Why does light meter affect shutter speed on flash exposures

[PeterB66]

As I understand it, the shutter speed is not part of the flash exposure triangle (i.e. ISO, aperture, flash power), yet the incident meter guides talk about taking a reading and then adjusting the aperture to your requirements which then adjusts the shutter speed.

But why would you adjust the shutter speed (leaving out ambient considerations for now; let's pretend, e.g. that we are using studio lights with the background totally knocked out) if it supposedly does not affect the flash?

 

[john isaacs]

As I understand it, the shutter speed is not part of the flash exposure triangle (i.e. ISO, aperture, flash power), yet the incident meter guides talk about taking a reading and then adjusting the aperture to your requirements which then adjusts the shutter speed.

But why would you adjust the shutter speed (leaving out ambient considerations for now; let's pretend, e.g. that we are using studio lights with the background totally knocked out) if it supposedly does not affect the flash?

You should be shooting in manual mode. If you don't care about ambient, then set your shutter speed to the sync speed for your camera.

If you do care about ambient, then set your shutter speed to get the ambient exposure you want (as long as it is below sync speed).

In studio, usually I set shutter speed to sync speed. I set aperture for the required DOF. I then use the flash meter to measure power on the strobes and adjust them to get the exposure I want.

 

[PeterB66]

You should be shooting in manual mode. If you don't care about ambient, then set your shutter speed to the sync speed for your camera.

If you do care about ambient, then set your shutter speed to get the ambient exposure you want (as long as it is below sync speed).

In studio, usually I set shutter speed to sync speed. I set aperture for the required DOF. I then use the flash meter to measure power on the strobes and adjust them to get the exposure I want.

Thanks, yes I know how to set it up, I was just curious specifically on the issue covered by my question...

 

[john isaacs]

You should be shooting in manual mode. If you don't care about ambient, then set your shutter speed to the sync speed for your camera.

If you do care about ambient, then set your shutter speed to get the ambient exposure you want (as long as it is below sync speed).

In studio, usually I set shutter speed to sync speed. I set aperture for the required DOF. I then use the flash meter to measure power on the strobes and adjust them to get the exposure I want.

Thanks, yes I know how to set it up, I was just curious specifically on the issue covered by my question...

You should be shooting in manual mode, not aperture or program mode. When you adjust aperture in aperture or program mode, the camera will adjust shutter speed to compensate. You don't want that.

 

[Ellis Vener]

As I understand it, the shutter speed is not part of the flash exposure triangle (i.e. ISO, aperture, flash power), yet the incident meter guides talk about taking a reading and then adjusting the aperture to your requirements which then adjusts the shutter speed.

If you are asking about the user manual for a hand-held meter which reads flash, , which to me it sounds like you are, you are probably reading a section covering an aperture priority mode. If that is the case You want to find the section covering standard manual mode where you set the shutter speed and then read the pop of the flash.



Which meter’s manual are you reading?

But why would you adjust the shutter speed…

which flash are you using? Is it an older flash? With some older flash units the flash duration (the length of time the flash lasts. While there are two accepted standards, t0.1 and t0.5, for determining how much of that flash pop to read, this doesn’t mean the total length of time light is being emitted ) in certain configurations -generally one head connected to a high capacity pack discharging at full power - those older units may be discharging light for a surprisingly long time. If that is the case, shutter speed might have an effect on the flash exposure if shutter speed is short enough.

--
Ellis Vener
A working photographer since 1984.
To see my work, please visit http://www.ellisvener.com
Or on Instagram @EllisVenerStudio

 

[MoreCowbell]

If you are in any mode other than MANUAL the camera will still think you want it to make some of the exposure decisions and is likely to change something. It will think you want adjustments made based on ambient light. It doesn't understand you only want to use flash.

--
https://www.castle-explorers.com

 

[PeterB66]

Hi, everyone - to clarify, if you take a reading on a light meter, and then click the buttons on the light meter to stop up and down, it adjusts both the aperture and the shutter speed.

Perhaps I neglected to mention it is an incident meter: the camera is not involved at all.

So what I don't understand is, when I take a flash reading, and click the buttons afterwards, the light meter still adjusts both. Yet the shutter speed should not affect a flash-only exposure (unless it is outside of sync limits), so I was curious why it would do that in this instance.

Similarly, if I set a 'preferred' shutter speed on my light meter before measuring the strobe, if affects the aperture that it outputs.

 

[Ellis Vener]

As I asked before: which meter? What page in the manual?

 

[PeterB66]

As I asked before: which meter? What page in the manual?

Polaris flash meter - you can see the manual here, and the process is really simple: cordless flash on page 3: https://www.bhphotovideo.com/lit_files/114529.pdf

 

[MoreCowbell]

Best WAG, you are in the measuring mode which includes consideration of ambient light.

 

[john isaacs]

Hi, everyone - to clarify, if you take a reading on a light meter, and then click the buttons on the light meter to stop up and down, it adjusts both the aperture and the shutter speed.

Perhaps I neglected to mention it is an incident meter: the camera is not involved at all.

So what I don't understand is, when I take a flash reading, and click the buttons afterwards, the light meter still adjusts both. Yet the shutter speed should not affect a flash-only exposure (unless it is outside of sync limits), so I was curious why it would do that in this instance.

