filters with digital

Discussion in 'Digital SLR' started by George, Apr 3, 2005.

  1. George

    George Guest

    Just got to wondering if anyone has found a particular filter that they
    can't "Photoshop" in? That is, any situation where you use a piece of glass
    in front of your lens because you either can't duplicate the effect
    digitally or it is too time consuming to do so?

    George
     
    George, Apr 3, 2005
    #1
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  2. George

    Jeremy Nixon Guest

    You can't Photoshop a polarizer.

    You can't duplicate a graduated neutral density filter, because if you lose
    the highlights they're gone. Or a regular neutral density filter, if you
    need it to use a larger aperture than otherwise possible.

    If you need to block infrared to get proper color rendition, a hot mirror
    can do it; Photoshop can't. Same with UV, though that's not as much of
    an issue with digital.

    Correcting tungsten light can be done in processing, but you'll get better
    quality if you do it with an 80A.

    Those are the only filters I have. I can't think of any others I see a
    need for.
     
    Jeremy Nixon, Apr 3, 2005
    #2
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  3. George

    Jan Böhme Guest

    Isn't this true only for an ephemeral subject? If the subject remains
    the same over a few seconds - like a landscape scenery - you must be
    able to take two different exposures and blend them in Photoshop.

    Jan Böhme
     
    Jan Böhme, Apr 3, 2005
    #3
  4. George

    Rob Guest

    Colour correction filters are still necessary just like film to preserve
    the dynamic levels.

    This is the short version:

    The camera is optimised for shooting in daylight irrespective of the
    clever dial and menus. When you make 3400 Kelvin white instead of 5000
    Kelvin then you place the blue channel into a very narrow range where
    noise becomes closer to the optimal signal level.

    By using a blue filter in a camera set for daylight and then shooting
    tungsten you preserve the dynamic range of the blue channel and change
    the light from yellow to daylight blue.

    You may note that yellow opposes blue on the colour wheel ..... so when
    yellow is normal ..... blue suffers.

    Since you camera is based on RGB pixels it means you are using RG fully
    and chopping down B when you dont have the filter. No electronics can
    compensate for physics.
     
    Rob, Apr 3, 2005
    #4
  5. George

    JPS Guest

    In message <RRM3e.90$>,
    You can *NEVER* photoshop a filter exactly; some let you get closer than
    others, but it really depends on your standards.

    You can't do a polarizer at all in photoshop, short of painting away all
    the glare manually. Even if you did that, you'd be subtracting glare,
    and the image would be dark (an under-exposure).

    You can't do IR, either. All you can do is bogus pseudo-IR.
    --
     
    JPS, Apr 3, 2005
    #5
  6. George

    McLeod Guest

    A graduated neutral density filter is one of the easiest filters to
    correct for. You make two exposures, one for the sky or lighter area
    and one for the ground or darker area of the scene.

    One of the beauties of digital is the ability to change your white
    balance for every shot if need be-an 80A filter can't adjust down to
    individual degrees Kelvin like my software can. You won't get better
    quality with an 80A filter.
     
    McLeod, Apr 3, 2005
    #6
  7. George

    JPS Guest

    In message <>,
    That gets you back to "white light" balance, but the digital cameras are
    not white-light-native, so an 80A is just an arbitrary correction, at
    the most basic RAW level.

    With a Canon 20D, an 80B actually works better than an 80A for tungsten,
    giving RAW exposure that is balanced for red and green, and only a stop
    short in the blue channel. Special filters really need to be made for
    digital cameras, with their different sensitivities in different
    channels. Even sunlight would require a magenta or orange-magenta
    filter for proper RAW color balance. In direct sunlight, the red
    channel is a stop darker than the green, and the blue lies somewhere
    in-between, with the 20D RAW data.
    --
     
    JPS, Apr 3, 2005
    #7
  8. George

    JPS Guest

    In message
    Not really. I don't know that there are any digital cameras that would
    expose a white card under the white sun with balanced RAW data for the
    three channels. According to the data in dcraw.c, open source for RAW
    conversion, the linear ratio of color channel sensitivity goes well over
    a stop for some cameras.
    --
     
    JPS, Apr 3, 2005
    #8
  9. George

    JPS Guest

    In message <>,
    You might. Using a filter on the camera changes the relative
    sensitivities of the three color channels in the RAW data, and this
    might improve the signal-to noise ratio for problem channels, or prevent
    clipping of a certain channel.
    --
     
    JPS, Apr 3, 2005
    #9
  10. George

    Alan Browne Guest

    I'd guess the most difficult would be a polarizer and I don't believer
    there is such in PS. Once the image is captured on film/sensor there is
    no means of determining the polarity of the light, so no way to filter
    for it.

    I'm not sure about neutral density filtering. If the original were
    recorded without an ND and then some 'dimming' were applied afterward,
    then the highlits would no longer be highlights. OTOH, if the
    'exposure' setting in Adobe RAW is used, then I believe the highlights
    are conserved (within reasonable change). This is not in the Filters,
    but in RAW import for digital.

    Same to lesser degree for grad ND filters.

    May be more.

    --
    -- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
    -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
    -- slr-systems FAQ project: http://tinyurl.com/6m9aw
    -- [SI] gallery & rulz: http://www.pbase.com/shootin
    -- e-meil: Remove FreeLunch.
     
    Alan Browne, Apr 3, 2005
    #10
  11. George

    Bubbabob Guest

    UV and IR block or pass filters and polarizers.
     
