Exposure question - confused !

Discussion in 'Photography Forums' started by py6km, Sep 12, 2007.

  1. py6km

    py6km
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    Just been reading Understanding Exposure, and there's something I don't get. In the section on low light/night photography, Bryan explains on most images that he meters from the dusky sky - fine. However, he meters at an aperture of, say, f/2.8 to get the corresponding correct shutter speed. He then recomposes the shot, but adjusts the aperture (to f/22 for example) and adjusts the shutter speed by a corresponding amount.

    For example, if he meters off the sky at f/2.8, but then composes the shot proper at f/22, he adjusts the shutter speed given at f/2.8 by 7 stops/positions to give the correct shutter speed at f/22 (since f/22 is 7 stops from f/2.8).

    My question is: why not just meter off the sky at the aperture you want to use int he shot, e.g f/22 ? Would that not give the same result ?
     
  2. Tobers

    Tobers
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    Good question there. I'd hazard a guess that the camera meters more accurately the more light it has hitting the light meter therefore meter at 2.8 then close the aperture to what you want the shot to look like.

    I think he says a few times about metering wide open then adjusting accordingly (too much maths for me though - not quite worked it all out yet).
     
  3. py6km

    py6km
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    Ok, yes, that makes sense. It would be interesting to see if the same results were achieved using his method and 'my' method
     
  4. Cadire

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    I think that his camera did not have a shutter speed that corresponded to f2.8 @ 1/4 at f22 (30 seconds), so the exposure time could not be calculated if the metering was done at f22.

    Metering first at f2.8 meant that he could calculate the correct shutter speed (for f22), and time it manually with his camera set to 'Bulb'.
     
  5. Liquid101

    Liquid101
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    This concept is a bit dated I think. There was a time when meters were more accurate when the lens was wide open - in fact some early lens/camera combinations required it.

    Not sure that modern cameras and lenses would suffer this kind of thing. Might be worth experimenting though. That said, any subtle differences that these readings give would be meaning less when shooting in RAW.
     
  6. py6km

    py6km
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    So you think that metering at the aperture you want to use to take the shot would work, and his method is an artefact of his knackered out old camera/lenses?
     
  7. T0MAT01

    T0MAT01
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    yeah dont worry about it. The lens is still wide open unless you press the depth of field preview button anyway.
     
  8. Tobers

    Tobers
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    Well, I meter at the aperture I want so I think you're right - a historical throwback. Might be worth a few experiments though and it'll certainly get your brain around the aperture/shutter calculations.
     
  9. Cadire

    Cadire
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    It depends if your camera has a shutter speed that long. If a 'correct' exposure was 30 seconds @ f22, and your camera had a maximum shutter speed of 15 seconds, how would you ever meter it? Using his method allows you to do that.
     
  10. py6km

    py6km
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    Cheers everyone - I'll give it a go.

    While I'm on the subject of exposure, and for those familiar with the book - why does a shot taken using his 'Mr Green Jeans' approach need to be exposed at -2/3 below the reading you get when you meter off any greenery ?

    I've not really figured out how to do that on my D200 yet - does anyone have any experience?
     
  11. Liquid101

    Liquid101
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    The exposure compensation button just below and the right of the shutter release. It's marked with a +/- hold this down and use the command wheel to set the compensation required. I always shoot at -0.7 - I feel the D50 exposes a little bright for my tatses. I think it roughly uses 3rd stop steps.
     
  12. py6km

    py6km
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    Would you adjust the exposure to -2/3 before or after you manually meter? I'm concerned that by changing the exposure manually, it will affect how the camera meters (i.e. it will try to compensate for the reduction I've made)
     
  13. Liquid101

    Liquid101
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    I'm pretty sure it wouldn't make any difference. If you set it before it will expose with the comp - set after and it will adjust the exposure.
     
  14. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    I woudl think that modern dslr actually work out exposure with the aperture wide open then they work out the maths themselves based on your set fstop. IE When you push the button for exposure lock the aperture doesn't close down through the viewfinder......it only closes the aperture when you actually do the exposure.

    Gordon
     
  15. py6km

    py6km
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    @ Liquid101

    Maybe I'm getting mixed up here, but wouldn't that therefore mean that there was no point in adjusting the exposure ?

