This text is from https://www.networkwebcams.co.uk/blog/2020/01/30/cctv-camera-lens-guide

I found this to wildly be the best article I’ve seen and since great articles have a habit of disappearing on the web, I’ve reposted it here. Thank you to the authors.

CCTV camera lens guide

If you’re new to security cameras you may think all lenses all the same… but they’re not. This is why choosing the right one is an important consideration, as this impacts your security footage.

There are actually a wide variety of lenses which produce different scenes, known as field of view. CCTV camera lenses are designed to address specific surveillance needs.

Before we delve into your different choices, let’s go back to basics on field of view…

What is field of view?

In short, field of view is the area of coverage that the camera can ‘see’ within a scene. This all depends on the size of the lens – the higher the number, the narrower a view you have.

This simplified diagram below shows you the sort of coverage you expect from different lens sizes.

A word of warning, field of view alone cannot guarantee you’ll be able to monitor a subject. Other considerations like camera placement and where your subject is, are also important.

What lens do you need?

This is a tricky question to answer as it depends on what you want to monitor. To help you choose, it’s handy to know what sort of view you’ll get from a lens.

Here’s a look at the most common lens types with examples to show you the field of view you’ll get.See lenses for IP cameras here.

2.8mm for a 90° view

With a wide field of view, the 2.8mm lens is a popular choice for static camera surveillance. It can cover a wide area with just one camera.

The 2.8mm lens is very popular indoors. This is because you only need one camera mounted in a corner to monitor a whole room. Though they’re also effective outdoors for areas like train station platforms.

Here’s a sample image of a 2.8mm view taken from anAxis P1455-LE.

If you’re looking for a wider field of view, then180° or 360° panoramic camerasare available. These models are ideal to provide an overview of your location. Please note with ultra-wide cameras there’s often some warping of the image.

4mm for a 60° view

Although it has more clarity at a further distance, the 4mm lens shows a similar scene to the 2.8mm. The main difference is the reduced angle of view which gives the illusion that objects are closer to the camera.

A 4mm lens is useful for detection & identification.

This lens is often used in outdoor areas like car parks, were detail is important. It’s also helpful when you want a narrower view to avoid monitoring unnecessary areas – an aisle in supermarket, for example.

6mm for a 45° telephoto view

The 6mm lens gives a narrow, but detailed distance view. A telephoto lens can also be used to monitor a specific object like tills.

Customers often use this lens for monitoring the entrance to a car park, or a doorway. It’s also useful for a corridor, especially when combined with corridor format/view technology.

Here’s an example of a 6mm lens.

Footage from three lens sizes

The video below shows the above three lens sizes, with the subject at a 16m distance from the camera.

What type of camera has which lens?

You’ll find these lenses on most of the cameras we stock in store.

Static cameras have a fixed lens, while a varifocal lens can be adjusted during installation. Pan tilt zoom models, also have a lens range and the view can be adjusted as required.

You may want to read our article on thedifference between fixed and varifocal cameras.

Summary

Your choice of camera field of view impacts what you’re able to monitor. For example, a 2.8mm lens produces a wide view which is great for general surveillance. While a narrower, telephoto lens is more suitable for looking at specific object.

Hopefully this CCTV lens guide has helped you to decide which would be most suitable to monitor your site.

Published on January 30th, 2020 by Network Webcams

https://en.wikipedia.org/wiki/Automatic_number_announcement_circuit

The current use of exchange prefixes for each area code is listed by CNAC;[8] if an exchange changes from “plant test” to reclaimed or active, any former test numbers with the associated prefix are invalidated. Commonly-used test numbers for major carriers include:

555-0311 Rogers (403 Alberta, 519 613 Ontario)
958-2580 Bell Canada (519 613 705 905 Ontario, 450 418 438 514 579 581 819 873 Quebec)
958-ANAC (958-2622) Bell Canada (416 Toronto) (invalid from WIND Mobile)
958-6111 Telus landline (403 780 Alberta, 250 BC)
959-4444 Manitoba Telecom Services (204 MB) (959 is used since 958 is a regular Winnipeg exchange, not a test prefix)
958-9999 Bell Aliant (506 NB, 709 NL)
958-2222 Eastlink
Additional plant test codes may be in use locally in some areas:

403: 555-0311 Alberta (GroupTel – may work in other parts of Canada – untested)
604: 1116 British Columbia (Telus)
604: 1211 British Columbia (Telus)
819: 959-1135 Most of Outaouais region (Bell Canada)
Occasionally, a number in an existing, standard local exchange in the area is used. These will incur a toll (and might not work) outside their home area. Some may be announcing caller ID, which is not the same as ANI. As standard local calls, they are not accessible from ADSL “dry loop”, inbound-only or unsubscribed lines:

403: 705-0311 Calgary, Alberta (Allstream – gives “call cannot be completed as dialled” in other parts of Canada, identifying as Allstream, active November 2019)
416: 477-0034 Toronto, Ontario (Fibernetics – Verified January 2021)
416: 477-0035 Toronto, Ontario (Fibernetics – verified January 2021; this number allows you to leave a message for reasons not yet determined)
416: 981-0001 Toronto, Ontario (verified July 2010) (busy, September 2015)
418: 380-0099 Quebec City (Vidéotron – verified 2017)
905: 310-3789 Mississauga, ON (Now no longer includes loop line or ringback. In NPAs where Bell Canada is incumbent, 310-xxxx is assigned as a pseudo-tollfree exchange which may be called at local call rates from an entire area code.)
In Bell Canada territory, +1-areacode-320 was formerly reserved for 320-xxxx test numbers; these were moved to the 958-xxxx range and 320-xxxx reclaimed for use as a standard exchange. The use of N11 prefixes (such as 3-1-1) for test numbers is also deprecated as 3-1-1 now often reaches city hall or municipal services while 2-1-1 is local community information.

This post serves as a place to collect information to deliver Multicast audio on IP Phones. The testing is done on Yealink phones however we will be doing required testing on Cisco/Linksys SPA series phones as well.

To change the format and codecs of the audio, I’m using the open source tools – Audacity.

The codec and audio formatting that work is:

Codec PCM S16 LE (s16l)
Type Audio
Channels: Mono
Sample rate: 8000 Hz
Bit per sample 16