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Trev: About your answer in the Metering manual flash with softbox

StephenStephen Member
edited January 2012 in flash & lighting
Hi Trev,
You recently replied to somebody's question in this blog post:
http://neilvn.com/tangents/2009/05/18/metering-manual-flash-softbox/

I don't understand your answer.

If I understand Neil's post...
Neil meters the ambient light (no flash used). The meter reads 1/200, f4, ISO 200.
Neil underexposes the ambient by one stop. So the settings become 1/200, f5.6, ISO 200.
Neil turns on the flash and sets it to manual. He dials the flash power until the flash meter reads f5.6.

Is that correct? Thanks.

Comments

  • TrevTrev Moderator
    edited January 2012
    Stephen,

    Yep, correct, perfect.

    You have to have the flash light up the subject you are shooting at the same power [f5.6] because the camera's aperture is f5.6, otherwise the lit subject will be still under-exposed.

    As I explained in that post, just forget for the moment that 'flash' was used, let's pretend that the overall scene is metered at f4.0 like Neil said, but suddenly, a bright beam of sunlight falls directly on the subject, do you still meter at f4.0, no, not if you want the subject to be correct, and if by some chance of fate, that sunlight happens to measure at f5.6, then that's what you set the camera's aperture to.

    Still in the 'sunlight' scenario, we have the camera set to f5.6, but then the sunlight goes away, and since the scene really needs f4.0 to be exposed properly, but, Neil likes the effect that was created, a stand-out subject from the 1 stop under-exposed background. What do you do? Add artificial lighting, flash.

    Flash is now introduced, therefore it now must meet the same exposure value of the 'sunlight' that the camera was set at, f5.6, forget about the rest of the image, the ambient will 'fall' away from the spot of light the flash lights up at the same exposure value of f5.6, exactly like the beam of sunlight did.

    I know it sounds simplistic, and that's just the point, it really is that simple. :)

    The power needed from the flash must, always, meet the camera's aperture setting, does not matter whatsoever of the ISO, Shutter the camera is set at, those values were there for the ambient, and if the flash is set in manual it has to power enough light to meet the camera's aperture.

    Obviously different ISO/Shutter/Aperture readings can be used to get to that -1 stop under-exposure Neil wanted, but the method of lighting remains the same, flash must meet the aperture value the camera is set to.

    Easy enough to test yourself of course.

    Edit: When I say the power of the flash must, always, meet the aperture value, I naturally am talking about dominant flash here, NOT fill flash, in case someone queries this, as obviously fill flash can be up to -5 stops under [Nikon] or -3 Canon to just tweak shadows on face, etc. It's meant for the scenario that was outlined in the original Tangents post, background under, subject lit to stand out.

    Trev.
  • Trev,
    Thanks for confirming my understanding of your answer in the post. I haven't done any manual flash yet, but I am considering getting a Sekonic lightmeter this year to practice some manual flash experiments.

    However, this leads to another question.

    Neil said that manual flash is controlled by:
    - aperture
    - ISO
    - distance from the light source to subject
    - and flash power setting

    But you said in the previous post that the flash power is tied to the camera's aperture.

    If the ambient exposure was 1/200s, f4, ISO 200, could you get 1-stop underexposure by doing 1/200s, f4, ISO 100?

    If so, suppose I have 1/200s, f4, ISO 100 as a 1-stop underexposed image, but I don't want to change the light source's distance to subject. That leaves flash power setting. Does that mean I can't use a light meter, and I just have to adjust the power myself until I think the camera histogram is acceptable? I have never used a lightmeter before, but your statements lead me to believe that lightmeters give you an aperture value (i.e. you dial in an ISO value into the lightmeter and fire a test shot. The lightmeter gives you an aperture value to tell you what your camera should be set at.)

    I will have to test all of this when I get a lightmeter.
  • TrevTrev Moderator
    Stephen,

    Yes, you will get -1 stop under with ISO down to 100 from 200, but, you would still need to have your flash to meter at f4 now, since same thing is required, the flash power needed to expose correctly, on the subject, is now f4.

    All four items mentioned help determine the ambient PLUS flash power needed, when flash is the *dominant* light source.

    Flash is controlled by all four in respect to how much power is needed and these are the key words here: 'power needed'.

    Actually, you really don't 'see' the first 3 items, as they are working behind the scene so to speak, and they have already influenced your flash's power without you doing anything, so the last thing is just setting that 'power needed'.

    It's the 'how' part that relates exclusively to the last item. [Hence my previous post] Power Needed. In this case the power needed is to match the camera's aperture to get correct exposure, on the subject, regardless of the background/ambient.

    How to: . . . get that 'power needed' all you do is meter the flash, at the subject you are lighting, adjusting the flash power until it matches the aperture of the camera. Simple, as I said previously, nothing mystical with that, it really is that simple. Power Needed.