    I mean, if you set it before, and then meter, the camera meters taking into account the exposure compensation; set it after, and the camera adjusts the shutter speed (or whatever) to maintain the exposure at the metered level - in other words, the camera works to negate the effect of the exposure compensation you've specified in order to get what it thinks is still correct ?

    Would you not need to meter, and record those values, then set the exposure compensation to -2/3 and use the values for exposure obtained prior to your compensation in such a way that the camera is not able to adjust anything to try to get back to 0eV...?

    :eek:
     
  16. Liquid101

    Liquid101
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    When you say 'manually meter' what exactly do you mean?
     
  17. py6km

    py6km
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    Possibly entirely the wrong way to describe it - I mean pointing the camera at the sky (for example) at a particular aperture (for example) so that the camera adjusts the shutter in order to meter from light coming off the sky, rather than allowing the camera to meter in the 'normal' way from the actual thing you want to take an exposure of.
     
  18. Liquid101

    Liquid101
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    OK, so your exposure will only hold if you hold the exposure lock button (AE-L). In this case you will need to set your exposure compensation before you taking your reading (unless you want to press lots of buttons at once)

    As soon as you release this button the exposure meter will take a new reading of the scene in the frame.
     
  19. grantsteve

    grantsteve
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    I think what Bryan suggests in his book is metering off the sky (by that he means pick your aperture, point the lens at the sky and adjust the shutter speed until the camera shows a correct exposure i.e. on the '0' mark), then you recompose your shot and shoot with the same combination of aperture and shutter speed.

    That's how I read it anyway.
     
  20. py6km

    py6km
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    I think that's what's getting me confused Liquid. In aperture priority, when I meter from something and press the AE-L button, the shutter speed that the camera has calculated is locked. BUT, when I then adjust the exposure using the +/- button you mentioned before, the camera changes the shutter speed, even though the AE-L light is still on (i.e. the exposure is still, so say, locked). I must be doing something wrong.

    My camera is set up so that the exposure remains locked (when the AE-L button is pressed) until a shot is taken, so I had assumed that me changing the exposure after I'd locked using AE-L wouldn't have allowed the camera tomake any adjustments to shutter (or aperture).

    Might be time to get back to the manual and read up a bit more.
     
  21. py6km

    py6km
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    Sure - my problem is now with his green metering technique. I'm simply having an issue setting my camera up in the way he suggests :suicide:
     
  22. Liquid101

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    This sounds like it's doing exactly what your asking it to do. What would be the point of making an exposure compensation if it had no effect on your exposure. I'm not sure what your expecting it to do?
     
  23. py6km

    py6km
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    I thought Bryan's technique was that you get the 'correct' exposure (by metering from the green in a scene), but then take the shot at -2/3 of an eV below that correct exposure. Might have misunderstood
     
  24. Liquid101

    Liquid101
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    That's exactly what you will be doing. You take your reading - lock it, then adjust the compensation by -2/3. To make this adjustment the cameras has to change the shutter speed from the 'metered' reading.
     
  25. grantsteve

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    I think the difference between what he suggests and you describe is that he is in manual mode, thus setting both aperture and shutter speed himself according to the meter reading from the sky, whereas you said you were using aperture mode and letting the camera set the shutter speed for you.
     
  26. py6km

    py6km
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    :rolleyes: Just after I wrote the last post, I realised this. The camera has to change something to get the exposure down. God - what a moron !

    Thank you for being so patient and helpful !
     
  27. Liquid101

    Liquid101
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    No problem :smashin: It can be a confusing subject when you have 3 variables on the go at once.

    Wait until you start playing with colour management :D
     
  28. py6km

    py6km
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    Don't ! I've already tried to calibrate my monitor - things look ok on it, but when printed look rubbish ! I've also completely messed up with CS3 and embedded profiles - I have no idea what to do ! I've got decent Martin Evening book on CS3 which has a substantial colour management section - I just have to get my head around it.
     
  29. Liquid101

    Liquid101
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    Lets not go there tonight though :rolleyes::D
     
  30. py6km

    py6km
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    Agreed. No amount of coffee will help !
     

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