Page 3 of 6 FirstFirst 12345 ... LastLast
Results 21 to 30 of 54

Thread: light meter calibration

  1. #21

    Join Date
    Dec 2009
    Location
    Pacifica, CA
    Posts
    1,710

    Re: light meter calibration

    Reading reflected light off a gray card won't equal an incident reading. Unless you hold it at a certain angle, which you can discover by reading with the incident meter and then adjusting the angle you hold the gray card until the readings agree. My favorite standard is the palm of my hand which I "place" on Zone VI. That seems to agree fairly well with incident meter readings.

  2. #22
    Land-Scapegrace Heroique's Avatar
    Join Date
    Nov 2008
    Location
    Seattle, Wash.
    Posts
    2,929

    Re: light meter calibration

    Quote Originally Posted by Kevin J. Kolosky View Post
    In other words, if a given amount of light hitting the meter should read f 16 at 1/400 of a second because the meter was originally calibrated to correctly give that exposure for that amount of light hitting it, then I want to be able to check to see whether it is still correctly doing so.
    I'm not sure whether you're suffering from "meter-worry," but I can tell you what cured me of it a long time ago.

    Every field trip when it's possible – and this is actually fun – I take occasional readings of middle-toned subjects in sunny-16 conditions, because I already know what the "correct answer" will be for any given ISO.

    As long as my meter gives me the "correct answer" (after any meter-quirk adjustments) and it falls within a practical photographer's reasonably tight tolerance, then as far as I'm concerned, I'm being sufficiently vigilant about its good behavior. (BTW, you might want to mention the degree of tolerance you have in mind.)

    The clear north sky in the middle of the day is a popular subject for this type of reading.

    I still enjoy visiting those two shops in post #8 – who doesn't like a free and friendly expert evaluation every now and then?

  3. #23

    Join Date
    Sep 2001
    Location
    Dunedin,Otago,New Zealand
    Posts
    191

    Re: light meter calibration

    You already have a reference standard light source at your disposal, and it is free ,and reliable.
    Full Sun, in a cloudless sky,between 10am and 2pm, should give you an incident reading of F16 at 1/125 sec at 125 iso, with the sensor pointed at the sun. Any variation you may choose to give for a specific film emulsion,is a result of the choices you will make when you choose your developer,the film you are using,the tonal trade-offs you are willing to make, and the visual effect you wish to render.
    So - get the meter to a reliable state, and then -wing it.

  4. #24
    Kevin Kolosky
    Join Date
    Jun 1999
    Posts
    791

    Re: light meter calibration

    "I'm not sure whether you're suffering from "meter-worry,"

    I am not suffering from anything concerning my meters. I want them to peform as they were engineered to perform.

  5. #25
    Jim Jones's Avatar
    Join Date
    Aug 2006
    Location
    Chillicothe Missouri USA
    Posts
    3,065

    Re: light meter calibration

    I agree with Bill Burk's posts 19 and 21. Keep in mind that neither grey cards nor north blue skies are consistent. The surface of florescent tubes have similar intensity to the north blue sky, but can also vary. I haven't compared CFLs to these other standards. I have several selenium meters that consistently have sufficed as secondary standards for several decades. In the Weston and Norwood meters, an unbalanced armature can be more difficult to compensate for than varying meter sensitivity.

    As Kevin suggests, it would be nice if everything performed perfectly. However, perfection is a difficult and expensive standard to maintain. The human mind compensates well for imperfection. For many of us, good enough is good enough.

  6. #26

    Join Date
    May 2007
    Location
    New York City & Pontremoli, Italy
    Posts
    883

    Re: light meter calibration

    I'll take a stab at it since I need to keep (at my job) approx. 85 meters reasonably well-calibrated.

    I have set up an 18% Gray Card on a Copy Stand; Two lights illuminate it at 45-degree angles.
    If the meters have a Zero-adjust, I check it and set it if necessary.
    With a spot-meter, measuring from above, I check to make sure that the illumination is even all over the card (center and all corners).
    All meters are then placed and aimed at the center of the card from the same position (where one would normally attach the camera on the copy-stand).
    I take the reading and check; (I write down the reading on a spreadsheet with each meter's serial number so that I can refer to it later). Almost all of the meters give me the same reading. If any one meter deviates by 1/3 or more, I send it to Quality Light Metric for recalibration. My personal Pentax Digital goes to Richard Ritter (nice guy!).

    This set-up has worked very well for me.

  7. #27
    Kevin Kolosky
    Join Date
    Jun 1999
    Posts
    791

    Re: light meter calibration

    another question then.

