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Thread: A couple of Alpa questions ...

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    Workshop Member Wayne Fox's Avatar
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    A couple of Alpa questions ...

    So far I've enjoyed the challenge of using my new Alpa system with my p65+ back. Experience is the best teacher, but to move myself along more quickly I hope to get some assistance from those more experienced with Alpa, and technical cameras in general.

    I was intrigued by the Alpa and the stitching adaptor, and I hope to maximize some images using it. My question is regarding lens cast calibration. Am I correct in assuming that lens cast is lens/shift specific, and not really affected by other factors? If I were to create a calibration shots for each increment of shift for a lens, all at the same time, would that suffice? Or is it just easier to have the white plexi with you and just shoot them as you shoot?

    Considering I am doing landscape work, and not architectural, I'm also wondering if I could create fewer calibrations, and just use something close to what I shot with in the field. So perhaps a 10mm shift would work fine with a 9mm calibration?

    Which leads me to my last question (for now), do you normally log this info as you shoot, (lens, f-stop, shutter speed, shift) and if tips and tricks to make this efficient.

    Any links to primers for technical cameras or blogs about real world experience using them would be very much appreciated.

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    Re: A couple of Alpa questions ...

    If I were to create a calibration shots for each increment of shift for a lens, all at the same time, would that suffice? Or is it just easier to have the white plexi with you and just shoot them as you shoot?
    this is almost a "religion". One party makes a set of color cast calibration shots that they use all the time (i.e. a stock of calibration files). The other party always shoots the white reference shots in the respective lighting… that's what I do.
    Why? I simply experienced some strange color casts I couldn't correct with my "stock" LCCs (I first thought I can do it that way...). Since then I am always shooting the white reference in the scene.
    Others may have different experience…
    That said, I do infact shoot a lot of architechtural stuff and color cast can be very obvious under certain circumstances on large uniform surfaces.

    However I use a set of "stock" LCC files to correct my Contax lenses

    Considering I am doing landscape work, and not architectural, I'm also wondering if I could create fewer calibrations, and just use something close to what I shot with in the field. So perhaps a 10mm shift would work fine with a 9mm calibration?
    definitley won't work! the white reference must be shot with the exact same lens setting (this is why you push the exposure for the LCC shot by increasing the shutter speed, not the aperture)… every little change in foucs, aperture or lens-/back-movement will exhibit color shift.

    Which leads me to my last question (for now), do you normally log this info as you shoot, (lens, f-stop, shutter speed, shift) and if tips and tricks to make this efficient.
    as to the LCC shot I don't care to note the movements as I shoot them in the respective scene … so they show up in C1 with the correct image number and in the correct order.
    But I do note movements as I use the Alpa Lens Corrector software where you have to type in the movements.
    My workflow is to shoot all the actual captures first and afterwards I shoot the LCC reference shots with the respective movements.
    For stitching meanwhile I always use the largest movements (or simply 10mm or 15mm) and crop the image afterwards. I do compose on the groundglass and know which part I actually want to to use for my image… but it makes things somehow easier if you simply use movements that you can remember even if you did not not note the movements… makes sense?

    Try out what works best for yourself. You are going to vary workflow anyway after some time…

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    Re: A couple of Alpa questions ...

    Quote Originally Posted by thomas View Post

    definitley won't work! the white reference must be shot with the exact same lens setting (this is why you push the exposure for the LCC shot by increasing the shutter speed, not the aperture)… every little change in foucs, aperture or lens-/back-movement will exhibit color shift .
    My workflow is to shoot a white reference (LCC shot) for each shot I take .
    I take the "real" shot first and then the reference shot .
    I always follow that self given rule .
    In my digital back's users guide it says , that the reference shot must be taken with exact the same lens setting as the "real" shot . That is exactly what Thomas says .
    Doing so , the shots I receive are all too dark and PHOCUS software gives me a (exposure) warning .
    Therefore I prolong the exposure time by one EV step . For example , instead of a 1/125 s I set the shutter speed to 1/60 s for the reference shot . I am aware , that this might not be a 100% correct , but it works .
    But I stll find a slight CC when using the DIGARON-S 28mm lens .
    Is there any reference or rule for prolonging the exposure time ? ? ?

    Jürgen

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    Re: A couple of Alpa questions ...

    Quote Originally Posted by jotloob View Post
    But I stll find a slight CC when using the DIGARON-S 28mm lens .
    Is there any reference or rule for prolonging the exposure time ? ? ?
    shoot the white reference shots at an exposure time one or two stops longer than the actual image.
    The white reference shot should not be too dark nor too bright. It works flawlessly when the tonal values of the reference shot are in the right hand third of the histogram.

    like this:
    Last edited by thomas; 28th July 2010 at 01:00.

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    Re: A couple of Alpa questions ...

    My practice is to make the capture and follow that immediately with the LCC shot by increasing the shutter speed 2 stops. Most often, about 2 stops longer will work fine for the LCC shot, however, I will check the histogram and make several exposures if need be for the LCC.

    The LCC shots are lens specific and movement specific and my naming convention for the LCC shots reflects this (47mm_12mm_Rise, for example or 47mm_12mm_Rise_StitchTest) so over time I have built up a catalogue of calibration shots which I do use on occasion as well as taking individual LCC shots for each image. When the shot is important to me I will always take the LCC shot along with the image.

    I have a pad and pencil in my case and I write down info after shooting which I can use for reference. I don't always make notes but the LCC shot is always right after each image so I know where to find it.

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    Re: A couple of Alpa questions ...

    Quote Originally Posted by jotloob View Post
    In my digital back's users guide it says , that the reference shot must be taken with exact the same lens setting as the "real" shot . That is exactly what Thomas says .
    Doing so , the shots I receive are all too dark and PHOCUS software gives me a (exposure) warning .
    Therefore I prolong the exposure time by one EV step . For example , instead of a 1/125 s I set the shutter speed to 1/60 s for the reference shot . I am aware , that this might not be a 100% correct , but it works .

    Jürgen
    This is 100% correct as far as my understanding goes.

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    Workshop Member Wayne Fox's Avatar
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    Re: A couple of Alpa questions ...

    Thanks for the input. Looks like the plexi goes in the camera bag with me. I appreciate the tip on the two stops using shutter speed.

    I'll begin the learning curve this weekend

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