Focusing through a red filter

Discussion in 'Large Format' started by carsten_benni, Jan 31, 2013.

  1. Hello,
    I am asking for your help because I have a feeling there is something tricky I am not able to solve...
    I am going to shoot a still life through a red gelatin fliter that I put before my lens.
    Do you think I will have focusing problems? And any suggestion about how to avoid them?
    Thank you so mùch,
    Best regards
  2. Nothing tricky. Focus without the gel and then put the gel in place and shoot. If you focus with the gel in place your ground glass image will be darker.
  3. Thank you Brian,
    So you mean no out of focus because of some chromatic aberrations created by the red wave lengtht or similar...
    I am shooting with 5.6 max. 8, so no depth of field.
  4. Focus through the filter and then focus without the filter. Is there a focus shift?
    If you are using an optical grade filter it should not make a difference. But if it is not optical grade or if it isn't flat it might.
    Since you will be focusing on your ground glass what you see is what you will get. The biggest problem might be your light losss, especially if you are not using a Fresnel on your GG.
    That is why you want to test focus with and without to see if there is a focus shift.
  5. Thank you Bob,
    it is not an optical filter, is just the gelatins you use to colour up lights and flashlights.
    I just shoot a 6x6 film, focusing with and without flter and then adding the red gelatin just before triggering.
    Tomorrow I will get the results and then shoot the 4x5.
  6. Do the experiment, but you won't see a difference.
  7. p.s. I've done the experiment with both Kodak gelatin filters and Rosco filters. Light loss is the only real effect.
  8. Question: Are you using this as a contrast control filter or as a gel for adding red to one specific lighting effect within the image?
    If the latter, it's a definite WON'T make any difference situation so focus without and then introduce the gel for the part of the exposure when it is needed.
    However, if it is the former, and this is just a piece of lighting gel material that is not optical grade gel like the old Kodak Wratten gels filters, you may and probably will experience focus shifts. It may be overall or it may be in one or more selective areas due to any curvature in the gel or due to the relatively poor manufacturing techniques. Definitely do the test of focusing with and without the filter to carefully observe any differences.
    Even better, use a very bright light such as a painters quartz light to focus with the gel in place which will help to overcome the light loss from the filter. Be sure you examine every square inch of the scene on the viewfinder to check focus. Turn the light off for your actual exposure if everything checks out.
    Best of all, forget the gel and get an actual optical filter so you can be confident as a gel will change shape (curvature) every time you use it.
  9. Thank you!
  10. Focus shift is a function of thickness, and is greater using a glass filter (even a perfectly flat, plan-parallel one) than when using a rather thin, curved and badly made gel.<br><br>Focus shift using a deep colour filter also depends on the degree of correction of longitudinal chromatic aberrations of the particular lens used.<br>The thing to do is indeed what was suggested: test. It may not be a problem at all.
  11. Update about my issue.
    So, no out of focus, but generally slighly overexposed.
    Apparently my red gel filter is not stealing light.
    Thank you for your help,
  12. If you are layering the exposures with this as just part of the background illumination, your exposure for that portion is totally predicated on the output of your light that is used for that portion of the image. It has to do with the filter density ONLY in terms of the power output for that light, so raising the power makes it a lighter red, lowering the power makes it deeper.
    Try to find information about "chromazones" as defined by the late Dean Collins. He was the absolute master of controlling lighting and balancing brightness ranges like you are working with. His materials are still available on line and are totally worth the investment to learn true control of whatever you with to produce with light and lighting.
  13. Tim,

    "shoot a still life through a red gelatin fliter that I put before my lens".

