Megadensity of lens ?

Discussion in 'General Dash Cam Discussion' started by Lola, Sep 24, 2017.

  1. Lola

    Lola Well-Known Member

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    Can some one explain how/what the mega density of a lens has to do with the lens/camera.

    I want to put an aftermarket lens in my Mobius 1 camera and do not know the relationship between the camera and the lens mega density? I see values of 2/3/5/ and up (Lens) on this but can't figure it out. Does it have something to do with CMOS relationship with the sensor or the design of the Mobius ?
     
  2. kamkar1

    kamkar1 Well-Known Member

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    The mobius use a 2 - 2.1 megapixel sensor as that's the minimum you need to make 1920x1080 footage, so a 2 megapixel lens will do just fine.
    You can also get away with using a lens made for analog cameras with 1/4 the resolution of a 2 megapixel camera, i think the 8 mm lens i used before on the x camera was for analog cameras.
    But i would go for a proper megapixel lens myself, as aside for pixel count going up all the time in cameras ( well not so much in dashcams ) then a lens will never get obsolete or wear out.
     
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  3. Dashmellow

    Dashmellow Well-Known Member

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    I highly recommend that you spend some time watching this interesting and excellent presentation, titled, "Debunking the Megapixel Myth".
     
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  4. c4rc4m

    c4rc4m Active Member

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    Not sure where "megadensity" comes from. The correct term would be resolving power. The megapixel rating is an indication of the level of detail the lens can resolve from a standard resolution test chart. Megapixel when used in the context of the lens, is not really a true term in so far as it's not scientific.

    Basically though, the higher the resolving power, the potentially sharper the image. Personally, I think you can never go too sharp. The worst case scenario is the lens can resolve more detail than the sensor can record - the result is the picture will be maxx'ed out for sharpness and detail, with some headroom in the lens to spare. However, get too a low resolving power, and the image won't be as sharp or detailed as it could be. The only downside to buying too much resolving power is cost, but provided lenses are a similar price, as I see it, you have very little to lose going for the higher resolving power.

    Note as well here, lenses aren't just about resolution alone. Low distortion often through the use of an aplantic lens, can be just as important as most cheaper lenses are not as sharp all the way across the lens and have distortion towards the edges that can affect the picture quality for non central objects.

    If looking for lenses, have a look at the Backbone website: https://www.back-bone.ca/ There maybe a lens there you could potentially use. Again I'd say concentrate on low distortion as well as resolving power.

    Also, one final factor, not talked about on here usually is the relationship between the number of sensor pixels and identifiability of faces / objects. To this end the higher pixel count sensor is king. In CCTV circles, as a rule of thumb the aim is to achieve 80-100 horizontal pixels across a face so as to achieve recognisability. The relationship this has to cameras is:

    1. The wider the lens, the more area you are spreading the sensor's pixels out over, so the lower the pixel density

    2. The higher the sensor rating in pixels, the more pixels your likely to have in any given area eg over a face or number plate increasing readability

    3. The distance from the lens also plays a part because as you enlarge the image to make it recognisable, you effectively reduce the pixel density.

    That's my understanding at least. A little simple guide here from a CCTV manufacturer:

    https://www.axis.com/files/feature_articles/ar_perfect_pixel_count_55971_en_1402_lo.pdf

    Must admit, to the end that sensor pixel count has an influence on picture recognition quality, I am a little surprised now that cheap 4K CCTV cameras are out there, that we haven't seen 4K sensors finding their way into dashcams. Maybe the sensors are just still a little too expensive for this purpose.

    However, I believe there are potentially gains to be made from a higher quality lens.
     
