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Photo Blow-up Question

Explorer ,
Feb 11, 2024 Feb 11, 2024

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Hi everyone! I have a quick question about the limitations of a photo blow-up for a poster. I can't share screenshots since the content is proprietary, but I will do my best to describe:

 

1. Boss wants an 5.5" x 8.5" photo to be blown up for a poster that is at least 30" on one side

The specs for the file I was given are: 1650 px x 2550 px (5.5" x 8.5"), Resolution is 300 and file size is 12M

 

2. I didn't think that would work because that's over double the size of the original so my assumption would be that the image would start to pixelate.

 

3. In Photoshop, I resized the image to be 30" on the long side and the new specs are:

Image size: 200M

Width: 12.413"

Height: 30"

Resolution 300

 

4. The image looks great when I open it and the resolution seems optimal based on that image size box.

 

My question is: would this actually print correctly if we send it to the printer? How is that resolution staying so stable and why is there no pixelation if it has been blown up to over twice its size? 

 

Thank you! I hope this isn't an overly basic question, I am just trying to understand as much as I can about image size/resolution/dimension 

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Adobe
Community Expert ,
Feb 11, 2024 Feb 11, 2024

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Note that the Resample box is checked. With this, Photoshop adds or removes pixels what an image is resized, setting the pixel dimensions based on the print dimensions and resolution.

 

When the Resample box is not checked, the resolution is set based on the pixel dimensions and the print dimensions.

 

There are several resampling methods available. Automatic is a good general purpose one, but they all have their uses.

 

It's a good idea to discuss this with the printer you use, as they can recommend what would work best on their press.

 

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Explorer ,
Feb 14, 2024 Feb 14, 2024

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Thank you for your answer!

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Community Expert ,
Feb 11, 2024 Feb 11, 2024

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You may not need 300 PPI, it depends on viewing distance/conditions and the variables of the printing method.

 

Without resampling, you would only have an effective resolution of 85 PPI for 30 inches on the longest edge... So some resampling is probably required to avoid visible pixels.

 

The final image quality will depend on the source image content/quality and the resizing interpolation method used.

 

When you say it "looks great", what viewing % are you evaluating this at?

 

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Explorer ,
Feb 14, 2024 Feb 14, 2024

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Thank you for your answer! I was just viewing it on my Mac desktop, so probably not a great assessment I am guessing

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LEGEND ,
Feb 12, 2024 Feb 12, 2024

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Posters can be printed in the 90-150ppi range and usually look pretty good. Photoshop has some built-in upsampling (in the Camera RAW module, and there is a neural upsampling filter) and there are numerous third-party solutions. I'd just try it in PS and see how it looks.

The printer should have a wide-format printer. Ask if they have a printer profile (to help with color matching.)

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Explorer ,
Feb 14, 2024 Feb 14, 2024

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Thank you! I will definitely ask my printer

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Community Expert ,
Feb 12, 2024 Feb 12, 2024

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The above is entirely right. You do not need 300 ppi for large format printing. The bigger it is, the farther away it will be seen from, so the optical resolution stays the same. The eye wants to take in the whole image. You won't stick your nose into it.

 

Just for demonstration, look at a traditional monitor at 95-120 ppi. How close do you need to go before you see pixels? That's right, very close. For a wall-sized banner, as low as 15 - 20 ppi may be good enough.

 

300 ppi is for books and magazines to be seen at arm's length or less. Even so, there's nothing magical about the 300 number. It's not even a sharpness limit - it's a smoothness limit. With a standard halftone screen at 150 lines per inch, 300 pixels per inch is the theoretical upper limit beyond which no individual pixels can be detected. But it can still be eminently sharp at lower ppi.

 

ppi3b.jpg

ppi2.png

 

 

All that said. 2250 x 1650 pixels is a small file to begin with. That sets limits to reproduction size under any circumstances. At 30", you will end at 75 ppi. While not disastrously low, it's still not very good.

 

Generally, all resampling disrupts the pixel structure and degrades the image. This is especially true for jpegs, which will never look good upsampled. In most normal circumstances, the image will look best as-is, without any upsampling. But sometimes you have to, to avoid visible pixels.

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Explorer ,
Feb 14, 2024 Feb 14, 2024

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This is so helpful, thank you for taking the time to explain these concepts!

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Community Expert ,
Feb 12, 2024 Feb 12, 2024

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'I am just trying to understand as much as I can about image size/resolution/dimension'

 

Resizing can be done two ways.
First without resampling. So your 8.5" x 5.5"  300ppi image becomes 30" x 19.4" 85ppi. Overall it has the same number of pixels 2550x1650 =4.2 million pixels and the image is unchanged.  If it is viewed from a greater distance so that it takes up the same field of view as the original viewed closer then the two will look identical. Only if you move closer will you see the pixel structure.

Second with resampling. If you resize and resample to 300ppi then your 8.5 x 5.5 300ppi image now becomes a 30" x 19.4" image at 300ppi and measures 9000 x 5824 = 52.4 million pixelspixels. You will not see any pixel structure, but those additional 48 million pixels had to be invented by the software based on the initial 4.2 million real image pixels. This 'invention' of image data can introduce artifacts.

