[libcamera-devel] [RFC 6/7] libcamera: properties: Define 'lens' properties

[Andrey Konovalov]

Hi Jacopo,

Nice to see how fast the areas which libcamera covers grow. Fantastic work!

Just I am somewhat confused with the terminology used.
Could you please elaborate a bit on that?

As considerable part of libcamera developers might not be familiar enough with optics and photography,
guess it would make a lot of sense to create, or to refer to some existing document which would provide
enough introductory type information on all that stuff?

On 18.12.2019 17:50, Jacopo Mondi wrote:
> Define properties that describe the optical characteristics of the image
> sensor.

Afraid the 'lens' properties aren't the optical characteristics of the image sensor.
The same image sensor can be paired with a bunch of different lenses..
(the family of ov5647-based cameras for RPi by waveshare, DSLRs, etc etc)

> Signed-off-by: Jacopo Mondi <jacopo at jmondi.org>
> ---
>   src/libcamera/property_ids.yaml | 34 +++++++++++++++++++++++++++++++++
>   1 file changed, 34 insertions(+)
> 
> diff --git a/src/libcamera/property_ids.yaml b/src/libcamera/property_ids.yaml
> index 9e2333cb7206..fdadfb8b1da4 100644
> --- a/src/libcamera/property_ids.yaml
> +++ b/src/libcamera/property_ids.yaml
> @@ -208,4 +208,38 @@ controls:
>           The range of supported ISO sensitivities, as documented by the
>           ISO 12232:2006 standard
>   
> +  - LensApertures:
> +      type: float
> +      compound: true
> +      description: |
> +        The available lens apertures, expressed as f numbers (the ratio between
> +        the lens focal distance and the diameter of the pupil aperture).
> +
> +        If the sensor

- s/sensor/lens ?

> has a fixed aperture, the property transports a single
> +        value.
> +
> +  - LensFocalDistance:
> +      type: float
> +      compound: true
> +      description: |
> +        The available focal length distances, expressed in millimeters.

Sorry, this sounds confusing.
The "focal length" is the characteristic of the lens: https://en.wikipedia.org/wiki/Focal_length
If this is what you meant, then the property name should be "LensFocalLength", and the description should use
"available focal lengths" (no "distances" here).

"Focus distance" is the distance at which the lens is currently focused at.

> +
> +        If the sensor is featured with a movable lens mechanism this property
> +        reports the focal lengths associated with each discrete step.

- this is a zoom lens, right?

> For
> +        sensor with fixed-focus lenses

"fixed-focus" means that the focus, the distance at which the lens is focused, can't be changed
(vs "auto-focus" or "manual focus").
So "fixed-focus" shouldn't be mentioned here.

>, a single focal length should be instead
> +        reported.
- this is exactly the term used at https://imaging.nikon.com/lineup/lens/f-mount/
   Other common names are "fixed focal length lens" or "prime lens".
   (and the "fixed-focus" is completely different story)

> +
> +  - LensHyperfocalDistance:
> +      type: float
> +      compound: true
> +      description: |
> +        The hyperfocal distance of the sensor.

s/sensor/lens ?

> The property is particularly
> +        meaningful for fixed-focus devices.
> +
> +  - LensMinimumFocalDistance:

The correct term which matches the description is "minimum focus distance".
See e.g.
https://imaging.nikon.com/lineup/lens/f-mount/zoom/normalzoom/af-p_dx_18-55mmf_35-56g/spec.htm

> +      type: float
> +      description: |
> +        The shortest distance in millimeters from the lens surface in which an
> +        object could be brought into sharp focus.

It might be not very precise (the "from the lens surface" part), but otherwise good description.

> +
>   ...
> 

Will continue in the reply to the next patch.


Thanks,
Andrey