Convert camera meter to incident meter?

Discussion in 'Photography' started by Martin, Oct 2, 2005.

  1. Martin

    Martin Guest

    Having read several articles on the web about the benefits of incident light
    metering over reflected light metering - and not possessing an incident
    light meter - I wonder is it feasible to place a translucent dome eg half a
    table-tennis ball or similar, place it over the camera lens and, after
    suitable calibration, use it as an incident light meter?

    I had a quick stab using Google to see if there is anything on the market
    that already does this, but I found nothing.
     
    Martin, Oct 2, 2005
    #1
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  2. There used to be do-dads like that, but I don't know if they're still
    available. You're better off with a handheld meter.
     
    Randall Ainsworth, Oct 2, 2005
    #2
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  3. I know of at least one point and shoot digital camera that does
    white balance in exactly that way, using a white somewhat
    translucent lense cap.

    Regardless, you can take your pick of something to place over the
    front of the lense, and very quickly do a few experiments that will
    provide the needed correction factor.

    A good starting point would be to take a reflected through the
    lense reading on an 18% gray card, or some "scene" that you
    believe the camera will meter accurately, and then put a
    translucent cover over the lense, position it where the gray card
    was and take a reading with it pointed at the light source. The
    difference between the two readings is probably going to be
    within 1/2 f/stop of correct, and two or three series of images
    under different lighting conditions using that correction factor
    and bracketing in 1/3rd stop sequences will quickly give you an
    idea of whether it will work for you or not.
     
    Floyd Davidson, Oct 2, 2005
    #3
  4. Martin

    Colyn Guest

    Wrong...

    There is a difference between incident and reflected light
    measurement.

    Reflected light is the light reflected from the subject while incident
    light is the light falling on the subject.

    There will be a difference of more than 1/2 stop..
     
    Colyn, Oct 2, 2005
    #4
  5. Martin

    John_H Guest

    One such device is the Wallace Expo/Disc (it's been around for thirty
    years or more). Dunno if it's still available but google will no
    doubt find it.
     
    John_H, Oct 3, 2005
    #5
  6. Martin

    Martin Guest

    Thanks. That was the sort of thing (the grey card) I had in mind by way
    of calibration except that you run into that old argument about 18% grey or
    12 % grey etc. Of course, with access to an accurate incident light
    meter, one could carry out a true calibration.

    I presume that another calibration point would be in circumstances when the
    reflected light equals the incident light and I guess this condition could
    be simulated if the (digital) camera lens was shrouded by a tube pointing at
    a light source (the reflected light metering) and then the translucent cap
    were placed over the lens (incident light metering) and another reading were
    taken down the tube at the same source.

    At least no-one shot the idea down as plain wrong, so I might play with this
    idea as well as setting the right exposure by other methods such as
    spot-metering and zonal methods, at least till I find a method I'm
    comfortable with.

    I really appreciate all the comments.

    Thanks

    Martin
     
    Martin, Oct 3, 2005
    #6
  7. Martin

    dadiOH Guest

    If the meter used is a true incident meter - not a jury rigged add on as
    asked about by OP - and the reading with the reflected meter is from an
    18% grey card as specified by Carlson, there should be *no* difference
    between the two.

    --
    dadiOH
    ____________________________

    dadiOH's dandies v3.06...
    ....a help file of info about MP3s, recording from
    LP/cassette and tips & tricks on this and that.
    Get it at http://mysite.verizon.net/xico
     
    dadiOH, Oct 3, 2005
    #7
  8. Martin

    -- Guest

    Probably true...


     
    --, Oct 3, 2005
    #8
  9. Martin

    Colyn Guest

    Taking a light reading from a gray card with reflected light will give
    a reading of approx 2-3 stops higher than the same reading with
    incident light.

    Example: Using my Minolta Autometer Pro and metering a gray card with
    the meter set for reflected light I got a reading of 1/[email protected]/8-11
    Setting the meter for incident light gave a reading of 1/[email protected]
    Note the reading between the indicated f/stop.

    Doing the same test with my Weston Ranger 9 both incident and
    reflected gave the same readings.

    BTW: Both meters are in calibration..

    Reflected light meters will read light reflected from the subject
    which is based on most meter calibrations to be approx 18%. This meter
    is pointed at the subject for readings. Zone 6 reflectance is 32%.

    Incident light meters read the whole light falling on the subject and
    is pointed at the camera from the subject position not pointed at the
    subject.

    There was and may still be a cone sold which attaches to the front of
    the camera lens which will convert the cameras meter to incident light
    metering but if the reading is taken pointed at the subject the
    reading will be wrong.
     
    Colyn, Oct 3, 2005
    #9
  10. What is wrong? Nothing you say below indicates anything I said
    above is wrong. It does suggest you either didn't understand it
    or haven't expressed yourself particularly well below...
    Do *you* understand what the significance of that is?
    Dead wrong.

