Newbie: Why are my file sizes increasing

[tealduck]

I have to say that I'm a little confused by this.

I open a scanned JPEG file, crop the image and then export at the highest quality back to JPEG. I would think that the file size would go down but instead it increases sometimes by a factor of 10.

What the heck am I doing wrong?

Version: 1.6.2

[Wikinger]

Bonsoir. 

 

Avez-vous en dire un peu plus? Dimensions en pixels, taux de ré-échantillonnage.

Si vous n'imprimez pas vos images, un ré-échantillonnage de 300 ppp (par exemple) pour l'affichage écran, est inutile.

Essayez de choisir un ré-échantillonnage égal à la résolution de votre écran.

En tous cas, ce peut être une cause.

[tealduck]

" Good evening.
Do you have to say a little more? Dimensions in pixels, resampling rate.
If you do not print your images, 300 dpi resampling (for example) for the screen display is unnecessary.
Try to choose a resampling equal to the resolution of your screen.
In any case, it can be a cause. "

So here is an example:

Input file: JPEG, 1648x2152x24bits, 200dpi, size: 230,877 bytes - mostly black and white as it is just an old marriage license.

Open, Rotate, Crop, Export JPEG, 2031x1617, Resample: Lanczos 3 Separable, Quality: 100%, Don't export layers = True, New Size: 1.43Mb

I just don't understand why making an image smaller in pixel size should increase the file size. My best guess is that when the program decompresses the file and then I save it at 100% I don't get the compression that the original file contained. Which then begs the question how do I match the exact compression of the original file as I do not want to lose any more detail.

[Alfred]

Welcome to the Serif Affinity Forums, @tealduck.

 

From the version number which you quoted in your OP, I gather that you’re on Windows. IrfanView (free for non-commercial use) will tell you the amount of compression used in a JPEG file; I think it’s only an estimate, but it seems to be pretty reliable.

[Pšenda]

18 hours ago, tealduck said:

Quality: 100%

It is "uncompressed" JPEG = super big file.

For example:

  

 

You can also find the quality of the source file by storing it in different quality in an AP without cropping. By comparing the file size or your image quality, you will find the corresponding compression level.

85% is standard - good quality and adequate file size.

 

Edit:

"uncompressed" does not mean lossless, but minimal lossy compression and best quality.

Lossless jpeg is specific mode (JPEG-LS), which Affinity unfortunately not offer.

[tealduck]

Thank you both for your help with this. I used the method described by Pšenda so I didn't have to install more software.

[Pšenda]

tealduck - this method gives an approximate result (repeat jpg save = repeat lossy compression).

Try online application "https://www.imgonline.com.ua/eng/determine-jpeg-quality.php". I tried her with an AP image, and selected compression (45, 85, 100%) found correct.