A question about image()

[gratings]

When I use "image(c)", where c is a matrix containing uint8 data and ranging from 0-255, 0 is completely black, 255 completely white.

Now I use

c=uint8(ones(256).*a);
image(c)
colormap gray

I increase "a" from 0 to 255. Strangely the picture becomes completely white when "a" increases to ~50 or 60, but not 255 which I guess. Could anyone give me some hints about this?

Thanks a lot!