Posted: Tue Dec 20, 2005 2:15 pm Post subject: Overall dimensions
Alright, someone asked me over the weekend how the dimensions of an image affect (effect?) the amount of pixels. I was stumped, simply for the sole reason that I forgot how to come up with the numbers. Simply put, I couldn't (still can't) for the life of me remember how to figure up a pictures size based on dpi, and size in inches, ig. an image at 72dpi that's say 39 in by 59 in is how many mb's? More over, could you (I) use the width and height pixels along with the dpi?
Here, this is much easier: What's the formula for figuring out the dimensions of a picture (image, etc.)? _________________ All gave some, some gave all.....Lest we forget that war produces veterans, wounded both mentally and physically, and it is our job to help them now, as they have already helped us all in ways we will never know, and in ways that we take for granted every day.
I wrote a whole thing describing the math... tossed it.
(I'm so not qualified to talk about math!)
As a base: One pixel in a grayscale pic is one byte. One pixel in an RGB pic is 3 bytes.
So as a rough rule of thumb: find the total numbers of pixels (in your example- 72 X 39 height, 72 X 59 width, then multiply those two numbers). The file size (uncompressed) will be about three times the number of pixels in your RGB (24-bit) image.
so-
72 X 39=2808
72 X 59=4248
4248 X 2808=11.9 million pixels
11.9 X 3 = (about) 34.7 MB file.
you can check this by going in PS File>New and typing in those factors.
(PS shows 34.1 MB - that's because a KB is a little more than 1000 bytes and a MB is a little more than 1000 KB)
and this is all for describing open file size. Compressed files is another animal.
EDIT:
And as a Post Script (Because I get so frustrated with ppi/dpi misconceptions)
The term ppi (“dpi” expresses a printer’s capabilities) only describes the size of the pixels when printed. It has NOTHING to do with image resolution.
“PPI” describes how big the pixels will be printed. Period.
When you see the terms 72 ppi, 300 ppi, 1200ppi. Just think of those numbers meaning: pixel size of 1/72 in, 1/300 in, 1/1200 in, WHEN PRINTED.
Imagine you have a 1 X 1 pixel image. (just one lone pixel)
Now, lets go to Image>Image size and change the resolution.
72 ppi?
300 ppi?
1200 ppi?
All you are changing is the SIZE of the pixel when PRINTED. Nothing more. Now extrapolate this concept to other images. You see that the real resolution information lies in total number of pixels at capture, not in “ppi”.
Nicely put cbadland. Again, I was stuck on printing and kept (still) used the terms dpi instead of ppi. I know the difference, it's just that I print nearly everything, so I'm constantly stuck on dpi this and dpi that, with a dpi here and a dpi there, everywhere a dpi....stevealmighty was a dpi.....kidding, you can sing that to the old mcdonald had a farm song. Again, my fault with the dpi instead of ppi.
I'm guessing that I (you, we, whomever) could plug in the same numbers on a camera that captured 300 ppi right? Man, to many numbers, and I'm less qualified that you are to talk about math stuff. I'll have my 12 year old son explain it to me tonight when I get home!
Thanks again!
EDIT: BTW, I had the jist of that figured out, I was neglecting to multiply by 3. Obviously threw everything off. _________________ All gave some, some gave all.....Lest we forget that war produces veterans, wounded both mentally and physically, and it is our job to help them now, as they have already helped us all in ways we will never know, and in ways that we take for granted every day.
I’ve given up my fight against the dpi vs ppi term use (a losing battle). I meant it only as a mention.
My real soapbox rant followed: trying to get people to think about ppi as “pixel size when printed” and not as the resolution of an image. (Another uphill battle for sure!)
So many get caught up in the “ppi” of an image. True image resolution is determined by the number of pixels (among other things), not the pixel size when printed. An image’s ppi is just tad of metadata that travels with the image file describing how big it will be when printed.
As an electronic file, the ppi of an image is practically meaningless.
I’ve given up my fight against the dpi vs ppi term use (a losing battle). I meant it only as a mention.
My real soapbox rant followed: trying to get people to think about ppi as “pixel size when printed” and not as the resolution of an image. (Another uphill battle for sure!)
So many get caught up in the “ppi” of an image. True image resolution is determined by the number of pixels (among other things), not the pixel size when printed. An image’s ppi is just tad of metadata that travels with the image file describing how big it will be when printed.
As an electronic file, the ppi of an image is practically meaningless.
