How to reduce image size for prints

[JeffreyW]

I have a JPG that is 20"x30" and would like to print the picture in 12"x18", 10"x15", and 8"x12".

Do I simply go to Image --> Image Size and set the width/height and then export the picture to be printed?

Thx

[D Fosse · Accepted solution]

You could do that, but know what you're doing first.

Photoshop doesn't work with physical sizes, that's not a property of the file. Photoshop only works with pixels. Pixels don't have size, they are just data points. The pixel dimensions is the file.

Size is set arbitrarily later. Any image can be printed at any size. The bigger you print, the lower the pixel density - as you can imagine. The pixel density is expressed as pixels per inch, ppi. It means exactly what it says.

You don't want the pixel density to drop too low, then you get poor quality. So there's a limit to how big you can print, depending on the pixel dimensions of the file.

What I'm getting at is this: If you use the Image Size dialog, make sure the "resample" box is unchecked. You don't want to change the number of pixels! This is important.

Of course, if you make copies of your original file, you can do whatever you want with those. But don't resample the original.

[JeffreyW]

TYVM for the reply and information as that was informative.

So to print at a lower ratio like 12"x18" I would want to first make a copy of my picture and them set the size accordingly making sure to uncheck the "resample" box to perserve quality,

I am trying to avoid any clipping when printing at a smaller ratio with my 20"x30" picture as I don't believe a majority of printing services will autoscale the picture for me.

Thx again

[D Fosse]

As long as it's a copy, not the original, you can do whatever you want, including resample. I just needed to prevent any resampling of the original. People will do that, we just had a case two days ago with someone who accidentally destroyed his original by resampling and had to rebuild it.

There are instances where resampling to make a print copy is an advantage. For one thing, you can sharpen the image optimally. This should always be done at final print size and resolution. But you don't have to do it. You can just uncheck the resample box and set the new print dimensions. This will increase the pixel density, and so the ppi number goes up.

The important thing here is that your original is sacred. Always keep that as your master file, and make copies as needed.

[JeffreyW]

Thx again!

[JJMack]

Yes un-check resample then set the width or height you want to print. Photoshop will set the correct print resolution and other side length.  All that happens is the size pixels are printed with changes.  That what Print resolution ppi is. Physical pixels size. All prints will have the same pixels and colors. The one difference is the size pixels are printed. You print different size 3:2 aspect ratio images with the same pixels.

20"x30", 12"x18", 10"x15", and 8"x12" have the same 2:3 Aspect Ratio.

Changing Aspect Ratio would require cropping, distortion's, or boarders.

[JeffreyW]

TYVM for the verification

[JeffreyW]

JJMack,

Just to revisit this as I have three more questions if you don't mind:

1) If my original JPG file is 18x24 @ 72 PPI and I want that to be at 18x24 @ 300 PPI, I simply set the PPI to 300. Do I need to select resample in this case?

2) I'd like to resize the 18x24 JPG that is now at 300 PPI to 12x18. Do I simply set the WxH to 12x18 and then click OK with out needing to select resample as I selecting a lower print size?

3) Finally, say I have a JPG at 12x18 and set it to 300 PPI and then want to set a new print size of 8x24 - to increase the quality/sharpness would I then select resample as I am moving to a larger print size?

Thx again

[D Fosse]

You're going about this the wrong way. Pixels per inch is not file resolution, it's pixel density. The pixels are given. Ppi is a secondary, derived measure. It determines how big any given file can be printed at a certain pixel density.

With Photoshop, you always need to think in pixel dimensions. How many pixels, that's how you define file resolution. That's the quality of the file, not ppi.

You cannot increase number of pixels, except by inventing them. That's what resampling does. If you increase pixel dimensions this way, you're ruining image integrity. It can only cause damage, it cannot improve anything. Upsampling is last resort, and I mean last. It's normally not necessary.

Resampling to decrease pixel dimensions is much safer. You're still throwing out data, but not in a way that destroys image integrity.

Never touch your original. Don't upsample - instead use the ppi number to determine how big it's possible to print your file.

To understand this better, read the following literally: pixels per inch. It means exactly what it says, no more, no less. There's no hidden meaning, it's a simple relationship. Those three words give the whole thing away.

[JeffreyW]

Thx again as I believe I'm starting to wrap my head around this concept.

So my two use cases are as follows:

1) I have a file whose dimensions are 1383 x 1796 and I'd like to print that at 16 x 20, but checking the pixel dimension at https://www.nationsphotolab.com/pixelchart.aspx it looks like 11 x 17 is the largest I can print for decent quality (125 PPI), but if I wanted to try and print at 16 x 20 I would need to make a copy of the original file and then resize in PS with resampling selected, correct?

2) I have a file whose dimensions are 5400 x 7200 and I'd like to print at 16 x 20 (or any size lower than 18 x 24 for that matter), I would make a copy of the original and then resize, entering the new size of 16 x 20 without resampling, correct?

Thx so much!

[D Fosse]

1) Nothing stops you, but it will look terrible. That file simply isn't big enough for that print size. It doesn't have enough pixels.

2) Yes, enter the new dimensions with resample unchecked. That doesn't resize the file at all, the pixels are the same. The pixels are just printed smaller, at higher pixel density.

Again, pixels per inch. Those three words are the whole secret.

[JeffreyW]

Thx - so even resampling has its limits when trying to print a picture at a larger size than it can support as it can only do so much with the picture adding additional pixels.

Thx again for all of the help as it's greatly appreciated!

[D Fosse]

It's not that resampling "has its limits". It's that it doesn't help at all. You're not improving anything.

An upsampled file usually looks worse than the original, at the same print size.

[JJMack]

Think about it you bought an expensive digital camera expensive high quality lens  and learned to use the camera to properly focus an set a proper exposure to capture the best pixels possible. You should preserve those pixels there the best you have. Resampling them up or down in number just create a new image. A lower quality image for the pixels were generated by a computer algorithm from the high quality pixel you had from your camera. Details you had had to be discarded if you lowered the number details you did not have had to be materialized if you increase the number.  None of your camera high quality pixel exist in the new interpolated image