Hi! There are several issues I need to mention:
(1) About the magnification: We prefer using “Field of View” over “Magnification.” A digital photo could be zoomed in and out on a screen of any size, so it’s hard for us to give a strict definition of how much it’s magnified. Field of View, on the other hand, defines the area of the scene you want to capture, and is usually expressed in the degrees of angles.
For example, Let’s assume the fingerprint of your index finger is 20mm wide, you want the fingerprint to cover the whole picture’s width, and you are shooting the picture from 40mm away. From that setup we will be getting a triangle with a bottom width of 20mm and a height of 40mm, which leads to a Horizontal Field of View (HFoV) of 28 degrees. From 60mm away, it will be 19 degrees to cover the whole fingerprint with 20mm width.
(2) The focus issue: Actually most fixed-focus (FF) cameras are focused to infinity with a limited minimum object distance (M.O.D). For example, the official Raspberry Pi cameras claims to offer a focused viewing distance of 1m to infinity.
Since you are talking about shooting from 40-60mm, what you are possibly looking for is a camera whose lens focus could be tuned – in other words, adjustable focus – because you cannot “move finger close or far until it is in focus” on a fixed-focus camera.
Adjustable focus comes in two types : Manual Focus (MF) and Auto Focus (AF).
(3) The depth issue: The depth of field (DoF) of a picture is at its maximum when focused to the furthest, so when you re-focus the camera to a closer object, the depth of field drops. When it comes to a 40mm level, the depth will be really shallow.
(4) The Distortion issue: Most embedded camera lenses come with optical distortions, except that the low-distortion lenses produce minimum distortion.
If you’ve read through the explanations above, here are some more details that I need to know:
a. Do you need your fingerprints to cover as much of picture as a possible, or do you accept that the fingerprint only takes a center section of the image as long as it’s identifiable?
b. Do you have strict requirements of image distortions?