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Thread: Exposure triangle ....

  1. #1

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    Exposure triangle ....

    Hi All,

    I came across an image that was taken inside of a restaurant. The photographer positioned the plate and she composed her shot. The floor was a dark carpet. She states that she did minimal PP on the raw photos. When I asked how she achieve a near black floor, she response surprised me.

    I have (I hope) a fairly good understanding of the exposure triangle. When the photographer took a photo of the plate, she boosted her ISO so that she could have a faster shutter (I'm good with that), but to also darken the floor (starting to not understand).

    My thought would be that if you are to increase the ISO, the entire image will be affected by the change. Even if she increases the shutter speed shouldn't her plate be underexposed if her goal was to bring the floor to near black?

    Erik

    P.S. I will need to test this out for myself as well, once I occupy my 5yr ;-)

  2. #2
    herbert's Avatar
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    Re: Exposure triangle ....

    Hi Erik,

    She probably was using flash and simultaneously altering the shutter.

    The note about the high ISO and faster shutter is confusing. If the aperture was the same and no flash was involved then a higher ISO,faster shutter combination will look the same.

    Alex
    Last edited by herbert; 20th December 2012 at 10:02 PM. Reason: Removed errors from first post reply

  3. #3
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    Re: Exposure triangle ....

    Quote Originally Posted by cichlid View Post
    Hi All,

    I have (I hope) a fairly good understanding of the exposure triangle. When the photographer took a photo of the plate, she boosted her ISO so that she could have a faster shutter (I'm good with that), but to also darken the floor (starting to not understand).

    My thought would be that if you are to increase the ISO, the entire image will be affected by the change. Even if she increases the shutter speed shouldn't her plate be underexposed if her goal was to bring the floor to near black?

    Erik

    P.S. I will need to test this out for myself as well, once I occupy my 5yr ;-)
    You are correct. doubling the ISO has exactly the same effect as doubling the time the shutter is open, or opening the lense an additional stop.

    However, it would make sense if there was some processing involved. if the initial exposure was too dark, you could do any of those three things to lighten it, to expose to the right. In postprocessing, you could then pull the shadows down to darken them. I sometimes select nearly black areas and slide the levels to zero to get full black.

  4. #4

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    Re: Exposure triangle ....

    I believe that the preceeding explanations are partially incorrect/misapplied.

    Boosting the ISO and closing the aperture creates the same level of brightness for both objects exposed by ambient light and object exposed by flash. Closing the aperture allows less ambient light, but boosting the ISO amplifies that light so that the resulting brightness is the same...not darker. Only by closing the aperture, leaving ISO alone, and boosting flash power would the ambient portion of the image be darker than the areas illuminated by flash.

    Also, the common guide for flash use is that aperture controls the flash, while shutter controls the ambient. Flash power is calculated by dividing the GN by the aperture to get distance, or by the distance to get aperture. Shutter doesn't factor into it. You can usually adjust your shutter speed without affecting the flash exposure (within reason...as shutter slows and ambient exposure increases, flash exposure starts being affected.)

    When it comes to flash, boosting ISO requires a smaller aperture, regardless of shutter speed. The short flash duration (1/800s to 1/40,000s) means that the flash always delivers all of its light before the shutter closes. That's why shutter doesn't really affect flash exposure.

    In any case, as to the OP's question...I would have to see the actual image to figure out the lighting. Generally, when you need dark areas in your image, increasing ISO is the last thing you do because it amplifies all the light.

  5. #5

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    Re: Exposure triangle ....

    In 'manual' mode boosting the ISO will reduce the flash output when you have your flash-mode set to TTL. This is mostly what darkens the background. A lot of people don't realise that when they leave their flash in TTL-mode, the onboard meter will still compensate exposure even if while most of its hands are tied behind its back.

  6. #6

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    Re: Exposure triangle ....

    Quote Originally Posted by Hero View Post
    In 'manual' mode boosting the ISO will reduce the flash output when you have your flash-mode set to TTL. This is mostly what darkens the background. A lot of people don't realise that when they leave their flash in TTL-mode, the onboard meter will still compensate exposure even if while most of its hands are tied behind its back.
    I just tried that and it didn't work (which is what I was expecting.)

    When you increase ISO, you amplify all light, regardless of whether it comes from the flash or ambient. Yes, when ISO is increased then TTL mode will reduce flash power to maintain appropriate exposure on the subject, even in manual exposure. But areas of your photograph that are only illuminated by ambient light will actually be brighter if you do nothing but increase ISO.

  7. #7

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    Re: Exposure triangle ....

    Quote Originally Posted by herbert View Post
    When using flash, the aperture controls the ambient light.
    Um, no.

    Aperture affects ALL light sources. Assuming not in a HSS mode then only shutterspeed affects / controls ambient light to flash ratio.

  8. #8

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    Re: Exposure triangle ....

    Just briefly ...

    If you want to darken a background then in essence what you're wanting is minimal ambient light and thus a foreground lit mostly with flash.

    The flash will have minimum impact on the background because it's further away.

    Other than that, you simply want to get the highest flash to ambient ratio that you can and the only way to do that is via shutterspeed; changing the aperture or the ISO keep the ratio of the two light sources the same (although one may well want to vary the ISO and aperture for other reasons such as DoF).

  9. #9

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    Re: Exposure triangle ....

    I've said it before and I will say it again, CiC is the best forum I have ever been a part of.

    Thanks all for your comments and help. It has also helped me to better reverse engineer another photo I have witnessed being made. Though, the photographer refused to respond to my metering question.

    I look even more forward to trying to recreate the original image seen. For sure I will post here.

    Thanks again everyone.

