Chris J said:
I have taken a photo on my camera (6MP), saved it to my PC (it can ONLY be a Jpeg) and then emailed it to someone - they say it is low res... but I've taken it with the highest quality I can.
The image I've given them is 2848 x 2136
That is a high resolution image. The people you are dealing with appear to be a little dumb.
If I open this in photoshop and change the resolution to 300 pixels - then it will be (largest)
9.5" x 7.12"?
Errr. no. It doesnt work like that, really.
I hit save as - to create a new image then email it to them and they will be happy?
Quite possibly. They are dumb. Who knows what will make them happy?
Pixels per inch is one of the most misunderstood things about photos. Its important for the
printed image but it is largely irrelevant as an attribute of your image file.
Your image file has a certain size - in pixels. It also has an attribute that you can set in your image-editing program called "pixels per inch". But altering this on its own does not necessarily do anything at all to the image. The number of pixels is the same - all you have done is stuck a virtual post-it note on the image saying "Dear printer; if you can, print this at so-many pixels per inch".
Photoshop also lets you specify an actual size for the image. But this can be confusing, as the image doesnt have a "size" - its an array of pixels, not a physical thing. (And, as far as I know, physical dimension is not a parameter of your image file - it only keeps a note of the pixels/inch). Clearly, if you have three parameters - pixel number, physical dimensions and pixels/inch - they wont all necessarily match, and something has to "give". The thing that gives is the actual number of pixels - your image editing program will delete data, or interpolate data to make the number of pixels match your specified physical dimensions and pixels/inch.
The only parameter about your image that is actually important is the overall number of pixels. Think of the pixels/inch as an "advisory" virtual post-it note, and the physical dimensions as something that is used to allow your editing program to decide whether to change the overall number of pixels.
When preparing photos for Speleology, I only ever use the actual pixel size. If you send me an image that is, say, 2848 x 2136 and I want to print it three inches wide, I will reduce it to 1050 pixels wide which at 3" is 350 dpi which is our standard resolution that we work to (it allows for some leeway in sizing amongst other, rather technical reasons). But I dont do that by altering the dpi or the size - I manually change the number of pixels because that's what Im actually interested in: I want the ultimate control over what Im doing to the pixels.
I would never say to you "send me an image at 350 dpi", because that ... well, it just doesnt make sense if I dont tell you the size Im going to print the image as well!
You may be wondering why it is necessary to re-size the images "manually" at all. Well, I dont
need to: If I just paste your 2848 x 2136 image into my DTP software, and make it look 1" wide, what happens at a later stage of the process (e.g. when I make a PDF or when the publisher outputs the file to his plate-maker) is that the additional pixels in the embedded image are thrown away. You'll see this if you paste a whole lot of photos into Word and then make a PDF. The PDF is usually a lot smaller than the Word document. So why do I bother to manually re-size and throw the pixels away myself? The answer is that it gives me a much better control over the operation. Some processes such as the unsharp mask, which is an essential part of the operation, only work well if they are applied to an image that is at its final printing size. Another reason, of course, is that the files are smaller if theyre the correct size. You wouldnt fill your web page with 1MByte photos and rely on the visitor's browser to re-size them (which it will happily do). Instead you reduce them to a sensible size to start with so that they download quicker.
Where this leaves you, Im not sure, because you have been given a request that is basically nonsensical. If they mean 300 pixels/inch then applying that setting to your image does not, in itself, do anything at all. If you apply it at the same time as specifying a physical size then the number of pixels in your photo could be altered (irreparably) to fit the request. Clearly if you said to PhotoShop "300 dpi and 2 inches wide, please" then the photo would be re-sized to be 600 pixels wide and, if they want to print it at 6" wide, its going to look a little sub-standard.
Of course they might
really have meant 300 pixels but if that's to be printed on paper, that's a very small image! 300 pixels on a computer monitor is a decent size. If your monitor is 1280 x 1024 for example, its about 30% of the screen width.