Lens Focal Length Tutorial

By IPVM Team, Published Jul 10, 2019, 11:44am EDT

Camera specifications often list focal lengths, e.g., 3mm, 6mm, 2.8 - 9mm, 5 - 50mm, but what do these specs really mean?

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These metrics are important in determining the correct camera coverage and Field of View (FoV). In this tutorial, we look at:

  • What lens focal length impacts
  • The relationship of imager size
  • Common lens length ranges and their uses
  • Limitations of using focal lens length
  • Manufacturer variances between focal length and AoV
  • Fixed vs. varifocal lenses
  • Fixed focal length options
  • Focal length impact on camera size
  • Lens limitations (low light performance and detail loss)
  • Picking the right lens length

Focal **** ****** ** *** / ***

***** **** ****** ** the ******** ******** ******* the **** *** *** sensor / ******. ** is ********* ** ************ because ** ** *** primary ****** ** *** FoV ******* / ***** of **** (***) ** a *****. ***:***** ** **** (***) Tutorial.

Lens ****** ******

*** ****** *** ****, the ******** *** ***** of **** / **** captured; *** ******* *** lens, *** *****. **** is * ******** ********* deriving **** *** ******** between *** **** *** the ******, ** *** image ***** *****:

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*** ****** *** **** is, *** ******* ** is **** **** *** imager, ********* ** * narrower ***.

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********, ** *** **** is *****, ** **** be ***** ** *** imager *** ******* * wider ****, **** **:

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Impact ** ****** ****

****** *** ***** **** length ** *** ******* driver ** *** ** surveillance, *** **** ** imager **** (*.*., */*”,*/*”,*/*”, etc.) **** ******* ****. Everything **** *****, *** larger *** ******, *** wider *** ***** ** view **** ** ** the **** ***** ******.

**** ******* **** **** imager **** ***** **** close, **** ** */*.*" vs. */*.*" **. */*", the **** ****** ******* in *** ** ****. This ***** ********** ** imager **** *** ****** in ******* ******* ********** in ***** ** ****, leading ** ***** ** narrower ***** ** **** than ********. ** **** distances, **** *** ****** to ******* **** ** more, ***** ** ***** ranges ** *** **** no ********* **********.

*** *******, *** ***** below ***** *** *********** in *********** ***** ** view ******* */*" *** 1/2.7" ******* ***** ******* lens *******:

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** ***** ***** *******, differences *** **** **********,**** the */*.*" ****** ********* a ******* ***** ***** of **** **** */*", which ***** ****** ** lower **** ******** *******.

*** ***** ************ ** your *** ******** **** **********.

Common **** ****** ******

** ********, ************ ******* typically **** ** *** of * **** ****** ranges:

  • ******* / ***** ***:******** ***** **** ***** views *** ***** **** distortion, **** *** **** 'fisheye'. ***** ****** ********* require ********* ******** ** provide * **** / corrected *****. ***** ** can ******* ********* ******, implementation *********** ******. *** our ***** ** ********* Panoramic / *** ******* cameras.
  • '******' / *** – 10mm:*** ***** ****** ** most ******* **** ** this ***** (******* ** to ** ******* ***).
  • ********* / **** – 80mm:***** **** ******* ** no **** **** ****** in **** *****, **** of ******** */** ****** *** be ********* *** ********* to ******* (********* *** form ******)
  • ***** ********* / *****+:****** ** **** ***** are ********** **** **** to *** / ******* objects **** *** **** (typically ***+ **** ***** 5kms ** ****)

Warning: ************ ***** ****** / *** *********

****** ************* ********* ******* AoV / *** ********* by ******* * ***** length. *** *******, ************* may **** *** *********** to **** **** * camera ** **** ***** or ********* **** ** show * ****** ** telephoto.

*******, ****** ************'* ***** lengths ** *** ****** map ** ** *********** / ************ *** *** , ********** **** ******* focal *******. ********* ** is ******* ** *** much ** *** ****** or **** ** **** or ******* *** ************ of *** **** ****** from *** *********** ************.

********** ** *** *****, the ******** ** ** always **** ******* / lenses ***** ** ***, not ***** ******. **** makes **** **** ********** ********* ** ****** models, **** ******** ****, in *** **********.

