Car Dash Cams – The Main Forward Facing Camera

In this article we consider the various things that make a dash cam’s camera better or worse than others.

If the camera is no good, nothing else really matters.  So we consider this point first and in careful detail, before progressing on to other aspects of choosing a dash cam.

Read more in our series on Dash Cams

Future articles to follow shortly :

  • Legal Issues
  • Entry and mid level dash cams reviewed
  • High end dash cam review
A screen grab from a Garmin dashcam complete with location and speed information as well as typical time information.

Okay, so of course, a dash cam necessarily has a forward looking camera.  But not all such cameras are created equal.  There are a number of important features to consider.  Alas, not all these features are well understood, and not all are well explained by manufacturers.

Resolution

How many pixels of picture does the camera provide?  The more pixels, the clearer the picture, and the easier it will be to see important details (or to be able to confirm that things alleged by the other party to your accident were not present/did not happen).

Resolution is often described as 720P or 1080P or HD or 4K or 2MP or some other term rather than an exact resolution statement such as 1920×1080.  The term “HD” in particular seems to take multiple meanings.  Make sure you understand exactly what the resolution is in both dimensions, and multiply the two numbers to understand the total pixels being recorded.

The larger each of these numbers is (horizontal resolution, vertical resolution, total pixels), the better the picture quality.  But if you had to choose just one number as being the most important, then it should be the horizontal number, and look for as many horizontal pixels as possible.  These days 1920 pixels should be the absolute minimum, over 2000 is good, and 3840 (ie “4K”) is excellent.  We expect that, within a few years, 4K will become commonplace and even higher resolutions will inevitably start to appear.  The more pixels, the merrier.

We also need to correct a potential misunderstanding.  Some of the dash cams show a picture that has rectangles to indicate how much picture a 1080 and a 4K image respectively capture, suggesting that the 4K picture is twice as wide and twice as tall.  This is theoretically correct, but actually totally misleading.  The amount of image that is captured depends on the camera’s angle of view (see next point), not on the camera’s resolution.  The only (but important) difference between different resolutions is not the amount of picture, but the quality of the picture captured.  The greater number of pixels means more detail, not a larger picture area.

Of course, more detail is very valuable, especially if you and the police are trying to read a registration number plate so as to identify a vehicle that perhaps didn’t stay at the scene of an accident.  We would definitely be willing to pay more for extra resolution, and this is usually a difference you would actually see (particularly if you had to zoom in on part of the picture).

Frame Rate

Sometimes camera manufacturers will also specify whether the camera is recording 30 or 60 frames per second.  It is desirable that these frame be recorded progressively rather than in interlaced mode (shown as “P” rather than “I”), but other than that, there’s no real value in 60P compared to 30P.  Not an awful lot of extra relevant detail can happen in a single 1/60th of a second – indeed, at 60 mph, your vehicle travels only about 18″ in 1/60th of a second.  So at 30 frames per second, and 60 mph, you are essentially getting pictures every 3′ as you drive along the road.  Additionally, with human reaction time from the first subtle appearance of some notable thing to your response being somewhere from 1/4 second to one second or longer (and usually on the longer end of this scale) your camera has probably captured 20 or more frames of what is happening before you’ve even started to react.

60P gives smoother motion, but that really isn’t an important thing.  If it is an option, by all means choose it, but it is not something to pay more money for.

Bit Rate

This is an important piece of data, but is seldom/never disclosed.

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Angle (Field) of View

This relates to how wide an angle the camera has.  You might think that “more” is better than “less”, but the difference is not as important as you might think.

A “standard” camera lens (say a 50mm lens for a 35mm camera – sure, there are not really any film cameras these days, but interestingly, their lens focal lengths remain as a standard) probably has about a 40° field of view horizontally (and less vertically – about 27°, because a normal camera usually has the width of the picture greater than the height, what is termed landscape mode).  A moderately wide-angle lens (28mm in 35mm camera terms) goes to 65°.

Because the field of view in our eyes is much greater than that (it is hard to measure our eye’s field of view, but for simplicity, it can be considered to be somewhere around 120°) and also because most photographers always want to “get more picture” into their camera frame, we have instinctively come to value wider angle lenses, and it is easy to understand how the concept of “sees more” is valuable for a dash cam that ideally should see “everything” that is happening.

