Jpeg File Sizes Are Enormous

[superherobyday]

So I'm trying to create image files that I can sell as digital prints to be printed out as posters.

As such, their dimensions are quite large (18" x 24") and the resolution is 300ppi. (Neither of these attributes can be sacrificed).

It seems no matter what I try, the resulting jpegs I save from photoshop are enormous. Anywhere from 11mb to over 40mb. That's video size!

I realize that such large dimensions and high resolution will create images in the MB, but I've purchased many digital print image files with the same dimensions and resolution that weighed in around 3 mb, so I know it can be done.

Here is what I've tried so far:

• Flattening/Resterizing/Merging layers
• Using 'Save for Web' feature, removing copyright info/meta data
• removing thumbnail preview
• resetting photoshop settings

Given that these files will be used to print posters, I can't sacrifice the size or the image quality. What else can I do to drastically reduce file size?

I'm using CS6

Thanks in advance!

[D Fosse]

11 - 40 MB are extremely small file sizes for those pixel dimensions! 7200 x 5400 pixels is quite big. That reduced footprint is only possible through jpeg data compression - which, BTW, I'd be very careful with if I were you. Jpeg compression is lossy, destructive and above all cumulative.

Work in PSD / TIFF, and only save out a jpeg copy at the very end.

And you should just start to get used to file sizes twenty times as big as what you have now.

[superherobyday]

Thanks for the reply!

The thing is, I have purchased/downloaded multiple digital image files at large dimensions (24x36) with high resolution (300ppi) that are all between 2-5mb.

In fact, on etsy (where I'm planning to sell digital prints) one is not even allowed to upload files larger than 20mb.

And given that these files generally contain multiple images, a file size of <5mb per image seems not only possible, but very common.

[D Fosse]

First, get used to thinking in terms of pixel dimensions. That's all that matters to Photoshop - print size/resolution is just slapped on afterwards. It's not a property of the file.

7200 x 5400 pixels print out at 18 x 24" - if you set ppi to 300. If you set ppi lower, it prints that much bigger. Pixels per inch.

The thing is, I have purchased/downloaded multiple digital image files at large dimensions (24x36) with high resolution (300ppi) that are all between 2-5mb.

In that case they are jpeg-compressed to within an inch of their lives. Or the resolution is lower.

[superherobyday]

Excuse my unfamiliarity with the lingo, but when you say that the resolution of their images is probably lower, you are referring to the ppi right? They are all 300. I've checked them in photoshop.

In regards to compressing images to within an inch of their lives, how exactly does one accomplish this? And what are the side effects? Is image quality sacrificed? If so, how could the sellers I've purchase images from compress the images while retaining 300ppi?

Cheers

[Derek Cross]

You could probably get away with a resolution of between 200 and 250PPI, it would be worth experimenting and with a JPG compression of around 8.

[superherobyday]

Hey Derek

Most online printing services require that images be at least 300 ppi, so I can't really sacrifice resolution.

Cheers

[Derek Cross]

I think you'll find the difference in quality to 250PPi from 300PPI will be imperceptible, especially if the poster is to be viewed from a distance. Don't be too precious!

By the way, IMO don't use Save for Web for this purpose, it's been deprecated for good reason. Use Dave As and select JPG from the drop-down format menu.

[superherobyday]

You're right. Personally I can't notice a difference between 250 and 300. But two things on that

First, it doesn't change the file size enough. My images are saving anywhere from 12-40mb. I need them down around 3mb. Changing the ppi to 250 or 200 doesn't even cut the file size in half.

Second, online poster printing services won't even accept images that are less than 300ppi. That seems to be the industry standard. Even if the difference is negligible, I'd rather not have to explain to my customers why my images don't meet the requirements of printing services.

Cheers!

[davescm · Accepted solution]

To add to what the others have said - file size from jpeg compression is image dependant. Some images compress really well, others result in bigger files.

Use Export save for web (Legacy) and adjust the compression slider whilst looking at the image in the preview. Watch the quality in both fine details and smooth gradients such as sky or backgrounds. You will quickly see when  compression issues result in unacceptable artifacts.

Dave

[superherobyday]

Hey Dave, thanks for the response. When you say that file size is image dependent, what factors of the image determine the file size? If two different image has identical ppi and dimensions, what further aspects could cause one to be 40mb and the other 3mb?

Also, excuse my newbness, but I don't see any sliders or anything labelled as "compression" in the Save for Web window. Could it be called something else?

Cheers

[davescm]

Hi

Large plain areas compress much more readily than areas of fine detail. Try it with a few images.

In Export - Save for Web (Legacy) click on the down arrow next to the quality setting quality setting and a slider appears.

Use the "2-Up" view and you can compare the original to the compressed version. If you look at the bottom of the compressed version you will see the estimated file size.

