How to understand EV range? Please

Discussion in 'Lighting' started by dxphoto, Dec 7, 2005.

  1. dxphoto

    dxphoto Member

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    I have a question of the light meter. The Ev range of a ligte meter. When in the specification, the EV range usually defined such ev 1-19 (ISO 100, F1.4 1 sec - F16 1/2000). Why it has something to do with the ISO? someone tells me if the iso is 400, then the smallest EV is not 1. Is that right? Please help
     
  2. Lee L

    Lee L Member

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    Last edited by a moderator: Dec 7, 2005
  3. Helen B

    Helen B Member

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    EV defines a set of shutter speed/aperture combinations that result in the same exposure. That's all. To make an EV indicative of a lighting level, you need to quote the EI or ISO. So a handheld meter that reads down to EV 1 at ISO 100 will only read down to EV 3 at ISO 400.

    If you are specifying the metering range of a TTL stopped-down system, such as the Leica M6, you need to quote either the EV, ISO and aperture, or just the shutter speed and ISO. This is because the aperture of the lens affects the apparent sensitivity of the meter.

    The Pentax digital spot meter readout is in 'EV' but the number is only correct at ISO 100.

    Zero EV is one second at f/1.

    Best,
    Helen
     
  4. dxphoto

    dxphoto Member

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    I am trying to understand the documents. Thanks.
    More particularly, the problem I have is a meter has the ev range I described as above. I ran a test on it and the correct EV is 10 (ISO 400, F2.8, 1/125).
    So Tv=7, Av=3, Sv=7, Bv=3.
    Now I slow down the shutter speed to 1/60, the meter says "Over" and I keep going. When the speed goes to 1/2, the meter says "under". The explaination I got from the dealer is its out of ev range. (all users have the same problem).
    I did the same tests with different ISOs at the same brightness (EV=10).
    film speed | f-stop/meter reads correct exposure "o" | speed where meter start to read "under"
    25 | f2.8 | 1/8 | 1/1000
    50 | f2.8 | 1/15 | 1/2000
    100 | f2.8 | 1/30 | B
    200 | f2.8 | 1/60 | 1
    400 | f2.8 | 1/125 | 1/2
    800 | f2.8 | 1/250 | 1/4
    1600| f2.8 | 1/500 | 1/8
    3200| f2.8 | 1/1000 | 1/15

    Bear in mind, the meter is correct when its in its range. the dial goes around (which means after B is 1/2000s)

    I read a little bit about the APX.pdf and I am still cannot explain my test. Can you help me? Thanks..
     
  5. Helen B

    Helen B Member

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    "I did the same tests with different ISOs at the same brightness (EV=10).'

    Remember that EV is not a measure of brightness unless it is accompanied by an ISO or EI.

    It would help if we knew which meter you are using. It sounds a bit like it is a camera meter. Each film speed setting will correspond to a shutter speed - the lowest the meter will work at. At speeds slower than that the meter will tell you that it is out of range.

    Your ISO 400 example:
    At 1/125 and f/2.8 the meter says 'That's the right exposure'
    At 1/60 and f/2.8 the meter says 'That's going to be over-exposed'
    At 1/2 and f/2.8 the meter says 'Never mind how much light there is, that setting is under my sensitivity range, don't trust me'

    All the combinations display the same sensitivity range.

    How does that sound?

    Best,
    Helen
     
  6. dxphoto

    dxphoto Member

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    Hi Helen,

    Thanks for the help. It is a camera meter (Bessa R). I just don't understand why 1/2 f2.8 at ISO 400 is out of EV range.
     
  7. haris

    haris Guest

    Because 1/2 f2.8 at ISO 400 means really dark scene. And it is supposed that if someone shoot at those conditions, he or she will use tripod and handheld light meter. Because camera manufacturer usualy think that meters included in camera will be used for camera handheld shooting. And in ligtning conditions in your example it is impossibile to shoot holding camera only in hands. Thus, there is no reason for including in camera meter which can measure those lightning conditions. And that maybe make meters cheaper or smaller which makes them easier to include meter in camera, and make overall price of camera more afordable. More sensitive meter = more expencive meter = more expencive camera.

    Atleast that is my opinion, which, of course, can be, and probably is, completely wrong :smile:

    Regards
     
  8. Lee L

    Lee L Member

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    That does appear on the face of it to be out of spec. The R should read EV 1 through EV 19 at 100 ISO, which would be EV 3 at 400 ISO for the lower limit. That should be down to 1 second at f:2.8.

    However, I just tested my Bessa R2, which has the same metering specs as your R, and it will not meter any light level with 1/2 sec shutter speed at 400 ISO. It does read correctly at 1/4 sec with 400 ISO.

    Lowest shutter speed bounds appear to be:
    ISO shutter speed
    100 1 sec
    200 1/2
    400 1/4
    800 1/8
    etc.