    Bubbabob, Apr 3, 2005
    #11
  12. George

    Ken Ellis Guest

    I use a polarizer and a nd filter. I believe you can duplicate both
    effects in ps. It's alot easier and less problematic (consistency
    issues) to do it with glass (although polarizers fall prey to flare
    easily). But...pixels is pixels and i think it's just how good you are
    in ps.

    my 2 cents; but hey, i'm new at this so what do I know
    rgds
    Ken
     
    Ken Ellis, Apr 3, 2005
    #12
  13. The two obvious cases are polarization, and graduated neutral density.
    And of course the grad can be done if the rules of the contest (and
    the scene) allow you to take *two* photos.

    However, people should note that extreme color-balance corrections,
    like tungsten to daylight, are actually *better* done with filters if
    circumstances allow. Essentially, white balance changes that large
    end up running one of the color channels at a higher gain than the
    others, and produce corresponding amounts of extra noise in that
    channel. Doing it with filters is inconvenient, makes for a dark
    viewfinder, requires longer exposures (and if you have to go to a
    higher ISO, that gives you more noise in *all three* channels, and
    you've just lost any theoretical gain), etc., so I end up using white
    balance mostly; but I can see the noise in the blue channel really
    clearly!
     
    David Dyer-Bennet, Apr 3, 2005
    #13
  14. Check. Although you can sometimes mask and darken the sky, at least.
    You can't reasonably get rid of complex reflections, though.
    Check. Although taking *two* exposures lets you take care of this.
    You probably can correct the color pretty well; it may just be really
    picky boring long work.
    Color balance in the camera is much better than just photoshop. But
    yes, you get increased noise in the blue channel generally.

    However, the 80A may require you to up the ISO, in which case you get
    increased noise in *all three* channels.
    Well, various fluourescent correction filters; you can at least cut
    down on the green.

    Split-field closeup lens?

    I've never used any of the "effects" filters, stars and such.
     
    David Dyer-Bennet, Apr 3, 2005
    #14
  15. Doh! I should have thought of ND -- given that I pine for them far
    too often. I need them to let me use a wider aperture or a longer
    shutter speed -- both of which control things that can't be
    photoshopped too effectively (depth of field and motion blur).
     
    David Dyer-Bennet, Apr 3, 2005
    #15
  16. George

    Jeremy Nixon Guest

    I do it all the time. It works great in some situations (and only when
    the scene isn't moving and you can use a tripod), is tricky in others,
    and doesn't work at all in some.
    Yes, you will. Look at what happens when you adjust for tungsten light
    in the raw conversion -- which is the same thing as doing it in-camera.
    You're boosting the blue channel by at least a full stop. So you're
    drastically hurting the signal-to-noise ratio in that channel to correct
    for the fact that it wasn't fully exposed in the first place. You'd do
    better to balance the exposure in all three channels at the time of
    capture.

    It doesn't matter that you won't be exact. Minor color adjustments are
    not going to hurt you. It's the big jump that hurts. The filter gets
    you close enough that the final adjustment is painless.
     
    Jeremy Nixon, Apr 3, 2005
    #16
  17. George

    Jeremy Nixon Guest

    Sure, but the data bears out the theory. Under low color temperatures,
    the color channels aren't balanced; the blue channel is drastically
    underexposed, and is boosted in the white balance process.
    I'd rather use an 80B, too, if only for the fact that the loss is just
    one stop. However, B+W only seems to make 80B in MRC (multicoated)
    versions to-order, and special-order filters from them take about six
    weeks (I've gotten a couple), so I just got an 80A instead. After all,
    you don't need to be exact in the correction -- just get it close enough
    that the adjustment doesn't degrade your image. The extra 200 degrees
    of correction is unimportant.
    If you want to be perfect, then yes, you could do that kind of testing
    and filter appropriately. Nikons seem to lose the red channel first
    in most situations, which actually brings you right back to the 80A
    for help when you know that's going to happen. It sounds like the
    Canon does exactly the opposite, which is interesting. It's very easy
    to blow the red channel on a Nikon. Maybe I should instead say that
    an 80A can be useful to a Nikon shooter? :)

    On the other hand, you also have to balance it with the fact that the
    extra glass in front of the lens can only have a negative effect on
    your image, too. For minor corrections I'll stick to Photoshop.
     
    Jeremy Nixon, Apr 3, 2005
    #17
  18. George

    JPS Guest

    In message <-b.net>,
    Not exactly. You generally get a better signal in the channel that was
    originally weak if you up the ISO setting. Very little noise is caused
    by a high ISO setting itself; it comes mainly from the low
    signal-to-noise ratio. A blue channel underexposed by 2 stops at ISO
    100 is far noisier than the a blue channel exposed properly at ISO 400,
    because the posterization of the under-exposure increases the variance
    of the noise, relative to the signal (even though the sensor noise is
    the same in both situations).
    --
     
    JPS, Apr 4, 2005
    #18
  19. George

    Lionel Guest

    I shoot a lot under tungsten, & have had hassles with the red channel
    blowing out in otherwise good exposures, so I've considered using a
    correction filter myself. The one thing I'm worried about is attenuation
    of the high end of the spectrum - have you found much of a difference?
     
    Lionel, Apr 4, 2005
    #19
  20. George

    Lionel Guest

    Yes, I've run into red-clipping many times, & I also like the idea of
    being able to recover more latitude in that channel.
     
    Lionel, Apr 4, 2005
    #20
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