Fixed ***** **. ********* ******

** ************ ***** *** two *** **** *****, fixed ***** *** *********.

  • ***** *****: ***** ***** lenses *** ****** **** one ***** ******, *.*., 2.8mm, ***, ****, ***. Their **** ********** *** simplicity (***** ** ****** adjustment ** ********) *** size (********* **** *****), though **** *** **** flexible **** ********* ****** and ****** ** **** tuned ** *** ******** desired ****** ** ****.
  • *********: ********* ****** ***** adjustment ** *** ***** length, ****** ******** ******** using *** ******** ***** or **** * ********* mechanism. ********* ****** *** specified ***** ***** ***** length *****, **** ** 2.8-10mm ** **-****. ********* models *** **** ********, as **** *** ** adjusted ** *** *** field ** **** *******, but **** *** ****** and **** *********, ********* bigger ********.

*** ******** ** ******* which *********** ******* *** varifocal (*** *** **********). *******, **** ******** depends ** * ***** integrator's ****** ******(*), **** those ******* ** ***** end ******* ***** **** fixed ***** ****** *** to ***** (*********) ***** cost.

Multiple ***** ***** ****** *******

************* *** ******* ***** focal ****** ****** ***** offer ******** **** *******. While **** ** *** as ******** ** ***** varifocal ****** ***** ***** fine, ** ****** *** some

*** *******, ******* ** a********* **-**********-*(* *** **** ****** camera), ** ** ******* with * ********* **** options, ******* **** **** angle ** ********* (****** **** ** ***** models):

  • *.***/***° ***
  • ***/**.*° ***
  • ***/**° ***
  • ***/**° ***
  • ****/**° ***

**** ****** ***** ** select *** **** ***** best **** ***** *****, e.g. ***** *** *.*** lens *** **** ******** of * ***** ***** or ***** *** **** model *** ******* ******** of * ******* *** entrance.

***** *** ***** ************ offers *** ***** *******, many ** *** ***** cost ****** ********* **, such *******,******'* * ******,********* *****/***********,*******, ***.

Focal ****** ******* ****** ****

****** ***, ***** ***** length ****** **** ** less *****, ******** ****** housings ** ** ******* than ****** ********* ** motorized ******.

*** *******, *** ***** below ***** * ***** focal ****** *** ****, typical ** ***** **** in******** *********** ** * *** camera **** **** ********. The **** ** **** lens ****** ******** *** cameras ** ** ******.

*** ********* **** ****** ******** **** ********** ** size ******.

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Lens ***********

*** ****** *** **** length, *** **** ****** three *********** **** *****:

  • ******** ***** *******: **** ******, ********** varifocal **** ****** ** surveillance, ********** **** ******** image ******* ******** ** what ***** ******* / ppf ******* ***** ********. If *** *** ******** / ***** **** **** ***********, you *** **** ** compensate **** ***** ***** density. ***:* ***** **** ** Long ****** *** **** Found

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  • ******** *** ***** ***********: **** **** ****** (as **** ** **** short ****) ********* **** significantly **** ***** **** typical ****** ** *** 3mm ** **** *****. Specifically, ********** **** ***** equal, *** ****** *** lens *** ****** *** F ******.****** *** * **** tutorial *** *******.

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  • ***** ** ***** ******: ************ ******* **** rarely **** ***** ** field ****** ***** ******* appear *** ** *****. This ** ******* **** usually *** ********** ***** lenses *** **** ******** that *** **** **** 5 **** **** **** the ******. *******, **** long **** **** ***** problems **** *** ********** hard ** ******* *** night **** ******* ** with ****-**** ******. ****** our***** ** ***** ********.

Picking *** ***** **** ******

*** ******* *** **** length, *** **** *** camera *** *******. *************, the ***** *** ******* area, *** **** ******** is *** ********** ****** captured. **** ** *** people ***** *** ******* when **** **** ** a ***+ ******* *******. Since ** ** **** resolution, *** ******* "******" be ****** *** ***** the *** ** ** huge, *** *** ******* can **** "*****" **** at ***** *****.