    There is no "layering the exposures with this as just part of the background illumination" etc.
  14. Yes Q.G.. Exactly.
    I have to take care not to curve the gel filter not only to avoid out of focus, but even to avoid reflections or mirroring the
    lens itself...
  15. Q. G.
    I got the impression that Carsten might be using the filter as part of a background only color layering during exposure, so I wanted to contribute a bit on how to do that if it were the case. The use of a gel filter for that purpose would be perfectly fine regardless of curvature whereas using a non optically flat one in front of the lens for a straight forward exposure mightcause potentially serious focus shifts. Just covering the bases.
  16. " I have to take care not to curve the gel filter not only to avoid out of focus, but even to avoid reflections or mirroring the lens itself..."
    You would not be able to see the lack of flatness that can interfere with the performance of a lens. You would not even be able to measure it without special equipment.
  17. " whereas using a non optically flat one in front of the lens for a straight forward exposure mightcause potentially serious focus shifts"

    Again: no, it wouldn't. Much too thin.
    The chance of that is "greater using a glass filter (even a perfectly flat, plan-parallel one) than when using a rather thin, curved and badly made gel".
  18. Actually, after having checked my negative better with a top notch loop, I noticed that is not really sharp.
    I mean I shot the same subject with the unfamous red gel filter and without, working on the stops of my lens.
    At first sight it seemed to me that red pictures where slightly overexposed, I over estimated the light loss caused by the
    filter. And they looked quite sharp, but changing the loop I noticed that they are not as sharp as the non filter pics.
    Then I remembered that the last 2 red pics where made curving the gel around the lens and, yes, there where reflections,
    but they are not softer than the red ones I shoot with the filter kept flat.
    Tomorrow I will try again and next week I will have the tests.
    Thank you again to all of you!
    PS and I think it is the red wave length that ineterfers with the optical system of the lens. Is not the thickness of the filter, no matter if glass or gel. I think that focusing through a red colored something interferes with the way lens are done or something like that.
    I don't know if it is related with infrareds, but something is happening on my film.
  19. Q. G.,
    My experience is entirely my own and therefore anecdotal and not scientific. However, I have tried both a gel Polarizer designed for use on lights in front of the lens and also a deep red Rosco theatrical gel in a shoot through situation and found the results with both to be far less than ideal sharpness. I did both trying focusing prior to placing the gels and also focusing after the gels were in place with equally poor results.
    Conversely, using Kodak Wratten optical gels were perfect and dead sharp on the images.
    I do not wish to dispute your assertions, only to relate my own experiences.
    I therefore shy away from using anything other than materials designed to be used in front of the lens with the exception of colored gels when creating overall background tones with layered multiple exposures by by means of lights; or halo effects around a commercial subject, again with the layered lighting techniques , neither of which has a thing to do with need for lens sharpness.
  20. Tim,<br><br>Thin gels, as Bob also wrote, do not cause a discernable focus shift. They, quite simply, can't.<br>Focus shift is a function of thickness: light is refracted when going from one medium to another having a diffrent refractive index, continues on that refracted path, until it exits that second medium again (usually - as when using filters - back into the original medium). The length of the path traveled along the refracted path determines the displacement of the ray, and when not considering a single ray, but converging beams, the displacement of the place the rays in that beam meet in a point.<br><br>Typically, the maximum shift is about 1/3 of the thickness of the filter. Gel filters are too thin to cause a discernable shift.<br>Glass filters, however, are thick enough to make a noticable difference, though only in close-ups, where the beams/pencils are less paralel.<br><br>What they can do, glass and gel filters alike, when not perfectly clear, is diffuse the image. Work as a soft focus filter.<br>Two different things.
  21. Strange enough I agree with both of you.
    I experienced the soft focus Tim is talking about, but I see a point in Q.G. answer.
    I have some kodak wratten filters, but none red.
    Let's put it this way, in your experience what happens with a non red coloured filter? Loss of sharpness too?
    Not in my experience. My question was about the specific red colour. I faintly remember that filtering with red leads to a
