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Red alert. Article in AP coverdate 25th june

Discussion in 'AP Magazine Feedback & Suggestions' started by Learning, Jun 19, 2011.

  1. Learning

    Learning Ethelred the Ill-Named

    Lee Frost comments with respect to a converted camera "And there's no need to worry about adjusting focus to account for the fact that infra red light focuses on a slightly different point to visible light." Is this really the case? When a camera is converted to work with IR is the focus sensor also converted?
    He also wonders why there are IQ problems with some zooms. Could part of the problem be that the lens is not being focused properly?
    It would be interesting to know how those problem lenses behave on a converted camera which can be focussed in live view.
     
  2. beejaybee

    beejaybee Marvin

    If you're focusing using a visual viewfinder screen or a rangefinder then you most certainly need to allow for the focus shift (unless the lens happens not to require it). If you're focusing using "live view" then what you see is what you get. If you're using autofocus, "consult your camera manual" as they don't all work the same way.

    A lens is computed to work optimally at one wavelength (usually in the green) - and a zoom at one focal length (somewhere in the middle of the range). By the time you are working in infra red, as well as the chromatic variation in focal length (which is what the focusing correction is for), the computation for spherical aberration, field flatness etc. will be a considerable distance out & the performance of the lens will be impaired to some extent. This effect is much more likely to be noticed with a zoom, where the tolerances have to be relaxed to some extent to get acceptable performance across the focal length range. (Especially with "superzooms" - which are probably best avoided when working in infra red!)

    For this reason it's wise to stop down 2 or 3 stops from max aperture when working in the infra red. Stopping down improves the tolerance to focusing errors as well as reducing the effect of aberrations when working well away from the wavelength the lens was designed to work at.

    But don't stop down too far - if your lens is showing diffraction softening at f/16 when used normally, diffraction softening will be setting in by about f/11 when working in the infra red - longer wavelengths have more diffraction.
     
  3. P_Stoddart

    P_Stoddart Well-Known Member

    I can only draw on my film days here. All the lenses I owned back then had IR focus markings.

    Usually about 1mm off the white marking for focus.

    You would focus the camera using the split image OVF then mark the position on the lens barrel and shift it to the red mark.

    In general for landscape you could risk not doing it if like has been said you was using a high f stop like f11/f16.

    But now you have contrast AF. That might come into play of the camera is doing IR. IMHO
     
  4. beejaybee

    beejaybee Marvin

    The issue here is that the spectral response of the AF sensor may not match that of the imaging sensor - and the imaging sensor is unavailable for AF until the shutter has opened.

    If you've modified the spectral response of the imaging sensor (by removing or substituting the IR blocking in the anti-alias filter normally fitted immediately in front of the imaging sensor itself) then you won't have modified the spectral response of the AF sensor so there is likely to be a serious mismatch & the focus selected by AF is likely to be wrong.

    Nevertheless we're ignoring the real issue of using a normal DSLR for IR work (or an EVIL/CSC for that matter) - which is that only the "red" pixels will register an image after visible light has been filtered out, resulting in loss of 75% of the incident light (low sensitivity / increased noise) and reducing the effective resolution of a Bayer matrix one-shot colour camera by 50% in both horizontal and vertical directions i.e. reducing the effective pixel count by 75%.
     
  5. MickLL

    MickLL In the Stop Bath

    I seem to be questioning your statements a lot today - sorry I don't mean to be difficult.

    My memory tells me that 'normal' lenses (achromat) were supposed to be computed so that they brought two wavelengths to the same point of focus. Then the big selling point of APO (apochromat) lenses was that they focussed three wavelengths correctly - thus bringing all the ones that they didn't correctly focus much closer to optimal.

    This article seems to confirm my memory.

    MickLL
     
  6. beejaybee

    beejaybee Marvin

    You are quite right provided you're talking about chromatic aberration. Other aberrations (spherical, astigmatism, coma & field curvature) are computed for a single wavelength, in the green for lenses intended for normal use.

    Apochromatic correction of telescope objectives does result in much less variation of focal length across the visual range (about 1/5 across the range 420-680nm compared with an achromat using standard glasses, though modern lenses including low dispersion crown elements can improve the performance of an ordinary achromat substantially the "ED doublets" are still well short of the performance of a proper APO triplet) but there is usually considerable variation in the ultraviolet and infrared even with an APO triplet using exotic glasses.
     
  7. El_Sid

    El_Sid Well-Known Member

    I have looked into converting a camera to IR and part of the conversion procedure includes recalibrating the AF system to account for the offset focus of IR light. IIRC they request that you supply a lens with the camera body, preferably the one you use most, for the calibration procedure. I suspect that other lenses may not necessarily correspond exactly to the same calibration - especially if they contain anomalous dispersion glass or moulded elements that are not present in the lens used for calibration. It could be this that affects some zooms.

    I do some IR with an unconverted camera (Nikon D50) and R72 filter mainly using old manual lenses. With wide angle lenses I tend to use a hyperfocal method setting the infinity mark to f8 or 11 which is enough to give acceptable sharpness with landscape type shots. Longer lenses are trickier and then it's better to focus normally, note the position on the focus scale and set that to the IR index. Even then I tend to set the aperture to one which includes the IR index within it's DOF scale marks. These methods work fine with prime lenses it seems harder to acheive acceptable results with zooms - you have to be really careful not to change the focal length as you repostion the focus to the IR mark, assuming of course the zoom actually has such a thing. With zooms I really close down the f stop to try and compensate for any such errors...
     
  8. Learning

    Learning Ethelred the Ill-Named

    I raised the issue in ignorance and am watching the thread without writing much but with much interest. I have a much underused D200 and I am tempted to have it converted.
     
  9. Benchista

    Benchista Which Tyler

    It's certainly true that some lenses seem to work much better than others on my converted 30D, but having tried adjusting focus manually by very small amounts, it doesn't appear to be that. I also find that with the 830nm conversion, exposure latitude is very limited.
     
  10. spinno

    spinno Well-Known Member

    Shureley the title of the thread is incorrect...should be "Infra Red alert......"
     

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