12MP is not really 12MP, 24MP not 24....

Started Dec 5, 2008 | Discussions thread
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Bernie Ess
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12MP is not really 12MP, 24MP not 24....
Dec 5, 2008

Yesterday a photographer of the Sony a900 sent me kindly a link to download a few RAW files shot with the Sony top model, shot with very sharp lenses (partly with Zeiss).

When viewing them, I found them to be very sharp even in 100% view, combined with great colour too.

Then I made the downsize + re- upsize test. Sampled the 24MP file (6000x4000px) down to 12MP (roughly 4200x2800px) and then re- upped it to the original dimension.

I was pretty surprised that there was no discernable difference. In other words: A 24MP DSLR - even a sharp one - is not worth 24MP of real detail, but roughly half of it: 12MP.

I then made the same experiment with some of my old EOS 5d files. I selected sharp pictures and to make a long story short, 6MP was enough in most cases, only the very very finest lines on single pixel level seemed a bit blurred I doubt that this is visible in a print smaller than A1 or A2. 8MP is the absolute maximum in fine detail that can be seen - I used the very sharp 50mm/1,8.

So what is going on there? First I thought of Bayer interpolation, out of 12MP only 6MP are green the rest is shared by red and blue, but this is not luminance (detail) information, but colour info mainly. Yes there is the AA filter, which blurrs fie detail, my Kodak SLR-n (no AA filter) which gives a 13,5MP file seems to have around 8MP of real detail.

My final explanation would be that those 50% are mainly lost on their way through the lens. No lens is able to resolve the scene 1:1 - the smaller the structures, the less accurate, in simple words.

So if you believe your camera is certainly more than half of the pixels worth, it is easy, give it a try: Take a reasonably sharp file and make the down + re-uprezzing test and see how low you can go before you start to loose detail in 100% monitor view.

I believe that in print things would be even worse, probably a 12MP file can be downsized to at least 5MP without loosing visible resolution in the print.

BTW, I have tried the same test with the Fuji f100fd (12mP) compact in summer the real resolution came out around 4-5MP

So, what do you guys think?
--
regards,
Bernie

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