is 68MB. I believe the slightly smaller PSD might come from PSD using RLE compression.

And because this is about the pure math of it, you will get those same file size differences from any other application, Adobe or not, that can convert a raw file to RGB TIFF/PSD.