A Major Flaw in Long Lenses and PTZs Found

Author: Ethan Ace, Published on Aug 26, 2014

Theoretically, long lenses should let you capture faces and license plates very far away.

For example, over 900 feet away from a 1080p camera one should get over 40ppf from a 100mm lens.

But how well does this work in practice?

In this test, we took 3 long lens cameras, including:

We ran tests from 100 to 500 feet away from the cameras, comparing the theoretical PPF to the actual achieved.

Can one really deliver such distances or are there quality issues for long lenses?

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Comments (31)

The best I can tell this has been going on for years. It is something we have seen but never formally tested / proved.

This major quality degradation has been masked by offering longer lens length. Of course, longer lens lengths with degraded resolution is a poor tradeoff, as...

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John, I am curious as to why do you give Dahua such visibility within IPVM when their cameras hide behind OEM labels? If so, then it would be helpful to know who the OEMs are. As you say within other IPVM articles, the three most value-centric camera lines appear to be Dahua, HIK Vision, and Samsung. Of the three I am least likely to ever procure Dahua because of their non-presence in...

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Thanks for the feedback.

We include Axis a lot because they are the favorite camera, by far, amongst IPVM integrator members. I agree with your point about margins, also with Axis' massive availability. Despite that, it's undeniable that they are broadly used.

As for...

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Linear density is determined by the lens aperture.
Was convinced.

Can you expand on your comment?

A Fujinon 8-80mm varifocal lens with a 720p Samsung SNB-5004 camera

Was the Samsung actually tested at 720p?

It's native resolution is 1280x1024, and since your ppf is based on horizontal pixels, you...

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"Picking up" vertical pixels does not change the image quality. It would simply increase the vertical area captured by the camera.

This is a really just an example of 4:3 1.3MP vs 17:9 720p, e.g., Aspect Ratio...

Would be interesting to see the same test with a Canon ES lens which is a DSLR lens that appears to have better glass than traditional lenses. I believe the Avigilon Pro Series cameras have the Canon ES lens for the 8, 12, 16 & 29 MP have the Canon ES lens.

I suspect Canon ES lens as well as some MP machine vision lenses would do better. The key limits one encounters there is cost (5x,10x, more) and size (which is an issue for be ******** *** ********** ***.

Hi Ethan, thank you for your excellent article. By any chance do you have a long, fixed lens which could be used to compare the quality of image against the varifocal lens on the Samsung SNB-5004?

Can you say at what length of lens, and below, image degradation becomes a non-issue? Does this vary much depending upon the brand of lens or other factors? Thank you for your information.

Hey Luke, we do not have a long fixed lens, and they're actually not so easy to come by. There are a couple options which are 50 or 75mm, but they're generally machine vision lenses, not IR corrected, and more expensive than even the 8-80mm we tested, in the $300+ range.

Lens manufacturer definitely makes a difference in shorter focal length lenses, but it also changes from model to...

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Thanks Ethan, I remember the HD lens shootout and have found it very useful in selecting lenses. I also could not find long fixed lenses.

"For the stadium, we used a 5-megapixel camera at 75-feet wide and 34 pixels per foot (ppf). Then we opened up to a higher resolution: 10-megapixels at 110-feet wide at 33ppf. We tried a 25-135mm lens that performed great with the camera and became one of the lenses they ultimately used in the installation. After seeing several different...

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Jeremiah, It's worth differentiating between how well it worked and how well it performed relative to PPF theoretical calculations.

All the cameras in our test worked 'well' at some level. You could see details very far away, relatively speaking. The issue is that they did not work 'as well' as PPF predicts.

Btw, we've tested the model they cite, i.e., "10-megapixel...

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Isn't some of the quality degradation explained by motion blur, which is intensified as you zoom? Could be due to unseen vibrations, e.g. due to nearby machinery/natural harmonics of the pole, etc. I would try putting your finger on the camera enclosure and check if it has any impact on the directionality or intensity of the blur.

