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Wednesday, March 9, 2011
Dispelling Scott Bourne’s Misinformation – PPI Doesn’t Matter

I’m not sure why this bothers me so much, but it’s seriously getting on my nerves, so I feel like I should write about it.

Yesterday, Scott Bourne posted to twitter:

If you’re interested in free image critiques send your 72 PPI 640 Pix wide pic to: thepwspi@gmail.com #@ThePWSPI

Many people correctly pointed about that 72 PPI is useless information. If an image is 640 pixels wide, it’s 640 pixels. Pixels per inch is a meaningless number unless you’re printing something. He went on to make a blog post with confusing reasoning why it actually does matter. Except he’s wrong, and he knows it, but can’t admit it. He then posted a couple of updates that further confuse things and serve no purpose but to defend himself and shift the blame to his audience.

He claims PPI affects file size, but it does not. He showed two images, one saved at 72 PPI and one at 300 PPI, and that the 300 PPI one takes up much more space than the 72 PPI one! Proof! Except he fails to point out that the 300 PPI JPEG file is undoubtedly higher pixel dimensions (I would call this “resolution”, but unfortunately Photoshop uses that word for PPI), and almost definitely wider than 640 pixels, than the 72 PPI image. Basically, he took the original image, scaled it down, and said, “look, smaller file size!”

Then, his updates:

UPDATE: I see where some of the confusion is. In the Photoshop resize box there is a button that is checked by default that says RESAMPLE image. This is why the file size is smaller. Given the context of my original Tweet – I hope this makes more sense. If not – move on. Just trying to make sure people understand file size matters.

I don’t know what version of Photoshop he’s using, but in CS5 it lets me change the pixel dimensions of the image when I resize it. I can completely ignore PPI and print document size, and type in 640, regardless of the number in the PPI box. In fact I can only do this if the Resample Image box is checked, which he claims is causing the confusion.

His second update:

UPDATE #2: The goal here is to show that file size can be impacted by PPI. If you don’t UNCHECK the resize image box (that is checked by default on most post-processing software and used by most JPG conversion utilities) then the PPI you set for the image is going to impact the final file size. Can you get around this? Sure. Just don’t resize the file (resample.) Then the PPI does NOT matter. But the problem is – in the world I live in where most people are new at this, they don’t know to uncheck that box so they don’t know to save space. Just trying to help.

He admits PPI doesn’t matter! But he still ignores the main point: The pixel width for the final image you’re going to save is right there in the resize box! If it says a number much larger than 640, then of course the file size will be bigger! If you leave resample checked, and change the PPI, you can see the changes to the pixel width right there in the resize box. Bottom line: PPI has no bearing on the file size of an image with fixed dimensions. 640 pixels is 640 pixels, period*.

I did the same experiment he did, taking one of my images from an original 300 PPI .DNG file. I resized them both, and saved one at 72 PPI and one at 300 PPI. The difference is when resizing them I made sure the pixel width was 640. Guess what?! The files take exactly the same amount of space! (Actually, the 72 PPI one saved a few bytes bigger for some reason, but not significantly.)

So to sum up: If you ask for an image that’s 640 pixels wide, the PPI has no impact on file size. It is a meaningless number until you want to print the image. Maybe Scott Bourne should a) have a little more faith in his audience to understand what 640px means, and b) when corrected, try to clear up confusion instead of defend himself and make things more murky.

*For the web, and digital formats, of course. As I’ve said, PPI does matter for printing.

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Posted by alan to photography at 8:35 pm PT | Link | Comments (0)

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