Metering, how does it really work?

Discussion in 'Digital Discussion & Q&A' started by homeostasis, Jul 4, 2007.

  1. homeostasis

    homeostasis TPF Noob!

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    To start off, I'm cognizant of what metering is (this isn't an aperature and shutter thread).

    What I can't figure out is how it works. What reads the metering image on a DSLR? Because your image never makes it to the CCD.

    You can understand, this became an important question when I was looking at the process of putting a non CPU lens on my nikon DSLR, and I began to wonder.. What information does a lens exchange with the body? ( As a consequence: ) Why doesn't metering work with a non CPU lens?

    Best Regards,
    Scott
    D70s

    PS: it won't even show the exposure level.
     
  2. Garbz

    Garbz No longer a newbie, moving up!

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    Well where it happens is easy. When light comes into the camera it bounces off a mirror straight up. At the top is what is known as a penta-prism, a piece of glass which uses various angles of light rather than a reflective surface like a mirror to split the beam. Some light bounces up onto a small (modern CCD (554pixels in the D200 if I recall, or in dated cameras a photo sensor). That reads the value of the light.

    Starting form the old cameras:
    non-AI Nikons: This is easy just read the light that hits the sensor and determine the exposure based on the ISO level set when you insert the film. If you stop down the lens the image gets darker in the viewfinder I believe. I haven't used non-AI cameras so I am not quite sure if this is 100% accurate.

    AI Nikons: It reads the light through the view finder and calculates the exposure based on ISO and and AI (Aperture Index). The Aperture Index is transmitted to the camera by way of either a little metal latch which connects to the body, or using a slight step on the outside of the lens (you can see this easily, this turns a ring on the lens mounting to give the camera the current aperture value.

    D type lenses:
    These were needed for matrix metering. Matrix metering works by reading a small CCD and reading all the brightness levels in various zones. The lens transmits information such as focal length and subject distance to the camera. If the camera supports it it also electronically transmits the aperture index, otherwise it uses the old mechanical AI ring. The matrix system then measures the image it gets along with all information and changes the weight of the image for the subject. Say if the subject is on the left of the frame, a centre weighted metre would expose for the background, but matrix metring would realise that the left of the frame contains the subject since it is in focus and metre for that.

    Modern 3DMatrix and Eval metering:
    This generates a low resolution luminance map of an entire scene as the lens focuses. The lens I believe transmits more information to the camera. I am not quite sure how a G or DX lens differs electronically from the old D lens but it does provide 4 I think extra electrical contacts. I believe that during focusing the camera builds a virtual low res luminance map of the scene and then compares the result to a database of pre-defined images to establish how it should provide weight to certain parts of the scene. But I am sketchy on the details of exactly how this is done.

    When you put a non-cpu lens on a Nikon body (AI lenses, non-AI lenses will not work and I believe some are even large enough that you can crack the mirror if you mount one and fire it), it only works with spot metering and centre weighted average metering. Setting matrix metering causes it to revert to centre weighted average.

    If you don't know all metering methods convert the scene to a grey value. 18% I believe is the standard quoted number but there's some confusion if it is in fact 18% or 12% (thanks Ansel Adams and Kodak). However this value is chosen based on weights. Matrix meters have multiple zones which are weighted, the exact number escapes me now. Centre weighted average as the name implies gives more weight to the centre. I think the Nikon FE manual quoted something like 80% of the weight to the middle 20% of the frame. Spot metering reads only the centre point (or on a modern camera it reads the selected focus point), and applies 100% weight to it.

    I think that's how it all works.

    Now regarding your specific problem. I believe non-AI lenses are not compatible at all. Not surprising either since AI lenses came out in the 70s you must be trying to mount some ancient equipment. The camera can not tell what aperture is selected. Not sure about the D70 but the D200 manual states non-AI lenses can NOT be used at all. If I mount an AI non-CPU lens on my camera the A indicator (A and M being the only usable modes) blinks telling me I need to specify the focal length and maximum aperture so the camera can function.

    Also as I said above this is important so I'll say it again. You have to check before mounting old lenses in your manual as some like PC Nikkors or reflex lenses are supported by the camera, but certain specific models extend too far back into the lens mount and depressing the shutter could cause you to damage the mirror.
     
  3. homeostasis

    homeostasis TPF Noob!

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    Thanks so much for your involved answer. I'm a travelling student so I'm looking to go real cheap. I'm looking at adapting M42 lenses, no automatic anything. So, all you can do is take pictures over and over and adjust the shutter to your aperture setting, because there is no exposure measureing show the +/-? It would just have to assume that there's no AI (the aperture doesn't change). Is there any way to edit firmware to do this on a D70s? no one seems to mod these things.
     
  4. Garbz

    Garbz No longer a newbie, moving up!

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    A Pentax lens on a Nikon? That's an interesting thought.

    The problem with a non-AI lens is not just metring, but I am also not sure if the camera will allow you to trigger the shutter when it's mounted. I have no idea how you would adapt these lenses.

    If you can get it to fire the shutter the camera may still be able to take good photos in Manual mode which isn't too bad if you have time to play with it.

    Just another suggestions but you could look at selling the M42 lenses (you won't get much for them) and buying some old manual focus AI-Nikkor lenses (you won't pay much for them :) )
     

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