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Thread: Metering and an exposure calculator.

  1. #1

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    Metering and an exposure calculator.

    I haven't yet mastered all of the nuance of metering on large format. I use a spot meter, a Minolta Spotmeter F to be exact.

    Basically, trying to meter a tone in the scene, correct the indicated exposure to place it in the appropriate zone, apply bellows factor and/or filter factor compensation, and apply reciprocity failure compensation all in my head, I have a tendency to either forget something or get one of the corrections backward, and more often than not I screw it up and end up with an ugly sheet of film. I also tend to shoot more slide film than black and white, so it's even more critical.

    I know a lot of people use exposure records to keep notes about their shots, and that looks like an excellent way to work through exposure calculation as well as keep records for development compensation and notes for the future.

    In that spirit, I started working on an iPhone app to help me calculate exposure and serve as an exposure record. Right now all I have is the calculator part, but I have some interesting ideas for the future of the app.

    Here's the current calculator UI. Note that it's ugly and quick, the final version hopefully will be MUCH more polished than this.



    The way the calculator works at this moment :

    1) spot meter the 'key' tone in the scene (with ISO set on meter to your desired EI and with meter set to read in EV instead of aperture/shutter)

    2) input EV from meter and zone you would like to place it in

    3) input filter factor and bellows factor in stops

    4) calculator shows the EV that would result in a proper exposure.

    5) slider allows you to select paired aperture and shutter combinations. The aperture and shutter will only ever show in standard stops, from f/0.5 to f/128 on the aperture side, and any number of whole seconds if greater than 1s, or 1/2, 4, 8, 15, 30, 60, 125, 250, 500, 1000, and multiples of 1000 on a log2 scale on up as necessary. It then calculates what EV is represented by the whole shutter stop and whole aperture stop, and finds the amount of error, and then displays that as a positive or negative third stop on the aperture side.

    The idea was that my shutters only allow me to select whole stops on shutter speed, but the apertures are continuously variable.


    Right now the big thing I'm missing is reciprocity failure. Obviously the big challenge there is that it's very film specific. Since the final app is going to be an exposure record, I am probably going to try to have a user-configurable list of films, and for a given record, you will select a film and that will set the EI and affect the reciprocity curve. The problem is that there's not exactly a standard database of reciprocity characteristics, and I don't know how I'm going to go about allowing the user to alter that curve. I'll probably use a t = a + x^c type equation, and i guess I can allow the user to set a and c for a given film, but I dunno how many people are going to have the desire or ability to determine and tweak those kind of curves. I think for the purposes of using the calculator right now, I'm going to use a simple t = x^1.48 type curve for anything over 1s.


    As I expand the app to include more of an exposure record type functionality, I'll add the ability to record multiple spot meter readings and place them in zones, and to designate one reading as the 'key' reading that determines exposure. The rest of the readings will be used to show an overall contrast in stops and to indicate compensation requirements for development.

    Another interesting thought I had for the final version of this app: many exposure record forms I've seen have an area to draw the scene. On the iPhone, the user could use the camera to take a picture of the scene which will be attached to the record, and if it's a 3GS, the exif data in the photos that it captures contain ISO/aperture/shutter info, and I can display what EV the iPhone's camera thinks the overall scene is, and use that as a sanity check against the recorded and calculated readings.


    Anyway, I'm mostly rambling about ideas now, but if anyone has any thoughts on how this can be more useful, please feel free to let me know. Also, if anyone with an iPhone/iPod touch/iPad is interested in playing with it, pm me and I can set up an Ad Hoc build for you to beta test. Just bear in mind that the state of it at this point is by no means sexy, polished, or bug free

    Ian

  2. #2

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    Re: Metering and an exposure calculator.

    some thoughts, questions and other rambling

    What have you assumed about film speed? (specifically, you cannot suggest a shutter speed and aperture based only upon the light value...)

    Read the order of your instructions and then look at your ui....seems like they do not agree. I suggest having the light meter reading at the top of the ui.

    bellows factor...so many people have asked how to calculate this and so many answers have been given and some answers aren't even correct....instead of a field for filter factor (leaving the user to his own devices to figure it out). Have fields like focal length of lens and bellows extension and then calculate the bellows factor.

    What if the user's meter does not read in EV?

    What benefit does this offer over a simple notebook and pen? other than the "gee, that's a neat toy" effect? I see a notebook and pen as having a great deal more flexibility and offering more features...without tying my hands. I'm not knocking your idea...just trying to push you into thinking about ways to make this a more viable and useful product. Forexample...can you incorporate an image taken with the iPhones digi-imager thingy? How would that be useful...move beyond these basics and give me something I don't already have.

  3. #3

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    Re: Metering and an exposure calculator.

    Brad,

    all great thoughts. You're right, the UI right now is not exactly in the same order as how I described the flow, and I know the UI is messy and terrible. It's mostly a proof of concept to enable me to get the math working.

    Right now the input EV values are assuming that your meter is putting out EV, which is already corrected for ISO. As it stands, it requires that your meter be set for the proper film ISO in order for metered readings to be correct. A thought I had earlier was to give the user the ability to specify a "meter ISO" and a "film ISO". This will support both users whose meters only read in one particular ISO (probably 100) as well as users who would rather just leave the setting alone and let the app handle conversions for different film speeds.

    I admit I am not familiar with a great many types of meters, so I'm mostly going by what I have. What types of meters don't read in EV, and what do they give you instead? If it's something like LV, as I understand it, that's essentially EV at ISO 100, and so the system I described above would handle users with that type of meter. Are there meters that display in pure luminance or illuminance? About the only other thing I could think of would be a meter which only displays an aperture/shutter combo, and that's just a longhand form of EV, and would likely be too cumbersome for inputs in a UI.

