Subscriber Discussion

What Does 2.8mm Lens Means

UD
Undisclosed Distributor #1
Feb 20, 2017

when we talk about 2.8mm lor 12 mm lens what does it actually means ? Does it mean the lens thickness is 2.8 mm or 12 mm. ? or does 2.8 mm lens have some other meaning ? 

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Ari Erenthal
Feb 20, 2017

It refers to the distance from the image sensor to the lens. The further away from the sensor the lens is, the narrower the angle at which light hits the sensor, making the field of view narrower. We explain it further here.

UD
Undisclosed Distributor #1
Feb 20, 2017

what makes the light rays hit the sensor in narrow angle when the focal length is increased .why is it so ?

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Ari Erenthal
Feb 20, 2017

The function of the glass in the lens is to bend beams of light so it converges on a single light. We call this focusing the light, and adjusting the lens in order to change the angle of convergence is to focus the lens. The point at which the rays of light meet is called the focal point. Adjusting the focal length changes the focal point so that the angle is narrower or wider. 

If you'd like a more in depth explanation, watch this video:

 

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UM
Undisclosed Manufacturer #2
Feb 21, 2017

Not a joke. It's because your mirror work.

http://www.physicsclassroom.com/class/refrn/Lesson-5/Refraction-by-Lenses

U
Undisclosed #4
Feb 21, 2017
IPVMU Certified

It refers to the distance from the image sensor to the lens.

Disagree.  

Perhaps if you had a lens made from a single thin piece of glass, but with actual lenses that one buys and uses in cctv cameras, no.

Rather, the "focal length" stated by mfrs is really the "effective focal length", which is best thought of as a measure of how strongly the lens can bend light, based on the equivalent length of an idealized single thin lens.

Measure the actual distance between the glass (front, back or middle) of a 2.8mm lens and the image sensor.  

You may be surprised just how much air there is :)

U
Undisclosed #3
Feb 21, 2017

I find weird things to waste time thinking about. Look at the VB770 equipped with a SEL2470Z lens and housed in the UNI-ED2G8 enclosure. Datasheet says 74° HFOV with this combination. Datasheet on the SEL2470Z states 84° HFOV on any E-mount 35mm Exmor sensor.

VB770 datasheet does not show dimensions of enclosure front panel glass height from bottom plane or it's width. Enclosure data shows 3.06"H x 3.65"W. Now what happens when we use the SEL2470Z? Do we throttle back some of our HFOV because our VFOV is seeing the bottom and top planes from the inside of the enclosure?

So if you built a hamster wheel out of 9 40° cameras in the center pointed outward you can have a 360° view stitched together. The horizontal aspects only mesh at equal lengths on the outer edges of the wheel, aka the edges of your monitor.

The question is how many cameras will it take if you use the UNI-ED2G8 enclosure that is deducting from each Vertical FOV?

Note: The VB770 is an awesome camera.

Now imagine a 9 wedge pie chart instead of the armature below.

UM
Undisclosed Manufacturer #2
Feb 21, 2017

I don't know if it help or not. VB770 sensor size is 37mmx24mm.
Tested SEL28F20, SEL35F14Z, SEL057FEC, SEL24240 & Canon EF 50mm F1.4 USM w/o any issue.

(1)
U
Undisclosed #3
Feb 21, 2017

Thanks for you input UD#2!

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