Digital camera images and What can you see

WSchick at aol.com WSchick at aol.com
Wed Mar 14 15:42:03 EST 2001


In a message dated 3/14/01 6:52:09 AM Pacific Standard Time, 
pxpst2 at dlt.pitt.edu writes:


> I do not understand what you mean by "30-40 grayscales", could you 
> please elaborate?
> 
> I am hard pressed to believe that significant differences could exist on 
> a image that the eye could not detect.  Possibly on the low end, but if 
> it (the pixel grayscale value) is close to background, one would have a 
> 

This test is from a 256 gray scale test pattern on any software program that 
can adjust BWG  or Black White Gamma settings of the monitor.  Some gel 
analysis software has brightness and contrast settings which are similar.

The 256 gray scale pattern shows 16 lines of 16 individual grayscale boxes.  
On most monitors, with BWG at 0 and 255 (the gray scale value for black and 
white on an 8 bit image), all viewers claim to see 16 lines--the gradation 
from, say 42 to 43 grayscale is so fine that our eyes can't distinguish the 
difference.

If you adjust the grayscale range to 30 or 40 (say 20black and 60 white, or 
120 black and 150 white) then most viewers can distinguish the difference in 
gray scale and count 30 or 40.   This means that the grayscale steps are now 
8-10 times greater than single grayscale increments, and at that point, we 
can distinguish them.

In practical terms, then, we could only be able to visibly determine if one 
band changed from another by this difference.  Software quantitation, of 
course, can calculate the differences of one grayscale to the next.

As you point out, on the low end you can't "see" the difference.  But by 
moving the grayscale downward, and reversing the image from white on black 
(EtBR) to black on white like in a textbook, you can see the faint bands that 
you might overlook on a Polaroid print.

If you wish, I can email you jpg files of a TIF CCD image I modified using 
AlphaEase software.   In one case, this saved redoing 40 PCR reactions in a 
96 well typing experiment.  So this image enhancement tools can save most 
labs time and effort on repeated experiments.

Or if you have a TIF file you have trouble seeing, send it to me and I'll try 
to improve it.  Please reduce the file to under a MB if you are using a 
scanner or high resolution CCD...my server might reject it.

Walt Schick
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