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Thread: Understanding flash, aperture and shutter speed

  1. #1

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    Understanding flash, aperture and shutter speed

    Hi everyone...new on here so forgive the naivity....

    Could anybody please explain:

    a) is shutter speed important when using a flash? If so in what way?
    b) why, given that my built in flash always sets the correct exposure, do I get very different results depending on the aperture I set (under/over exposed)?

    Many thanks in anticipation

    Adrian

  2. #2
    herbert's Avatar
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    Re: Understanding flash, aperture and shutter speed

    Hi Adrian,

    There is an upper limit on shutter speed when using flash. This is known as the X-sync speed. This is because at fast shutter speeds the camera only has part of the sensor exposed to the light. A focal plane camera has two shutter curtains. One opens and exposes the sensor, the second curtain closes and covers the sensor. These only move at one speed. So to get a very fast shutter speed the camera opens one and closes the other just after it, sweeping a narrow slit across the sensor. If you use flash in this situation you will get a photo that looks black except for a slit that was exposed.

    There is a way around this problem where the flash can pulse at a low power level continuously. This allows a faster shutter speed but since most of the flash light is wasted shining on the shutter curtains the effect is that the flash does not seem as bright.

    The sync speed on most cameras will be 1/200 or 1/250 of a second. That is your upper limit.

    As for your camera setting the exposure it will work for foreground subjects. The camera will see the amount of light in the scene and know what exposure to use. It may fire a pre-flash to light up the scene to help it set the exposure. The flash will bounce off the subject and so that will be correctly exposed. However the background is so far away that the flash does not effect it. So the background will not be as bright since it is only lit by the natural incident light.

    As you change the aperture the flash should be able to compensate and make the subject look the same. However as the aperture gets smaller (higher f number) the shutter speed will get higher and there will be less incident light. So the background will look darker.

    In summary:

    You can use flash up to 1/250 s shutter
    As you increase shutter speed the background will look darker.

    Regards,

    Alex

  3. #3

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    Re: Understanding flash, aperture and shutter speed

    Just a couple of extra points, Adrian.

    Some external flash units have a high speed flash option which enables you to use a speed which is higher than 1/250.

    When using flash on aperture priority setting (Av) exposures can be longer, as Alex mentioned, so you need to be careful to avoid camera shake problems if these get slower than say 1/60 for example.

    Personally, I find that using shutter priority (Tv) can also have problems as your camera usually tries to set the aperture wide open which may not suit the scene in question. The same applies to the auto or semi auto setting (P).

    When any of this looks like being a problem I prefer to set my camera manually to choose a suitable shutter speed (not above 1/250 unless using high speed flash) and a suitable aperture. Then choose an ISO which fits with the previous settings, or use Auto ISO, and adjust the flash output compensation as required.

    But this method does require a bit of experimentation to get everything synchronised together and may need readjustment if the conditions change.

  4. #4

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    Re: Understanding flash, aperture and shutter speed

    Wow guys, thank you, very detailed answers. If I'm honest whilst they've helped a bit with my a) question they don't seem to address my b) which is really confusing me at the moment. Cheers again

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    Re: Understanding flash, aperture and shutter speed

    What are your exact settings when shooting in Av mode? This mode can give varied results depending on the exact amount of light which you are metering, and your ISO etc.

    I think Alex's comments here refer to the chief problems

    As you change the aperture the flash should be able to compensate and make the subject look the same. However as the aperture gets smaller (higher f number) the shutter speed will get higher and there will be less incident light. So the background will look darker.

    Both the foreground and background areas affect the overall exposure particularly if using a general exposure mode, like Evaluative.

    It is possible to shoot the exact same scene, with or without flash, and get different results dependent on changes in the ambient lighting or the exact area from which you have taken your exposure readings.

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    Re: Understanding flash, aperture and shutter speed

    Oh dear me. I typed a reply then went to fetch a link and what I already typed has disappeared. Does anybody else ever have problems like this?

    Anyway, have you read this CinC Tutorial?

    http://www.cambridgeincolour.com/tut...mera-flash.htm

    Now my previous typed reply has appeared.

  7. #7
    shreds's Avatar
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    Re: Understanding flash, aperture and shutter speed

    Quote Originally Posted by Geoff F View Post
    Oh dear me. I typed a reply then went to fetch a link and what I already typed has disappeared. Does anybody else ever have problems like this?
    Now my previous typed reply has appeared.
    Geoff,

    Yes occasionally, on long replies. Probably better to compose off line and cut and paste the reply when you are ready in such cases.

    I think it is something Sean can influence, with the way the site is configured, although presumably it is at an optimal setting, but I prefer to be safe having lost replies in the past.

