Lens Focal Length Tutorial

Author: John Honovich, Published on Nov 17, 2014

3mm, 6mm, 2.8 - 9mm, 5 - 50mm, etc.

Camera specifications often list lens lengths but what do they mean?

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
  • Picking the right lens length

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Comments (30)

This is one of the best articles on this topic that I have ever read (and I have read a lot of them).

Great article. My name is Dan Davies from Tamron USA.

Focal length is often misunderstood in the security industry, and it's nice to see it explained here. The focal length of a single lens element can be described by the distance between the lens and the point where parallel light rays are focused together by the lens. However, for a lens system (many lens elements), this distance can and almost always is controlled to a specific value through the design of a lens. For example, commonly-used CS-mount lenses all have the standard 12.5mm distance between the lens mount and the sensor, regardless of focal length.

I think focal length can be accurately described as how strongly the lens system brings light together. A lens system with a shorter focal length brings light rays together more strongly than a system with a longer focal length. This lens can therefore bring light rays at wide relative angles together to create a wide angle view. A lens with a longer focal length does not strongly bring light rays together, and captures only light rays that have small relative angles, providing a narrow angle of view.

Despite the different focal lengths, it is still possible to design the two lenses so that their images are focused at the same distance away from the lens, allowing them to be used interchangeably on the same camera.

Daniel, thanks for the explanation. Very helpful!

John:

Focal lens length is the physical distance between the lens and the sensor / imager.

Daniel:

...focal length is actually not a physical distance anywhere in or on the lens.

Does John agree to Daniel or Daniel agree to John? Or both disagree to agree?

There has to be some physical connection. For example, all 1000mm lenses are far physically longer than a 3mm lens.

Novice question:

This sketched picture look like has camera box+lense outline. But i have also this Avenir (la cheapa) lens that say 2.8mm fixed focol length.

Problem is front glass to back glass is 40mm. Mucha more than 2.8mm. Other problem, camera not even attached on it yet, so more distance to imager still to come.

So when they say 2.8mm, where is it can be actually measure on real lens?

Good question. The 2.8mm focal length is actually not a physical distance anywhere in or on the lens. It is a measure of the "light bending power" of the lens system. 2.8mm is relatively wide (bends light strongly), and should give about a 90-degree angle of view on a 1/3" sensor. The manufacturer's specification sheet will have details on the exact angle of view the lens will provide.

Thx. Mr. Dan Davies, you tell it like 'tis!

Ok, if not real measured distance, but is it basically how thick glass would be if was one piece solid lump lens?

So physical lens length usually longer or shorter or neither/both than manufactured focal length, for complexed lenses?

That helps with this quester too.

a good paper to read is http://pages.cs.wisc.edu/~dyer/ah336/papers/02_lenses.pdf

understanding the lens is detialed in chapter 2 page 13..

Great Introductory reading and information!

Hi Brian
Isn't AoV the same as FoV?

Isn't AoV the same as FoV?

Good question. They are definitely closely related and often used to describe the same thing.

We try to use AoV primarily to emphasize when we are talking about the Angle of View, i.e., the angle covered in degrees. This is because the AoV ultimately dictates the Field of View seen, generally expressed in feet / meters.

AOV and FOV can often be used interchangeably*, though technically FOV refers to the linear width of angle (subtend) at a given distance.

So, FOV width (w) depends on AOV (a) and distance (d) as shown here:

w = 2d * tan(a/2)

therefore for an AOV of 80°, some valid FOVs widths are

83 ft @ distance 50 ft
167 ft @ distance 100 ft

*AOV, FOV used here actually refers to the HAOV (horizontal angle of view) and the HFOV (horizontal field of view), though height and VAOV and VFOV can be substituted if desired**

**per IPVM style guide

I know with broadcast cameras you have a back focus adjustment, do any of the available security lenses require this adjustment?

Can you explain why same spect lenses offer different angles?

