file size?

[Jelleke]

question from a beginner, when I express a file as jpeg in the best quality, then the size of the file doubled, how is this possible?

 

[thomaso]

The file size of image file formats always depends on the selected compression rate, which is usually selected as the quality value.

When you mention a file size is "doubled" then you need to know with what "half" it is compared; you didn't mention that "before" state.

To compare the impact of JPEG compression on file size have a look at this site of Jeffrey Friedl: Just scroll to an image and hover over the different compression rates to compare details/loss and file size simultaneously:   http://regex.info/blog/lightroom-goodies/jpeg-quality

[firstdefence]

Although it states 100% quality, there will be some loss of detail but for the most part there is no quantisation of the image which contributes to the majority of the compression seen with JPEG files, reducing the quality to approx 93-95% will half the file size of the image to JPEG

[thomaso]

> reducing the quality to approx 93-95% will half the file size of the image to JPEG

In my experience, it is rather 75-85%, which halve the file size compared to 100%. However, it also depends on the amount of image details.

[firstdefence]

7 hours ago, thomaso said:

> reducing the quality to approx 93-95% will half the file size of the image to JPEG

In my experience, it is rather 75-85%, which halve the file size compared to 100%. However, it also depends on the amount of image details.

Either way it's no mean feat to retain that level of quality and at the same time halve the file size  When you look at the math that enables this it's like WTF.

https://www.ijg.org/files/Wallace.JPEG.pdf

[Jelleke]

Thanks everyone for the comments.
Forgive my stupidity but I don't get it ... I just switched to digital photography. My original jpeg photo has a size of 15.1 mb, I only now see that when I save it as an affinty file the size becomes 46 mb and after editing and export it is 29.39 mb. What is the reason that after editing a file gets bigger and what is the difference in data information? Is this normal with affinty? Usually a jpeg file becomes smaller after editing or am I missing something?

[thomaso]

> Usually a jpeg file becomes smaller after editing or am I missing something?

Not at all, or, it depends: Often you edit an image file to increase its amount of information (details). For instance you might edit an image to increase its sharpness or to get more visible items in lights or shadows. That would result in more details = more different pixels compared to the origin, and therefore result in a larger file size, even with same compression rate. Vice versa it means you need to reduce the amount of different pixel to achieve a smaller file size when re-saving a JPG. Such a reduction accordingly can be achieved by blurring the image or reducing its total pixel amount (= dimension).

The special situation where you re-save a JPG without editing it also results in different pixels because even 100% quality (0% compression) would slightly compress the file again. That way every save action would reduce the file size – but you would need hundreds of re-save actions to make this effect visible for your eyes, not to mention obvious.

The different file sizes are partially also related to the document format. 46 mb:  Affinity apps use a format of their own with specific data which aren't inside non-Affinity files, for instance a larger preview image with no or less or different compression.

[Jelleke]

hi, thank you, i'm beginning to understand,