Compressing pics larger than needed before printing

[Ring]

I have around 500 pics taken on my S45 in Australia that I want to print. I took them all in RAW mode. The jpegs I made from them average around 2.2MB per picture. I'm wondering how much I can safely compress them and not notice the difference with 4x6 prints.

Why? Because it will take forever to upload over 2GB of pics to an online printer. There are guides for what minimum resolution to use to print at X size, but they're no help -- even if I compress a jpeg to a quarter of its size (using CompuPic), the resolution is still the original 2272x1704. So the picture is being degraded without affecting resolution.

How much can I compress these pics and still have them print well at 4x6? And what metric can I use for compressing them, since I've ruled out resolution?

[atleypnorth]

Why have you ruled out resolution ?

If you take 300 DPI as what you need (you can go lower than that)
then you only need images 1800x1200 pixels which is a fair bit smaller
than 2272x1704.

[stopbath]

Since the format of the original is not the same as the format of the print, if you don't resize and just compress, you'll be relying on the printing shop to crop your images for you.

I would suggest cropping and resizing yourself. Save at the suggested resolution in a lossless format, the resave for printing in jpeg (if needed). Use the best quality first, and see what sizes you get.

Why not take a few samples, and get them printed at high quality and at less qualities. Then order the whole batch at the compression you want (depending on how critical you are.)

There is no 'best' setting, as each image will compress differently, and each person has their own criteria of what makes "exceptable" image quality... Jpeg works best with gradual colour changes, and works worst with sudden colour changes...(A red balloon in a blue sky will render worse than a blue balloon in a blue sky...) A 'good' level of quality on one shot does not mean all the others will still be as good...

[Ring]

Why have you ruled out resolution ?

If you take 300 DPI as what you need (you can go lower than that)
then you only need images 1800x1200 pixels which is a fair bit smaller
than 2272x1704.

The utility I'm using to compress is CompuPic. It gives a JPEG Quality slider that goes from 0% to 100%. No matter what I set it at, the resulting picture is still 2272x1704, the resolution of the original. So I can't manipulate actual resolution with this utility. I don't have Photoshop (I do have BreezeBrowser). Is there another utility I could use that would let me compress by actually adjusting resolution?

[Ring]

Since the format of the original is not the same as the format of the print, if you don't resize and just compress, you'll be relying on the printing shop to crop your images for you.

Why not take a few samples, and get them printed at high quality and at less qualities. Then order the whole batch at the compression you want (depending on how critical you are.)

I shouldn't have said I'll be printing at 4x6 -- it'll actually be in the proper electronic photo aspect ratio, which will prevent the need to crop. That leaves just the file size issue. Thanks for the suggestion to experiment with several compression levels, which I just may try.

[atleypnorth]

try irfanview for resizing just to give things a go

[stopbath]

The utility I'm using to compress is CompuPic. It gives a JPEG Quality slider that goes from 0% to 100%. No matter what I set it at, the resulting picture is still 2272x1704, the resolution of the original. So I can't manipulate actual resolution with this utility. I don't have Photoshop (I do have BreezeBrowser). Is there another utility I could use that would let me compress by actually adjusting resolution?

One thing that JPEG stresses, is that 100% is useless setting. It ends up amplifying the file size for extremely small gains. They stress 95% as maximum. They also stress that sliders and click boxes between applications necessarily do not compare (even between versions of the software, the compression may change...) What is 75% on my program, may be closer to 50% on another, which may be almost the same as a Best click box on another program. Also, how one program compresses may be different from another program so 95% is not always the same... There is lots of different ways to play with quality and file size in jpeg. Much more than a click box or slider can represent...

Perhaps 100% on your program does not increase the file size beyond and 100% is actually 95%... Test to find out (load a good quality jpeg, save at 100% did the file size increase hugely?)