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The title of my question may seem misleading; I don't intend to ask how a proper exposure is calculated. I know the exposure triangle of ISO, aperture and shutter speed.

But in several articles I've read something confusing to me; that is a photographer using a light meter is told which aperture to shoot at for a specific exposure.

When I choose an aperture, I take into consideration my desired dof, available light, motion... but mostly DoF is what I think of as my main reason for choosing a particular aperture.

In this video on using lighting and shadows for depth in portraits; Creating Depth, Shape and Form in Portrait Photograph at 25 minutes in the speaker states that he chose an aperture of f/10, I think it was, because the light meter told him so for that particular exposure.

And in this article on high speed sync; "High Speed Sync Flash Understanding High Speed Sync Flash and Shutter Curtains" the author discuses that although his lightmeter suggested to shoot at f/16, he chose to shoot at f/2 to achieve his desired DoF, and this lead to shooting a a shutter speed of 1/5000 and needing HSS...

So my question is both photographers allowed the light meter to chose an aperture for their exposure; this seems backwards to me. Especially in both of the circumstances (portraits), where it seems more intuitive to choose your own aperture and allow the light meter to select your shutter speed for you. Why did they do this the other way around?

  • ok, with everyone's answers i think i know now what i didn't know i didn't know before; both of the described situations were ones using a flash. and since they were using flash, the shutter speed compared to the flash duration is going to be much longer, so it doesn't really matter. so the meter tells you what aperture you need to get a proper exposure. – user74091 Sep 24 '14 at 22:19
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    Related: Be aware of the effect of ambient light in photos with motion in. Your camera may expose the scene correctly for ambient light and so use a low shutter speed. The flash level will then be adjusted automatically and depending on mode used, the ambient may be signfiicant. This can lead to halo'd shots with sharp flash core and a blurred overlay from motion and ambient. You can experiment with this effect to achieve interesting results - but you don't want it accidentally. – Russell McMahon Sep 25 '14 at 2:29
  • The aperture and shutter speed and motion effect can lead to interesting results. My profile picture is an example. Many people think it is a blurry mess (and much of it is) and at least one person here was extremely rude about it [Their SOP so no problem :-) ] but it's purposefully done. In this case its background is essentially contentless but this does not have to be so. Larger version of blurry mess here** zz – Russell McMahon Sep 25 '14 at 2:38
  • Sometimes in special cases shutter speed is critical. Television pictures usually work best at some multiple of frame rate/2. eg 1/25th. 1/50th second in 50 Hz frame rate systems. Plasma TVs have internal clock rates that are best synced to to avoid some otherwise astounding results. – Russell McMahon Sep 25 '14 at 7:04
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Yes, a flashmeter only gives aperture. The shutter speed on the camera merely has to be long enough to ensure the film or sensor are uncovered for the duration of the flash (1/50 sec for a Leica M3, 1/250 for modern SLRs and up to 1/500 for a leaf shutter) and doesn't affect the exposure, assuming ambient light is insignificant compared to the flash output, which it usually is.

If you want a wider aperture you turn down the flash power. Most have 1/4 or 1/2 settings.

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    Most of this doesn't seem relevant to the situations being described in the question, in particular outdoor portraits - turning down the flash power is entirely not what you want to do as you're trying to get the flash to be on the same magnitude as the sunlight: that's why you need a faster shutter speed and high speed sync. – Philip Kendall Sep 24 '14 at 21:07
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When we talk about flash photography; this is because the shutter speed does not contribute to the exposure from the flash. A flash will output a burst which last maybe 1/1000s, so changing the shutter speed won't affect the exposure from the flash but form the other continuous light sources.

And since the light meter used in the first video you linked seems to be connected to the flashes, it tells him what aperture to use to get a correct exposure from the flash.

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You've only got the three variables to work with -- ISO, aperture, and shutter speed. When shooting in a studio environment, shutter speed doesn't really matter because you're relying upon the lights and their limitations, so you often need to work at 1/60 sec. You input your desired ISO into the light meter, and take a reading. The only variable left is the aperture. So yes, the aperture is being set by the light meter.

Of course, that's just one scenario. And now that you've got all three values, you can shift them around however you like (-1 aperture stop and +1 ISO stop, for example) to maintain correct exposure.

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When using regulable flashes or countinuous lamps, the lightmeter does not tell the photographer what aperture to use, instead it tells which aperture the current lighting is set up for.

The photographer first decides what aperture to use, a suitable ISO and exposure time. The meter is then used to find a flash or lamp setting that is good for the aperture you selected at the selected ISO and exposure time. Normally, a couple of setting would be tried and measured before getting to the desired power setting.

When working with flash only (continuous light is not needed/desired as part of the exposure) then a shutter speed close to the current camera/flash sync speed is to be used. Most DSLR from whithin a decade old are around 1/200 - 1/250.

The actual pulse from the flash units is in the range of 1/1000 seconds, so if no continous light is present, any shutter speed can be used, as long as it is no faster than sync speed.

When continuous light is needed as part of the exposure then shutter speed is more relevant and the photographer decides on this based on wheter moving subjects are to be "frozen" or motion-blurred and to what extent. (I.e. according to the speed at wich the subjec is going to be moving).

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You would let the light meter choose the aperture if you did not care about that choice. That's the short answer.

I think you are thinking about it correctly. You need to make a choice of the aperture for DOF reasons or perhaps you care more about a specific shutter speed to capture motion (or not) the way you want.

But the light meter has to tell you something for all three to express the exposure. I do not usually use a hand held meter so I usually set the aperture and then meter in the camera which then chooses the shutter speed then I consider if I want to change the ISO to effect the shutter speed.

The author you quote

the author discuses that although his lightmeter suggested to shoot at f/16, he chose to shoot at f/2 to achieve his desired dof,

Is doing what many of us do with the light meter reading except he has to manually calculate the change to the shutter speed. But I am pretty sure that most hand held meters allow you to set the aperture you want to use so it calculates the shutter speed needed.

  • ok so to the point of my question, i am not incorrect to assume that most photographers would do what i imagined: that they would chose their own aperture for their own artistic purposes of the shot and use the light meter to give them a more precise shutter speed for their desired aperture? in these instances the other considerations like dof, motion, light, were not relevent to the shot so shooting at f/10 or whatever was just as good as any other aperture/shutter speed combination? – user74091 Sep 24 '14 at 20:32
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    Most of the situations in which you'd use an external light meter already have the shutter speed constrained. If you're shooting cine (video or film), your shutter speed is usually determined by the frames per second you need. When shooting with flash (other than high speed sync), your shutter speed is limited to the X-sync speed of the camera/lens. When you're shooting ambient, you can use the controls on most meters to change the shutter speed or ISO until you reach the desired aperture (if available). But the aperture is the most freely variable setting most of the time. – user32334 Sep 24 '14 at 21:32
  • Correct @user74091 I think you have it figured out. – Ian Lelsie Sep 26 '14 at 16:45

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