[Jim Royal]

Well if I take an image that's 8" x 10" and 72 ppi/dpi and print it on my Xerox Doc 12 it will without a doubt not look like an 8 x 10 300 ppi/dpi image printed on the same machine. I can tell you which one will look good and which will not before I ever hit the print button.

That's because you first reduced the pixel dimensions of the image. As you well know, the output quality is determined by the total number of pixels in the image combined with the print resolution. However, that is not the issue in this thread.

The original poster was asking if changing the DPI value embedded in his image would affect the output quality, and the answer to that is no. The DPI setting in a JPEG file is pretty much meaningless.

That's because you first reduced the pixel dimensions of the image. As you well know, the output quality is determined by the total number of pixels in the image combined with the print resolution. However, that is not the issue in this thread.

The original poster was asking if changing the DPI value embedded in his image would affect the output quality, and the answer to that is no. The DPI setting in a JPEG file is pretty much meaningless.

What he could do is just lower the quality of the jpeg when doing a save as. Just don't ever, ever reduce the original file always make a copy. I go to print more than I ever go to web and I will admit to having a problem with file sizes there. Going to press or to a digital output is so second nature it sometimes scares me.
When a client asks what we need for output I usually just tell them to set the camera on max resolution and I'll take care of the rest.
When I put something on the web I just set Photoshop to low or medium quality


I tink my keeboard es broka agin. It kepes speling thins wrong. Nede knew keeboard.

[Ben_Edict]

Well if I take an image that's 8" x 10" and 72 ppi/dpi and print it on my Xerox Doc 12 it will without a doubt not look like an 8 x 10 300 ppi/dpi image printed on the same machine. I can tell you which one will look good and which will not before I ever hit the print button.

I think here is a slight misunderstanding between you and Jim. Changing the dpi in Photoshop has no effect on the print quality, as the total pixel count will not be altered. Photoshop simply makes the print dimension larger, when the dpi goes down. So a 4000 x 3000 pixels image at 300 dpi will be scaled to 13 x 10 inches and when you change the dpi setting to 72dpi, the print size will automatically be blown up to 56 x 42 inches.

If changing the dpi setting one must keep the output size constant, in order to reduce the total pixel count. That just needs to be observed.

Ben

I think here is a slight misunderstanding between you and Jim. Changing the dpi in Photoshop has no effect on the print quality, as the total pixel count will not be altered. Photoshop simply makes the print dimension larger, when the dpi goes down. So a 4000 x 3000 pixels image at 300 dpi will be scaled to 13 x 10 inches and when you change the dpi setting to 72dpi, the print size will automatically be blown up to 56 x 42 inches.

If changing the dpi setting one must keep the output size constant, in order to reduce the total pixel count. That just needs to be observed.

Ben

Changing the dpi/ppi changes the SIZE of the pixels. Not the quantity. Printing at 72 abd 56 inches will yield a low quality output when viewed up close.

[jeffkrol]

Unfortunately, smaller than what is usable for web viewing.
It will stop the casual thief.

Isn't that what we really only do anyways? Even w/ "pro" anti-piracy?

BTW: How about just sending a flash link? I find it a bit tricky to strip out images...

[Marc Sabatella]

Changing the dpi/ppi changes the SIZE of the pixels

Changing the *actual* dpi does this. Changing the meaningless number stored in the EXIF does nothing of the sort. In fact, it does nothing of *any* sprt - it does nothing at all. It's as relevant to the discussion as changing the file-modified date. That won't *really* make the picture go back (or forward) in time any more than chanigng the EXIF resolution figure will change the *actual* resolution of any printed image. The actual resolution is number of pixels divided by size in inches - very literally, pixels per inch. Nonumber stored in any EXIF will ever change this. The only way to change to resolution of an image to be printed at a given size is to actually change the number of pixels. You can't divide 3000 by 10 and ever get anything but 300. If you want something other than a 300dpi image in a 10" print, you're going to need something other that 3000 pixels.

Changing the *actual* dpi does this. Changing the meaningless number stored in the EXIF does nothing of the sort. In fact, it does nothing of *any* sprt - it does nothing at all. It's as relevant to the discussion as changing the file-modified date. That won't *really* make the picture go back (or forward) in time any more than chanigng the EXIF resolution figure will change the *actual* resolution of any printed image. The actual resolution is number of pixels divided by size in inches - very literally, pixels per inch. Nonumber stored in any EXIF will ever change this. The only way to change to resolution of an image to be printed at a given size is to actually change the number of pixels. You can't divide 3000 by 10 and ever get anything but 300. If you want something other than a 300dpi image in a 10" print, you're going to need something other that 3000 pixels.

Yeah I'm talking about in PhotoShop under image size. Not in EXIF

[Marc Sabatella]

Well if I take an image that's 8" x 10" and 72 ppi/dpi and print it on my Xerox Doc 12 it will without a doubt not look like an 8 x 10 300 ppi/dpi image printed on the same machine.

