XnView modified photo has much smaller file size

[flanneryd]

I use XnView Classic to view and modify my pics. Why do I lose significant file size when editing my files? For example, a 16MB file becomes a 2.3MB file after adjusting alignment. An 11.7MB file becomes a 2.1MB file after adjusting black/white points. Thanks.

[cday]

Could you say which file format you are using, and if you are setting a particular compression option for the saved file?

I don't have Classic installed as I'm in Linux, but you may be able to find a menu item, or toolbar button, that displays the properties of an opened file enabling you to see how the saved file differs from the original file.

If you are converting a colour or grayscale image to black and white, and saving the resulting image using default compression options, then with some file formats a large reduction in filesize would be expected.

[flanneryd]

Could you say which file format you are using, and if you are setting a particular compression option for the saved file?

I don't have Classic installed as I'm in Linux, but you may be able to find a menu item, or toolbar button, that displays the properties of an opened file enabling you to see how the saved file differs from the original file.

If you are converting a colour or grayscale image to black and white, and saving the resulting image using default compression options, then with some file formats a large reduction in filesize would be expected.

I'm using JPEG format and not aware of any compression setting. Perhaps there is some default compression set - I'm looking but haven't found it so far. When I open a photo and look at Properties within XnView, the original file shows the original file size. As soon as I make a modification (such as ctrl-alt-l, or adjusting the black/white points on the histogram) and Save, the saved file size is reduced significantly.

[flanneryd]

I figured it out. Instead of overwriting the original file, after mod's, I need to "Save As" a new file. When I do that, there is an "options" box that can be clicked. That takes me to several Write options, one being saving with lowest to best Quality. The default was set to 80, with 100 being best quality. There also is a box called "Use estimated original quality if possible," which is defaulted to unchecked. Saving at 100 Quality & selecting "Use estimated original quality..." results in a file size almost as large as the original.

I'm not so sure I can actually see the difference between an original 10MB photo and a modified 2MB version of it, on my laptop screen. But my concern was mostly for future prints.

[cday]

With the JPEG format there is unlikely to be any visible gain in image quality for quality values greater than about 80 or 85, only an increase in filesize. In my experience, images with large pixel dimensions can also often be saved with significantly lower qualities without obvious degradation. Depends how important filesize is, for example with a large image collection, and how important ultimate quality is the way the image will actually be viewed.

Note that the 'Use original quality when possible' will in general determine the quality value used, rather than the set quality level.