What's new

# Focal plane distance formula for varifocal camera

#### Oppiie

##### n3wb
Is it possible to compute the distance of the focus point (distance at which objects are sharpest) using the Zoom and Focus parameters shown in the interface ?

The camera I'm using is this one:
12MP IR Vari-focal IP Bullet – Dahua North America

I see that the parameters ranges are [0, 852] for zoom and [0, 1025] for focus, however even after some research I can't quite figure out what the distance of the focal plane is.

#### Beemer

##### n3wb
Hyperfocal distance is dependant on the aperture. Generally it results in a point 1/3 distant from the camera lens to the farthest required point of focus.

#### Oppiie

##### n3wb
Hyperfocal distance is dependant on the aperture. Generally it results in a point 1/3 distant from the camera lens to the farthest required point of focus.
After more research I was told that what I'm looking for is basically not possible (i.e. computing distance from focus) since those ranges I mentionned are most likely step-motor units and no full model of the camera is available. If this is wrong, please let me know

#### Beemer

##### n3wb
After more research I was told that what I'm looking for is basically not possible (i.e. computing distance from focus) since those ranges I mentionned are most likely step-motor units and no full model of the camera is available. If this is wrong, please let me know
You are using the term "focal plane". This is the point on an optical system where the image height and width is formed and nothing to do with any point distant. I'm curious as to why you would need to know this distance which is from the nodal point of the lens to the sensor?

#### Oppiie

##### n3wb
You are using the term "focal plane". This is the point on an optical system where the image height and width is formed and nothing to do with any point distant. I'm curious as to why you would need to know this distance which is from the nodal point of the lens to the sensor?
Yes I was wrong; I didn't know much about photography in general (and I still don't really). I meant focus distance. Sorry for the confusion.

#### Beemer

##### n3wb
Yes I was wrong; I didn't know much about photography in general (and I still don't really). I meant focus distance. Sorry for the confusion.
I assumed that is what you wanted which is why my first reply mentioned hyperfocal distance. To explain further, camera lenses have a characteristic that means that not all distances can be in focus at the same time. If you want a distance range to be acceptably in focus then it is better not to focus on the closest, or the middle, or the farthest point. Instead you should focus on a subject that is one third (1/3) the distance from the lens to the farthest point you want to cover. In practice this applies to near subjects, e.g. say 6 feet to 50 feet so you don't need to work out 1/3rd point to the horizon! Just for completeness , landscape photographers refer to tables or data engraved on their lenses to tell them the hyperfocal distance. In this case everything from infinity to a stated close point will be in focus.