Image Ratio of 1:1

P
Posted By
pjmacjr
Sep 12, 2003
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1370
Replies
6
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Closed
I have a number of photos in which I need to make the image on the computer (or printed image) equal to the actual size of the object.

For my research I am looking at a number of tooth photos and need to measure their surface area. In each photo I included a centimeter scale. So in photoshop I am able to calibrate the measuring tool to reflect the size of this scale and thus the actual size of the object. I know the calibration works because I am able to get the same linear measurement in photoshop as I did on the tooth itself. (Just for reference, my centimeter scale seems to be equal to 10cm on the rulers included at the top of the image in photoshop).

What I need to do now, is to find a way to have the image on the computer, for example my centimeter scale, equal to an actual centimeter. Or in other words, be able to resize the image to a 1:1 scale. I need to be able to print images that are the actual size of the object. Or better yet, determine the 1:1 ratio and then be able to enlarge the photos (before printing) so they are exactly 5x the size of the object.

Sorry for any confusing language, I am new to photoshop. Any help on this matter would be greatly appreciated.
DM
Don McCahill
Sep 12, 2003
Check your printer manual. If your printer is 300 dpi, then you need to use a resolution of 300 ppi. If the printer is 600, then set the resolution of the image to 600.

I know of no way to ensure that an image will be actual size on all monitors, since each monitor uses a different resolution. Compound this by the fact that in default mode Internet Explorer will shrink the image to fit within whatever size window is open, and there are more problems, if this is part of your situation.
Y
YrbkMgr
Sep 12, 2003
Slightly confusing. But if I have you right, your image is already 1:1; inches in your image will equal inches. What I think you’re asking is a way to show the scale as in a legend, with a centimeter scale. If that’s the case, here’s what I’d do.

Right click on your rulers above and make them display centimeters. Use the measure tool to create line that is the length of centimeters that you want. Snap guides to each end of the line made by the measuring tool. Draw your horzontal line using shift so that it’s straight. If you want graduations, use the measuring tool (with shift so that’s it’s straight) and draw your "halfway" points, then snap guides to those endpoints and draw vertical lines to show the graduations. In essence, you are creating a centimeter ruler.

When you print, no matter what resolution you use, provided your document dimensions remain the same, it will be one to one. Of course, if you lower resolution keeping the image size the same, you lose quality. That nothwithstanding, you’ll print 1:1.
JS
John Slate
Sep 12, 2003
First of all, do not set your resolution to 600 ppi for a 600 dpi printer. The final resolution can be a range of values and that range is determined by the final printing process, whatever it is. But printer resolution does not dictate image resolution, printer resolution is what renders dots and type, not photograhic imagery. 300 ppi is a good rule of thumb for high resolution (2400 dpi), and for lower res printers, like 600 dpi, obviously even less image resolution is needed. Of course using lower image resolution would imply that the lower rez output is the final use of the file.

Second, I don’t think that P. James is concerned about a same size display, rather wants to set the image size to print out at actual size to the real objects, and has photographed a ruler as a reference.

That being the case, use the measure tool to measure the 10 centimeters on the ruler that is within the photograph (naturally with your units & rulers preference set to centimeters). Then, do the math to figure the scale percentage you need to apply to the image to make that ruler the right size. Divide 10 by whatever measurement your read, and multiply by 100 to give the proper scale percentage. Then (unless your answer was 100%), go into image>image size, uncheck resample, and in the "print size" section, set either the height or the width scroll-field to "percent" instead of "cm", type in the scale percentage your figured above and say OK. It should now be the correct size. Whether or not the resolution is now sufficient (or too much) for your needs remains to be seen, but if that needs adjustment, you can re-enter image>image size, check resample, and change the resolution only, leaving height and width alone.
Y
YrbkMgr
Sep 12, 2003
Then, do the math to figure the scale percentage you need to apply to the image to make that ruler the right size.

This presumes that his image is larger than his media (paper). If his image fits on the paper, then no re-scaling is needed is it? When you draw a 10cm line, it’s going to print 10cm long.

If I scan a 6" ruler in at 600ppi, it’s huge for on-screen display. But it’s only 6" long and will print 6 inches. So if what he’s trying to do is show scale, he only needs to draw an object and measure it using the rulers (in this case, in centimeters).

What am I missing?

Peace,
Tony
JS
John Slate
Sep 12, 2003
Partly what you’re missing is that on display size has nothing to do with what he’s after. True-to-life print size is his goal, whether or not the image will fit on his paper.

Sure, if you scan a ruler it will be the correct size already, unless you applied a scale in the scanning interface. On the other hand, if you photograph a ruler, it will not necessarily be true to size. More than likely, if it takes up a good portion of the image length in a digital camera shot, it will come in very large at 72ppi.

To simply open an image, then draw a ruler on it does not make the image true size, just the ruler you have drawn.

Remember, he has included the ruler in his photos. Naturally, the ruler would have to be on the same focal plane as the teeth in order to be an accurate reference.
Y
YrbkMgr
Sep 12, 2003
Got your point; I forgot that we’re looking at teeth, probably magnified, so the tooth, even though the picture is 8×10 or whatever, is much smaller. <slaps forehead>. Thanks John.

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