So, for a dash cam, a wide angle of view is better.  The practical maximum for any camera lens is 180° degrees, which means all the way to the left and all the way to the right.  But doing this creates a distorted “fish-eye” perspective, and is also difficult to engineer, so it is uncommon to see anything more than 170°.  (Note that these days better cameras will automatically compensate for and adjust the fish-eye effect to make it less noticeable.)

On the face of it, you might think that the wider the field of view, the better.  There’s some sense in that, but there are compromises involved.  The wider the field of view, the more you are squeezing into each video frame, and so the fewer pixels there are to detail each item.  The picture quality goes down as the angle of view goes up.

And while it might be desirable to have as much side/peripheral vision as possible, even to the point of seeing things that you’d not see yourself, the reality is that a narrower field of view sees almost the same, but just from a bit further back.  This point in particular is extremely counter-intuitive, and so we did some trigonometry to work out how close to your car different fields of view actually pick up.

If we assume that you mount your dash cam in the center of your dash-board, facing directly ahead, and if we say that the side of the road that interests you is 8ft from where the dash cam is mounted, we can calculate a distance ahead – let’s call it D, that the field of view will see to this point  by knowing the stated angle of view (let’s call it “a°”) of your dash cam.  Obviously the shorter the distance, the better.

This formula is D = 8 x tan((180-a°)/2).  If you wanted to see something closer to the camera or further away, you’d simply change the 8 for whatever other distance in feet, and the relationship is linear – double the distance one way, and the distance doubles in the other way, etc.  So therefore :

If a = 180°, D = zero
If a = 170°, D = less than one foot
If a = 160°, D = 1.4 ft
If a = 150°, D = 2.1 ft
If a = 140°, D = 2.9 ft
If a = 130°, D = 3.7 ft
If a = 120°, D = 4.6 ft

Let’s say your car is traveling sedately at 30 mph.  The time it takes to travel 3.7 ft is less than one tenth of a second.  In other words, the difference between how soon a camera with a 140° angle of view stops seeing something to the side and when the 170° camera stops seeing it is less than 1/10th of a second.

If your concern is a car that comes down a sideroad, fails to give way, and hits you, and assuming you are traveling at similar speeds, any camera with greater than a 90° field of view will pick it up.  If the other car is going twice your speed, you need a camera with at least a 128° field of view.  If you are going faster than the car coming at you from the side, then you only need less than 90° field of view.

In other words, it seems likely that for most purposes, any dash cam with at least a 130° field of view will be adequate, and certainly, 140° starts to get as good as you’re likely going to need, most of the time.  So, for product comparison purposes, seek out a model with more than 130° but don’t place any additional value on much over perhaps 150°, and realize that when you start to go much over 160°, the loss in video quality is starting to outweigh the benefit of the extra picture coverage.

One other point.  It is perhaps just as well that there’s not a lot of real world difference between reasonably similar angles of view, because it seems some manufacturers exaggerate their claimed angle of view.

Contrast Capability

This is an important but hard parameter to compare, because most dash cams don’t give you a meaningful number.

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Low Light Capability

A related capability is for the camera to be reasonably sensitive in the dark and able to record helpful information, even at night.

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Image Stabilization

We’ve seen a couple of dash cams claiming they have some type of built-in image stabilization feature.  In theory, it is easy to understand the benefit of such a feature, because we’ve all seen more than our fair share of jerky hand-held video scenes.

But in practice, being mounted on the car windshield is actually reasonably stable, and in any event, you’re not filming in the hope of winning an Academy Award.  You just want to capture a clear enough depiction of what happened to help establish who did what and who is at fault.  Some jerkiness is acceptable.

This is not a feature we’d pay extra for.

Continuing On

Phew!  Who knew there was so much to consider with just the front camera alone.  But now that you’re an expert on that topic, please continue on through our article series.  Click the gold up arrow on the right hand side of the page to go back up to the menu, or if you wish to continue reading in sequence, the next suggested article is our one about extra dash cameras and how to mount them.  You might be pleased to know that it is shorter than this article!

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