Photoshop - Feathering Guidelines for Area of Interest Tools (Marquee and Lasso Tools)

It is permissible (and legally acceptable) to use area of interest tools to
isolate specific areas within an image and enhance those specific areas.
The tools that are typically used to isolate specific areas within the image
include the rectangular and elliptical marquee, the lasso and polygonal lasso,
and the magic wand and quick select tool. When isolating specific areas,
it is strongly recommended that you must feather (or blend) the enhancement
into the rest of the image to avoid causing visual inconsistencies.
Typically, it is not required to feather a selection made using the Magnetic
Lasso or the Magic Wand as those tools are used to select an area of interest
based on a difference in edge contrast.


For high-resolution images, such as latent prints, the amount of blending is based
upon the calibrated resolution (PPI) of the image.

If the calibrated resolution is less than 1500 PPI, multiply the number of hundreds by three
(3). For example, if the resolution is 1252, multiply the number of hundreds (i.e., 12) by 3, which
will give you a feather radius of 36. Enter that value as the Feather Radius in the Feather
Selection dialog box.


If the calibrated resolution is greater than 1500 PPI, multiply the number of hundreds by four
(4). For example, if the resolution is 1632, multiply the number of hundreds (i.e., 16) by 4, which
will give you a feather radius of 64. Enter that value as the Feather Radius in the Feather
Selection dialog box.

NOTE:
To keep it simple, I typically round the PPI up to the highest whole number in
my head, and then use that value to calculate my feather radius. For example, if my resolution is
1232, I simply round the number up to 1300 in my head and then multiply 13 by whatever
multiplier I use. Whether or not you use 3 or 4 is not all that critical. Remembering to feather the
selection is far more important to avoid creating artifacts that do not appear in the original
image. Not only does it avoid creating artifacts, but it also makes the enhanced image more
balanced and more professional.


For lower-resolution images, such as footwear impressions or tiretread impressions,
the amount of blending is based upon the size of the area selected rather than
the calibrated image resolution. Unfortunately there is not as clear cut
a recommendation for these types of images as there is for smaller items, such
as fingerprints, questioned documents, firearms, etc. When feathering a
selection for low-resolution items, it is recommended that you use a feather
radius that is between 50 pixels and 100 pixels. Use a feather radius
that is closer to the top end of the range when blending a selection with a
lower resolution and a larger selection; use a feather radius that is closer to
the bottom end of the range when blending a selection with a higher resolution
and a smaller selection. If an edge  becomes visible after you have
enhanced the selected area, step backwards (Ctrl + Alt + Z) to undo the last
enhancement, then increase the feather radius and repeat the enhancement
process.