    Lee
     
  9. Helen B

    Helen B Member

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    A Bessa R. It sounded like a TTL stop-down meter. As I mentioned in my first post, the lower limit of a TTL stop-down meter can be defined by a simple set of ISO/shutter speed combinations. This is the same for the Bessa R and all the Leica TTL metering models like the M5, the CL (and Minolta CLE), the M6, the M7 and MP.

    These rangefinder cameras meter at the working aperture, so stopping the lens down reduces the effective sensitivity of the meter (the meter gets less light).

    So, irrespective of the aperture, the longest shutter speed the Bessa R will meter at is one step shorter than the ones you quote for displaying 'under', ie

    100 | 1
    200 | 1/2
    400 | 1/4
    800| 1/8
    1600| 1/15
    3200| 1/30

    You will notice that this corresponds to the range you quote: that at ISO 100 it will meter to 1 second. The misleading information in the spec is the suggestion that it will meter to EV 1 at ISO 100. It will only meter to EV 1 at ISO 100 if the lens is set to f/1.4. If you stop down to f/2.8 it will only meter at EV 3 - ie 1 second at f/2.8 (still at ISO 100).

    At ISO 400, the meter will be out of range at 1/2 second at f/22, or EV 10, for example.

    Whatever the lens aperture, the limit is always set by the shutter speed at any given film speed setting.

    Therefore the Bessa 'knows' that if you set a shutter speed longer than the limiting one, the meter will not be sufficiently sensitive to measure reliably, so the electronics tell you that, irrespective of what the meter is reading at the time.

    Does that explain it?

    Note: all the above applies to TTL stopped-down metering only.

    Best,
    Helen
     
  10. dxphoto

    dxphoto Member

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    Well explained! Thanks so much.
     
  11. Ed Sukach

    Ed Sukach Member

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    The "EV" - Exposure Value - is independent of film speed. It indicates a combination that will allow a specific amount of light to pass. If you are familiar with reciprocity (n.b. NOT "reciprocity FAILURE"), an exposure of 1 second @ f/22 will allow the same amount of light to pass as an exposure of 1/60 second @ f/2.8 - both -- I'll check the Hasselblad lens scales -- have the same "EV" - 9. 1/60 second @ f/5.6; 1/8 second @ f/8; 1/4 second @ f/ 11 ... All allow the same amount of light - and all are EV 9.

    The sensitivity of an exposure meter is represented by the lowest and highest EVs it can read at a given film speed; therefore, @ ISO 100 - a given meter may be capable of reading EV 2 - 2 seconds @ f/2.8; to EV 18 - 1/500 second @ f/22.
     
  12. Helen B

    Helen B Member

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    "The sensitivity of an exposure meter is represented by the lowest and highest EVs it can read at a given film speed; therefore, @ ISO 100 - a given meter may be capable of reading EV 2 - 2 seconds @ f/2.8; to EV 18 - 1/500 second @ f/22."

    Unless it is the meter in a TTL camera that reads at the working aperture, which is what this thread is all about...

    Best,
    Helen
     
  13. Ed Sukach

    Ed Sukach Member

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    As I understood it - and I HAVE re-read, the question was "What does EV (Exposure Value) Mean?" It was framed as, "I see the sensitivity of exposure meters given as 'X' EV at a certain ISO".
    EV alone does not define sensitivity, or the method of metering, or have anything to do with film speed. One could easily substitute, "This meter has the capability of measuring the amount of light ranging from that requiring an exposure of 1 second @ f/2.8, given a film speed of ISO 100, to that amount of light requiring an exposure of 1/500 second @ f/22, given the same film speed of ISO 100."
    It is more economical - takes up less space in the brochure - and is widely accepted, to give the sensitivity as "EV3 - 18 @ ISO 100".


    I think it is of great usefulness to keep our terms and definitions straight: It does no good to confuse "f/stop" with "Effective f/stop", or the much more utilitarian term "T/stop".
     
  14. Lee L

    Lee L Member

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    Ed,

    That was the original question, but that turned into "why won't my Bessa R meter at settings that are within the specified meter sensitivity range in EV?". This is the question that Helen has answered completely and accurately.

    Helen's point is that the statement of a pure EV sensitivity range isn't a complete statement of the metering capacity of this particular camera, and that it actually can't meter at all combinations of ISO/aperture/shutter speed that fall within the stated EV sensitivity range. That failure to meet the stated standards is a function of stopped down TTL metering.

    Lee
     
  15. Ed Sukach

    Ed Sukach Member

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    Ah! I've been "hoist on the petard" of the shifting topic.

    I apologize .. in my zeal to clarify, I must have come on as an an unconscionable know-it-all, which I assure you, I DON'T.

    In a different life I used to work in a place making sophisticated, and I mean sophisticated custom optical systems. One of my tasks was calibrating "light meters" - mostly Cascade Photomutiplier types ... so topics bordering on that subject tend to fire zealous neurons. Possibly those neurons were sensitized from a few accidental zots from photomultiplier primary power supply circuits. Real eye-openers!

    The history of the means of controlling the amount of light - the exposure of film - and why we call varying sized apertures - whether produced by individual holes or sliding blades - "stops" is in itself fascinating.