how to perform image quality comparison at different resolutions?

Question

My objective is to compare two photos of the same object at different resolutions and be able to tell how much more detail does the larger image contain than the smaller one.

The point of this is to find the minimal picture size or resolution at which the camera can preserve the maximum detail.

The manual procedure is the following.

1. take a photo using the lowest resolution, then another at the max resolution,
2. resize the small one to the same resolution as the bigger one and compare.
3. if the bigger photo has more details then repeat using the next to the lowest resolution.
4. if the images contain the same detail level then the image at the lowest resolution is the max detail level the camera can capture using the lowest resolution possible.

I wonder if this can be done using software or another easier procedure.

any help is really appreciated. thanks.

Answer 1

take a photo using the lowest resolution, then another at the max resolution,

A camera will take the underlying RAW image at only one resolution : the native resolution of the sensor.

Different resolutions are obtained by scaling the image in a combination of hardware and firmware inside the camera.

resize the small one to the same resolution as the bigger one and compare.

In doing this you will only be comparing the software used to scale the large image to the firmware used to scale the camera's version.

if the bigger photo has more details then repeat using the next to the lowest resolution.

Even if you ignore everything I've said, how do you tell real detail from artifacts of different scaling algorithms ?

How do you measure detail objectively (or numerically) ?

if the images contain the same detail level then the image at the lowest resolution is the max detail level the camera can capture using the lowest resolution possible.

I don't think you're doing that at all.

Answer 2

Assuming you understand all that StephenG wrote, and perhaps would like to experiment with, say images taken in and out of focus, or with image-stabilization turned on and off, let me recommend reading up on the NIIRS scale of image evaluation. It's by no means perfect but it is at least a reference standard that is used by many communities, not just the military intel groups.
Quoting from the link referenced,

The aerial imaging community utilizes the National Imagery Interpretability Rating Scale (NIIRS) to define and measure the quality of images and performance of imaging systems. Through a process referred to as "rating" an image, the NIIRS is used by imagery analysts to assign a number which indicates the interpretability of a given image. The NIIRS concept provides a means to directly relate the quality of an image to the interpretation tasks for which it may be used.