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Thread: Large Format Focal Plane Meter - New Product

  1. #11

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    Re: Large Format Focal Plane Meter - New Product

    This is just me thinking aloud, but how much does that control box do that a smartphone running an appropriate app & plugged into the the probe couldn't? If this was feasible, I could see it being possible to integrate your metering with relevant documentation - ie how much + or - development etc - & even automate that process to a certain extent, perhaps incorporating tested film curves in the app to enable you to tailor exposure to specific films in a convenient manner. The ultimate stage would be to integrate this with an electronic in-lens shutter, with the smartphone as the controller, but that's perhaps getting away from your design goals...

  2. #12

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    Re: Large Format Focal Plane Meter - New Product

    Quote Originally Posted by interneg View Post
    This is just me thinking aloud, but how much does that control box do that a smartphone running an appropriate app & plugged into the the probe couldn't? If this was feasible, I could see it being possible to integrate your metering with relevant documentation - ie how much + or - development etc - & even automate that process to a certain extent, perhaps incorporating tested film curves in the app to enable you to tailor exposure to specific films in a convenient manner. The ultimate stage would be to integrate this with an electronic in-lens shutter, with the smartphone as the controller, but that's perhaps getting away from your design goals...
    Interesting. I don't know a lot about smart phone apps - I'll have to find someone that does.
    The only bit in my box that might be problematic in a smart phone is the front end stage. The signal range is huge to cope with exposures from hours to 1/2000s or so; putting that directly into the smartphone would inply an input sensitivity in the sub microvolt range and effective accurate analogue to digital conversion of 24 bits resolution. Therefore I think I'd have to put the front-end analogue logarithmic amplifier outside the phone. However this could be made tiny and integrated with the sensor if I can get a bit of power out of the phone to drive it.
    Another interesting job to put on the increasing list of things to work on!

  3. #13

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    Re: Large Format Focal Plane Meter - New Product

    Quote Originally Posted by Dave Gooding View Post
    I am not sure why readings should be lower at the edges and corners of the image unless there's any vignetting by the camera optics and in any case you should be metering the light that the film will actually see, unless there's some behaviour of the ground glass scattering for off-axis illumination.
    There is SEVERE light FALL-OFF with some lenses -- more with wider lenses, and more with lenses used opened up. But even if that is not an issue, the light that hits the edges of the film plane (GG) will be less that what hits the middle.

  4. #14

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    Re: Large Format Focal Plane Meter - New Product

    It is an interesting project, but wouldn't a traditional spotmeter accomplish the same thing, be easier to use, and be a lot more versatile?

  5. #15

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    Re: Large Format Focal Plane Meter - New Product

    Quote Originally Posted by faberryman View Post
    It is an interesting project, but wouldn't a traditional spotmeter accomplish the same thing, be easier to use, and be a lot more versatile?


    Do you mean a spot meter of the scene or a spot meter of the ground glass?

  6. #16

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    Re: Large Format Focal Plane Meter - New Product

    Quote Originally Posted by xkaes View Post
    There is SEVERE light FALL-OFF with some lenses -- more with wider lenses, and more with lenses used opened up. But even if that is not an issue, the light that hits the edges of the film plane (GG) will be less that what hits the middle.
    But isn't it the light actually hitting the film plane that you want to measure, since that is what the film records? I mean the whole point of metering in this way is to measure the actual amount of light at that point that the film will be exposed to, surely?

    I can see that the ground glass would give a different scatter, reflection and transmission away from the lens axis and that would have to be accounted for, and we are going to plan some tests for this to see how much difference it makes and how well we can capture both the directly transmitted and forward-scattered light. Backscatter and speculation reflection is of course light that is just lost and cannot be captured when metering off the viewing side of the screen.

    What sort of accuracy do you need in the metering? Is one-third or one-quarter of a stop sufficient? Doing better than this over a full exposure range of, say, 1/1000s to an hour will be challenging.

  7. #17

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    Re: Large Format Focal Plane Meter - New Product

    Quote Originally Posted by Dave Gooding View Post
    But isn't it the light actually hitting the film plane that you want to measure, since that is what the film records? I mean the whole point of metering in this way is to measure the actual amount of light at that point that the film will be exposed to, surely?
    You are absolutely correct, and it is absolutely a challenge.

    And using a spot meter of the scene (or any meter of the scene) does not account/adjust for light fall off due to the aperture and the lens design. At least metering off the the GG can attempt to do that -- but the photographer needs to decide where in the scene to expose for (or use other methods such as central ND filters, dodging/burning, etc.).

  8. #18

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    Re: Large Format Focal Plane Meter - New Product

    Quote Originally Posted by xkaes View Post
    Do you mean a spot meter of the scene or a spot meter of the ground glass?
    Spot meter of the scene. This seems like a solution in search of a problem.

  9. #19

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    Re: Large Format Focal Plane Meter - New Product

    Quote Originally Posted by faberryman View Post
    Spot meter of the scene. This seems like a solution in search of a problem.
    So metering a scene using a 50mm lens with 300mm of extension with a spot meter and running a bunch of calculations is somehow easier than just reading the light on the groundglass? Maybe in your world.

  10. #20

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    Re: Large Format Focal Plane Meter - New Product

    Quote Originally Posted by xkaes View Post
    So metering a scene using a 50mm lens with 300mm of extension with a spot meter and running a bunch of calculations is somehow easier than just reading the light on the groundglass? Maybe in your world.
    I do not know of any 50mm lens for large format, certainly none that would focus with 300mm of bellows extension. But that is just my world.

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