How to count EFFECTIVE number of pixels in a photo

Started 5 months ago | Questions thread
OP il_alexk Senior Member • Posts: 2,879
Re: How to count EFFECTIVE number of pixels in a photo

ProfHankD wrote:

il_alexk wrote:

Is there a SW tool there that can scan a photo and advise to how many mega-pixels to scale down while keeping all the details intact?

Good question. Not really, although I easily could make one...

How?

Compression algorithms don't really benefit from having fewer pixels when there really wasn't extra detail there.

Thus, if you are talking JPEG or HEIF files, reducing the file size is unlikely to make much difference in file size after denoising. This is because the frequency-domain compression (e.g., in JPEG, it's an 8x8 DCT) is quite effective as long as you don't have lots of high-frequency content.

Before denoising, things are different because noise tends to be random, so it doesn't compress much at all...

Just trying to understand if I really need 44Mp sensor, PP in Lightroom started to take way too long with their latest AI denoising.

So I am considering to downgrade to A7III with 24Mp pixels, which both would reduce the PP time and improve the AF. Just want to make sure I get enough resolution for 50-30% cropping. Low ISO isn't a brainer, A7R3 is unbeatable, however at ISO 4000-6000 where most of my photos are things are less than trivial and A73 might be equally good.

So, how would you "make one"?

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Cheers,
Alex

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