    When you use a light meter measuring right from the subject, and you have already determined your camera aperture at f5.6, and flash power is reading f4 or f8, that will mean the actual flash power is going to be -1 stop under-exposed or -1 stop over-exposed in relation to camera's aperture of f5.6.

    If you cannot get enough power to correctly expose the subject, you then need to re-evaluate the first 3 criteria, Aperture, ISO, Distance, and re-configure all, 2 or just move flash closer if able. Of course the first two will affect your ambient also [shutter speed is a given for ambient].
    Stephen said: If so, suppose I have 1/200s, f4, ISO 100 as a 1-stop underexposed image, but I don't want to change the light source's distance to subject. That leaves flash power setting. Does that mean I can't use a light meter, and I just have to adjust the power myself until I think the camera histogram is acceptable? I have never used a lightmeter before, but your statements lead me to believe that lightmeters give you an aperture value (i.e. you dial in an ISO value into the lightmeter and fire a test shot. The lightmeter gives you an aperture value to tell you what your camera should be set at.)
    Of course you can use a Lightmeter [flash in manual, cannot measure in TTL].

    Yep, if you don't want to change distance, you need to increase flash power, nothing else. Unless your flash is already then at full power, no room to move, you then need to re-evaluate as I said above.

    Stephen, if you don't have a light meter, then you need to take a test shot, zoomed in [or move closer making sure you don't block flash light] to just cover the subject, then evaluate the LCD/Histogram. Then recompose, remember camera and flash are set manually so composition, zoom, does not alter the background/subject exposure.

    Yes, you are correct with how lightmeters work [in flash mode].

    Trev. [sorry for long post]
  • Hi Trev,
    I appreciate the information you are providing.

    I looked over my SB-900 manual, and you cannot directly dial an aperture number into it. There is a section on using a formula and guide numbers to get the flash output to where you want, but it's over my head. I need to get a lightmeter or just use the histogram method and get some practice in.

    I'll be back in the future with further questions. :-)

    Thanks again.
  • TrevTrev Moderator
    Stephen,

    You don't dial an aperture into the flash, that's not it mate. You can't dial aperture or ISO except the power. Even though you can see an f-stop, and a range, those are merely telling you what the camera has been set to and indicates how much range it can reach, if it's attached to body or I presume in slave mode.

    You can use those as a guide, but, I thought we were talking about off camera flash, if you have on camera flash, simply dial in the camera's settings first, then you will see that the back of the flash will indicate 2 things, the fstop number and a range of numbers, set your flash to manual, and then adjust the flash power, until the numbers correspond roughly to the distance you are standing away from subject. Take shot, adjust until happy with result.

    If you are using an off camera flash on stand, and it's set to slave you can do similar with it. Or just keep adjusting power on it until you are happy with the shot on back of camera LCD/and or histogram.

    Trev
  • Trev,
    We are talking about off-camera flash. I switched my SB-900 to manual flash, and I see a power setting from 1/1 to 1/128. I didn't realize the manual was talking about on-camera manual flash. Sorry for the confusion.

    I'm interested in off-camera manual flash, so I will specify that in future questions.

    Thanks for helping me with this. I am sorry for the bother.
  • TrevTrev Moderator
    Stephen,

    I have to ask. Since you are shooting like I see you are by your website, you are already obtaining those types of images, with main part lit perfectly, and fading back in background ambient.

    They are really nice images, nothing to worry about there, keep shooting like the way you are. Great exposure, color, oomph! I personally like them.

    Trev.
  • StephenStephen Member
    edited February 2012
    Trev and Lou,
    Thanks for the compliments! All of my work you see on my photography website is done in TTL flash with one main light (and rarely a second light).


    Trev,
    I got interested in manual flash, because there a few situations where I felt that maybe manual flash would be better. In those situations, I would have to mess with the FEC a lot. For example, these photos of mine:

    http://www.flickr.com/photos/tangst/5709290258/
    http://www.flickr.com/photos/tangst/5318577311/

    In both cases, the ambient light level was very low, reaching almost total darkness. I had to attach a SB-900 on my PocketWizard FlexTT5 to use the AF-Assist light to acquire focus. I shot that with TTL flash. In the second photo, I had to modify the FEC a few times to get the photo into a tonal range that I could accept.

    So, in those cases, I was wondering if I could have shot that in manual flash using a lightmeter. That is why I brought this thread up in the first place.

  • just my two cents guys, however, I am seeing the practicality of TTL flash mode if you are in events who does not have time for taking light meter values for example wedding receptions, engagement dinner, etc however, if doing portraiture in or out of studio using a light meter to have a starting point is helpful if not ideal. IMO. As with Stephen's very nice photos, if its staged then you have an option and the luxury to use Manual flash to get your desired exposure. And yes using a light meter in that situation may shorten your value adjustments.
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