ok- this may take a little explaining, so please bear with me.
there is a subtle difference between how photoshop and after effects apply effects, and i need to know if i can get round this.
in photoshop, if i have a heavily feathered selection and apply a filter such as "blur", the amount the filter effects the image is gradually reduced as i reach the outer edges of the selection. so if the "blur" filter has a value of say 100px, then where the image is completely selected it will blur by 100px. but as you move towards the lesser selected edges of the selection, it may only blur it by, say, 50px, right the way down to where no blur is applied at all. so you have a gradual transition between the area of the image thats heavily blurred to the part of the image that is not blurred at all.
now, in after effects it is different. because i cant apply a filter to a "selection" in the same sense that i can in photoshop, to achieve the same effect i would take 2 duplicate layers, apply a heavily feathered mask to the upper one, then apply a 100px blur filter to it. the difference here is that towards the middle of the feathered selection, you are not getting an image with a 50px blur applied to it. instead you are getting an image with a 100px, overlaid at 50% opacity onto an image with no blur filter applied to it.
the difference here is small, but the smooth transition you get between blurred and un-blurred areas in after effects is no where near as nice as if you do it in photoshop. this is relevant to a lot of common filters taht both ae and photoshop use.
i would like to know if i can make a feathered selection in ae apply a filter in a graduated way, as you can in photoshop.
To control the size of the convolution kernel you have to use Compound Blur or similar third-party effects. Your assumption of Photoshop doing so is not correct. Even with a selection, all pixels are blurred uniformly. The difference is in how both programs treat Alpha information. AE works with straight Alphas internally all the time whereas PS uses a different scheme. As you may come to realize, blurring pixels that were "transparent" to begin with yields different results than blurring a fully opaque pixel and re-introducing a blurred Alpha (as AE does things in simple terms). If you will, in AE Alpah calculations are decoupled from RGB calculations.