    loss of sharpness because of its wavelength. But it is just a shadow of memory and in the end, I don't really know what
    I'm talking about.
  22. But you do know, Carsten.<br><br>Focus shift also depends on how well a lens manages to bring the wide spectrum from deep blue to deep red to a single focus. When we focus, we find the optimum setting for the difference between blue and red focus, whether that be a big difference or a minute one. Doing so, we also favour our own vision's dominant part of the spectrum, so will ignore both the blue and red and of the spectrum a bit.<br>When you then take out most of the spectrum by putting a narrow band filter in front of the lens, the optimum, the compromise between the extremes of the spectrum used when focussing, will shift, will be invalidated. (Again: by how much, and if by an amount we could detect, depends on how well a lens is corrected for chromatic aberrations).<br>A deep red filter, only allowing light from one extreme of the visible (and actinic) spectrum, combined with a lens that isn't that well corrected, will indeed cause a shift of focus and a need to refocus, compared to the optimum focus using the full spectrum.
  23. Well said Q.G.,
    since my negatives came out a little soft, now it is about how to correct this aberration.
    As far as I can see there is not a proper rule to follow to avoid this issue.
    So I have just to try, using the max depth of field I can, taking care not to curve the gel.
    I did this test with a Hasselblad, but the real job will be shot with a Linhof 4x5, with a Schneider lens.
  24. If the lens you use does indeed show a noticeable shift in focus, there's only one thing you can do: focus through the filter.<br>Which is also how you can determine whether a lens does show a focus shift you have to take into account when using that lens with deep colour filters: compare focus with and without the filter.<br>You can only see so much on a focussing screen, and you will find out more examining negatives made with and without filter, focussed without, and focus left untouched when the filter is put on.<br>And since you can only see so much on a focussing screen, you may want to stop down a bit (which, by the way, could in some lenses cause yet more focus shift).<br>What you can't do is test a filter on one lens (the Zeiss/Hasselblad in your case) and rely on the outcome to be valid when using another lens (the Schneider). Each single lens you want to put the filter on has to be tested individually.<br><br>I lost track a bit: could it be that the softness you see is caused, not by focus shift, but by a diffusing/softening effect of a non-optical grade filter material used?
  25. Yes, I agree. In the end the soft effect could be a consequence of the gelatin quality.
    Thank you for your help.
  26. Carsten... what lens are you using that is potentially "chromatically defective"?
    I understand (at a basic level) the scientific-sounding discussion but my experience, as mentioned earlier, showed no focus shift or softness when using a Rosco material. I did not look with a microscope, just a regular 4x loupe though. Are you sure there isn't soemthing else going on?
    The lenses I've used in this manner were largely 1980s era Schneiders, which I believe were very well corrected. But I have used this kind of material with a 1912-era Bausch and Lomb. I have no idea how well that was corrected but the difference between a naked lens and Rosco gel wasn't really noticable except for the contrast impact of a red filter.
    In the end, it might be much easier to buy a Kodak gel and stop the madness. :) In the end, that is probably the right thing to do anyway if you even think there may be a problem... don't you think?
    But if it is a chromatic defect of the lens... even that should show a softness.
  27. p.s. Did you use a good shade with the filter? I find that a good lens shade is essential when using a gel filter on the lens.
  28. p.s. If I failed to mention already, the use of Rosco material versus Kodak gel on a lens is an emergency measure that I wouldn't really advocate for serious use.
  29. Dupe (again)
  30. The Hasselblad I did the test with has a good Zeiss lens.
    The softness is visible even with a normal loop, but first I thought that could be the contact print to give sofness to the red
    pics. Then I understood, checking the negatives, that the red pics were softer than the natural light pics. So I thought
    something fishy was going on, but on a scientifical way. I mean I thought I was ignoring some general and well known
    rule about filtering with red.
    Probably is a blur due to the non perfect quality of my red gel, maybe is not as trnsparent as I my eyes perceive it.
    Or is the lack of perfect flatness. Tomorrow I will try again and as soon as the tests are ready I post an update.
    Thank you,

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