  8. #8
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    Re: Understanding flash, aperture and shutter speed

    Quote Originally Posted by northlondon43 View Post
    Hi everyone...new on here so forgive the naivity....
    Could anybody please explain:
    a) is shutter speed important when using a flash? If so in what way?
    b) why, given that my built in flash always sets the correct exposure, do I get very different results depending on the aperture I set (under/over exposed)?
    In short, shutter speed is important as it controls the amount of ambient light
    hitting the sensor. Not sure what camera body you have, but most DSLR's have
    a max. shutter speed of about 1/200th when using flash (either the built-in or
    off-camera speedlight).

    Depending on what power setting you have it at, a flash fires in as fast as
    1/20,000th of a second...so your shutter speed (so long as it's under 1/200)
    will never have an effect on the amount of light from the FLASH, but it will
    affect the amount of ambient light that enters the shutter.

    The answer to your second question is that when you stop down the aperture
    you are reducing the size of the opening that the light comes through, and that
    will have an effect on your image.

    Just remember that your shutter controls ambient light, and aperture controls
    how much light from the flash passes through the shutter.


    I would suggest reading the tutorials on flash usage that are here on CiC, and
    also this video on how aperture controls flash:

    http://www.youtube.com/watch?v=lxmGg5GuDNc

    There are tons of YouTube video's that can explain almost every feature
    of your camera, so be sure to use that as an additional resource.

    Just keep practicing and asking questions...soon enough you'll be nailing the
    exposure just right...

    Mike
    Last edited by Dizzy; 26th December 2011 at 05:30 AM.

  9. #9

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    Re: Understanding flash, aperture and shutter speed

    Once again thanks to everyone for taking the time to try and answer and my questions, much appreciated - especially your last one there Mike as that was succinct yet very clear! :-)
    However, it still leaves me wondering why the EV is automatically going to 0 (correct exposure) no matter whether I adjust the aperture or shutter speed, yet I get very different results (obviously). I therefore seem to have no way of telling whether I actually DO have the right exposure. P.S. the same think happens if I connect the bespoke flash gun unit.

  10. #10
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    Re: Understanding flash, aperture and shutter speed

    Hi Adrain,

    What camera model do you have? I can only comment on how Canon cameras work since that is what is use. With a Canon camera in aperture or shutter priority mode the camera will meter as if there is no flash. With no flash as you change the aperture the overall exposure should be the same, but the depth of field will change.

    However if you have a flash on you will get different results when the aperture/shutter speed changes since your ratio of flash to ambient light will change. This contrast is most marked when the distance to the subject is low. Remember that flash controls the fill light on the subject, ambient controls the background. So the photo will look different since the amount of light on the subject will vary.

    Fill flash looks more natural with a -1 to -2 flash exposure compensation. In this case your subject will not look too bright.

    You can always check the histogram after the shot to make sure that the exposure is where you want it.

    Alex

  11. #11

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    Re: Understanding flash, aperture and shutter speed

    Hi Alex

    I have a Samsung NX 11 with a built in flash plus a Samsung SEF42A external A TTL flash. Both have an auto flash metering control. I use my camera in manual mode with the flash on A TTL (NOT manual) mode. The external flash manual clearly states that whilst I can set aperture and shutter speed in manual mode the amount of flash light is automatic (set by the camera/flash).

    I've read the tutorials on flash on this site plus gone onto You TUbe....

    Forgive me but I am still no nearer to an answer to my b) question in my original post. Yes I understand that aperture affects the amount of flash light going to the sensor and yes I understand that at the shutter speeds I am using (and the night time indoor scenes I taking pics of) there is very little, if any, effect of the latter on the exposure. But why is my internal light meter/EV showing as 0 no matter what combination of aperture/shutter setting in manual mode I use given the same focal length, distance to subject etc??? Surely, for any given aperture setting the camera/flash changes the flash intensity automatically (on the A TTL mode) so I get the correct exposure? However, this is not happening....to re iterate I always get a 'correct' exposure showing on my internal light/EV meter but I get different results depending on the aperture. Sorry but this is very very frustrating.
    Last edited by northlondon43; 25th December 2011 at 04:55 PM.

  12. #12
    inkista's Avatar
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    Re: Understanding flash, aperture and shutter speed

    The meter can only measure the light it sees. The flash hasn't gone off yet when you meter. So the metering is only relevant to the ambient exposure. You typically have to juggle a few stops in your head, imagining how much light is going to be added by the flash and where in the frame. A typical flash shooting setup is to underexpose the ambient by a stop (or two) to get the background properly exposed, and then bring up the subject illumination with the flash.

