DPI / PPI and Scaling Images for Large Print

[SBR0630]

Hello!

I am working with a client that has 72 DPI and 150 DPI images he wants blown up to sizes such as 30" x 30," 12" x 24," etc. I haven't printed at this scale, but I was taught to have an image at least at 300 DPI. Here is a blurb from him:

It is my understanding that the DPI number in the Properties section of the image file is a number that is set (somewhat arbitrarily) by the camera maker.  The important factor is the area resolution of the camera sensor expressed as the number of pixels in the width and height dimensions.  One can calculate the dimensions of an image at various DPI resolutions based upon this (72, 150, 180, 200, 240, 300 dpi) based upon those number of pixels.  Therefore, I think it is important to choose an image that has low image area resolution. 

I am now confused as his comments don't make a lot of sense. At a large print, I would think that 72 DPI and 150 DPI is too low. Am I crazy? If I am, please correct me. If I am not, how do I explain it to him? Is checking the DPI via viewing the properties (or popping it in Photoshop and viewing it there) an inaccurate way to find the resolution?

Thank you so much for all of your help!

[SBR0630]

Also, I forgot to mention, this is to be printed. THANK YOU!

[Chuck Uebele]

I used to print a lot of 30X40 inch prints. I used between 100 to 150 PPI. 300 is an overkill.

[gener7]

I agree, large prints are going to be viewed at a distance, so depends how far the viewer will need to be from the print.

Viewing Distance and DPI Chart - Design Resources

[Test Screen Name]

What matters is not the ppi in the file. That is indeed arbitrary, that makes perfect sense, so does talking about actual pixel count. What matters is the Effective resolution, be sure you know how to calculate it. Only the last sentence from your customer is confusing.

Your instinct might well be right, but do the math first. 

[D Fosse]

As I've said many times before:

Stop and think long and hard about what pixels per inch means, quite literally. This is not an abstract unit - it means exactly what the three words say they mean. How many pixels to one inch, times how many inches. That's how many pixels you need.

In this case, 150 ppi will do just fine, as per the table Gene posted a link to.

[SBR0630]

So if I have an image PPI that is 3072 x 2304, if I divide it by 150 PPI, the max size I can print it at would be 20.5" x 15.4," correct? How do I know 150 PPI is sufficient?

[D Fosse]

That's right. Resolution = pixels / inches, or inches = pixels / resolution.

The thing is - the eye wants to take in the whole image. You don't put a book up to your face to squint at the halftone pattern - you hold it at a comfortable distance. Same with an image hanging on a wall. You step in just close enough to take in the image, but not closer. The bigger the image is, the farther away you'll stand.

The key factor here is not absolute resolution, but optical resolution. How many degrees of arc in your total field of vision.

That's why a wall-sized banner at 20 ppi looks just as sharp as the same image in a book at 300 ppi. Because you're essentially seeing the same thing - the optical resolution is the same.

Look at your monitor. Unless you've invested in a new 4K monitor, you're looking at roughly 100 ppi. Do you see pixels? I bet you don't. Because you sit just far enough away.

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Even so, starting from the basic premise of 300ppi for books - there's nothing sacred about this number. All it is, is a theoretical upper limit beyond which no improvement is possible, not even hypothetically, because it's based on the commonly used halftone screen frequency of 150 lines per inch.

This upper limit is then determined by the point at which individual pixels cannot be discriminated. Not even in theory. But it doesn't say anything about sharpness! Only about individual pixels. It can well be sharp at lower resolution. In fact it can be razor sharp all the way down to 150. You may well see some pixels if you really put on your glasses and squint - but it'll be sharp.