  5. TonyM

    TonyM Well-Known Member

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    Watching - Since I want to do exactly the same with my M1
     
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  6. Lola

    Lola Well-Known Member

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    @Dashmellow Need some help, I decided to see if I could order a high quality lens from China through Alibaba. This place is extremely confusing in how they conduct business to say the least. I gave an order request of: M12 type lens, 1/3 sensor format, 8 to 10mm fp lens with cut IR filter (red), 1/4 aperture. With a side note that it had to be of low or no distortion and absolutely of very high quality. This was exactly what I said.
    However, when I received the returned quotes, their was always something wrong in what I said I wanted from their end. For instance when it came to sensor/image format (1/3) they would say 2/4 or 1/2.5 was what I was ordering, am I wrong because of knowledge are is something being lost in the translation from Alibaba to the lens dealers. One of the manufactures sent me a message through Alibaba that said they would like to have more information, so I sent them the Camera name and everything else I could think of.
    I received a note back (from Alibaba) that message could not be sent, because mailbox was full! That's when I just gave up and came here to ask you your opinion.
     
    Last edited: Sep 27, 2017
  7. Dashmellow

    Dashmellow Well-Known Member

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    Alibaba is a B2B (Business to Business) portal which is focused on providing larger quantities of goods to small and medium sized businesses. AliExpress is similar but is more focused on providing smaller quantities or individual items (B2C - business to customer). I'm afraid I have no experience ordering from either one of them so I can't offer you much advice or help but AliEpress is probably the place to find the lens you are looking for. (or possibly eBay if you can find a good vendor with the right product). If you want a high quality lens with the kind of specs you mention you might be better off buying from a vendor like Peau Productions or perhaps even M12lenses.com.

    Many members here have ordered on AliExpress so perhaps someone else here can share their experience and offer guidance.
     
    Last edited: Sep 27, 2017
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  8. c4rc4m

    c4rc4m Active Member

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    8-10mm fp is a very narrow field of view for Dashcam use. It's around 30 degrees on a 1/3rd sensor. I'd suggest that's more suited to drone use. Personally I'd look at going down to around 60 degrees minimum, and even then some might say that was too much.
     
  9. Dashmellow

    Dashmellow Well-Known Member

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    As is so often the case you hand out advice without having any actual hands-on experience with what you are talking about. Some of us have very good reasons to employ longer lenses in our dash cams and lenses up to 12mm in focal length can provide outstanding results.

    Check out some of the great work @kamkar1 has been doing recently with his 12mm lens experiments. Or look at some of the examples I've been posting using a varifocal set to 11m and 12mm. For one thing, telephoto lenses can make for an outstanding companion camera when used in conjunction with a typical wide angle lens dash cams or they can be used individually if you just want or need a highly detailed capture of two lanes of traffic in front of or directly behind your vehicle.

    @Lola has specific reasons for wanting a longer lens to use with a Mobius and has done her homework in selecting an 8-10 mm focal length.

    11mm varifocal capture (IR)
    NHroadscene.jpg

    11mm varifocal capture (IR)
    Bstreet.jpg
     
    Last edited: Sep 27, 2017
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  10. SawMaster

    SawMaster Well-Known Member

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    I've bought from AE twice and they are a great place to shop. They are different than most Chinese sellers; they have their own "buyer protection" service sort of like what Paypal has but which has to be selected and comes with a small cost- use that ;) They don't take Paypal either. They are not a store per-se like BG,GB, or the others, when you order your money goes to AE, not the seller, and AE holds it in escrow until the item(s) arrive and you respond to their delivery email. The seller doesn't get your money unless you say all is good or you do not respond after a period of time. This helps eliminate bad sellers and AE is active in banning sellers who have more than a few problems reported. Problems do happen but much more rarely than with the other Chinese sellers. Many friends on other forums have reported similarly. I've only heard of one instance where AE buyer protection was used, and it was slow but a full refund was given without negotiations and no product return was required.

    YMMV but I don't worry about my orders with AE though I do worry a bit with the others :cool:
    Phil
     
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  11. kamkar1

    kamkar1 Well-Known Member

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    I like a lens like this in the front, cuz i get usable footage of what i see a bit ahead of my car, a regular dashcam lens will just be a handful of pixels moving, where as a long lens you will be able to see if its a him or her out there, and the clothe they are wearing.
    A very nice compliment to a regular camera / lens combination.
     
