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  1. #1
    DPP Member Ryan Beato's Avatar
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    Flash and ambient light metering questions



    I'm still new into photogography. I am currently researching on flash photography when I came across one article that kinda confused me.. i know that there are a lot of experts around here so hope you could help me on this. Here is the qoute from the article:


    "In Av, night and Tv (shutter speed priority) modes the camera meters for ambient (existing) light and fills in the foreground subject using the flash. It does not assume that the primary light source is the flash, and therefore the shutter speed it sets is the same as it would set if you weren’t using flash at all."


    My question is, if the camera meters the ambient light separately or independently and sets the shutter speed (If using Av) as if it was not using a flash at all, then wouldnt it be overexposed if you add in the flash?

    I made some tests on my camera, Im not sure if I did it correctly but here s what i did.

    1. Without Flash, I set my camera to Av mode, set aperture to 1.8.
    2. Point the camera to a specific spot and take note of the meter reading(speed = 1/15).
    3. Press the shutter completely.
    4. Using the same aperture, i flip up the flash of my camera and take another meter reading on the same spot. The shutter speed is no longer the same (speed = 1/30).
    5. Press the shutter completely and the flash fires.
    6. I repeated number 4 to 5 but i block the flash using my hand so that no light from flash will reach the subject.

    Here's what I found out based on the output of my pics. Result of Item 3 exposure is almost the same as Item 5, but number 6 is a bit underexposed.

    Now my confusion is, the article said that the camera meters in Av, T or M mode as if the flash is not the main source of light, why did my meter readings without flash and with flash different? Did i understand the article correctly?



    Also can you guys enlighten me more on relation of flash to the ambient light or the relation of metering of camera(aperture and shutter settings) to the flash metering?How they work together to get correct exposure?

    thanks a lot.

  2. #2
    DPP Member darwinandres's Avatar
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    Quote Originally Posted by Ryan Beato

    My question is, if the camera meters the ambient light separately or independently and sets the shutter speed (If using Av) as if it was not using a flash at all, then wouldnt it be overexposed if you add in the flash?

    I made some tests on my camera, Im not sure if I did it correctly but here s what i did.

    1. Without Flash, I set my camera to Av mode, set aperture to 1.8.
    2. Point the camera to a specific spot and take note of the meter reading(speed = 1/15).
    3. Press the shutter completely.
    4. Using the same aperture, i flip up the flash of my camera and take another meter reading on the same spot. The shutter speed is no longer the same (speed = 1/30).
    5. Press the shutter completely and the flash fires.
    6. I repeated number 4 to 5 but i block the flash using my hand so that no light from flash will reach the subject.

    Here's what I found out based on the output of my pics. Result of Item 3 exposure is almost the same as Item 5, but number 6 is a bit underexposed.

    Now my confusion is, the article said that the camera meters in Av, T or M mode as if the flash is not the main source of light, why did my meter readings without flash and with flash different? Did i understand the article correctly?



    Also can you guys enlighten me more on relation of flash to the ambient light or the relation of metering of camera(aperture and shutter settings) to the flash metering?How they work together to get correct exposure?

    thanks a lot.
    When you use flash photography there are two kinds of metering working on your camera. Your exposure metering for your aperture and shutter speed combination and flash metering (ETTL or i-TTL) for your flash output.

    Your ambient light was being metered by exposure metering of your camera and that would give you your ambient light exposure. When you half-press your shutter there's a series of pre-flashes that flash meters your subject using TTL flash metering. Once you press fully your shutter your locked aperture and shutter speed exposure and TTL flash metered value will be used.

    Your TTL metering works like this: the pre-flash meters the right amount of light that the flash should fire during exposure based on the reflected light of the pre-flashes.

    For your items 2 and 4, even if you don't put up your flash your reading will still vary 'coz you're using Av mode. A slight displacement of your camera from its previous position will also affect the metering.

    The best way is to use Flash Exposure Lock button (FE lock) or the * button. Read the manual.

    Better yet, make the test using M mode and not Av mode.


  3. #3
    DPP Member darwinandres's Avatar
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    Flash photography is really tricky. Learn how to use Flash Exposure Compensation when doing flash photography. Compensating your exposure during low light condition can do you no justice. The recommended hand holdable shutter speed is 1/focal length and you'll probably be stuck to that speed. You should compensate your flash power output to get properly exposed subject..... assuming you're not using a tripod, but that's another story.

  4. #4
    DPP Member Ryan Beato's Avatar
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    Quote Originally Posted by darwinandres
    When you use flash photography there are two kinds of metering working on your camera. Your exposure metering for your aperture and shutter speed combination and flash metering (ETTL or i-TTL) for your flash output.

    Your ambient light was being metered by exposure metering of your camera and that would give you your ambient light exposure. When you half-press your shutter there's a series of pre-flashes that flash meters your subject using TTL flash metering. Once you press fully your shutter your locked aperture and shutter speed exposure and TTL flash metered value will be used.

    Your TTL metering works like this: the pre-flash meters the right amount of light that the flash should fire during exposure based on the reflected light of the pre-flashes.

    For your items 2 and 4, even if you don't put up your flash your reading will still vary 'coz you're using Av mode. A slight displacement of your camera from its previous position will also affect the metering.

    The best way is to use Flash Exposure Lock button (FE lock) or the * button. Read the manual.

    Better yet, make the test using M mode and not Av mode.

