This 300 PPI figure is a bit of hogwash that came from GATF — the Graphic Arts Technical Foundation — way back in the 1990s. If you reproduce a photo in a magazine or book, on coated paper, with a 133 or 150 line screen, you really only need about 200 PPI in the original photo file. The 300 PPI standard was created to give photo editors the ability to enlarge a file by 50% without degradation!

I confirmed this in 1998 when we started making digital halftones and color separations for memory books at my school portrait lab. We tested hundreds of photos at various resolutions, made press tests, and confirmed that 200 PPI would do for 133 and 150 line screens, both in black-and-white and four-color reproductions.

Using higher resolution than 200 PPI did not resolve more detail in our books! We saved a TON on storage space, scanning time, network bandwidth, etc. by doing this testing. All those things were EXPENSIVE back then!

BUT, in the photo industry, the professional rule of thumb (thanks to a LOT of Kodak research) is that the eye can only resolve about 240 PPI from an 8x10 silver halide photographic print, at the diagonal of its viewing distance, 12.8 inches (round up to 13). So your 6000x4000 pixel image will enlarge quite well at 240 PPI to 25x16.67 inches (round down to 24x16 — it's a lot easier to remember!).

Many pro labs use 240 or 250 PPI as their standard for file submission.

What is interesting is that for MOST subject matter, you can take a 240 PPI 8x10 image, enlarge it to 16x20, and view it from 26 inches with no degradation in detail. You can also enlarge it to 32x40, view it from 52 inches, and see no degradation in detail. Of course, if you view the 32x40 at 13 inches, you will see the original pixels, and they won't be pretty... (So, as a practical matter, I would not be nervous about making 50x33.33 inch (or 48x32) prints from most sharp images coming from that 24 MP camera...)

For this reason, many folks believe we are at a point of diminishing marginal returns, once cameras get to around 24 MP. Go to 32, 36, or 50 MP, and you gain little, except at the EXTREME margins of cropping and/or enlargement of subject matter with lots of detail. And at those extreme resolutions, you had better have the best glass in front of that camera body that money can buy! With cheap or very old glass, you won't reap the benefits of the highest resolution sensors.

Well over 90% of today's images wind up on the Internet, on HDTV and 4K screens, or they are never printed larger than 12x18 inches. Unless you make very large prints, almost any of today's midrange to high end dSLR and mirrorless cameras will satisfy. We met and exceeded the capability of 35mm film quite a few years ago, and most lens makers have had to redesign and reissue their lens lines to keep up with improvements in sensors and electronics.

Oh, and that PPI vs DPI confusion? A pixel is a value in a file. A dot is ink (or dye) on paper. Cameras and post-processing software create pixels. Printers create dots to reproduce pixels. I can't say it any clearer. THEY ARE ALMOST NEVER THE SAME. But older folks in the graphic arts industry tend to use the terms interchangeably. The professionals in the photo industry are not confused!

Yes.

• Resize to prevent unauthorized high-res reproduction.
• Resize to optimize upload time to the server.
• Resize to optimize download speeds, regardless of what the site is doing to the image

I'm sure there are quite a number of still valid reasons to resize images for use on the web. Not all sites treat images the same way, so it can be important to know their policies and customs and practices. For example, some sites allow you to create a profile photo for yourself, but they limit it to 100x100 pixels or so. Send a larger image, and they give you a wrist slapping error message.