Isn't the reduced quality due to anisoplanaticism across the image? The angular size of the pixels in these examples is 5 seconds of arc or thereabout. So one can expect atmospheric image warp to start to become a problem. Also in the movie (although there for illustrating depth of field issues), I think I see warping of the edge of the cones. I also think warp can be seen in the eyechart....

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Please help us out with this one. Does anisoplanticism refer to non-cartesian mapping of optical path lengths that occurs because of atmospheric diffraction which is different for each ray path?

Or, what is it?

Thanks.

Yes, isoplanticism means all ("iso") in the same plane ("planaticism"). So all of the central rays from each portion of the image are parallel and a good picture can be formed. Anisoplanaticism means that the image is not isoplanatic so all of the central rays are not parallel and a good image cannot be formed. Think of "heat waves" rising from a desert floor and the resulting "wavering"...

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Does the test hold true for PTZ at standard definition at these high focal lengths? I recall reading several years ago that the glass in the lens of most high pixel cameras don't support anywhere near the pixel densities of these compact cameras. There has to be good reason a digital SLR camera has a lens that is 10x the length and width of a lens used in a typical HD security camera.

Hi Robert, good question. We're going to get out a standard def PTZ and check that out.

As far as DSLR lens size, keep in mind that most of their sensors are much larger than surveillance imagers. A Four Thirds DSLR sensor is 9x larger than a 1/2.7" sensor common on HD cameras, and that's small for a DSLR. So with sensor areas that big, they simply need more glass and larger lenses....

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The important point is not the size of the sensor and the focal length of the lens, you have to think about the angular size of the pixel as it maps onto your targets. So it's the ratio of these two quantities that is important. Lenses are all about bending light and angles. The lense has no idea what sort of sensor you're going to put behind it. But really, I think the problem here is not...

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IPVM's tests covered a variety of conditions. If the deviation from theoretical was related to atmospheric distortion, how would you explain the observation that the effect is pretty consistent across a variety of distances? Passing through only 20% of the distorting media, would one expect significantly better performance at 100' than at 500'?

Hi Horace,

No. The effect is very local. This doesn't happren at all with satellite pictures, for example. If the light is bent away at great distances from the lens, then the light doesn't make it into the camera and there is no effect. So this happens (depending on the atmosphere, of course), in the first 100 yards (closer if the turbulence is high--could be 100 feet). You can also...

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As an integrator many of the discussion points regarding anisoplanaticism and isoplanaticisim made my head ache but the insight is invaluable. Especially when one comment / recommendation stands out so clearly: "the effect goes away very significantly if you get the camera off the ground." Thank you Rick!

The second critical reminder is the truism that the image is only as good as...

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Howdy Ethan! Do y'all have a link for any additional images/detail that go with this test?

Hi Jim,

What other images / details do you want?

John

I'm prolly mixed up, but I was thinking I read something about a 80mm fixed lens or a shorter focal tube shot closer to compare, but I might be thinking bout another discussion.

Maybe I jumped the gun there, I thought this search result was referring to the same test as in the Flaw Found one? Maybe supplemental info? Or an outline? But it says 'try back later once its posted', so I reckon I'm wondering when that will be...

(In case my...

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Howdy everyone!

It's now been over a week since I asked about any other image data on this test. First off as you can see above, John didn't say there wasn't any, but instead just asked me for exactly what I was lookin for. I obliged him generally, and then a little later I screen printed exactly what I seen in the search results to make me curious. By the next mornin the search no...

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We have two types of articles - 'reviews' and 'updates'. Ethan originally wrote this as an update. What you saw is an unpublished early draft that our search engine showed in the results. That was a bug (showing unpublished posts) which we fixed in the past week or so.

This report is the one and only official post on the topic.

The poor MTF at longer lens lengths is most likely source of the image quality issues.

Yes, it may very well be.

The 3MP fuji lens is listed at f/1.4 at 8mm, and although I can't find a published spec for it at 80mm, a conservative guess puts it at around at least f/3, possibly a good deal more.

Where the snb-5004's 1/3" image sensor looks...

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