Question About Image Sizing in ACR

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@soulthief wrote:

If my camera (Canon 5D4) is taking a full sized 30 MP RAW images, when I'm in ACR, should I not be using Default (16.9 MP) in Image Sizing and instead Resize to Fit Megapixels and change megapixels to 30 MP to get the best quality image?

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If Default is 16.9 megapixels, that is exactly what is in the file. But we know that the Canon 5D Mk IV is a 30-megapixel camera, so of course the question is, why did the camera put only 16.9 megapixels in that file?

The first thing to check is: Was the 5D set to capture full resolution raw, or one of the other lower-resolution “raw”-ish formats? Because if you look at the Canon 5D Mk IV specifications, 16.9 megapixels is exactly the stated size of a file when shooting as mRAW. So my guess is that the camera was set to save mRAW, which is 16.9 megapixels, and that is why that’s what you get in Camera Raw.

The way to prove or disprove that theory is to inspect that same photo file in any other program, such as the Get Info command in macOS Finder, or the Properties command in Windows Explorer, or Canon DPP, and look at what it says the pixel dimensions are. If it says 6720 × 4480, it’s 30 megapixels, and so the camera was set to save full resolution raw. If it says 5040 × 3360, it’s 16.9 megapixels, so the camera was set to save mRAW and that is all you have in that file. You can ask Camera Raw (or Photoshop or any other photo editor) to increase it to 30 megapixels, but that will just be making up pixels, so the quality of that semi-synthesized 30 megapixels will not be as good as if the camera was actually set to save all of the sensor’s actual 30 megapixels into that file.

You probably don't want the resize check box on at all. Then ACR will just use all the image data you've got.

That said, IF you knew you needed to interpolate UP (for say a bigger print than the native resolution of capture), you might find you get a tiny bit better quality doing the resampling UP here, from raw than after in Photoshop if that's what you'd normally use. 

Otherwise, I'd simply keep that resample check box off. Crop as you desire in ACR, render 16-bit, wide gamut, (ProPhoto RGB), and move on.