Maybe this is just a Hik issue, although I doubt it.

Model 1:
http://overseas.hikvision.com/en/Products_accessries_758_i6114.html
1/2.8" Progressive Scan CMOS
2.8 mm, horizontal field of view: 108°
4 mm, horizontal field of view: 86.4°

Model 2:
http://overseas.hikvision.com/en/Products_accessries_756_i6109.html
1/2.9" Progressive Scan CMOS
2.8 mm, horizontal field of view: 80°
4 mm, horizontal field of view: 63.5°

The CMOS has a minimal differente, but the horizontal FoV has massive difference.
I see this difference in many ranges and can't really find a good explanation for it.

Can you explain why same spect lenses offer different angles?

#3, good eye. This actually happens a lot across many manufacturers.

One common reason is that the manufacturer is using a larger sensor resolution than what is being streamed our (e.g., a 5MP 'camera' but it uses a larger 6.4MP lens) which means the actual streamed AoV will be less than the theoretical. Other reasons we have heard where variances in what lenses could deliver, etc.

We have seen this many times as we were building out our Camera Calculator we found many times the actual specified AoV was much different than what it 'theoretically' should be. It is one of the reason we list thousands of specific models with their specified AoV rather than the theoretical one.

Possibly combination of software/firmware not just variances in lenses or sensors. might also explain for actual differences. Just a thought.

The CMOS has a minimal differente, but the horizontal FoV has massive difference.

IMHO, the main reason for the difference lies in the fact that the CMOS sizes (1/2.8" and 1/2.9") are approximate diagonal measurements and the FOV (108 and 80) are given in horizontal measurements.

Since the CMOS size is only given in diagonal units, you can only infer its width by looking at the aspect ratio.

So we have a 1920x1080 vs 2560x1920 sensor, which are 16:9 and 4:3 ratios respectively.

Assuming that there is no cropping at max resolution, we have a much squarer sensor in the second case, which translates to a narrower HFOV.

I see this difference in many ranges and can't really find a good explanation for it.

Net/net, diagonal sensor sizes used to be good enough to infer AOVs, but now that it's common that sensors come in many different shapes, it is not enough.

See: Easy Question: What Is The Size In Mm Of a 1/3" Sensor?

So we have a 1920x1080 vs 2560x1920 sensor, which are 16:9 and 4:3 ratios respectively.

it's common that sensors come in many different shapes, it is not enough.

That's a really good point. Thanks for sharing!

Hi Brian,

The Table: Focal Length/Approx Angle of View shows 3mm : 77 deg.

In the Video recording slide, the same table shows 3mm : 85 deg.

Is this just a typo or am I missing something?

Regards,

Eddie

Hello Eddie:

Good catch! Sharp eyes, sir. The correct value is 77 degrees for 1/3" @ 3mm. I'll update the table used in the slides deck.

Thanks Brian... Result of spending too much of my life vetting supply agreements.. Excellent course so far... Taking a while to get my head back into the 'study-space'.

Nice article

Great information in a concise format.

I'm really impressed with the knowledge shared in the comments!

One thing I'm a little shocked to see missing from this tutorial is any reference to zoom, which focal length has a direct correlation to and is more easily understood by laymen. The article doesn't at all mention that longer focal length lenses will zoom in to a scene whereas short ones zoom out. Varifocal lenses are often referred to as variable zoom lenses as a result.

When I first started learning about CCTV this was one aspect that I found difficult to understand simply because few articles actually made this clear. Without the two photos in the examples above having different zoom levels, the article suggests that a longer focal length will only have a smaller field of view rather than a smaller and closer one.

Great tutorial for my first class!

Diffrent OEM cameras having same focal length and imager size have diffrent AoV .

Is there any other factor other than imager size and focal length effecting the AoV ?

This was a very insightful article I've learned a lot from it. Thank you.

An interesting article.

An interesting article/knowledge made simple for trainees to understand...Nice.

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