Tested: Video Quality vs Dome Bubble

Author: Ethan Ace, Published on Dec 11, 2014

Often, industry professionals will warn about three problems with using domes:

  • The dome bubble degrades image quality
  • The degradation is worse with megapixel cameras
  • The degradation is worse through the side of the dome bubble rather than the center

But is this true?

We tested six cameras from four different manufacturers in multiple scenes, through both the center and side of the dome to find out.

We answered these key questions:

  • How much practical impact do minidome bubbles actually have on image quality?
  • Do different regions of the dome vary in the resolution they can deliver?
  • Do they impact multi-megapixel cameras more than typical HD 720p/1080p cameras?

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Comments (19)

John,

That doesn't jibe with what I've found. There is a noticeable difference in fine focus with the bubble on versus with the bubble off in many applications. The difference is exacerbated with "long" lenses and often gets worse towards the edge of the dome. The focus can be restored by focusing through the bubble but that is a pain to handle.

Some manufacturers (including IndigoVision and Arecont) include a "focus tool", which is basically a cross section of a bubble. Using that to accomplish fine focus does help but it doesn't always guarantee perfect focus with the bubble on. I believe that may be due to slight differences in lens-to-bubble distance and radius of the dome versus radius of the focus tool.

Perhaps we're more "picky" than most but we try to optimize every aspect of our camera installations, including zoom and focus. It makes a difference when attempting to read card values and suits, gaming chips, currency and other things we absolutely have to see. I would guess that identifying faces is a bit less critical.

"There is a noticeable difference in fine focus with the bubble on versus with the bubble off in many applications."

Can you be more specific? You mention 'long lenses' and edge of dome? How long of a lens? What specific models have you found problems? Are you using factory bubbles or third party ones, etc.

On analog cameras, we use 1.8-3.6mm, 2.8-12mm and 5-50mm lenses. While there is little, if any, focus difference bubble on/off with the 1.8-3.6mm lenses, 2.8-12mm lenses exhibit progressively more variability as we zoom in and 5-50mm lenses are even worse. Also as we take the aim closer to horizontal (towards the rim of the dome), the focus issue tends to get more pronounced. In fact, as we near maximum zoom, the difference between bubble on and bubble off becomes very pronounced.

On MP cameras, we typically use Theia 1.8-3.0mm, Arecont 3.3-12mm and various 3.1-8mm and 9-20mm lenses. We have observed the same issues with the MP lenses. We haven't tried 5-50mm or other very long focal length lenses on MP cameras yet.

Obviously, this is not a problem with power zoom/focus lenses since they can be focused with the bubble on. All tests are with factory bubbles.

I certainly can see the issue with the non factory lenses, especially longer ones. Our tests were with manufacturer included lenses.

Btw, we have a discussion on that exact Arecont issue that I am sure you know about but for others - Solution To Focus Problem For Arecont Dome.

I think we agree then it is not typically the bubble itself that inherently degrades quality but the focusing of longer lens inside a bubble, yes?

JH,

I don't get your point about "factory lenses". We get the same thing with the integral 2.8-12mm S-mount lenses on Vitek cameras and have seen it on other domes with integral long focal length lenses. We even get it with the IndigoVision cameras with factory-provided 9-20mm lenses.

I really don't think this can be passed off as a lens issue. It is a bubble issue. I've had manufacturers acknowledge that, especially with longer focal length lenses, the bubble tends to act like an element of the lens.

On a related note: I've never seen this in a camera housing where the front glass is parallel to the front of the lens so my educated guess is that the curvature of the bubble acts like a lens element.

It is a combination issue - long lens + bubble.

Ethan, please check on our cameras what happens with the longest integrated factory lenses we have.

Also, are you claiming that it is inherently worse - long lens + bubble or that it requires a focal adjustment? The latter is what the manufacturers are claiming, e.g., Solution To Focus Problem For Arecont Dome

At minimum a focus adjustment but I've found that at certain angles with narrow FOVs, the bubble distortion can't be totally overcome. If we get one part of the FOV in focus another part is out of focus.

That may be that the lens is not perfectly perpendicular to the sensor. I have had lenses or lens holders where the tightening of the lock screws caused an internal shift which caused that effect.

Bob,

That wouldn't explain why focus is fine across the FOV with the bubble off. I have seen what you describe, though. In at least one case, the lens was cross-threaded into its holder.

Ethan- were the cameras refocused for the on and off shots?

To my eye the dome offs are slightly sharper in all cases than dome on. Same with center vs edge.

The dome bubbles will cause a slight focal shift which is why you see the focus aids.

Ethan- were the cameras refocused for the on and off shots?

Bump. Without knowing this, how can we draw any meaningful conclusions?

Yes they were.

Thanks!

Using test bubble, trial and error, or remote focus?

Great article! Coming from a company that helped design OEM housings for many large manufacturers it takes quite a lot of research to properly design the most optically "clear" bubble possible.

Many will find that the thinner acrylic bubble to be far superior to industry standard polycarbonate bubble. The polycarbonate bubble allows for a more ruggedized window and can acheive an IK10 impact rating. The thinner acrylic while optically superior lacks the impact capabilities of the polycarbonate. New dome materials that can help both these aspects and includes added bonuses such as their ability to be more scratch resistant as well as corrosion resistant exist; however, the price for the material is still 2x-3x that of the polycarbonate and acrylic.

Without getting too technical the keys to optimal optics is high quality uniform extrusions of polycarbonate/acrylic with the most uniform shape to the actual camera lens. The more uniform the overal bubble shape to that of the camera lens the better the overal image of the camera. Meaning the distance from the camera lens to the bubble should be identical across the entire lens. The downfall of this type of solution is you will have a noticeable edge at the very bottom of the dome curve resulting in poor image quality. This is mainly due to the extrusion process resulting in limited horizon degrees for optimal clarity. Acheiving beyond 10-15 degrees above horizon becomes extremely difficult with the degraded extrusion line.

Adding in a tint to the bubble further degrades image quality resulting in a full F-stop typically. Unless a new manufacturing process is implemented or material costs come down we will continue to see this issue regarding dome optics.

Pierce, thanks for the detailed feedback.

Can you comment about how bubble design / manufacturering has changed in the past decade? I am curious what improvements have occured.

Btw, as for tint, we have a test on that: Testing Smoked vs Clear Domes

Dear Pierce McCord,

If possible I would like to have your contact info.

Great test. Appreciated.

Many will find that the thinner acrylic bubble to be far superior to industry standard polycarbonate bubble.

Which type are the domes that were used in the test?

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