I've been reading a lot about image sharpness lately. The only thing I haven't seen discussed is photo size. I know that when you zoom in on a photo, it becomes pixelated quickly. So would the opposite also be true? Say a 2000 by 3000px photo is reduced to 1000 by 2000px. Would the smaller image appear to be sharper?
Say a 2000 by 3000px photo is reduced to 1000 by 2000px. Would the smaller image appear to be sharper?
No. Sharpness is limited by the display's resolution -- the number of pixels per inch. If you want to increase the make the displayed image seem sharper, you need to increase the display resolution. You can (sort of) do that not by making the image smaller, but by backing away from the display so that the pixels appear smaller. Or, you can get a display that actually has smaller pixels.
Note that this is different from changing the sharpness of the photo itself. When you adjust the sharpness of an image in an image editor like Photoshop, you're changing the image itself and not just the way it's displayed. I don't think this is what you're asking about, though.
No the opposite is the case, the higher the true resolution* the more details you've see, and the more details the more sharp the images appears.
Although sharpness is limited by the display's resolution, downscaling will still make the images appears sharper when it's higher resolution.
*true resolution is not just the resolution of the image, but the actually perceived resolution or level of detail and contrast.
The term sharpness is directly related to contrast - a detail (colour transition) being sharp means that the brightness ratio between adjacent regions is high. This one is not sharp because the brightness changes smoothly across the image. This one is sharp though.
Another pair of examples. This is sharp. And this, the version scaled down by 66% in the mode "bicubic sharper", is not.
It is a simple demonstration that an arbitrary image may only become sharper if recorded at higher resolution and may only get worse if scaled down. These images are demonstrating worst case and your photos will never look as bad downscaled because they do not contain the high contrast ortogonal details.
Now regarding your actual use case.
The only thing I haven't seen discussed is photo size. I know that when you zoom in on a photo, it becomes pixelated quickly.
It looks like you are viewing the photos on a screen with limited resolution. It means that you are always viewing the image scaled down and limiting the sharpness (as demonstrated), and a photo will never be as sharp downscaled as it is originally.
Now, the question is in the method of scaling which the viewing program applies. A program is forced to downscale the image and may do so differently: quickly and dirtily or a bit slower and make it look good. To make it look good (i.e. sharp) the program should apply some sharpening.
However, it does not make sense to do so because it may impose false impression on the photographer about photos being sharper than they are. Photo editing programs are not for exposing the result, they are made for managing the intermediate material.