    Thanks a lot Darwin.. Few more concerns here..

    I am using a canon 30D and based from the manual it is using E-TTL internal flash and not TTL. I dont know if Im wrong but based from what i read, E-TTL is different from TTL since it does not fire a preflash at halfpress shutter. TTL sends preflash when the shutter is pressed halfway but E-TTL sends the preflash after the shutter is fully pressed but right just before the first curtain opens. It is not noticeable since the real flash comes directly after the preflash, unless the camera is set to second curtain sync. Am I right guys?

    When i did the test, i did not notice any preflash being fired each time I press the shutter halfway.

    What I am more concerned here is on the ambient light metering and why it varies with and without flash as opposed to what the article above said. Maybe you are correct that slight movement could vary the meter reading but I tried doing the test several times on another location of different lighting condition and still could not get the same exposure reading with and without flash.



    By the way, If I use the FEL, wouldnt it affect only the flash exposure reading but has no effect on the ambient light reading? That means even if I use FEL, the aperture and shutter speed will not be affected since it is independent right?

    And also if I set it to Manual mode, then there is no point of metering the ambient light since the aperture and shutter speed is set already. There will be no difference of course in the meter reading with and without flash.I did the test in Av mode cause that is what i wanted to test because that is what is in the Article, Av mode.

    Hope im making sense here. still a newbie you know.

  5. #5
    DPP Member Ryan Beato's Avatar
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    correction pala, A-TTL pala is the one that's sending preflash during halfway press. TTL does not use preflash. E-TTL fires the preflash after shutter is fully pressed but before the first curtain opens.

  6. #6
    DPP Member darwinandres's Avatar
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    Quote Originally Posted by Ryan Beato
    Thanks a lot Darwin.. Few more concerns here..

    I am using a canon 30D and based from the manual it is using E-TTL internal flash and not TTL. I dont know if Im wrong but based from what i read, E-TTL is different from TTL since it does not fire a preflash at halfpress shutter. TTL sends preflash when the shutter is pressed halfway but E-TTL sends the preflash after the shutter is fully pressed but right just before the first curtain opens. It is not noticeable since the real flash comes directly after the preflash, unless the camera is set to second curtain sync. Am I right guys?

    When i did the test, i did not notice any preflash being fired each time I press the shutter halfway.

    What I am more concerned here is on the ambient light metering and why it varies with and without flash as opposed to what the article above said. Maybe you are correct that slight movement could vary the meter reading but I tried doing the test several times on another location of different lighting condition and still could not get the same exposure reading with and without flash.



    By the way, If I use the FEL, wouldnt it affect only the flash exposure reading but has no effect on the ambient light reading? That means even if I use FEL, the aperture and shutter speed will not be affected since it is independent right?

    And also if I set it to Manual mode, then there is no point of metering the ambient light since the aperture and shutter speed is set already. There will be no difference of course in the meter reading with and without flash.I did the test in Av mode cause that is what i wanted to test because that is what is in the Article, Av mode.

    Hope im making sense here. still a newbie you know.
    There are several kind of TTL (through-the-lens) metering. The older one is A-TTL for Canon which was the one you are talking about. It sends pre-flash when you half press. Yup, I was wrong there's no pre-flash when you half press the shutter on your camera (old school kasi ako). he he he your Canon uses E-TTL (just what I said in the first paragraph of my first reply)

    The ambient light will not vary whether you have flash or not assuming you have the same exposure value. When we say ambient light let's look at it as your background light. The area not lighted by your flash.

    About FEL, what I know about FEL is Flash exposure is being computed after the exposure reading. Meaning Flash exposure still depends on the locked aperture and shutter speed. Remember that just before you push your shutter button there's already an exposure value recorded. Your Flash meter will still depend on your exposure value, but will not change your exposure value.

    When using Manual mode you are still metering the ambient light using the graph for manual exposure (the pointer that goes from left to right -2 to +2) but it is only done manually.

    Your original question is whether there will be over exposure when you use flash while using an exposure value for ambient light. The answer is no. Why? Because the Flash meters differently. If you set your exposure value to your ambient light you will have a properly exposed subject. When you turn on your flash assuming you still use the same exposure value (that's why I told you to use Manual so the value will not change) the flash will meter your subject using E-TTL and will only give enought light to fill.

    Try this test:
    Meter your subject using Av or P mode
    Record the metered value
    Set the camera to M and use the metered value (so the value will not change)
    Shoot the subject w/out light
    Shoot the subject w/ light

    You will get differently exposed images, but not over exposed. The first one w/out fill light and the other one with/fill light. Now, try to change your shutter speed to different levels and shoot with flash. There will be no over exposure, but a change on your ambient light (background light) You will see that as you make your shutter speed faster your ambient light becomes darker, but your subject remains properly exposed.


  7. #7
    DPP Member Ryan Beato's Avatar
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    Thanks a lot darwin. You answered my questions quite well...

    Still Im puzzled as to why the shutter speed changes when Im using/not using flash while pointing on the same subject as opposed to what the Article said... Ive done it several times and keep on getting a one stop difference between shots with flash and without flash.

    well as youve said, probably the metering changes all the time with the slightest movement. ll just need to check again tonight and make a series of tests.

    Thanks a lot.

  8. #8
    DPP Member darwinandres's Avatar
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    That's why when I'm shooting in complex lighting I use in manual for greater control of my exposure.

 

 

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