Quantitative measure of degreee of focus across an image for se

Quantitative measure of degreee of focus across an image, for segmentation. Your thoughts?

I'm looking at various ways of segmenting images; one such idea I've had by 
looking at the images in question is to identify areas of the image that are 
in focus.

As such, I have thought of two ways of calculating some numbers.

Firstly, apply a sharpening filter to the whole image and then splitting the 
image into regions, say 10x10 pixels, and assigning each a value based on 
the difference between the original image and the sharpened image - the 
larger the number, the greater the difference and thus the less in focus the 
original image was in that region. I'd have to assume some things about the 
nature of the focus, and would probably start with the assumption of a 
gaussian PSF.

Secondly, applying edge detection across the whole image; I suppose that the 
object in focus will have a sharp edge against the background, so if I 
assign a value to every edge detected based on the gradient of the edge 
detected, and then again break the image into regions and assign a value per 
region based on the gradients of the edges detected (perhaps an average 
gradient of edge detected) I would expect to see high values at the edges of 
the object in focus.

By these two methods I hope to identify the region of an image that is in 
focus, and direct my segmentation efforts accordingly.

I'd be very interested to hear the thoughts on these from people with 
experience. I anticipate that there's nothing new I can think of that hasn't 
been done already, so if anyone can direct me towards the results from 
similar approaches I'd be very grateful.

'Chops