*** **** **** ** thumb ** ** **** the *** ** ***** than *** **** ** interest – *.*., **** what *** **** ** capture. *******, **** *** be ***** ** ****** want ** ******* * lot ** ****** (*.*., I **** *** **** in *** ******* *** plus *** ******* ****** of *** **** ******** it).

** **** ***** ****, Pixels *** **** (***) / ****** *** ***** are ******* **** ** help ********* *** **** one *** ** ******* making *** ******* ** the **** ******** ** fuzzy **** **** **** no *****. ********** *** *** ******* ********** ****. ********** with *** ****** ********** to ********* *** ******* trade *** ******* *** and *** *** **** specific ************.

[**** **** **** ******** was ********** ********* ** 2011, *** *** ******* in **** *** **** with **** ******** ******** and ********** *-**** *** iCS **** ***********.]

Comments (13)

Focal lens length is the physical distance between the lens and the sensor / imager. (Emphasis added).

in my experience it’s not, although longer focal lengths generally speaking are physically longer.

here’s a couple of examples:

first is a fixed focal Tamron 8mm.  without even adding in the back-focus distance (space from the rear glass to the imager), it’s easily 3x the marked measurement.

next is a computer 8-48mm vari, fully extended.

at least 2x more.

obviously, this is just a couple of lenses that I have kicking around.  I’d be interested in seeing what others find.

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then there are reverse cases such as the following fuji; 800mm focal length somehow in a 410mm box (including the sensor/connectors and housing)

imho, the number of millimeters of a given “focal length”, while meaningful when determining AOV, is unlikely to correspond to a physical length you can actually measure with camera/lens in hand.

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To clarify, it's the physical distance between the imager and the lens' point of convergence (shown below). So in the case of your two lenses, the focal length is simply somewhere in the middle of the lens. Most lenses are made up of several elements (also shown below) and the convergence point hits at one of these elements.

In the case of the Fuji camera, I'm not 100% sure what they're doing there. They could be using a telephoto converter in their lens, which magnifies the center portion of the image to increase the effective focal length. They're common in photography, though they are available in surveillance as well. We will see what we can find out.

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To clarify, it's the physical distance between the imager and the lens' point of convergence (shown below).

this diagram has the convergence point right at the sensor though, (if i’m reading it correctly).  i like the new diagram.

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Onetime I have asked professional optical laboratory to measure actual focal length of some very expensive FA lenses. Actual value varies up to 10%. What could we expect from those cheap used in cctv ? 

only exact AoV is valuable. 

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A year or so ago, someone posted a great video showing how to properly specify lenses and configure the FOV (depth of field?). The video showed a man walking a very long distance in a parking lot towards the camera - in the first poorly configured example, the man was indistinguishable until he was very close to the camera and the distance he walked looked really long.  In the properly configured view, the depth of the parking lot looked shallower and the man appeared to walk forever even though it was the same distance and the image at all depths was crystal clear.

If this sounds familiar, I'd love to see that video/link again.  I think the person who posted it was a Consultant.

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This is an excellent focal length primer. 

Probably not going to get as many clicks as the sexier news [Tariffs, etc] but good info.

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So, I have a question, with time, especially with geovision, an untouched camera, how does it get unfocused? No motorized lens, just a simple manual 2.8-12mm lens.

NOTICE: This comment has been moved to its own discussion: With Time, How Does An Untouched Camera Get Unfocused?

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Love the level of detail. Just watched the video after the reading assignment. I see why it's important to do the reading first. Makes the video tutorial a lot more understandable.

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Great explanation of all the lens types and there respective applications. The impact of the length of the lens is a much-needed consideration when selecting an area to install for a customer.

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what is the sentence "While this is not as flexible as using varifocal models which allow fine, it allows for some” supposed to say?

on this page;

Lens Focal Length Tutorial

thanks!

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hi, can you explain the camera calculator results (generic camera) in view of;

"Everything else equal, the larger the imager, the wider the angle of view will be at the same focal length."

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the 1/2.8” imager likely has an HD aspect ratio that makes its width greater than the 1/2.7”, at the expense of its height of course.

it’s worth also mentioning that the imager dimension, e.g. 1/2.8”, is a inexact diagonal approximation, see Easy Question: What Is The Size In Mm Of A 1/3" Sensor?

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