True, but that's because the only way you can have an image image that is 72dpi when printed at 8x10 is to start with an image that 576x720 pixels, whereas the 300dpi image needs to have 2400x3000 pixels. It's the fact that you have fewer pixels to work with in the first image that makes it look not as good, some some meaningless number stored in the EXIF. You could change the EXIF resolution of both images to 1000 or to 10 and it wouldn't change a thing about the actual resolution of those prints.

So there are two things going on here. One is that simply changing the EXIF resolution field has *no effect whatsoever on anything* - it's just a number stored in a place that never gets looked at even once during the printing process. The real issue is, how many fewer pixels do you need in order to discourage 8x10 prints. I'd concur with the idea that 576x720 won't look very good at 8x10. But you might be surprised at how good, say, 1000x1250 looks.

True, but that's because the only way you can have an image image that is 72dpi when printed at 8x10 is to start with an image that 576x720 pixels, whereas the 300dpi image needs to have 2400x3000 pixels. It's the fact that you have fewer pixels to work with in the first image that makes it look not as good, some some meaningless number stored in the EXIF. You could change the EXIF resolution of both images to 1000 or to 10 and it wouldn't change a thing about the actual resolution of those prints.

So there are two things going on here. One is that simply changing the EXIF resolution field has *no effect whatsoever on anything* - it's just a number stored in a place that never gets looked at even once during the printing process. The real issue is, how many fewer pixels do you need in order to discourage 8x10 prints. I'd concur with the idea that 576x720 won't look very good at 8x10. But you might be surprised at how good, say, 1000x1250 looks.

How big is a pixel?

[Wheatfield]

How big is a pixel?

It's this big..

How long is a piece of string?

[debbie]

How big is a pixel?

hey!! stealing my questions, are we??

[Wheatfield]

How long is a piece of string?

That one's easy.
It's the square root of a duck.

that one's easy.
It's the square root of a duck.

rofl .

[Ben_Edict]

Changing the dpi/ppi changes the SIZE of the pixels. Not the quantity. Printing at 72 abd 56 inches will yield a low quality output when viewed up close.

Sorry, your definition is sort of confused. You're mixing printing dots with image pixels. But that are two wholly different stories! I have been in the business of both, image acquisition and digital printing, for the last fifteen years and though I sure do make mistakes or do not know everything, I at least get my pixels right... You obviously mean printing dots. But at least with digital printing these do not have a defined final size. First of all the manufacturers don't state values, only ink volumes, not print dot sizes. Secondly during printing these dots get layered and/or dithered to form the final colour dot. The size of that dot is variable as it depends on the saturation and hue of the colour. Conventional printing (offset printing) is ofcourse an entirely different thing and we would talk about lpi and we would need to talk about screen ruling and screen angle.

Image pixels are much easier to understand, though I have never actually seen any statement about their size. I would guess it is a matter of esoterics, as the single pixel ideally is a point = infinitely small. In reality you have a finite number of pixel which you spread out over space as you like. As we can safely assume, that the image formation will not show gaps between the pixels (one difference compared to printing) we could simply divide the image width or height through the number of available pixels to calculate the size of the single pixel.

This is where resolution comes into this play: When the size of the pixel increases beyond the limit, where the single pixel gets visible for the naked eye at usual viewing distances, the quality breaks down. The average eye resolves detail down to just about one arcminute (really trained eyes might be better, down to 30 arcsecs). This is the reason, why generally "normal" printing sizes (for images viewd in a photo album or standard pictures on the wall) require an image resolution of 300dpi. If you have really large prints, which you view from a distance (posters, banners viewed across the stree), you can go down in resolution to perhaps something like 5dpi and the quality will still be adequate.

So, back to your above statement: As you can read out of my longish post above, changing the "size" of a pixel is a necessary consequence of reducing their number, if the overall size of the image (in units of length) stays fixed. You could perhaps devise a algorithm to have the pixel size fixed and would get something like a screen printed image, with lots of white space around each pixel. You could on the other hand preserve resolution and print quality, by simply making the print size smaller, after reducing the number of pixels:

If you reduce an image to 600 x 800 pixels, to make printing of high quality prints impossible, you have the choice to print an image with only 72dpi resolution at a size of 8 x 11 inches. 72 dpi would not look nice. But you could also print an image at 300 dpi, but the size would only be 2 x 2.7 inches - a bit small for most tastes.

Nevertheless many untrained viewers (Auntie and Grandpa) will accept images in most cases happily printed with only 150dpi, because the average viewer will concentrate on the contents and the colour, not so much on edge sharpness or fine tonal differentation. This is the reason, why a simple reduction in pixel numbers might not be a working solution to image protection, because many people would print low res images and would still be happy.

So, I personally think, there are two alternatives for protecting images:

- simply accept that people will print low res images and try to emphasize the advantages of buaing a high res print
- add a watermark of kind to the image

Ben