Resize JPG photo dilemmas

[ali_4619]

Hi, is there a simple way to resize photos of different sizes and dpi to 1 MB and 300 dpi? I have a lot of them and they are all different sizes & dpi. The online instructions are very complicated and there seems to be a lot of guesswork involved unless you are formally trained. As one example, I open a 637 KB photo in Photoshop and select 100% increase and 300 dpi. It exports at over 9MB in size. So I experimented with 50%, 100%, 200% and 300% using the original 637 KB photo and all are resizing at over 9MB. It doesn't make any sense to me. I tried originally with pixels width and height but I feel you need a degree to figure that out.

[Stephen Marsh]

Why do you want a target file size of 1mb compressed, regardless of image content and pixel size?

What file format and pixel width depth for your example image of 637kb  (before) vs 9mb (after)? Are you getting the file size of 9mb when opening in Photoshop (decompressing) or when saved to disk (compressed JPEG).

Are you using save as (save a copy), export as or save for web (legacy)?

The resolution PPI value is just metadata, it's the pixel width and height that is the key issue that will affect final file size on disk* when saved and viewed on disk.

*JPEG compression quality and excessive unwanted metadata will also affect final file size on disk.

[ali_4619]

Hi thanks for your help. I am very new to Photoshop and will be doing training but I’m on a deadline for a book so very grateful for your help. The printer told me I would need the photos to be 1MB and 300dpi to print well. Also, Adobe Express wouldn’t let me upload a photo into my design bigger than 5MB.

In Photoshop what I am doing is going to the Image menu, then Image Size then I adjust the dimensions and resolution before exporting to my computer as JPGs.

With all the various sizes and quality of my original photos, I wondered if there was an easy way in Photoshop to resize without experimentation or expert knowledge – eg to just type in 1MB & 300 dpi. I don’t want to oversimplify things and I know you need expertise to work well in Photoshop but I wondered if there was a quick way for deadlines!

But since I posted here, I have been experimenting with changing the pixels rather than percent (see screenshot attached to see what I mean by percent) and having much better results. I have been able to achieve 300 dpi for all of the photos with jpg file sizes ranging from 1MB – 2.5MB; including the 637kb one I mentioned in my original post that I was having trouble with. 

For clarity, when I talk about percent above, I mean as in the dialogue box next to width and height in the screenshot attached.

A related question is when it says image size 18.9M at the top of the dialogue box in the screenshot, what is M the measure of? I have googled and can’t find the answer. The photo the dialogue box relates to when exported is 1.1 MB.

[Trevor.Dennis]

The size you see when open in Photoshop has little relationship to the size of the saved file on the drive.  That obviouslt depends on the format. i.e. a 3000 X 2000 pixel image with lots of colour variation might be 10Mb saved as a TIFF or PSD, but only 500KB saved as a high quality JPG.

Tell us a bit about the book you are working on? How big will the printed images be?   If it is a fairly large book and the images are going to be printed 10 inches on their long side, then 10 X 300(dpi) = 3000 pixels

That's the important math.  If it was only 1000 pixels on the longs side, then its quality would be poor if printed at 10 inches.

[Bojan Živković]

You can achieve your goal by using Export > Save for Web, where you can adjust dimensions and utilize the Optimize to File Size function. As @Stephen Marsh  pointed out earlier, please address the questions mentioned above.

[NB, colourmanagement]

@ali_4619 

Where possaible, I would not be resizing a JPEG but a TIFF then after resizing save as JPEG

Why?

A few notes on Jpeg:
Jpeg files have compression applied, changing resolution or cropping and re-saving enhances the compression artifacts - this means that Jpeg is only really suitable for final file delivery/transfer - with the Jpeg created only once size and resolution (and any sharpening) have been completed. 
 
So, Jpeg is far from ideal for editing and not OK for archiving or for any file that may need to be resaved, resized or cropped down the line.
 
Jpeg is the worst possible format if you want to keep high quality - you should always archive a copy of your original, ideally with any adjustment layers intact - if you work with layers.
Jpeg compression (at any setting*) really is "lossy”, irreversible, and cumulative, so Jpegs should ONLY be used only for final delivery and only created from the original file format AFTER resizing & cropping to the FINAL size and crop.
To explain: any edits to size or crop, or even just re-saving a Jpeg file means further compression, potentially that’s very damaging.
The JPEG compression damage is not always immediately apparent, which is perhaps why it's still widely used - however, that compression will soon cause issues if you do further work and save again. That’s when you’ll see a Jpeg with some real issues.
 
*don’t imagine that selecting maximum quality for your Jpeg is preserving the original data, it’s still compressing a lot which discards information.
SO, don’t reuse Jpegs if any resizing or resaving is needed. Always go back to the PSD/ Tiff originals, Jpegs are essentially a 'use once and trash' file type.
 
I hope this helps

I hope this helps
neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management
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