Why 72 ppi in "Image Size"

Z
Posted By
Zal
May 11, 2004
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736
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8
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I’m very confused with the fact that 72ppi always there when you open the "image Size" to resize. Why ?
How to know what is the Raw resolution of the image , before I resize either for web or for outside printing ?

And another question: how to properly resize an image for 4×6 printing in outside lab ?

Thanks!
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W
westin*nospam
May 11, 2004
"Zal" writes:

I’m very confused with the fact that 72ppi always there when you open the "image Size" to resize. Why ?
How to know what is the Raw resolution of the image , before I resize either for web or for outside printing ?

Well, that’s the size stored with the file. This may

o come from an image source, such as a scanner or camera,

o be set by the user, or

o be a default value supplied by Photoshop.

<snip>

—
-Stephen H. Westin
Any information or opinions in this message are mine: they do not represent the position of Cornell University or any of its sponsors.
J
JJS
May 11, 2004
"Zal" wrote in message
I’m very confused with the fact that 72ppi always there when you open the "image Size" to resize. Why ?

It’s got to be something, so they (whomever they are) chose ’72’ probably because that was the average screen metric. You don’t need to know "dpi" anyway, just the total number of pixels and maybe your anticipated print/presentation size.
B
bhilton665
May 11, 2004
From: "Zal"

I’m very confused with the fact that 72ppi always there when you open the "image Size" to resize. Why ?

"ppi" doesn’t mean anything until you print. 72 ppi is just a placeholder, likely chosen long ago when many monitors had a 72 dpi pitch (now 96 is more common). You can ignore this until ready to print. If it *really* bugs you go to Preferences > Units and Rulers and in the ‘New Document Preset Resolutions’ change ‘Screen Resolution’ from 72 to a new number (which will be just as irrelevant). This is for CS, earlier versions might have this in a slightly different place.

Images imported from another source (as opposed to new files) will have a default placeholder from the capture device. For example, my film scanner uses the scan rez (usually 4000 ppi) and my digital camera uses 180 ppi, presumably because they think that’s the minimum rez for a decent print. But you can change either one easily enough.

How to know what is the Raw resolution of the image , before I resize either for web or for outside printing ?

Look at the dimensions in pixels, that’s all that counts. If using Image > Image Size just uncheck ‘resample image’ and you can keep the same pixel dimensions while changing the resolution.

And another question: how to properly resize an image for 4×6 printing in outside lab ?

Image > Image Size, uncheck ‘resample’ and put 4 or 6 in width or height, depending on the image orientation. This assumes your image aspect ratio is already 3:2. If not you should crop it to that aspect ratio first.

Bill
XT
xalinai_Two
May 11, 2004
On Tue, 11 May 2004 17:24:51 -0400, "Zal" wrote:

I’m very confused with the fact that 72ppi always there when you open the "image Size" to resize. Why ?
How to know what is the Raw resolution of the image , before I resize either for web or for outside printing ?

There is no such thing as RAW resolution. There are two numbers of pixels: length and width. Assume an image that is way larger than the monitor, 3 Megapixels, 2000x1500pixels in size.

Then there is the idea to print the image.

Now resolution comes into play – either as a requirement or as the result of a size specification.

As a requirement, you say "I want good quality" – then you might hear "That’s 300dpi" or you say "Average photo print quality is enough" – that means "Print at 200dpi".
This requirement will result in the above image being printed 6 2/3"x5" as 300dpi/high quality or 10"x7.5" as 200dpi/average quality.

If you have a size specification instead – like 20"x15" – you tell the person asking for that, the image will print at only
(2000pixels/20"=100dpi) and this isn’t really bad but is designed for a bigger viewing distance.

And another question: how to properly resize an image for 4×6 printing in outside lab ?

Make it 1200×1800 pixels and set resolution to 300dpi.

If the original aspect ratio is not 2:3 crop it to that ratio (you lose part of the image) or put the image on a background that has that ratio (you need to crop the borders later).

If you don’t do that, the lab will do either way and you do not control the process, very bad if they decide to crop the image.

Michael
JH
John Hewett
May 15, 2004
On Tue, 11 May 2004 17:24:51 -0400, "Zal" wrote:

And another question: how to properly resize an image for 4×6 printing in outside lab ?
I had this problem and found help as follows, click on the crop tool, in the crop menu above enter 4in or 6in in the width box and 4in or 6in in the height box (depending on whether you want to crop ‘portrait’ or ‘landscape’) enter 300 in the pixels per inch box. When you crop it will always be at 3×2 ratio for printing 6"x 4" photos.

I did a batch this way last week and the results were excellent with nothing being cropped by the processors.

John.
WA
Wilder and Wilder
May 15, 2004
"Zal" wrote in message
SNIPPED!
And another question: how to properly resize an image for 4×6 printing in outside lab ?

Thanks!
Agfa and Fuji digital minilabs resize your pictures and adjust colour, density and brightness unless the operator intervenes or you specify they do not.

I get excellent results with my postcards by sending the Agfa D-lab, images in 1200 pixel x 1800 pixel files. This results in a 300dpi, 6"x4" photo of superb quality.

All the rest of your stuff has been discussed by others. Some agree some don’t agree about dpi and pixels. The day a pixel has a measurable dimension is the day a printer or publisher will use it to define a size.

Until then the likes of ‘Bill Hilton’ will argue the leg off a wooden chair that pixels rule… Personally, I go for the dot because I can use one on my offset press. I tried to print a pixel once and had Santa up in arms about pixie abuse!

Wilder all the time!
—————————————-
T
tacitr
May 17, 2004
Until then the likes of ‘Bill Hilton’ will argue the leg off a wooden chair that pixels rule… Personally, I go for the dot because I can use one on my offset press.

Dots and pixels are not the same thing. A device like a printing press, which does not print continuous-tone images, prints dots, not pixels; a scanned image, which is a continuous-tone image, is made up of pixels, not dots. Using "dpi" to refer to a color digital image is incorrect; using "ppi" to refer to output from a device like an imagesetter or output printed on a press is not correct.

When a digital image such as a scan is printed on an imagesetter and run on a press, the continuous-tone image is converted to a halftone, and the pixels become dots. Normally, for maximum quality, the resolution of the image in pixels should be double the line screen frequency of the halftone dots; this means that if you are printing with a 150-line halftone–the dots are 1/150 of an inch apart–then the image should be 300 pixels per inch.

When a device like a digital camera takes an image, there is no resolution. The image is defined only in terms of pixel dimension–for example, my camera makes an image 1600 pixels wide by 1200 pixels deep, with no reference to how big each piixel is. Photoshop arbitrarily assigns a resolution of 72 pixels per inch when the image is opened, for historical reasons.

You control what the resolution is. a 1600×1200 pixel image at 300 pixels per inch will be a bit over 5 inches wide, and 4 inches deep. A 1600×1200 pixel image at 200 pixels per inch will be 8 inches wide and 6 inches deep. A 1600×1200 pixel image at 400 pixels per inch will be 4 inches wide and 3 inches deep.

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O
Odysseus
May 17, 2004
In article ,
"Wilder and Wilder" wrote:

… Personally, I go for the dot because I can use one on my offset press. I tried to print a pixel once and had Santa up in arms about pixie abuse!
I bet your problem is that you were using a CMYK press. Get an RGB press and, trust me, the pixels will be unlikely to complain about their working conditions.

—
Odysseus

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