    As for bellows factor, I did it the way I did it because my workflow involves using a QuickDisc to measure magnification ratio and reads directly in stops. I don't have an easy way to measure bellows draw on my camera, so this is quicker and easier for me. You're absolutely right that having a more sophisticated UI could offer more flexibility for people who do things somewhat differently; I'm just not down that road yet.


    As for the benefit over pen and paper... well that's entirely subjective. For me, it gives me less opportunities to screw up the math in boneheaded ways, and it gives me a more permanent way to store this information. You have no idea how often I make notes about a shot and lose the notes, or write down what film is in which holder and then leave those notes at home when I go out to shoot. I don't have that problem with my phone! Also, as I said in the original post, what's pictured above is just a quick and dirty mockup to get started, and there are a lot of pretty useful things that could be added to it.

  4. #4
    Mike Anderson's Avatar
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    Re: Metering and an exposure calculator.

    Quote Originally Posted by tmbg View Post
    ...On the iPhone, the user could use the camera to take a picture of the scene which will be attached to the record, and if it's a 3GS, the exif data in the photos that it captures contain ISO/aperture/shutter info, and I can display what EV the iPhone's camera thinks the overall scene is, and use that as a sanity check against the recorded and calculated readings
    I'd like to see something to allow one to operate without a separate meter, to use the iPhone's camera as the meter. It could augment traditional (separate) spot metering as well (as you say a sanity check), but allow one to leave the spot meter at home.

    ...Mike

  5. #5

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    Re: Metering and an exposure calculator.

    I have an iphone app that I purchased from the appstore which does function as a meter... it works, but it's clunky.

    Bear in mind that the only iPhone to date which can practically meter is the 3GS. It stores ISO, aperture, shutter speed, and focal length in EXIF data. Its predecessors don't.

  6. #6
    Wally Wally's Avatar
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    Re: Metering and an exposure calculator.

    I think an added benefit of doing these calculations in a device (aside from it not getting 'corrections backwards' is if it could be told of the actual performance of the shutters we use.

    If it knew the shutter on my 75mm lens was 1/2 stop slow at 1/30th, it could take that into account automatically.

    I imagine a shutter tester with a usb interface connected to a PC: You run each of your shutters through it, firing at each shutter speed 3 or 4 times. This will tell the average and a good hint at the deviation. Shutters that deviate too much should be serviced.

    The data correlating shutter speed setting to speed measured would be uploaded to the iphone app which could then take this into account along with your bellows and filter factors, and give you a scale of adjusted aperture/shutter speed combinations to choose from.

  7. #7

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    Re: Metering and an exposure calculator.

    I have been at this for 42 years.
    I know exactly what my lens + film +exposure are gont to do. I actually quit using a light meter about 20 years ago.
    My secret is I know what is going to happen because I stick with the film and same camera virtually all the time.
    You just have to track what you do each shoot. Then use whaat was successfull every time.

  8. #8
    Lachlan 717
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    Re: Metering and an exposure calculator.

    Ian,

    I like the idea!

    I'm not sure that the workflow is the one that I would use, though.

    I'd like to have presets as the starting point. From there, have preset film/filter choices (like the preset options that you have on those nasty DSLR things). For example, have the choice of 72mm, 90mm, 150mm and so on (as per your lens setup), Fuji Velvia 50 with a warming filter as "Custom 1", Efke 25 with red filter as "Custom 2" and so on.

    If a preset is not chosen, it would default to a manual input screen for lens/film/filter selection.

    Bellows factor would be useful as a background algorithm based on the manual input of distance (default would be zero, I would think).

    One thing that I would love to see is the ability to use the iphone's camera to record your spot meter readouts for a record. I would like to be able to take a shot with the phone, and use the touch screen to input my shadow/mid/highlight readings. For example, I'm taking a shot of a forest setting with a waterfall. There are ferns to the bottom left, grey rocks next to the fall, and blue sky with white clouds above. By touching the ferns, I can put in either Zone III or -2 (depending on your preset choice of Zone versus Stop). I then touch the rocks and set for Zone V or 0. The white clouds are put at Zone IX. Thus, a good visual record (like sketching the scene is). On closing this screen, I would like to be able to record the development details (expansion/contraction units).

    If you could hook in GPS metadata, that would also be good!

    I really like the app "PhotoBuddy", but its interface is very poor. Generally, we're visual people, so I'd like a more graphic (not data) presentation.

    Hope this is of some use.
    Lachlan.

    You miss 100% of the shots you never take. -- Wayne Gretzky

  9. #9

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    Re: Metering and an exposure calculator.

    Quote Originally Posted by Henry Larson View Post
    I have been at this for 42 years.
    I know exactly what my lens + film +exposure are gont to do. I actually quit using a light meter about 20 years ago.
    My secret is I know what is going to happen because I stick with the film and same camera virtually all the time.
    You just have to track what you do each shoot. Then use what was successful every time.
    I too often shoot without the benefit of a meter. Some years back I read something called The Ultimate Exposure Meter which talks about the limits of film speed, aperture, shutter speed, tilts and extensions, etc., and the varying nature of the light on a scene. It then goes on to talk about how the eye/brain can make a better determination of exposure than a meter, which is limited. The human response can make adjustments to the values seen whereas a meter can't. Of course in tough situations or where I'm unsure I still use a meter.

    Brian

  10. #10

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    Re: Metering and an exposure calculator.

    Looks really nice! I'd buy it!

    I'm lazy and stupid when it comes to zone stuff....this would be very handy.

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