Why is a 7Mb image downsized to <2Mb after being cropped when the selected area (what I kept after cropping) is more than 50% of the original?
The picture was taken with a Canon 40D and then cropped using gthumb, gwenview, nomacs and showfoto. I didn’t calculate the percentage of the picture I cropped with each program, but I am absolutely positive it is more than %50. My thought process was something like this: “Since I am keeping more than half the picture and it is 7Mb, I will still have 3Mb squares after cropping”. I was wrong.
I did try GIMP, darktable and RawTherapee, but I don’t seem to be as intelligent as my parents told me I was because my brain didn’t understand how to do something as simple as cropping an image with those tools.
Here is a screenshot of the thumbnails and the size for each.
There are a number of things that contribute to image size.
First is that a cropped image has a lot less pixels. E.g., if your original image is 10x10px, your total pixel count would be 100px. If you halve it to 5x5, you would have 25. 25 is a quarter of 100.
Second is that your file format settings, metadata and compression can make a (huge) difference too. It can take a long time to explain depending on how much you want to know about that.
For the moment, I would say that bitmap formats would be the largest formats but unfortunately they lack the metadata and preview support that say TIF and JPG files have. I suggest that you use 16-bit / channel TIF files with an embedded colour profile.
If you save an image as a JPG you also have to specify the amount of compression to be used. Which settings did you use in the various tests? This setting determines the amount of image information that is lost by the compression.
Even without cropping you can get changes in file size of a factor between 5 and 10, depending on the settings used. Changes in image quality are subtle and only visible to the eye on casual viewing for the strongest compression.
Just take an original in TIF format (uncompressed, i.e. lossless) and save it with e.g. RT as a JPG with various settings of compression and compare the results.
In your example you cropped from 5184×3456=17915904 pixels to 3456×3456=11943936 pixels.
So the cropped image has 67% of the pixels of the original image. Therefore, the image should shrink from 7.4 MB to ~ 5 MB. Because the complexitiy of the cropped areas seems not to be much higher than the rest of the image, you apparently saved the new image with less quality and higher compression.
You can test this, when you resave the original without cropping in one of the programs mentioned. The default settings in image viewers produce often smaller, higher compressed images than most cameras.