 

So it is all about striking a balance between the visibility of the image pixel structure and the introduction of image artifacts.

 

What becomes important in striking that balance is viewing distance. It is obvious really, but the closer you get to view an image the more detail you can see.

 

There is a handy formula, based on good eyesight, to work out what ppi is required in order to avoid seeing image pixels.

ppi required = 6878/viewing distance in inches.

So, for example, if your poster will be viewed from say 4 feet away.

The formula gives 6878/48 = 143ppi. At that distance a higher ppi is finer than the eye can resolve.

Going any higher than that is wasted and just increases the chances of introducing resizing artifacts.

 

I hope that help

 

Dave

 

Edit : D Fosse posted while I was typing. We are both saying the same thing 🙂

 

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Community Expert ,
Feb 12, 2024 Feb 12, 2024

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@davescm 

 

Putting that formula into a basic script:

 

2024-02-13_11-42-34.png2024-02-13_11-42-40.png

 

 

/*
Print Resolution Calculator Viewing Distance - Feet.jsx
Formula from @davescm
v1.0 - 13th February 2024, Stephen Marsh
v1.1 - 14th February 2024, Added canvas size to the alert
https://community.adobe.com/t5/photoshop-ecosystem-discussions/photo-blow-up-question/td-p/14416674
https://community.adobe.com/t5/photoshop-ecosystem-discussions/quick-question-how-to-you-tackle-image-size-regarding-printing-projects-distance-dpi/m-p/13273751
NOTE: This script assumes that the correct print size is set in the Image Size window
*/

#target photoshop

if (app.documents.length) {
    var theViewingDistance = prompt("Enter the viewing distance in feet:", "1");
    if (theViewingDistance < 1) {
        theViewingDistance = 1;
    }
    var feetToInch = (theViewingDistance * 12);
    var theResult = Math.round(6878 / feetToInch);
    alert("Poster Size Print Resolution Calculator: " + "\r" + "Viewing Distance : " + theViewingDistance + " Feet" + "\r" + "Calculated PPI Value: " + theResult + " PPI" + "\r" + "Document Resolution: " + Math.round(app.activeDocument.resolution) + " PPI" + "\r" + "Canvas Size: " + activeDocument.width.as('in') + " x " + activeDocument.height.as('in') + " Inches" + "\r\r" + "(Based on the viewing distance, the ability to resolve 60 line pairs per degree and current document print dimensions)");
    
} else {
    alert("A document must be open!");
}

 

 

Or for those of us who use the metric system:

 

/*
Print Resolution Calculator Viewing Distance - Metres.jsx
Formula from @davescm
v1.0 - 13th February 2024, Stephen Marsh
v1.1 - 14th February 2024, Added canvas size to the alert
https://community.adobe.com/t5/photoshop-ecosystem-discussions/photo-blow-up-question/td-p/14416674
https://community.adobe.com/t5/photoshop-ecosystem-discussions/quick-question-how-to-you-tackle-image-size-regarding-printing-projects-distance-dpi/m-p/13273751
NOTE: This script assumes that the correct print size is set in the Image Size window
*/

#target photoshop

if (app.documents.length) {
    var theViewingDistance = prompt("Enter the viewing distance in metres:", "1");
    // Metres to feet
    theViewingDistance = (theViewingDistance * 3.281);
    if (theViewingDistance < 3.281) {
        theViewingDistance = 3.281;
    }
    var feetToInch = (theViewingDistance * 12);
    var theResult = Math.round(6878 / feetToInch);
    alert("Poster Size Print Resolution Calculator: " + "\r" + "Viewing Distance : "  + (theViewingDistance / 3.281) + " Metres" + "\r" + "Calculated PPI Value: " + theResult + " PPI" + "\r" + "Document Resolution: " + Math.round(app.activeDocument.resolution) + " PPI" + "\r" + "Canvas Size: " + (Math.round(activeDocument.width.as('cm') * 100) / 100 + ' x ' + Math.round(activeDocument.height.as('cm') * 100) / 100 + ' CM') + "\r\r" + "(Based on the viewing distance, the ability to resolve 60 line pairs per degree and current document print dimensions)");

} else {
    alert("A document must be open!");
}

 

https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html

 

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Community Expert ,
Feb 12, 2024 Feb 12, 2024

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That's clever Stephen. 

Dave

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Community Expert ,
Feb 13, 2024 Feb 13, 2024

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Thanks Dave, however, that was simple compared to the original formula!

 

More could be done... Perhaps opening up Image Size with the calculated PPI value set, or just automating resizing a copy of the original with the calculated value. There are many possibilities depending on how far one wishes to take this!

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Explorer ,
Feb 14, 2024 Feb 14, 2024

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Thank you so much for this thorough answer Stephen_A_Marsh! I really appreciate the detail you have given here

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Explorer ,
Feb 14, 2024 Feb 14, 2024

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Wow thank you for taking the time to answer davescm!

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