Apple Preview—question

[clyde]

Using Apple Preview to view and edit photos.

Ran into problem, or perhaps I misunderstand its controls.

Problem is sizing an image. Using the Tools, Adjust size, I set the image to a particular size. Then I check the size of the image shown on screen. Image size does not match size chosen. The image size will match the desired size ONLY if I multiply the desired size by 1.48 (1.5 for ease of calculation).

My guess is operator error…

Whatt'ya think?

[pdq]

Yeah, depends on the monitor.

I suspect that Preview is assuming 72 dpi (or something like that, which I think used to be the "standard" monitor resolution.)

[clyde]

What's the resolution of a late 2013 iMac?

[Sarcany]

It's inches-at-resolution. Not just inches.

So, if the image is 10x10 inches at 72 dpi then when displayed at 72dpi you will see it at 10x10 inches.

If your display is 3840 by 2160 pixels scaled to 2560 x 1440 (Late 2013 27-inch Retina iMac defaults) then you're viewing at an approximate 109dpi.

At this scale, you're seeing your 72dpi image scaled down to roughly 66% of the size that it would have on a 72dpi display. Your 10-inch image is now displaying at only 6.6 inches.

[clyde]

Thanks for the info. In System Info the graphics display of this 21-inch iMac is said to be 1920 x 1080

I see that 2560/3840 = 0.66666 in the example of the 27-inch Retina display.

What numbers apply to this 21-inch monitor? If it's 1920/2560, that's 0.7245, but if the ratio is 1920/3840 the ratio is 0.500. Must be the former…

Or where did I go off the rails?

Thanks!

[Sarcany]

1920 by 1080 at 21.5-inches is 102.5dpi.

72/102.5 = 0.7 so your 10-inch image at 72dpi will display at 7 inches.

To bring it back up to 10 inches you'd need to multiply by 1.43... Not sure how you got to 1.48/1.5.