Wide angle lens dof expansion and speed perception.

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paulca

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Paul Campbell
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I had a very minor bump in the car, into a kerb, avoiding an idiot. I, probably stupidly, posted the footage. Most people said I was going too fast. I know for a fact I was not. I even used the stopping distance to work out, that if I was doing 20mph+ as people suggested my deceleration would have been 8.5G, enough to damage my eyeballs!

Obviously dash cams have very wide angle lens. I think mine is roughly 140* lateral.

So I wanted to do an experiment. Putting my Sony A6100 in the car, on it's tripod strapped to the side of the driver seat. Then drive around at various speeds of 10, 15, 20mph and compare how the footage looks and feels regarding speed compared with the dashcam.

Which lens setting would you use to best represent the eye (on an crop sensor camera)... 35mm, 50mm?
 
I did a bit of reading up.

So the Sony APS-C crop sensor has a conversion factor of 1.53, which means a 35mm lens should give the equivalent of a 50mm lens.
 
50mm is the accepted standard on a full-frame or 35mm camera, but I believe that the nearest focal length equivalent to the human eye is actually 43 mm. So, on a cropped-frame camera this should equate to about 28mm.
 
paulca said:
Most people said I was going too fast.
Click to expand...
They were foolish to voice an opinion, unless the speedometer was visible in the frame.

Tests have been run by psychologists on speed perception. These suggest that even experienced train drivers or police officers are unable to assess speeds to better than 10% and most people do well to get within 20%.

Instrument panel Hyundai i10 Panasonic TZ40 1000650.JPG
 
Last edited:
AndrewFlannigan said:
They were foolish to voice an opinion, unless the speedometer was visible in the frame.

Tests have been run by psychologists on speed perception. These suggest that even experienced train drivers or police officers are unable to assess speeds to better than 10% and most people do well to get within 20%.
Click to expand...

Indeed. That is what has been so frustrating about the whole thing. Everyone thinks I'm doing 20+mph. At the start of the video I AM doing 20+mph, based on how long it takes me to pass the red 107 in frames / framerate. It worked out about 22mph. But I'm slowing constantly on 2nd gear engine braking and when I measure the distance (as best I can based on kerb stones, lines and car lengths), the distance I stop in, once I apply the brakes is somewhere between 6meters and 10 meters. Running the reverse displacement calculations on that and given that 1G to 1.1G deceleration is the maximum available, I simply cannot be travelling faster than about 12mph when the hazard appears.... 15 meters back from the junction I know people who run faster.

When I spoke with the police and mentioned everyone who sees the video says I was going to fast. He said, "without even seeing it I can tell they will be wrong", he then explained that for dashcam footage were speed is a question, they will ignore any speed written on the video and send the clip to their cyber crime team to do analysis. He says that every single time the speed is much lower than perceived watching it.

I mean I drove down that road yesterday 3 times. Each time I glanced at my speed and it was, at the same point, descending through 15mph. Yet, to my eyes, this is very, very slow and I have huge amount of reaction time and distance.

I should just let it go, people are people and once they think they see one thing, you can't make them see anything else.
 
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