Is there a reliable method to determine if an image is blurry? No, it depends on the image and the intended subject and intended focusing.
What if the image has a sharp foreground subject, takes up less than half the frame, and the background is intentionally heavily blurred?
The algorithm(s) you use has to have some kind of threshold to determine the level of blurriness that you're willing to accept. Nothing is going to replace looking at each image yourself though.
There's a ton of examples on various algorithms
here in this post[
^].
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