    They don't measure the same thing, and there is no way to
    directly compare them in a way that has "a difference of more
    than 1/2 stop" and still makes sense. The *fact* is that you
    *must* use the right correction factor for the reflectance of
    the subject measured with a reflected light meter, and if you do
    there (by definition) is 0 difference.

    A "different" reading depends *entirely* on how reflective the
    subject matter is for the reflective reading. The fact that I
    mentioned an '18% gray card, or some "scene" that you believe
    the camera will meter accurately' should have clued you in.
    Depending on how one wishes to "calibrate" meters, that may be
    correct... or will be off by the difference between 12.5% and
    18% gray or by the amount that the "scene" varies from whichever
    "standard" one wishes to calibrate against.

    Which reading a person happens to prefer as the "correct"
    exposure is matter of taste, and you'll get no argument either
    way from me except to say that neither is wrong. The fact is
    that reflective metering *requires* the photographer to make an
    educated guess about the reflectivity of the scene being
    metered, and *cannot* provide accurate exposures of varied
    subjects. An incident light measurement does exactly that.

    The point is this "difference" is the fudge factor that *must*
    be used with a reflective measurement to account for the
    reflectivity of the metered object. If you get it right, there
    is *no* difference. And no matter what, it can't be claimed to
    be 1/2 stop in all cases.
     
    Floyd Davidson, Oct 3, 2005
    #10
  11. Then you are not properly using your tools.

    Given what you state below, it appears you are using a meter
    calibrated for incident metering to measure the light reflected
    from the gray card. That is okay if and only if the subject to
    be photographed is being illuminated by light reflected from the
    gray card.

    But nobody else is expecting to light up any subject with
    reflected light from a gray card, so I'm not sure why you would
    want to do it that way.
    Only if you don't have enough sense to realize the gray care *is*
    the subject which will be photographed, and is *not* being used as
    a source of light.
    But that doesn't seem to be what you were doing above.
    No kidding! Why would anyone take an incident reading of light
    reflected from a subject? That is absurd (and was not what
    anyone else has suggested). But that incorrect measurement
    technique does explain the strange readings you state that you
    got.
     
    Floyd Davidson, Oct 3, 2005
    #11
  12. Martin

    McLeod Guest


    There's something wrong here. They should give exactly the same
    reading if they're measuring the same light and your meter is set for
    the standard grey. My Minolta does, and theoretically yours should
    too.
     
    McLeod, Oct 3, 2005
    #12
  13. Martin

    dadiOH Guest

    And there is the source of your discrepancy...you are measuring the
    light falling on *both* highlight and shadow side. That is not the case
    when you are using a reflected reading on a grey card...you are
    measuring the incident light that is reflected from the grey card;
    consequently, take your incident reading from the light source.

    If you don't want to measure the incident light itself, then use the
    reflected meter to read the subject (both highlight and shadow) to get
    an *average* reading.

    Doing either of the above will result in congruent readings.


    --
    dadiOH
    ____________________________

    dadiOH's dandies v3.06...
    ....a help file of info about MP3s, recording from
    LP/cassette and tips & tricks on this and that.
    Get it at http://mysite.verizon.net/xico
     
    dadiOH, Oct 3, 2005
    #13
  14. Martin

    Whiskers Guest

    The quickest way is to find some 'standard subject' that you can place in
    front of the camera (ideally, in the position of the subject and facing the
    camera position) to 'take the reading from'. An 18% grey card or a
    'photographic white card' can give you that reference point; so can a clear
    blue sky or a patch of well-manicured green lawn. Perhaps the most easily
    carried around however, is the palm of your own hand.

    If you can think in terms of a 'zone system' such as the markings on a
    Weston meter, or a simplified version of Ansel Adams' famous system, then
    your standard subject will always be in the same 'zone' and you can take it
    from there to work out the actual shutter-speed and aperture to use. This
    is easiest with a hand-held meter, but it can be done with a TTL meter on a
    manual camera - with some gymnastics.

    An 'invercone' (which can be flat) attached to the front of an exposure
    meter, placed in the subject position and facing the camera position,
    should give precisely the same reading as pointing the same meter from the
    direction of the camera, without the invercone, at an 18% grey card placed
    in the subject position and 'square on' to the camera position and filling
    the meter's view.

    A hand-held meter, however simple, is well worth the expense and
    learning-curve. Some can be fitted to the camera's accessory shoe (if
    there is one and it doesn't assume that any accessory fitted is a flash
    gun) or attached to the neck-strap, for convenience.
     
