Yup. No easy answer. Image content is a huge determinant.
Erik Krause wrote:
> Am 25.08.2011 22:18, schrieb Ken Warner:
>> For practical purposes for web publishing, one can probably use less
>> than full size images and still have a good looking presentation.
>> But a hard and fast 70% reduction -- I'm not sure about that.
> As Sacha wrote: Reduce and enlarge again, overlay with the original,
> eventually use difference layer mode in photoshop to actually see what
> has changed. During one of the gigapixel discussions recently someone
> even wrote a script to do that automatically and determine the smallest
> size which contains (almost) the same information.
> However, there never will be the full information, since some is lost.
> There is no possibility to get it again, no matter how good the
> interpolation is. Imagine a fine diagonal line which hits the green
> pixels only. How should a software determine whether it is green or
> white? The effect is visible as moiré, btw.
> But there certainly are differences depending on the image content. I'd
> guess that a mostly black and white image will suffer more from
> reduction since all sensor pixels potentially contribute while an image
> that is evenly bright and has only color contrast could possibly be
> reduced to 50% without loss...