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Thread: Underexposed when using ext flash and zoomed in

  1. #1

    Question Underexposed when using ext flash and zoomed in

    I was taking photos for a friend. I find that when I zoom in, I get a underexposed picture. But when I use a shorter focal length, the picture is brighter.

    Both photos were taken at ISO400, 1/40s, f5.6. The only difference is degree of zoom.
    The darker picture is taken using a focal length of 135mm (202mm for 35mm equiv), the brighter is at 85mm (127mm for 35mm equiv).

    Am I missing anything? Thanks.

  2. #2

    Default Re: Underexposed when using ext flash and zoomed in

    Is your subject dressed in white in a darkened room?

  3. #3
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    Default Re: Underexposed when using ext flash and zoomed in

    Light skin tone and light coloured clothings may fool the camera metering to underexpose.
    Canon 80D|Panasonic LX3/LX5
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  4. #4

    Default Re: Underexposed when using ext flash and zoomed in

    It's a wedding. Bride is wearing light blue. Groom is black suit. I focus on their faces using spot metering. I would say the ambient light is adequate

  5. #5

    Default Re: Underexposed when using ext flash and zoomed in

    Quote Originally Posted by sentinel_006 View Post
    I was taking photos for a friend. I find that when I zoom in, I get a underexposed picture. But when I use a shorter focal length, the picture is brighter.

    Both photos were taken at ISO400, 1/40s, f5.6. The only difference is degree of zoom.
    The darker picture is taken using a focal length of 135mm (202mm for 35mm equiv), the brighter is at 85mm (127mm for 35mm equiv).

    Am I missing anything? Thanks.
    Is it because of the range of the flash? I think most flash can reach only about 120mm at most unless a hammer head flash is used?

  6. #6
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    Default Re: Underexposed when using ext flash and zoomed in

    Quote Originally Posted by elcap View Post
    Is it because of the range of the flash? I think most flash can reach only about 120mm at most unless a hammer head flash is used?
    Really?? I would have thought that most external flash have at least a GN of 40. Meaning it should have a reach of 40m when you zoom in.
    Canon 80D|Panasonic LX3/LX5
    35f2 IS|50f1.8|85f1.8|12-24f4|18-135f4-5.6 IS|28-75f2.8

  7. #7
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    Default Re: Underexposed when using ext flash and zoomed in

    Quote Originally Posted by sentinel_006 View Post
    I was taking photos for a friend. I find that when I zoom in, I get a underexposed picture. But when I use a shorter focal length, the picture is brighter.

    Both photos were taken at ISO400, 1/40s, f5.6. The only difference is degree of zoom.
    The darker picture is taken using a focal length of 135mm (202mm for 35mm equiv), the brighter is at 85mm (127mm for 35mm equiv).

    Am I missing anything? Thanks.

    Does your zoom lens has constant or variable aperture? If it is variable, then the effective aperture for the same aperture setting will be smaller when you zoom in.

    What flash mode did you use? TTL, auto or manual? Unless it is TTL you would have to take into account the reduction in effective aperture when you zoom in.

    Is the subject to camera distance same or different in both cases? If you had to zoom in because the subject is further then it would mean that you might have reached the flash's power limits.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  8. #8

    Default Re: Underexposed when using ext flash and zoomed in

    Quote Originally Posted by roygoh View Post
    Does your zoom lens has constant or variable aperture? If it is variable, then the effective aperture for the same aperture setting will be smaller when you zoom in.

    What flash mode did you use? TTL, auto or manual? Unless it is TTL you would have to take into account the reduction in effective aperture when you zoom in.

    Is the subject to camera distance same or different in both cases? If you had to zoom in because the subject is further then it would mean that you might have reached the flash's power limits.
    When I zoomed in, the aperture is still at set to f5.6. Flash is a Nikon SB-800 in TTL mode. The camera-to-subject distance is the same in both cases.
    Is there a thing such as effective aperture?
    Lens is Nikon Kit lens with the D80, 18 - 135 mm (F3.5 - 5.6)

    1st image:


    2nd (zoomed) image:


    Although both are underexposed, does the 2nd one seeme more underexposed?

  9. #9
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    Default Re: Underexposed when using ext flash and zoomed in

    not too much different. were the 2 images cropped and did you fire straight on? u might be too far away from stage and hence both pics are under. you could adjust the curves/levels to brigthen the pics. results are pretty good when I tried.
    Last edited by mpenza; 17th October 2006 at 12:10 PM.

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    Default Re: Underexposed when using ext flash and zoomed in

    Did you use built-in diffuser on SB-800 (or diffused dome)?

    What is your aperture used on both photos (lazy to check the exif)?

    Did your flash beep 3 times when you use it (not enough power to brighten the subject)?

    Regards,
    Arto.

  11. #11

    Default Re: Underexposed when using ext flash and zoomed in

    Quote Originally Posted by Artosoft View Post
    Did you use built-in diffuser on SB-800 (or diffused dome)?

    What is your aperture used on both photos (lazy to check the exif)?

    Did your flash beep 3 times when you use it (not enough power to brighten the subject)?

    Regards,
    Arto.
    I used the supplied diffuser dome with the SB-800. Both pics apertures are the same - f5.6. I guess next time I gotta to move closer.

  12. #12

    Default Re: Underexposed when using ext flash and zoomed in

    Quote Originally Posted by sentinel_006 View Post
    I used the supplied diffuser dome with the SB-800. Both pics apertures are the same - f5.6. I guess next time I gotta to move closer.
    with supplied diffuser, zoom is set at 14mm...cannot zoom out...so flash power is lower and diffused...

  13. #13
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    Default Re: Underexposed when using ext flash and zoomed in

    Quote Originally Posted by sentinel_006 View Post
    I used the supplied diffuser dome with the SB-800. Both pics apertures are the same - f5.6. I guess next time I gotta to move closer.
    That's the culprit .

    Use no diffuser (built-in or dome diffuser) if range is more than 3m. Or pump up ASA. Lens' Aperture, I think you can't open more (with kit lens). Move closer to subject.

    As theRBK mention above, diffuser dome will force flash coverage to 14mm (unless you do modification on flash), which is too bad decrease the flash GN (thus the range also decreased).

    Also, if both photos is taken in short time, the second photo may underexposure since the flash's capacitor may not full yet (but you still can take a picture with flash).

    Regards,
    Arto.
    Last edited by Artosoft; 17th October 2006 at 04:14 PM.

  14. #14

    Default Re: Underexposed when using ext flash and zoomed in

    Quote Originally Posted by Artosoft View Post
    diffuser dome will force flash coverage to 14mm (unless you do modification on flash), which is too bad decrease the flash GN (thus the range also decreased).
    actually, modification on the diffuser...cut a slit out so that the sensor on the flash body is not forced in when diffuser is installed...

  15. #15
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    Default Re: Underexposed when using ext flash and zoomed in

    Quote Originally Posted by theRBK View Post
    actually, modification on the diffuser...cut a slit out so that the sensor on the flash body is not forced in when diffuser is installed...
    Thanks for correction .

    Yes, the modification is on the diffuser. But don't hope any miracle. It is only improve GN slightly. Without diffuser is still the best GN power.

    Regards,
    Arto.

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