Vortex Radar's DR970X Plus Test & Review Notes

[Nigel]

There's also dashcams with f/1.5 lenses.

Not for an imx678 size sensor?

The bigger the sensor, the less depth of field even if the aperture doesn't change, and thus the more accurate the focus needed. Think of the big plate cameras with very shallow depths of field despite having fairly wide angle lenses.

but really you're working with a wide angle lens that's focused to infinity

Focusing to infinity is easy, but having done that, the depth of field limits how close it can focus, there is not much point having a dashcam that can't focus at less than 20 meters when the resolution limits reading plates to less than 20 meters. You can of course focus it for 10 meters, but then everyone will complain that infinity is out of focus.

So for a dashcam that must have excellent focus from 3 meters to infinity, there is a limit to how big the aperture can be, and a limit to how big the sensor can be, and with an f/1.7 imx678, you will have to be pretty accurate on the focusing, while an imx675 sized sensor with an f/2.8 aperture might almost be focused somewhat reliably by screwing the lens in x turns and not bothering to check the result!

but ultimately it needs to be done. We need to be able to properly record details to the right and left of our car as well.

Yes, nobody wants an out of focus dashcam, if you are going to sell an f/1.7 then you need an adequate focusing facility, with appropriate quality control.

 

[Vortex Radar]

Not for an imx678 size sensor?

The bigger the sensor, the less depth of field even if the aperture doesn't change, and thus the more accurate the focus needed. Think of the big plate cameras with very shallow depths of field despite having fairly wide angle lenses.

Indirectly, but not exactly. That was a common misunderstanding that became especially prevalent when DSLR's with cropped sensors started arising.

There's plenty of articles on the topic you can check out.

Understanding How Sensor Size Affects Depth of Field

Most photographers have started with a crop camera when digital photography was introduced. Only after a few years, the full frame cameras became affordable and that is the moment when all the discussions about the effect of depth of field and crop sensors started, often with the mentioning of...

fstoppers.com

In short, using a smaller / cropped sensor reduces how much of the lens' total view you can see. A fixed focal length and aperture lens does not change its DOF based on the size of the sensor behind it. However, because you see more or less of the image the lens projects based on the size of the sensor, it means that you either need to use a wider focal length lens (zoom out) or move forward or back (zoom with your feet) which will affect the DOF.

So there's some truth to it because it means manufacturers need to compensate for smaller sensors by using wider lenses, but DOF itself does not change based on the size of the sensor.

Focusing to infinity is easy, but having done that, the depth of field limits how close it can focus, there is not much point having a dashcam that can't focus at less than 20 meters when the resolution limits reading plates to less than 20 meters. You can of course focus it for 10 meters, but then everyone will complain that infinity is out of focus.

So for a dashcam that must have excellent focus from 3 meters to infinity, there is a limit to how big the aperture can be, and a limit to how big the sensor can be, and with an f/1.7 imx678, you will have to be pretty accurate on the focusing, while an imx675 sized sensor with an f/2.8 aperture might almost be focused somewhat reliably by screwing the lens in x turns and not bothering to check the result!

Yes, nobody wants an out of focus dashcam, if you are going to sell an f/1.7 then you need an adequate focusing facility, with appropriate quality control.

Do you have some sample numbers for a dashcam's exact focal length? I'm not sure off the top of my head, so I used a Canon 1.6 cropped sensor and an ultrawide 10mm lens.

Plugging it into an online DOF calculator, the hyperfocal distance at f/1.7 is just 10.3 feet. That means that focused at 10.3 ft at f/1.7, you can get everything from 5.7 ft to infinity in focus. An even tinier sensor which uses an even wider focal length lens would let you get objects in focus even closer than 5.7 feet like you said before, but that's already plenty to capture details right in front of your car.



So I don't think wide aperture lenses have a dramatic impact on DOF when you're shooting with such tiny sensors that are focused to infinity.

 

[Nigel]

A fixed focal length and aperture lens does not change its DOF based on the size of the sensor behind it.

In general that is not true due to the pixel size being smaller on smaller sensors, and pixel size does affect depth of field, but if we are comparing the 2K imx675 and 4K imx678, they have the same pixel size, so your sentence is correct.

You do need to change the lens focal length if you fit a larger sensor to your dashcam, and so you will end up with less depth of field for the same image. There is a significant difference in size between 2K imx675 and 4K imx678, so there is a significant change in focal length, and thus depth of field. One is 2K pixels wide, the other is 4K pixels wide, the pixels are the same size, so the larger sensor must be four times the area! "2K" isn't accurate of course, but 1/1.8" vs 1/2.8" is a significant difference.

Do you have some sample numbers for a dashcam's exact focal length?

Every dashcam seems to use a different lens with different FoV and different amount of fisheye, but an imx675 lens is going to be roughly 3mm, an imx678 lens is probably going to have a wider FoV due to it being a 4K sensor instead of 2K, so the focal length will not increase enough to give the same image, although looking at Panzer Platform's tests, the actual horizontal FoV of various dashcams seems a bit random, the 4K ones tend to be significantly wider than the FHD cameras, but they don't all fall where you might expect.

Plugging it into an online DOF calculator,

I think, if you want to use that calculator, you need to scroll right down the list of cameras until you get to the CoC values, and choose an appropriate value for when you want to inspect every pixel on your license plate when using a sensor with 2μm size pixels. Using a Canon 30D with a CoC calculated for print media does not give the result we want...

A guide to CoC; it doesn't mention licence plates, but it does explain why your DOF calculator doesn't give a sensible result for reading licence plates on a dashcam, unless you select a sensible CoC from the list:

Depth of Field: The Definitive Photography Guide | PhotoPills

All you need to master depth of field (DoF): meaning, inspiring examples, tips, DoF calculator and charts, DoF app, DoF vs aperture (and sensor size, focal length, subject distance), hyperfocal distance, diffraction, macro photography, circle of confusion, bokeh, preview DoF button – including a...

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