Photoshop - images under 300dpi resampled - help please

[Lawtz01]

Hi

Id like to give my (non-photoshop expert) view on dpi/ppi/resampling and then my situation and hopefully I can get advice.

My view: Where possible all images used in a design for print should be 300dpi (true resolution).  Images that are say 150dpi can technicially be made 300dpi by resampling the image in photoshop.  This I believe means photoshop will insert artificial pixels and makes a best guess on the colour of these pixels based on surrounding existing original pixels.  While this might be useful sometimes, the original image should always be 300dpi.

My situation: I have a client who advertises in quite a prestigous magazine (A4 printed).  They are having some trouble with images and I have given my explaination above of why they should not be using resampled images.  However it doesnt seem to be getting through and they've found it hard to grasp that why should not just resample images to twice the size.

1. Is my view right? Or do people hold views that technology/photoshop has come on far enough that resampled images are acceptable?  

2. How can I best explain the issues with resampling and what might happen in terms of quality on the final printed copy?

Thanks! 

[Stephen Marsh]

My view: Where possible all images used in a design for print should be 300dpi (true resolution).  Images that are say 150dpi can technicially be made 300dpi by resampling the image in photoshop.

By @Lawtz01

When viewed at arm's length, the final physical reproduction size for halftone printing should ideally have an image resolution (PPI) of x1.5-2 times the halftone line screen (LPI) value. So, a 150 LPI halftone would require 225-300 PPI image resolution at 100% print size. This also depends on image content.

The first thing to do is use Image > Image Size with resampling turned OFF to resize the image to the final desired 100% printed scale to see what the effective resolution of the image is. The effective resolution is the "true" resolution of the image when sized for print. This could be lower than the ideal target of 300, or it could be higher. Once you know the effective resolution, you can decide to leave it as is, downsample or upsample.

There are many ways to upsample, it depends on the resampling algorithm and the image content on the results.

[Bojan Živković]

"This I believe means photoshop will insert artificial pixels and makes a best guess on the colour of these pixels based on surrounding existing original pixels."

Good guess, Photoshop will introduce new pixels based on surrounding pixels, depending on the algorithm in use.

"They are experiencing some trouble with images, and I have provided my explanation above regarding why they should not use resampled images."

Resampling small images, like 300x200 pixels, will most likely result in a pixelated image, depending on the image content. That image is likely scaled down and saved, in most cases, for the web with low-quality settings for faster loading on the website. Resampling even larger images will often appear unacceptable as well. Resampling a 5000x3333px TIFF image derived from the original RAW file may not look bad; in fact, it should look acceptable, depending on many factors—do not misunderstand that any image will look good. Primarily, it depends on the image itself, its sharpness, the quality of details, and scaling factor. It really depends on what you have to start with; sometimes it's not acceptable, while other times it can be fine. Resampling down or up will always result in a loss of quality. Scaling down is almost always acceptable unless there are small details that must be visible, while upscaling can be more problematic. Sometimes it is acceptable, while other times it is not, which depends on many factors.

[Lawtz01]

Thank you both.  @Stephen Marsh  you lost me on the first paragraph to be honest! I know what halfprinting is but I will be looking up LPI!

I will attach the file details for each of the images - see if this helps.

Thanks guys!

[Per Berntsen]

The original placed in InDesign is 288 PPI, which will be absolutely fine.

If you know at what size an image is going to be printed, you can use the Image size dialog (with Resample unchecked) to see what the effective PPI will be.

 Photoshop does the math for you – pixel dimensions divided by printed dimensions in inches = PPI.

     

[D Fosse]

There's a lot of misunderstanding around the 300 number. It's not sacred.

It's usually treated as a minimum requirement, but it's not. It's a theoretical upper limit, above which any increase is totally masked by the line screen frequency (lpi).

Furthermore, it's not even a sharpness parameter. It's a smoothness parameter. It's the point where it's no longer possible to discern individual pixels. The sharpness, OTOH, the acuity, is determined by the line screen - so maximum sharpness is already achieved at 150 ppi, not 300 ppi.