    Whiskers, Oct 3, 2005
    #14
  15. Martin

    Martin Guest

    Thanks. Actually, I have a grey card and have experimented with it - and
    then noticed the 18% vs 12% grey card quagmire and the recommendation from
    the Kodak card (mine's a Jessops card) to open up by one stop - I don't know
    if that applies to all grey cards, but by that stage I was getting a bit
    cheesed off and decided to do my own 'calibration' based on some simple
    trial and error stuff. But when I posted my question, I was thinking in
    terms of landscape photography where, I assume, a grey card is not
    practicable and where incident light metering is recommended (by some people
    at least). Nevertheless, I have been reading various web articles on the
    zone system and I intend experimenting to find a system of exposure setting
    that I'm happy with. By the way, I have a digital Canon 2100 ultrazoom,
    so it's fairly basic and no such thing as an accessory shoe.

    The replies to my post are very interesting and informative and I'm very
    grateful for them.

    Martin
     
    Martin, Oct 3, 2005
    #15
  16. Martin

    Martin Guest

    Many thanks. I followed your suggestion and fhave ound
    http://www.expodisc.com/

    It seems designed primarily for setting the white balance but it does also
    say that it can be used to get the correct exposure. I'll now have a
    good read of the site.

    Thanks again.

    Martin
     
    Martin, Oct 3, 2005
    #16
  17. Martin

    Whiskers Guest

    Well, a grey card the same size as the landscape is a bit awkward to carry
    - but taking an invercone half way to the horizon isn't always practical
    either ;)) If you can stand in a place that has 'the same' lighting as
    the part of the landscape that you want to photograph, then either put the
    small A5 grey card (or the palm of your hand) in front of the meter, or fit
    the invercone and turn around so that the intended scene is behind you, and
    take your reading.

    The key point to remember, is that an exposure meter is designed to 'average
    out' the light it receives on the assumption that the scene is required to
    average to a 'mid tone' in the final picture. Scenes aren't always like
    that of course; snow is brighter than a mid tone, dark slate roofs are
    darker than a mid tone, and all-in-white brides get quite emotional if their
    dresses look as though they are 'eau-de-nil'. By using a standard subject
    such as your hand or a grey card, you can eliminate any exposure mistakes
    caused by a 'non average' subject, and can use the same 'calibration' every
    time.
    Good thread :))
     
    Whiskers, Oct 3, 2005
    #17
  18. Martin

    Colyn Guest

    Not if you are doing incident v reflected light measurement.
     
    Colyn, Oct 3, 2005
    #18
  19. Martin

    Colyn Guest

    I know the difference between reflected and incident light
    measurement. And I know how to properly use my tools.
    My test was done by measuring light reflected from a gray card using
    the reflected light method. I then measured the light falling on the
    gray card from the gray card location using the incident light method.
    Both meters are properly calibrated and give correct measurement
    regardless of which method used.
    You will have on average a 2 stop difference not 1/2 stop as you
    suggested.
    I've never known of a gray card being used as a light source.
    Actually I conducted several different meter tests.. I even used my
    daughter as a subject. I still got approx 2 stops difference between
    the two methods and both gave proper exposure
    That's a good question. I've seen people who claim to be pros use
    incident measurement from the camera position pointed at the subject.
    My readings are correct..
    My preferred method of light measurement is incident since it is more
    accurate than reflected. About the only time I use reflected light is
    to determine the overall lighting ratio of the scene..

    The point I am making is there is no 1/2 stop difference between
    incident and reflected measurement. If that is all you are getting
    then your meter is off.
     
    Colyn, Oct 4, 2005
    #19
  20. Martin

    McLeod Guest

    Neither reading is more "accurate". The reflected reading has the
    added responsibility of requiring the person working the meter to
    evaluate the reading.

    I decided to do a little test right here at my computer since
    everything you were saying went against everything I was taught and
    have been teaching for the last 3 years. I took out a Kodak grey
    card, my Minolta IV meter, and my Gossen Lunasix F meter.
    I placed the grey card right beside my keyboard, grey side up. My
    lighting in my office is dim so I set both meters to ISO 1000.

    I first took an incident reading with the Minolta laying flat against
    the card with the dome up and got 1 second at 5.6 plus .6 of a stop.
    I then put on the reflected attachment which takes a 30 degree FOV. I
    aimed the meter at the card from about 7-8" away and got a reading of
    1 sec at 5.6 plus .7 of a stop.

    I then held the Gossen meter upright so the dome was aiming straight
    up on the same plane as the card. I got a reading of 1 sec at 5.6
    plus 5/6 of a stop. (The Gossen only has markings at 1/3 of a stop
    but it was exactly halfway in between) I then slid the dome over and
    pointed it at the grey card at almost exactly the same position as the
    Minolta reflected reading. I took a reading and got 1 sec at 5.6 plus
    2/3 of a stop.

    What you are saying does not make sense and can be proved by anyone
    with a grey card and a meter.
     
    McLeod, Oct 4, 2005
    #20
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