Request for 300 dpi picture is in itself meaningless

oblivion

oblivion

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Richard
Edit My Images
Yes
A little confusing for me to read and wondered what you peeps reckon on this link I found? Is this true?

http://www.rideau-info.com/photos/faq.html

Snippet from page:

(there is no such thing as a 300 dpi, or any dpi digital photo). What they want is digital image of sufficient pixel dimensions to be able to deliver 300 pixels per inch at the dimensions of the printed image. So, for the "300 dpi" request to be meaningful, you'll have to know the intended output (paper) dimensions
 
for a picture thats printed out at 6x4 inches at 300dpi you'd need an image 1800x1200 pixels. dpi for a display is pointless
 
I recently applied for a photocard from Transport for London. You could upload your own picture but it had to be 300dpi. I just sent them a file large enough to print passport size.
I then sent an email asking what they really wanted as I assumed that they meant 300ppi but they did not give the physical size of the final photo, so was it OK to send a 150 pixel by 75 pixel jpeg as they could then print it 0.5" by 0.25" :D.
I received a reply saying that they were looking into my query and would let me know. That was 4 weeks ago and I am still waiting for the reply. (The photocard arrived after 2 weeks)
 
Yes it's all so confusing. All I do is size the photo the size I want it printed ie; 12x8" and always save from photoshop at 300dpi as this is the resolution the company's printer requires. This all seems to produce excellent quality.
 
Perhaps a better way of putting it would be for the printing firm to ask for a file of such a size that they can print to your desired dimensions at 300 dpi.
 
nex7photo said:
Yes it's all so confusing. All I do is size the photo the size I want it printed ie; 12x8" and always save from photoshop at 300dpi as this is the resolution the company's printer requires. This all seems to produce excellent quality.
Click to expand...

So in other words you send a 3600 x 2400 pixel image (look at the pixel dimensions):thumbs:
 
Think of the pixels as hundreds and thousands sweets.

Let's say you have 8,640, 000 sweets. When spread out you have 3600 x 2400 sweets.

Now what size do the take up on the floor? Really that depends how tightly or loosely you put them together and that would be sweets per inch (spi)

If you have 300spi then the space on the floor would be 12 x 8 inches. If you think about it. 3600 sweets aross divided by 300 =12 and 2400 sweets down divided by 300 =8.

Let's now say you want the area to be 24 x16. You spread out the sweets so there are less sweets per inch. In this case you would have 3600/24 = 150 and 2400/16 = 150

So you need 150spi to get that sized area.

Now swap spi for ppi and sweets for pixels.

Pixels per inch in itself is meaningless. Ppi ONLY REFERS TO PRINT SIZE for a set amount of pixels. So you can save an image for the web at 600 x 400 and if the ppi value is 72 or 300 it doesn't change the image. It ONLY CHANGES THE PRINT SIZE. @72ppl it would be 8.33 x 5.55 inches but at 300ppi it would be 2 x 1.5 inches.

The image hasn't changed at all. All you have done is take the pixels and squeezed them closer together.

Pixels (Px) (on each axis) = print size (PS) x resolution (R)

Can also be written as
R = Px / PS

Or
PS = Px / R

Try the calculation on each axis, that is why ppi doesn't matter when discussing a digital image. The only thing that matters is the number of total pixels. Wen sending to print you decide then how tightly you want those pixels.

Hope that helps.

Jim
 
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Dpi and ppi are generally referred to meaning the same thing but in fact dpi is different. Dots is physical thing like dots on a printed image . Pixels are what your image is made of so when discussing digital images try referring to the correct term being PIXELS PER INCH.

my printer outputs at 5760 x 2440 dots per inch. And I can send any resolution image I like to be printed at that resolution. Be it a 72ppi image or 300ppi. Think pixels not dots
 
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So I guess I confused people more? :D
 
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