Basic Cropping Question

R
Posted By
rvs053063
May 21, 2007
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291
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5
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Closed
With CS2, I cropped an image to be 5×7 at a resolution of 300. When I open the image, it is huge and doesn’t fit on the screen. I would have expected this image to be 5×7 inches on my screen, since that’s the size I cropped it to. Under Image Size, the document size is also 5×7. Why doesn’t this add up? Thanks.
Rick

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P
Phosphor
May 21, 2007
You just seem to be a little unclear on grasping a basic concept, that’s all.

First of all—and the most important concept to get under your belt—Photoshop (and indeed, every other raster-based image display app) only cares about raw pixel dimensions. When you see any other unit of measurement being used (e.g.: in, cm, mm, etc.) it is calculated on-the-fly and is only converted and displayed for the convenience of the user.

Then, consider this:
5 in × 7 in at 300 ppi = 1500px × 2100px.Sidebar: The accepted standard for dimensional reference always states Width first, and Height second.Now…What is the resolution of your monitor, in raw pixel dimensions?

Just for explanation let’s say it’s a big monitor set to display at a reasonably high 1920px × 1200px.

Even with no operating system or application interface elements taking up space, your image would not fit on screen at 100% (normal) magnification.

Then take into account that Photoshop will display an image at the largest size possible, respecting the location of whatever application palettes you have visible while you’re working.

So that shrinks the available display area to even smaller than the full pixel dimensions of your monitor.

So, the only thing you can do is reduce the magnification if you want to see that entire image on screen at one time—say perhaps to 50%. But keep in mind that the only way to display and view your image accurately is to view at 100%, where one image pixel is mapped to one monitor pixel. Any other magnification values will only permit an interpolated display of your image, wherein each image pixel is mapped to fewer monitor pixels (when zoom is <100%) or more monitor pixels (when zoom is >100%).

Additionally, when you must view at less than 100%, the best possible display will only occur when the magnification has ONLY even-integer factors, (i.e. and e.g.: 50%, 25%, 12.5%). These factor out as 2/100, 4/100 and 8/100, respectively. These views aren’t perfect, but if you must view a large image on screen, they’re the best values to use. See? No factors other than 2.

With magnification values that use odd-integer (or fractional) factors, the application isn’t as accurate at mapping image pixels to monitor pixels, i.e. and e.g: 66.7%, 33.3%, 16.7%, which factor out at 1.5/100, 3/100, and 6/100 respectively)…see the fractional and odd factors in these magnification calculations? These values make it difficult (you might as well say impossible) to interpolate and display a large image on screen.

Before we go any further…is this making sense to you?
R
rvs053063
May 21, 2007
Yeah, that makes sense. Thanks for the clarification. Really helpful. But then what’s the point of the measurement 5 x 7 inches if it has no bearing in viewing reality? Is it only then for printing? So if I was to print this image (that doesn’t fit on my screen) at its normal size, it would print out the correct dimensions? Put another way, are the dimensions by default then only relevant to printing, not viewing? Thanks.
P
Phosphor
May 21, 2007
"Is it only then for printing?"

Yup. That’s the only place where units of measurement other than pixels matter for an image.

However, units other than pixels will be of some help when doing page layout (say, for a PDF which will primarily be displayed on the web), if only because they’re measurements which we humans are more used to dealing with and imagining. But even with a PDF you still ultimately need to think in terms of raw pixel dimensions when deciding how big your pages and images will display when they are rendered at application-specific 100%. It can also come into play in helping you decide how small you’d like the file size to be for the entire document. The amount of compression that will be applied to the images in your PDF can be set in the dialogue you see when you go to export or save in the PDF format.
R
rvs053063
May 21, 2007
Thanks. Very helpful. Rick
P
Phosphor
May 21, 2007
OK, cool, Rick.

Glad to help.

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