    Flash requires that you think about light and exposure in a more complex way than just "needle there. OK."

    Think of a flash photograph as a combination of two exposures from two different sources of light: your ambient (all the light that isn't from the flash), and your flash. These two are related, but also independent, and any combination of these two can yield a "good" exposure--it's up to you to decide what effect you want and how you want to balance the flash against the ambient.

    Ambient is controlled (as you know) by iso, aperture, and shutter speed.

    Flash, however, is controlled by iso, aperture, flash power output, and subject-to-flash distance. (Shutter speed doesn't affect flash illumination because the flash burst is faster than your fastest shutter speed. Leaving the shutter open longer can only gather ambient light--it's already gotten the full light of the flash. The only other way shutter speed affects things is that HSS will rob you of power).

    TTL schemes work like this: the camera tells the flash to send out a "preflash" pulse of light of a known brightness. This is metered through the lens (TTL), and then the camera adjusts the flash's power level based on that metering. If the pulse was brighter than it should have been, the power's dialed down, and if it was too dark, the power's dialed up. So what metering mode you're working with can have a huge effect on how the flash power is set.

    Now, all this TTL metering assumes you only really care about your subject, and not so much your background. And in Av/Tv mode, the assumption is that you want to use the flash for fill (i.e., most of the illumination is going to come from the ambient light, and only a little from the flash). In P mode, the assumption (in low light) is that you want to use the flash as the main illumination for your subject, so your subject is lit the same, but the background illumination may vary a helluva lot, depending on how far away you are from the subject and what aperture/shutter speed combination you're using.

    So your judgement that something' under- or over- exposed is probably based on the background illumination. Look at your sujbect, not the background, and you'll see that TTL tries to keep it well-lit. You have to control the flash/ambient balance yourself, though, and being in an auto mode makes that tougher. My general recommendation is to be very very comfortable in M mode on the camera before you start heading into flash, because it'll help you understand what's going on a bit easier. And then using the flash in M mode, and dialing in the power levels yourself may also help you get a feel for what's going on.

    I'd recommend reading this article on dragging the shutter to get a feel for flash/ambient balance, and why you'd want to be shooting in M mode, rather than using exposure compensation and an auto mode with a flash.

  13. #13
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    Re: Understanding flash, aperture and shutter speed

    Hi Adrian,

    A quick look at your camera specs show the flash has the following modes:

    Flash modes: Smart Flash, Auto, Auto + Red-eye reduction, Fill-in, Fill-in + Red-eye reduction, 1st Curtain, 2nd Curtain, OFF

    It also has +/- 2 stops of flash exposure compensation.

    I do not know why the camera metering seems to turn off, I.e. always register 0, when you have your flash active. Perhaps a read through the manual will help you find out if you have one. It may be showing the flash exposure compensation. Try adjusting that and see if it moves off zero.

    I would try using some different flash modes. Have you taken the flash out of auto? Try using first curtain mode. That is the standard mode for cameras. See if any of the different flash modes get you back the use of the camera exposure meter.

    Sorry for not being a lot of help. Without the camera to touch and poke about it is hard to figure this one out.

    Alex

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    Re: Understanding flash, aperture and shutter speed

    Thanks again everyone....looks like I have lots of reading and practising to do :-)

    Alex - good idea ref the exposure meter, I'll give it a go...the same thing happens whether I use the built in flash or the external flash.

    Cheers everyone

    Adrian

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    Re: Understanding flash, aperture and shutter speed

    Hi Alex...it didn't work so hey...I'll just have to work with it...no idea what's going on

  16. #16
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    Re: Understanding flash, aperture and shutter speed

    Hi Adrian,

    At least we know that the exposure of zero is not referring to the flash exposure setting.

    As a last effort can you use the camera in a very dark place where the flash will not fix the exposure. For example outside at the night sky. In this case the flash will get lost in all the dark and there will be no foreground to bounce the flash back and get a good light reading. I suggest this because some cameras have an indicator stating if correct exposure was obtained after the shot was taken. You may find that the exposure meter indicates that the shot was underexposed, i.e. the flash could not fix it.

    A similar test but for overexposure it to shoot a white wall, preferably with a long shutter speed to really overexpose the shot. The meter may flash at the other end to show over exposure.

    Regards,

    Alex

  17. #17

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    Re: Understanding flash, aperture and shutter speed

    Hi Alex thanks for the suggestion.
    I've just tried shooting outside but the EV meter stays at 0; no indicator regarding exposure beyond that I'm afraid. Interestingly when I move to P mode to take a shot (back inside the house now) I can alter the EV value, i.e. dictate whether I want a shot over/under exposed by changing the value of the EV.....curiouser and curiouser

    Adrian

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