    Erik

  10. #10
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    Re: Exposure triangle ....

    Quote Originally Posted by cichlid View Post
    I've said it before and I will say it again, CiC is the best forum I have ever been a part of.

    I TOTALLY AGREE

    Though, the photographer refused to respond to my metering question.

    Perhaps the photographer knew what to do and how to do it but, not why he did what he did

    Then there is also a possibility that he could be suffering from what clinical pyschologists describe as "Being a big jerk"

    I look even more forward to trying to recreate the original image seen. For sure I will post here.

    Recreating images is a great way to learn the capabilities of your gear...

    Thanks again everyone.

    Erik

  11. #11

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    Re: Exposure triangle ....

    So, I got my hands dirty and child to bed (hmm, was actually the other way around).
    Here is the original photo that I was referring to:
    Exposure triangle ....

    When I attempted to recreate this photo, I was disappointing with my results. I did have a couple lazy oversights though. I didn't use the same lens and ISO. When I was reviewing the photos on the camera, the floor is very clearly visible. Import into LR4 the floor is gone. This is when I remembered the preview on the camera is a processed jpeg. Therefore, I actually did achieve my goal. Ordinarily I would tweak the images for better clarity. Below are two of my resulting images. (Flash comp. was at -3.0)
    [IMG]Exposure triangle ....[/IMG]
    [IMG]Exposure triangle ....[/IMG]

    This opens a whole new world. As Richard said: "
    Recreating images is a great way to learn the capabilities of your gear..."

  12. #12

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    Re: Exposure triangle ....

    Quote Originally Posted by cichlid View Post
    Here is the original photo that I was referring to:
    That doesn't look appetizing at all...anyways...

    Since the shadow of the plate is clearly visible, the light was not on the camera. Based on the position of the light's reflection, it looks like the light was in front and to the right on the photographer. As the floor is being illuminated by ambient light only, we're definitely talking about controlling the ambient areas of a flash exposure.

    If the photographer was trying to darken the floor, then that definitely would not be accomplished by increasing ISO. That would only be accomplished by increasing shutter speed or by using a more narrow aperture. I'm convinced that there was a communication failure between you and the photographer, and that the photographer did not fully or correctly explain all the aspects of the shot.

    I've attached an image I shot to demonstrate. The black area behind the statue consists of a white door and blue walls, about 6 ft back. That area had the same illumination as typical indoor lighting...I didn't do anything to the ambient lighting to make the background black.

    I made the background black by shooting at a shutter speed of 1/1000s. I have a Nikon D90 and a Nikon SB-600 flash. I can control the flash wirelessly, and I'm able to use a feature called High Speed Sync even when the flash is off-camera. So the flash illuminated the statue from the side, and anything illuminated by ambient light was completely suppressed.

    Regardless of what the photographer told you, the answer to your original question about the background is this...in flash photography, areas illuminated solely by ambient light can be darkened by using faster shutter speeds and/or narrow apertures. If you use a faster shutter speed, your flash power is usually not affected. If you use a more narrow aperture, your flash power, and your depth of field, will be affected. With a more narrow aperture, you will need to increase the flash power to maintain the exposure of the subject, and your depth of field will increase.
    Attached Images Attached Images

  13. #13
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    Re: Exposure triangle ....

    Quote Originally Posted by Colin Southern View Post
    Um, no.

    Aperture affects ALL light sources. Assuming not in a HSS mode then only shutterspeed affects / controls ambient light to flash ratio.
    My mistake. Not enough caffeine that morning.

    This is the way to remember it:

    SAAF = Shutter Ambient, Aperture Flash

    The shutter controls the ambient light. The aperture controls the flash, although only when the flash is at full power:

    http://www.peachpit.com/articles/art...81032&seqNum=3

    Alex

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    Re: Exposure triangle ....

    Quote Originally Posted by herbert View Post
    The shutter controls the ambient light. The aperture controls the flash, although only when the flash is at full power:
    Um, not really either I'm afraid ... well kinda, but only under specific circumstances ...

    1. You need to be under X-Sync speed. Above X-Sync the flash behaves like a constant light source and at that point only moving the flash or altering it's power will make any difference because it's an all in all drag race between the ambient and the flash.

    2. The flash doesn't need to be at full power - in fact, if the resulting exposure is to be correct - albeit with more flash and less ambient - then the flash needs to be capable of going brighter if increased flash component is desired. Works best in ETTL mode.

    To be honest - as much as I respect Syl, I really don't think he did the world any favours with his SAAF mnemonic. Easiest way I find to get the result I want is to just get the ambient where I want it with the flash turned off, and then use FEC from there.

    All SAAF is really saying is "aperture and shutterspeed BOTH control ambient light, but flash is unaffected by shutterspeed below x-sync speed" - so:

    1. Shutterspeed adjustment gets the ambient where it's wanted (from a given reference point)

    2. Aperture fixes up the change from the reference shot that changing the shutterspeed stuffed up, and

    3. The flash then has to "pick up the tab" on the amount of power required due to the changed aperture.

  15. #15
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    Re: Exposure triangle ....

    Thanks Colin. A better explanation than the SAAF mnemonic. Simple and easy to follow.

    Alex

  16. #16
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    Re: Exposure triangle ....

    A factor that has not been mentioned is the ratio of the flash to plate distance versus the flash to floor distance. The closer to the plate the flash is placed the darker the floor will be provided the exposure on the plate is consistent. It will be dependant on the type of flash head, it's beam focus and and any diffuser used.


    Oops - I just noticed that Colin did refer to flash distance but he did not elaborate on it.
    Last edited by pnodrog; 28th December 2012 at 07:12 PM.

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