    As I admitted, I am not well schooled in electronics. I assume that a cell in a meter converts light energy to electrical energy, which, in an electronic meter, is then calculated and a value given to it by a small computer.

    If that is true, then "calibration" is not doing anything to the cell itself. (correct me if I am wrong) Rather, calibration is changing the value that the computer assigns to the electronic signal it receives. So, for example, taking a Pentax spot meter. If one points it at an object and uses the "trigger" a number will appear. That, I assume is the computer giving a value to the amount or intensity of the light that hit the cell. That is the light meter part of it. Then, the photographer herself takes over and uses the exposure meter part by assigning that number to a certain place on a dial in order to calculate an exposure.

    My question is this. How are these little computers manipulated to calibrate the "light meter"? Obviously there is no keyboard on the meter to do this manipulation. Are there small buttons or points or contacts or whatever they are called that one somehow manipulates in order to change the readings for any given light reading.

    So, to be perfectly clear what I am asking. A person aims a light meter at a known light source. Based on the engineering of that meter, it should assign a certain number to that given reading, which can then be used for whatever purpose, including making a negative. But if person then finds that the number is incorrect, he then takes off the screws, opens the meter, and DOES WHAT to reprogam the little computer or chip or whatever to provide a different (correct) number for that given amount of known light?
    I am not complaining about the cost of having a meter calibrated. But I would like to know why it costs from $60 - $80. When I have an oil change I know that the technician drives the car into the garage, places the hoist, lifts the car, unscrews the filter, removes the filter, removes the oil plug after placing a container under it, lets the oil drain, replaces the oil , replaces the filter, lowers the hoist, checks the oil level, closes the hood, and drives the car out. This is all done for about $35.00 including the cost of the filter and oil. And it takes about 20 minutes.

    So, what are the steps with the meter. I would assume:
    1. turn on the special light source.
    2. make a reading with the meter.
    3. observe the number.
    4. determine whether the number is correct.
    5. if so, rebox meter and return to customer.
    6. If not, open meter
    7. Go to ??????? where computer can be changed.
    8. Change computer.
    9. retest meter.
    10. If correct, put meter together and return to customer.

    if this is not correct, what is correct?

    How long does this take? No parts involved.

  8. #28

    Join Date
    May 2007
    Location
    New York City & Pontremoli, Italy
    Posts
    883

    Re: light meter calibration

    I should have stated that, my re-calibration procedure is essentially a "Zero-ing" of the light meter as provided by the manufacturer for the end-user. Recalibration of a meter beyond those parameters or checking that the meters reads a certain EV/Foot-candle No. to a known source requires specialized testing equipment; that is my understanding.
    A meter may be off because the voltage from the battery is incorrect, etc.

    You might get a more precise answer from someone who repairs meters (including built-in camera meters) than from most of us mere mortals. It seems to me that we simply don't understand your question, or at best, don't know how to answer it.

  9. #29
    C. D. Keth's Avatar
    Join Date
    Apr 2006
    Posts
    2,089

    Re: light meter calibration

    You're getting bogged down in the tech details. Buy several brand new 100W clear glass lightbulbs. Get one out of the box and put the rest away. Wipe off the bulb with a towel and put it in a socket. With a measuring tape, place that socket 2 feet from a wall. At night, with only that one light on, hold your meter level with the filament and with its back against that wall. Record that reading. Repeat this procedure as often as you like. Lightbulb manufacturing is well consistent enough for you to check your light meter by. If that's not good enough for you, you either need to send your meter to quality light metric in hollywood and have them do it or you need to quit photography because you'll only ever obsess and will never actually get to making pictures.
    -Chris

  10. #30
    Kevin Kolosky
    Join Date
    Jun 1999
    Posts
    791

    Re: light meter calibration

    you ask for an explanation of the derivative and they tell you about the integral.

Similar Threads

  1. spot meter calibration
    By Joseph O'Neil in forum Gear
    Replies: 16
    Last Post: 21-Apr-2008, 20:24
  2. Spot meter calibration
    By John Fontana in forum Style & Technique
    Replies: 7
    Last Post: 23-May-2005, 15:15
  3. calibration drift in Sekonic L-608 light meter
    By Bruce M. Herman in forum Style & Technique
    Replies: 4
    Last Post: 14-Dec-2001, 22:56
  4. Spot Meter Calibration
    By William Lindley in forum Style & Technique
    Replies: 3
    Last Post: 13-Nov-1999, 04:19

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •