Ask a camera module supplier for a wide-angle lens and you will usually be offered something around 110° or 120°. For a camera on a shelf, that is generous. For a camera built into a crib rail 25 cm above the mattress, a 120° lens leaves about an eighth of the mattress out of the picture. The missing area is right next to the camera, not at the far side.
For a camera at the midpoint of a long rail on a standard US full-size crib mattress, a 120° rectilinear lens covers about 87% of the mattress at 25 cm mounting height and about 99% at 40 cm. Full coverage at 25 cm takes a fisheye-type lens of around 160°. The area a too-narrow lens misses is the two mattress corners on the camera’s own side, not the far edge. Three more things decide the result besides the angle on the datasheet: the lens projection, which way the sensor is turned (turning it 90° drops coverage to under 60%), and how fast resolution falls off toward the far corner, which in our model is about 7 times lower than at the near edge.
This is Part 02 of our series on designing a camera into a crib. The hub article shows why mounting height drives everything. This article works through the lens choice itself.
§01How much of the mattress a given lens actually sees
| Lens (diagonal FOV) | Coverage at 25 cm above mattress | Coverage at 40 cm above mattress |
|---|---|---|
| 90° rectilinear | ||
| 110° rectilinear | ||
| 120° rectilinear | ||
| 140° fisheye | ||
| 160° fisheye |
TABLE 01 · Share of mattress area inside the frame, best tilt for each lens. The best tilt falls between roughly 43° and 68° below horizontal, depending on lens and height.
The jump between the two columns is the first lesson. Fifteen centimetres of mounting height is worth roughly as much as moving from a 120° rectilinear lens to a 160° fisheye. If the crib design has any freedom in where the camera sits, that freedom is worth more than any lens catalogue.
§02The missing part is next to the camera
Most people expect a narrow lens to lose the far side of the mattress. In a rail it works the other way round.
A tilted lens projects a trapezoid onto the mattress. The far side of the trapezoid is wide, often wider than the mattress. The near side, right under the camera, is narrow. So the part that falls outside the frame is the two corners at either end of the camera’s own rail.
FIG. 01 · Top view, drawn to scale from our model. Dashed lines are the edges of the lens footprint. Each missed triangle is about 34 cm along the rail by 35 cm across, together roughly 13% of the mattress.
This matters because those near corners are not dead space. A sleeping child moves, and the ends of the mattress next to the camera rail are as reachable as anywhere else. A sample checked by placing an object in the middle of the mattress will look fine. A sample checked by placing objects at all four corners will show the gap.
§03The angle on the datasheet is not enough
Two lenses both sold as “140°” can cover noticeably different areas. Three properties sit behind the single number.
Projection
A rectilinear lens keeps straight lines straight. Past roughly 120° diagonal, rectilinear designs become impractical and most modules switch to fisheye-type projections, which compress the edges of the image to fit more angle in. That compression is why a 140° fisheye already covers 93% at 25 cm while a rectilinear lens would need about 144° just to reach the corners. The cost is curvature: crib bars on the far side bend, and the edges of the image carry less detail. Ask for the lens’s image height versus field angle table, sometimes called the distortion curve. The diagonal angle on its own does not tell you how the image is distributed across that angle.
Sensor orientation
The mattress is long along the rail and short across it. At 25 cm it spans about 139° along the rail and about 112° across. A 16:9 sensor has to have its long side running along the rail. Turn the same module 90° in the housing and coverage collapses:
| Lens · height | Long side along the rail | Module turned 90° |
|---|---|---|
| 120° rectilinear · 25 cm | 87% | 59% |
| 160° fisheye · 25 cm | 100% | 58% |
| 120° rectilinear · 40 cm | 99% | 71% |
TABLE 02 · Same lens, same height, same best-tilt search. Only the sensor orientation changes.
This sounds obvious, but it is a real mechanical constraint. A rail profile is narrow vertically, and a module board is often longer in one direction than the other. If the board only fits one way, the sensor orientation is decided by the rail, and it needs to be checked before the profile is final.
Aspect ratio
This one matters less than people expect. At 25 cm with a 120° rectilinear lens, a 4:3 sensor covers 84% against 87% for 16:9. Orientation is the bigger factor by far.
§04Resolution falls off toward the far corner
Getting the whole mattress into the frame does not mean the whole mattress gets the same level of detail. In a rail-mounted camera, the near edge of the mattress is very close and fills a lot of the sensor. The far corner is distant and compressed into a small part of the image.
We measured how many sensor pixels each centimetre of mattress receives, for a 150° fisheye at 25 cm on a 1920 × 1080 sensor (97% coverage in our model):
Linear pixel density at the mattress surface. A 10 cm wide object at the near edge spans about 330 sensor pixels. At the far corner, about 45.
That is a factor of about 7 in linear detail across one mattress, before any compression. And the sensor resolution is not what the parent sees. If the video is sent to the display at 1280 pixels wide, the far corner drops to about 3 px/cm. At 640 pixels wide, about 1.5 px/cm, or 15 pixels across a 10 cm object. Our article on sensor, stream and screen resolution explains why those three numbers differ.
So “1080p” on the box tells you very little about what a parent can make out at the far end of the crib. If far-corner detail matters for your product, specify it directly as pixels per centimetre at the far corner, at the stream resolution actually delivered.
§05Run the calculation at every mattress position
Most cribs let the mattress support be lowered as the child grows. The camera stays where it is on the rail. So the distance between camera and mattress changes over the product’s life, and the lens has to work at every position.
The rule that falls out of the numbers:
- Choose coverage at the highest mattress position. That is where the camera is closest and the lens needs to be widest.
- Check detail and night illumination at the lowest position. That is where the far corner is farthest away.
- Check how much of the frame is wasted at the lowest position. A lens wide enough for the high position sees a lot of rail, bars and room when the mattress drops.
| Lens · height | Mattress covered | Share of frame that shows mattress |
|---|---|---|
| 150° fisheye · 25 cm | 97% | 72% |
| 160° fisheye · 25 cm | 100% | 64% |
| 160° fisheye · 40 cm | 100% | 51% |
| 140° fisheye · 40 cm | 100% | 66% |
| 120° rectilinear · 40 cm | 99% | 61% |
TABLE 03 · Mattress share of the image area, best tilt for each case.
With a 160° lens chosen for the 25 cm position, half of the image is something other than mattress once the support is lowered to 40 cm. That is not a failure, but it is resolution being spent on crib bars. Sometimes the better answer is 150° with 97% coverage at the top position, accepting a small slice of the near corners, in exchange for more usable detail. That is a product decision, and it should be made explicitly rather than by whoever picks the module.
§06What not to do
Moving a narrow view around the mattress means most of the mattress is off screen most of the time. Inside a crib that defeats the purpose. Background in our PTZ article.
One camera per half of the mattress removes the extreme angles, but doubles module cost, IR and wiring, and needs split-screen or stitching on the display. A real option for premium designs. See single vs multi-camera.
The most effective fix if the crib allows it: from 25 to 40 cm, a 120° lens goes from 87% to 99%. How high it can go is a crib structure and safety question, covered in Part 05.
§07What to ask for in the lens specification
Crib camera lens · RFQ items
These turn “a wide-angle lens” into something a module supplier can quote against and you can test.
- Horizontal, vertical and diagonal field of view, all three, measured on the sensor you will use.
- Projection type and distortion curve: image height versus field angle.
- Image circle versus sensor size. If the image circle is smaller than the sensor diagonal, the corners of the frame go black.
- Sensor orientation in the housing, long side along the rail, confirmed on the board layout.
- Mounting tilt and its tolerance, since the best tilt is steep, roughly 50–60° below horizontal at 25 cm.
- Coverage at every mattress position, checked with test objects at all four corners.
- Far-corner detail in pixels per centimetre at the delivered stream resolution.
- Night performance of the lens under the IR wavelength used, since focus can shift between visible and infrared light on lenses that are not designed for both.
Night performance is where lens choice meets the next problem: lighting a mattress that is four times farther away at one end than the other. That is Part 03.
Where the numbers come from. These figures come from the same model as the series hub, on standard mattress sizes, with the assumptions stated in the method note. They describe what the geometry requires. They are not test results for any specific lens or product, and a real lens should be confirmed on a sample in the actual rail.
If you have a crib drawing, we will run the same calculation on your rail and mattress positions before discussing modules. Our Camera & Video Monitoring page covers the embedded project type.
§08Questions buyers ask
Is a 120° lens wide enough for a crib camera?
It depends on mounting height. In our model of a camera at mid-rail on a US full-size mattress, a 120° rectilinear lens covers about 87% of the mattress at 25 cm above it and about 99% at 40 cm. At lower mounting heights, full coverage needs a fisheye-type lens of around 160°.
Which part of the mattress does a narrow lens miss?
The two corners on the camera’s own rail. A tilted lens projects a trapezoid that is narrow near the camera and wide at the far side, so the far edge is usually covered while the near corners fall outside the frame.
Does sensor orientation matter?
Yes, a lot. The sensor’s long side must run along the rail. In our model, turning a 16:9 module 90° reduces coverage from 87% to 59% with a 120° lens at 25 cm, and from 100% to 58% with a 160° fisheye.
Why is the far corner less detailed than the rest of the image?
Because it is much farther from the camera and is compressed into a small part of the image, especially with a fisheye lens. For a 150° fisheye at 25 cm on a 1920 × 1080 sensor, our model gives about 33 pixels per centimetre at the near edge and about 4.5 at the far corner.
What if the crib has more than one mattress height?
Choose the lens for full coverage at the highest mattress position, where the camera is closest, then check detail, night illumination and wasted frame area at the lowest position.
Choosing a lens for a crib camera?
Send the rail drawing and every mattress position. We will return coverage, near-corner gaps and far-corner detail for the lenses you are considering, before a module is ordered.
Send Your Crib Drawing → Coverage model · Lens selection · Module integration · PCBA