What sizes are detectable from 1m using a fairly good cmos camera? I haven't ordered a CMOS camera module yet, but I am planning to use it to detect a small object from ~1-1.5 meters.
Just too be specific, say I am using the following CMOS camera (although if you know there is better with around the same price I'm open for suggestions):
its 1920*1080p, 120fps(this may reduce the resolution), I plan to use it ~90 degree viewing angle (I had the impression that its adjustable). Lens is "2.8-12mm varifocal". I have no idea about DPI.
Considering other factors are satisfied (such as illumination, colors), what size of objects will I likely to be able to detect in image processing? Would a 2cm object be detectable for example? Is there a way to estimate what size would correspond to a pixel in 1m distance?
 A: With a little math you can get an excellent approximate answer.  Pretend that the lens is a pinhole.  You have a triangle formed by one side of the object, the other side of the object, and the pinhole, and you know how far away the object is.  Another triangle is one side of the image, the other side of the image and the pinhole, and the distance between the image and the pinhole is the focal length.  These two triangles are similar.  Thus $$\frac{s_o}{d} = \frac{s_i}{f}$$
Where $s_o$ is the size of the object, $s_i$ is the size of the image, $d$ is the distance to the object, and $f$ is the focal length.  This equation is approximate, but it is a very good approximation in most cases. So the size of the image on the focal plane array is $$s_i = s_o\frac{f}{d}$$
and you know everything on the right hand side.  Now all you need to know is how many pixels are covered in the distance $s_i$.  If that number is about 10 your image will look like a blob.  If is about 100 it should look pretty good, but "good" is in the eye of the beholder.
So you do need to know the size of a pixel, or the pixel pitch (almost the same thing) or the number of pixels per inch.  The data sheet for the focal plane array will have that information.  Since you know the resolution, you can get to that number if you know how big the entire array is.  As an estimate, you can take a wild guess that the pixel pitch is 20 microns.  