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  12. Dashmellow

    Dashmellow Well-Known Member

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    Some of the images I've captured of cars in front of me with 11-12mm are so detailed that I can sometimes read the expiration date on the 1 inch wide registration sticker on their license plate. Other tiny unexpected details jump out all the time.

    At this point I don't plan to travel without the telephoto lens camera operating in my truck. I can't go back to the way it was before with only a wide angle forward facing lens. :)
     
    Last edited: Sep 28, 2017
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  13. kamkar1

    kamkar1 Well-Known Member

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    If you pause my dual footage video in the 12 mm lens experiment post at 2:20 you will see ( in the 12 mm camera below ) a guy on the right hand sidewalk walking a little dog.
    In the GC footage you can see the guy and that's about it.
    Also at the same time in the 12 mm footage on the left sidewalk way up ahead yoy can see another guy walking, and he appear to wear dark colored jeans and a black or dark brown jacket.
    In the GC footage you pretty much cant see him at that time.

    So if either of those 2 guys had done something stupid at that time in the footage, i would be able to document if it was on their own accord, or something forced them to do something.
    The GC would just give me the result of their theoretical deeds when i got up there.
     
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  14. SawMaster

    SawMaster Well-Known Member

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    My thoughts exactly (y) Wide-angle cams show the whole picture well and for driving you need that. The trade-off is a lack of details which even the narrow-angle cams being sold aren't so good with. As has been seen in other threads, with the right lens swap on a compatible cam you can see details at almost a professional-grade level :D"OMG, that girl 4 cars away has spinach stuck in her teeth- it's here on the video!" :ROFLMAO:

    Phil
     
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  15. c4rc4m

    c4rc4m Active Member

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    No I'm fully aware of the implications and the benefits of using a narrower angle. The OP has never indicated a specialist use and to that end, a wider view than 30 degrees is better for most to ensure you capture front side impacts. I'd fully agree that if you're running multiple front cams there are benefits to having a narrow angle 2nd cam to get a better plate view / distance view.

    If you read just one or two posts above, I've mentioned a CCTV article which explains how the pixel density, which is the sum of several of parts including fov from the lens, is a major factor in the identifiability of objects such as faces (and text).
     
    Last edited: Sep 28, 2017
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  16. c4rc4m

    c4rc4m Active Member

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    Further on pixel density, there are a couple of good videos on Youtube that demonstrate pixel density through both fov and sensor pixel count changes vs readbility of text, but I won't complicate the thread by posting them unless asked. They do show interesting results though. In 1 test the fov is changed vs signs placed at 10m intervals. In the other a car is at the same distance from the camera, but the camera is changed to a different one with a higher pixel sensor each time and then the picture zoomed in on the plate (much as we might try and do after an accident). In both you can see the changes to the readability of the signs at different distances and the number plate. The sign one also includes tests at night under IR conditions.
     
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  17. TonyM

    TonyM Well-Known Member

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    Are there legal implications regarding invasion of privacy if you get TOO close? (This may vary between states and countries) e.g. Being able to see what someone is doing inside the car in front, when they might reasonably expect that no-one can see them
     
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  18. kamkar1

    kamkar1 Well-Known Member

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    Not here, only the privacy clause that say you cant film over the hedge or in thru the windows on ones home, everything else are fair game and also why Denmark have full google view coverage.
    Its the sharing in public of footage that might be a problem here.
     
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  19. Dashmellow

    Dashmellow Well-Known Member

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    No legal implications here either. If you are driving around in a car with glass windows all around, on the public roads and highways, there would be no "expectation of privacy". Whether someone in another car has a telephoto lens dash cam or someone is sitting right next to you in traffic it would be the same thing.
     
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  20. SawMaster

    SawMaster Well-Known Member

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    You have no reasonable expectation of visual privacy here in SC if any person acting normally while not on private property could see you. Wikipedia's page on dashcams has some info on this under the 'legality' section https://en.wikipedia.org/wiki/Dashcam

    Phil
     
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