# Global shutter QQQVGA - lens

In order to choose an appropriate lens (focal length/FOV) for my application I am trying to use a lens calculator like this one Lens Calculator | Focal Length | Working Distance | Field of View.

-Working distance is adjustable between 35 -43 mm
-h x v of flat object to be observed 32 x 20.4mm

I need to acquire images in QQQVGA Format.

Using the calculator to calculate the appropriate focal length for my application if I choose the MT9V034 1/3" sensor I cannot use 32x20.4 as FOV in input to the calculator because that would restore a focal length valid for VGA mode of the sensor.

The calculator restores Horizontal 4.5mm and Vertical 2.9mm when choosing the global shutter sensor.
QUESTION → Is it correct to divide by 8 to determine the equivalent sensor H and V in QQQVGA mode? If not how to obtain those values?

In the calculator if I insert the working distance above and the h x v of my object in the field of view I get a focal length of 0.6 to 0.7.
QUESTION → Does this approach seem reasonable?

Finally , when S-mount lens have for example focal length of Xmm is that length valid if the Lens is screwed all the way into the lens holder only?

You are overthinking this. Unless you know how in detail the camera sensor works you are wasting your time. For example, in qqvga mode the camera bins pixels. Then, we resort to cropping to further reduce the resolution. If you have a grasp of exactly what our driver does then this is something you can approach.

Fair enough. So i should just go by trial and error with a bunch of different lenses?

Can you suggest a good starting point?

Not really, you just have to try a lot of different lenses. That’s what I did. I searched AliExpress and bought every different type of lens from one manufacturer.