Magnification
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How do I know what magnification do I have?
I scanned my picture, the whole image size is 2464x3432 pixels. I cropped it to remove excess background to 1260x1260 pixels. Then I selected a 296x296 pixels and duplicate that to open in a separate window. I need to know the magnification of that window. Does anybody know how to measure it? Thanks! -- View this message in context: http://imagej.1557.n6.nabble.com/Magnification-tp5002383.html Sent from the ImageJ mailing list archive at Nabble.com. -- ImageJ mailing list: http://imagej.nih.gov/ij/list.html |
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Hi Esther,
Here's the problem. Suppose that you have a photo of a 25 foot tree, and you scan it with your scanner. Then, suppose that you crop your photo so that all you see is the height and width of the tree. Since you know that the height of the true tree is 25 feet and its height on the screen, on which you project it while on your visit to a friend's in a far-away place, is 5 feet. The true magnification then is 1/5 or 0.20X. Since you know the height of a tree, and have named the image file of the tree, "photo of 25 foot tree at home", you will always be able to calculate the magnification of the tree using your knowledge of the true height of the tree. Now, imagine that time has passed - ten years - and the tree has been trimmed to look just as it appeared in your old photo of the tree when it was 25 feet. You call a tree surgeon - who has the appropriate liability insurance - and ask her/him to trim the tree at its current height, which is 30 feet, so that it appears just like the tree appeared 10 years ago. S/he does a great job, so much so that a new photo of the tree now looks almost identical to the old photo, and you place them on a page in your year's-end report to your family such that each image is the same size. For the sake of argument, assume that both trees are the same height on the page - 5 inches, what magnification would you apply to each. For the younger tree, where the height was 25 feet 10 years ago, the magnification would be 5 inches divided by 300 inches (= 25 feet) which = 0.017X). For the older tree, where the height is now 30 feet, the magnification would be 5 inches divided by 360 inches (= 30 feet) which = 0.014X). No, suppose that you wanted to present the trees so that they would show the change in ten years. The difference in height would be first based on the page height of the 30 foot tree - say, 5 inches. The younger tree would have to be shorter by what factor. 1. The height of the 30 foot tree, on the page, is 5 inches. 2. The height of the 25 foot tree, on the page would have to be: 25 feet divided by 30 feet = 0.8333333333+ = 83.333% of the 5 inch height of the 30 foot tree, or 5 inches times 0.8333333333+ = 4.167 inches. Its magnification, relative to the actual 25 foot tree would be, 0.14X (NOTE!!!!: This is the same magnification of the younger 30-foot tree which is 5 inches on the page. Hmmmmm!!!!! The lesson here, I hope, is that if you know the actual dimension of one object in a photo, you can always compute the actual magnification of any copy of the image. Without an actual dimension at the time of image capture, the magnification an object in the image can only be determine IF on has a scale bar attached to the image at the time of capture OR a notation of the actual area, width, or height of the space captured in the image. Even a notation like, "John on the day he was 5 feet tall" will enable you to calculate the magnification of any derivative image in which that image of John is included - even if the original has been cropped. Magnification is not to be trusted as much as 'scale' or recorded dimensions of image objects AT the time of image capture. "That was taken when john was 10," will not help you with magnification UNLESS you kept careful records of his growth - OR know the width and/or height of the garage door that was right behind him. Hope this helps, Fred Monson Frederick C. Monson, PhD Technical Director Center for Microanalysis and Imaging, Research and Training (CMIRT) West Chester University of Pennsylvania Schmucker Science South - Room SSS-024 MailDrop: Geology-Astronomy 750 South Church Street West Chester, PA, 19383 610-738-0437 [hidden email] HomePage: http://cmirt.wcupa.edu (with link to instrument Scheduler) -----Original Message----- From: ImageJ Interest Group [mailto:[hidden email]] On Behalf Of esther31 Sent: Monday, March 25, 2013 12:44 PM To: [hidden email] Subject: Magnification How do I know what magnification do I have? I scanned my picture, the whole image size is 2464x3432 pixels. I cropped it to remove excess background to 1260x1260 pixels. Then I selected a 296x296 pixels and duplicate that to open in a separate window. I need to know the magnification of that window. Does anybody know how to measure it? Thanks! -- View this message in context: http://imagej.1557.n6.nabble.com/Magnification-tp5002383.html Sent from the ImageJ mailing list archive at Nabble.com. -- ImageJ mailing list: http://imagej.nih.gov/ij/list.html -- ImageJ mailing list: http://imagej.nih.gov/ij/list.html |
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