Flash compensation, 1st/2nd curtain, etc.

Discussion in 'Beyond the Basics' started by battletone, Nov 28, 2009.

  1. battletone

    battletone TPF Noob!

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    So I have reached the point of wanting to learn to use my flash in a manner where I am in control and so I know what it is going to produce.

    I have been reading up on flash compensation and it seems that it is generally something to enhance or reduce the flash power, but from everything I have read it sounds like more of an after the fact adjustment for a follow up shot. Is this correct?
    In general I have felt my flash is too much rather than not enough, so I am going to shoot a bit with it at -2. But it doesn't seem that -1 or -2 is quite the same amount of exposure variation as actually exposing by a stop or two. Are these also considered stops?

    Next is 2nd curtain. First thing I noticed is at slow shutter speeds there are two flashes. Are there two at faster speeds also?

    For my last question right now, I don't have a speedlite, but do they allow you to set your flash for the end of the exposure?
     
    Last edited: Nov 28, 2009
  2. Garbz

    Garbz No longer a newbie, moving up!

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    Yes they are considered stops, but remember the flash is separate from the rest of the exposure. While the flash may fire two stops darker it means it will be more balanced by ambient light so it won't actually look 2 stops darker necessarily (unless your exposure is otherwise completely black).

    There is only one flash for the exposure in rear curtain. This is what you would see if you shot in manual. What you are actually seeing first is the TTL preflash which meters the scene to determine it's final output. For front curtain flash or for short shutter speeds the TTL preflash is instant before the exposure and main flash. Many people don't notice that. I blink rather hard from that preflash so it's hard to get a shot of me with my eyes open because of it. But in essence unless you're shooting manual your flash always fires twice.

    When the speedlight is mounted on the camera it still takes instructions from the camera. All your exposure compensation and flash settings (redlight included) can still be camera controlled.
     
  3. battletone

    battletone TPF Noob!

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    This isn't making sense to me.
    With 1st curtain, it fires at the beginning, but not the end. With second curtain it fires at the beginning and the end. But why does it need to meter the scene in 2nd curtain prior to opening the shutter but not in 1st curtain?
    I am not referring to the metering/focus flash from a half press of the shutter button. It does that on BOTH modes, but still fires a flash at the start of 2nd curtain, for as you say, a meter reading.
    I also noticed that in the dark, with 2nd curtain, after the metering/focus flash, I have to shoot almost immediately or else it will not allow me to take the shot.
     
    Last edited: Nov 28, 2009
  4. Garbz

    Garbz No longer a newbie, moving up!

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    No not beginning and end, before and beginning. The TTL Preflash is not part of the exposure. It also happens in front curtain exposures, just so quickly you don't see it.

    So in high speed this is what happen:
    Front Curtain:
    1. TTL Preflash fires, camera analyses returning light.
    2. Front curtain opens.
    3. Flash fires once front curtain is fully open.
    4. Exposing.
    5. Rear curtain closes.

    Rear Curtain:
    1. TTL Preflash, cameras meters returning light.
    2. Front curtain opens.
    3. Exposing.
    4. Flash fires just before rear curtain begins to close.
    5. Rear curtain closes.

    I'm not sure what you mean about the waiting thing? What do you mean you need to shoot immedaitely?
     
  5. KmH

    KmH Helping photographers learn to fish Supporting Member

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    I think he may be seeing the AF assist illumination? I don't think there is a 'focus flash'.
     
  6. battletone

    battletone TPF Noob!

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    Okay thanks a lot. This is making a lot more sense now. Though it brings me to one more question...

    How is the 1st curtain process different with red eye reduction. Now that you typed it out in steps, I always though something like that was happening to constrict the pupil prior to the shot.

    (this seems to be in 2nd curtain)
    If I half press the shutter button, and it pre-fires a flash out to focus, it seems that if I wait more than a second to take the shot, it won't let me. Maybe it thinks it needs to refocus or something. I will have to play with that more.
     
    Last edited: Nov 29, 2009
  7. Plato

    Plato TPF Noob!

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    1st/2nd curtain is totally unrelated to red-eye. Some cameras have a setting that fires the flash before the first curtain to cause the subject's pupil to close down.

    There is no such thing as a "pre-flash" for focus purposes. The focus assist lamp has a relatively lengthy duration. It MIGHT assist with redeye but I doubt it. In my case, with the camera, it's not bright enough. When I use my flash, the focus-assist lamp is IR and that doesn't close down the pupils.
     
  8. battletone

    battletone TPF Noob!

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    That wasn't what I meant. I was referring to this description of 1st curtain:
    Front Curtain:
    1. TTL Preflash fires, camera analyses returning light.
    2. Front curtain opens.

    3. Flash fires once front curtain is fully open.
    4. Exposing.
    5. Rear curtain closes.

    I was asking how this differs from the cameras red eye function where I am under the impression it fires a flash prior to opening the shutter to constrict the pupil.

    I think we are talking about different things here. I am referring to pressing half way on the shutter button and it firing a couple busts of flash to obtain focus. Once focused, you can press the shutter button to take the shot. I was not saying that has anything to do with redeye. But I am unaware of what this IR focus assist is. I will have to look that up. I just know in really dark areas the flash will fire off to help it focus, because the flash doesn't fire off in MF.
     
  9. Derrel

    Derrel Mr. Rain Cloud

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    Battleone,
    THose "flash bursts to obtain focus" you speak of--that probably is a tip-off that your camera uses a white light AF assist system, not an infrared assist system. Does your camera perhaps have a small, round, white lamp on the front side of the camera body, right beside the lens mount? The earlier Nikon and FujiFilm d-slr bodies which were based on the Nikon N80 film body used that type of AF assist system. The fact that it appears the flash is firing to help obtain focus makes me feel pretty sure that your particular system is using a white light AF assist system, which is somewhat obnoxious, but is very effective--on many types of surfaces, like animal fur, fur coats, and many synthetic fabrics, IR assist systems flat-out fail, whereas the white light system is almost fool-proof.
     
  10. battletone

    battletone TPF Noob!

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    Its an XSI. I do have that round white bulb looking thing, but I just put the camera out in front of me with the lens cap on, popped up the flash, and tried to focus. I got hit with bursts from the pop up flash, not that other thing. I put the flash down and tried again, but this time nothing from either.
     
  11. Derrel

    Derrel Mr. Rain Cloud

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    Yes, the Canon XSi has a white-light AF assist system and it uses the flash to provide the AF assist bursts. I just confirmed that with dcreview's XSi review. Knowing the exact camera model one has is very helpful in these types of troubleshooting scenarios; I assumed you had an older d-slr.

    Although a lot of people complain about white-light AF assist, I've found that it is very effective. People who do nature photography at night often have problems with the IR systems and animal fur not working so well with the IR beams that are less-visible, but which actually don't work quite as well.
     
  12. battletone

    battletone TPF Noob!

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    Thanks. I see there is a Custom Function for turning it on/off/external flash only.

    After reading the manual...
    The round white lamp off to the side of my lens mount is a redeye reduction lamp. Its a yellow color when it lights up. It doesn't appear very bright but it says people need to look at it for it to be most effective, also to wait for the LED in the view finder to count down. I guess I was wrong on a flash to constrict the pupil. Don't know where I got that from.

    A side not, it also is the remote timer light. It rung a bell once I saw the white looking lamp turn yellow.
     
    Last edited: Nov 29, 2009

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