Dave

[superherobyday]

Oooooohhhhh. Ok gotcha. It just looks a bit different on mine.

So here's where I'm at now.

If I adjust the compression slider, the file size changes at a commensurate amount. However, when I save the image, and open said image in photoshop to check the specs, the resolution drops to 72 and the dimensions increase significantly.

As an example, if I take an image at 18"x 24" & 300ppi, and save for web at a quality (or "compression level) of 20ish, the resulting image is 75"x100" and only 72ppi.

I'm so confused lol

[davescm]

Hi

The 72 ppi and 300 ppi are just stored in metadata in the file. They are used in printing, so that the print driver can lay down a physical print size,  so make no difference to the actual image pixels.

To ensure that the metadata shows 300ppi then you have two choices:

1. Use an Exif editor to edit the metadata after use Save for Web or (Export As).

2. Use Save As. Unfortunately the Save as quality (compression) slider does not use the same scale a Save for Web but you can check preview in the Save As dialogue to see the target file size. So use the preview in Save for Web to check what maximum compression is acceptable for your image and check the target file size. Then use Save As and adjust the slider to get as close as possible to that file size.

Note if you have a maximum allowed file size (e.g. from Etsy)  - then aim at that.

Dave

[superherobyday]

Thanks for all the help thus far, it is much appreciated.

Ok, so my next question is, how can I distinguish between an image's actual pixels, and its metadata pixels? And which am I seeing in the Image>Image Size window in photoshop?

Also, I'm still a bit confused as to why the image changes dimensions and ppi when compressed.

If I'm taking an image that shows in the Image Size window as being 300ppi & 18x24, and am then compressing it and saving it (with the save for web feature), and the result is an image that shows as being 75x100 & 72 ppi in the Image Size window, what exactly has happened to the image quality? And how can I get it back to 18"x24" and 300 ppi?

Thanks again

[davescm]

Nothing has happened to the pixels. 300ppi is just a value stored alongside the picture info as is 72 ppi.

Here is an example :
If you have an image that is  2000 pixels wide and 1000 pixels tall. That is 2 million pixels.

At 300 ppi that image will print out 6.67 inches by 3.33 inches - but will still be made of 2 million pixels.

At 72ppi the image will print out 27.7 inches by 13.89 inches - and again will be made of those 2 million pixels.

The image quality has not changed - it is still made of 2 million pixels. At 72ppi the individual pixels are printed bigger than those printed at 300 ppi - but the resulting larger print will be viewed from further away.

Dave

[superherobyday]

Ok gotcha that all makes sense. So the total # of pixels always remains the same, while the ppi fluctuates with the dimensions.

So my new compressed image shows as 72 ppi and 75"x100". Will this cause any issues when trying to get it printed as an 18x24? I understand that the different sizes are identical images with an identical number of pixels, I'm just wondering if a print shop requires "300ppi", does my image need to actually be formatted and saved in a size that makes it 300ppi?

Also, seeing it as 18x24 and 300ppi in the image size window would be much more satisfying (and this is how all prints I've purchased arrive), but if I change it and re-save, it jumps from 4mb to 9mb. Why photoshop? Why? lol

[D Fosse]

Actually it all explains itself:

pixels per inch

OK? Take a minute, and think about what that means. Pixels per inch. So you have your pixels, and that's all there is in the file. But you get to decide how many of them per inch of printed paper. You get to decide how big each pixel is - and, putting all those pixels together, how big the print gets to be.

----

Jpeg compression is something else. That's data compression. If these pixels have the same value, can we save some space by just recording x amount of pixels at y value, starting here, instead of spelling out each one separately? That sort of thing.

[superherobyday]

Ok I get it now. I was thinking that the ppi and size were independent of one another. Now I see that the ppi and dimensions are inversely correlated, and that total number of pixels always remains constant.

So my new highly compressed jpeg seems indiscernible from the original. Even though I set the quality to only 30. Does compression affect the image quality at all? Even while highly zoomed in, both images are equally sharp.

Cheers

[D Fosse]

superherobyday  wrote

Now I see that the ppi and dimensions are inversely correlated, and that total number of pixels always remains constant.

Yes! That's it. They are inversely correlated. You can set it up as an equation, and like all other equations, it can be rearranged so that knowing two, you can always calculate the third.

A jpeg compression of 30 is really crunching the file. I can almost hear it. Set your zoom ratio to 100%, one image pixel per one screen pixel. Look closely at edges where solid areas meet, that's usually where it hits hardest.

In most cases a single save as jpeg doesn't do too much damage. But this happens every time you resave it, and it quickly builds up. The funny thing is that if you do this, the file tends to become bigger with each save, even at the same compression rate. That's because all those artifacts build up. As Dave pointed out, high-frequency detail doesn't compress as effectively as flat areas.