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Thread: Digital Center Filter

  1. #1

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    Digital Center Filter

    As a new 4x5 format photographer working on both a financial and energy shoestring, I am a little daunted by the idea of needing to use a center filter for wide angle lenses. I am looking for feedback from anyone who has used John Walker's "pnmctrfilt" digital soulution or who has a position on its efficacy. Here is what he says about the problem and his solution:

    "With the wide latitude of present-day film and the colour (or grey- scale) depth of film scanners, it's possible to simulate the effect of a centre filter after the fact by applying an equivalent transform to a raw image taken without the filter. pnmctrfilt applies a centre filter transform to a grey scale or RGB image, allowing you to specify the density, power of the density curve, image circle radius, and optionally shift the centre of the filter to compensate for camera movements."

    For those of us working digitally, once we get the chrome or negative this looks to be a real viable solution.

    You can find more information at: http://www.fourmilab.ch/netpbm/pnmctrfilt/

  2. #2
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    Digital Center Filter

    It's probably OK, but bear in mind that the digital effects are applied post-exposure and if you use negative film, you may not have enough detail at the edges for the digital filter to act on. Not sure what problems underexposre with positive materials would cost you. There's also the problem of calibrating things for your lenses--maybe you could reuse settings on all photos from the same lens, but it might be more complex than that & if so your energy investment would skyrocket.

    I'd say you'd be better off with the CF, but since this filter is free, it's definitely worth trying it to see if it works for you.

    My 2¢

  3. #3

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    Digital Center Filter

    This appears to be a Unix based project. You will need a fair amount of programming experience to get it to work on Windows or Mac (and even some experience to compile it on a Linux box) unless of course, someone has already compiled it and can supply binaries. A quick Google did not show anything else.

    I saw a Photoshop plugin to do this sort of thing mentioned some time ago - sorry can't remember where as I wasn't paying much attention at the time (currently 100% wet process here ('tho that will change if I take to colour)).

    Cheers,

  4. #4
    Whatever David A. Goldfarb's Avatar
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    Digital Center Filter

    There are a few ways of handling this using the radial gradient tool in PhotoShop. A proper glass center filter will produce better results, but if you can expose the neg/chrome to be sure you get some detail in the darkened corners without losing highlight detail in the center of the frame, and if you're printing digitally anyway, the digital approach isn't too bad.

  5. #5

    Digital Center Filter

    If you shoot B&W you might be able to do this with some sucess. For color work though you will need the filter. Color film renders color differently with changes of exposure. So that big uniform blue wall (or red or whatever color) will actually be different versions of blue wherever the exposure is different. If you are doing something that is critical in this regard you'll NEVER get it exactly right in the computer.

    Of course every scene is different and many times you won't need a center filter as you may like the effect of exposure falloff. But for those scenes where you want to have even exposure you need the fliter.

    Buy a center filter.

  6. #6

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    Digital Center Filter

    Not all "wide angle" lenses need center filters. I never found a need for one with my 90mm Super Angulon and so far my testing doesn't show a need for one with my 80mm Super Symmar XL either. But some people say they need a center filter for both of these lenses. So I wouldn't take as gospel everything you might read about the need (or lack of need) for a center filter with any particular lens, instead check the lens out yourself and only after that start worrying about the need for a center filter if necessary.
    Brian Ellis
    Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
    a mile away and you'll have their shoes.

  7. #7
    Ted Harris's Avatar
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    Digital Center Filter

    Let me add to Brian's comments. Very few people do work that requires a center filter with a 90 mm lens. I routinely use a 75 mm lens without a center filter and without noticable vignetting/darkening at the corners. I do own one but never use it.

    you should try your lens before worrying about a center filter. If it is a 90 mm you are talking about chances are that you will never need a center filter.

  8. #8
    Whatever David A. Goldfarb's Avatar
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    Digital Center Filter

    With a 90mm lens on 4x5" with landscapes you can sometimes use the falloff to your advantage by using front rise and repositioning the camera, kind of like a built-in ND grad.

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