I agree. I also disagree. Makes sense, but doesn't make sense. I was with you until you said that very last sentance. The size of the image is made of the total number of pixels, not the ppi, yes, I agree. But, as an electronic file, the ppi of an image is pratically meaningless....I'm not to sure about that one. That may stand true for web graphics, but for photography, most images are shot at 72 ppi (jpeg), while most pros shoot in a .tiff format or raw format which captures 300 ppi. Big difference in the file quality and size. Images of (roughly) the same overall size (say 19 mb) would be different sizes (in inches) when opened in PS or printed; example; an image that is 19.7 mb and is 72ppi, is 3216 pixels wide by 2316 pixels tall and is 44.667 inches wide by 29.667 inches wide. An image that is 300 ppi is 4288 pixels wide by 2848 pixels tall and is 14.293 inches wide by 9.493 inches tall and is 35mb in overall size. The actual size (in inches) is what photographers are concerned with for printing, followed by the ppi of the image. I prefer to capture images at 300 dpi as they tend to be slightly sharper when enlarged and/or altered. 300 ppi for the internet would be overkill. The size difference comes from the ppi of each image, as there's a big difference between 72 ppi and 300 ppi, specially when it comes to printing. Not to mention that the 300 ppi image is smaller (in inches) than the 72 ppi image. Or does this all relate back to "...describing it when printed"? Just wondering, and letting you know how I look at it. Certainly not trying to argue with you, as you should know by now that I do have great amount of respect for your knowledge of PS and all things related! But I'll only get smarter if I discuss and ask (stupid?) questions! _________________ All gave some, some gave all.....Lest we forget that war produces veterans, wounded both mentally and physically, and it is our job to help them now, as they have already helped us all in ways we will never know, and in ways that we take for granted every day.
I would imagine a lot of people would.
It’s a sit around and have a coffee (or beer?) discussion.
My reasoning is that cameras don’t capture at any particular ppi. That is just a description added to the file after capture. The size difference you see between the JPEG (72ppi) and the TIFF (300ppi) is due to the JPEG compression, not the ppi number. (well in your example the overall pixel dimension of the JPG is less, but it doesn't have to be.)
Do this experiment:
Shoot a TIFF image, and shoot a JPEG image. (at identical number of pixels) now open the two in Photoshop. Go Image>Image Size and look at the opened file size at the top of the Image Size dialog. They will be the same, even though one picture is 72 ppi and the other 300 ppi.
Beer. Definately. I'm old enough to drink, so if you're not, you can be my DD.
I see what you are saying. Yep, beer it is. LOL!!! _________________ All gave some, some gave all.....Lest we forget that war produces veterans, wounded both mentally and physically, and it is our job to help them now, as they have already helped us all in ways we will never know, and in ways that we take for granted every day.
Hey cbadland, ever toss in interpolation as a factor? Or is that not really a factor for web design and stuff? Just curious. Some cameras deal with it better than others, and on average, .tif and .raw formats deal with it better than .jpg's do. _________________ All gave some, some gave all.....Lest we forget that war produces veterans, wounded both mentally and physically, and it is our job to help them now, as they have already helped us all in ways we will never know, and in ways that we take for granted every day.
I was not considering interpolation in my little diatribe (on purpose).
If you are not interpolating pixels (ie Resample is left unchecked in the Image Size dialog) then you can type various ppi in the Resolution input ‘til the cows-come-home and NOTHING happens to your file. 72 ppi, 300 ppi, 6000 ppi, 2 ppi. Makes no difference to your file, it stays just as it was. (Which is why I said ppi in an electronic file is meaningless).
When you interpolate, Photoshop actually does something to the image. (you have a choice of algorithms) I will almost always downsample for web (say turn a 1000 X 1000 px image to a 100 X 100px image). In that case I use Bicubic Sharper. (For upsampling use Bicubic smoother. In PS6, I think you only have Bicubic as a choice.)
Photoshop then has to figure out how to convert a picture made up of one million squares look right with just 10,000 squares. No small feat.
Quote:
Beer. Definately. I'm old enough to drink, so if you're not, you can be my DD.
They stopped carding me a decade or so ago at the beer store.
Beer. Definately. I'm old enough to drink, so if you're not, you can be my DD.
They stopped carding me a decade or so ago at the beer store.
Good....I'm not alone
Does it make a difference in the file size if you resample it one way or the other? ig. Does bicubic sharper make it smaller than if you did it bicubic smoother? Is one better for web use while the other is better for printing? _________________ All gave some, some gave all.....Lest we forget that war produces veterans, wounded both mentally and physically, and it is our job to help them now, as they have already helped us all in ways we will never know, and in ways that we take for granted every day.
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum You cannot attach files in this forum You can download files in this forum