Similarly, if I set a 'preferred' shutter speed on my light meter before measuring the strobe, if affects the aperture that it outputs.

I have a Polaris meter, and it doesn't work like that in flash mode. And this is verified by the user manual.

In Ambient Mode f/stop, once you take a reading it will change aperture and shutter speed as you press the up/down buttons.

In Ambient Mode EV, it only reports the EV and the up/down buttons have no effect.

In Flash Mode, you set the shutter speed and it reports the aperture for the combined ambient and flash exposure. The up/down buttons reset aperture to zero.

In Multiple Flash Mode, you have to hold the MULTI button down while pressing the buttons, and then press up/down buttons to change the number of flashes and it displays the aperture required for those flashes.

If aperture is changing as you press the up/down buttons, you are in Ambient Mode f/stop and are not measuring the flash (even if you fire the flash). The measurement is made at the time you press the trigger button.

 

[Ellis Vener]

As I asked before: which meter? What page in the manual?

Polaris flash meter - you can see the manual here, and the process is really simple: cordless flash on page 3: https://www.bhphotovideo.com/lit_files/114529.pdf

Do you mean this?





The makers of the meter do not know whether you are inside or outside and do not know what the ambient light conditions are.

As I think you understand, when you use flash, unless it is the only light source illuminating the subject and there is no ambient existing light , think of a flash exposure as a double exposure. One exposure is made with the light of the flash and the second is from the ambient light. The two are additive and you control the balance between the two by adjusting the shutter-speed.

Would you like that illustrated?

--
Ellis Vener
A working photographer since 1984.
To see my work, please visit http://www.ellisvener.com
Or on Instagram @EllisVenerStudio

 

[PeterB66]

Hi, everyone - to clarify, if you take a reading on a light meter, and then click the buttons on the light meter to stop up and down, it adjusts both the aperture and the shutter speed.

Perhaps I neglected to mention it is an incident meter: the camera is not involved at all.

So what I don't understand is, when I take a flash reading, and click the buttons afterwards, the light meter still adjusts both. Yet the shutter speed should not affect a flash-only exposure (unless it is outside of sync limits), so I was curious why it would do that in this instance.

Similarly, if I set a 'preferred' shutter speed on my light meter before measuring the strobe, if affects the aperture that it outputs.

I have a Polaris meter, and it doesn't work like that in flash mode. And this is verified by the user manual.

In Ambient Mode f/stop, once you take a reading it will change aperture and shutter speed as you press the up/down buttons.

In Ambient Mode EV, it only reports the EV and the up/down buttons have no effect.

In Flash Mode, you set the shutter speed and it reports the aperture for the combined ambient and flash exposure. The up/down buttons reset aperture to zero.

In Multiple Flash Mode, you have to hold the MULTI button down while pressing the buttons, and then press up/down buttons to change the number of flashes and it displays the aperture required for those flashes.

If aperture is changing as you press the up/down buttons, you are in Ambient Mode f/stop and are not measuring the flash (even if you fire the flash). The measurement is made at the time you press the trigger button.

Oh, my word, then I totally misremembered my last session then - will set up the lights and check it out.

Much appreciated, glad to know it isn't some aberration of logic, but just an aberration on my part.

 

[PeterB66]

As I asked before: which meter? What page in the manual?

Polaris flash meter - you can see the manual here, and the process is really simple: cordless flash on page 3: https://www.bhphotovideo.com/lit_files/114529.pdf

Do you mean this?



The makers of the meter do not know whether you are inside or outside and do not know what the ambient light conditions are.
As I think you understand, when you use flash, unless it is the only light source illuminating the subject and there is no ambient existing light , think of a flash exposure as a double exposure. One exposure is made with the light of the flash and the second is from the ambient light. The two are additive and you control the balance between the two by adjusting the shutter-speed.
Would you like that illustrated?

No thanks - that part has always been clear to me. Thanks for taking the trouble though - I think John Isaacs has put his finger on it, that I simply misremembered the behaviour of the meter in practise.

 

[PeterB66]

Hi, everyone - to clarify, if you take a reading on a light meter, and then click the buttons on the light meter to stop up and down, it adjusts both the aperture and the shutter speed.

Perhaps I neglected to mention it is an incident meter: the camera is not involved at all.

So what I don't understand is, when I take a flash reading, and click the buttons afterwards, the light meter still adjusts both. Yet the shutter speed should not affect a flash-only exposure (unless it is outside of sync limits), so I was curious why it would do that in this instance.

Similarly, if I set a 'preferred' shutter speed on my light meter before measuring the strobe, if affects the aperture that it outputs.

I have a Polaris meter, and it doesn't work like that in flash mode. And this is verified by the user manual.

In Ambient Mode f/stop, once you take a reading it will change aperture and shutter speed as you press the up/down buttons.

In Ambient Mode EV, it only reports the EV and the up/down buttons have no effect.

In Flash Mode, you set the shutter speed and it reports the aperture for the combined ambient and flash exposure. The up/down buttons reset aperture to zero.

In Multiple Flash Mode, you have to hold the MULTI button down while pressing the buttons, and then press up/down buttons to change the number of flashes and it displays the aperture required for those flashes.

If aperture is changing as you press the up/down buttons, you are in Ambient Mode f/stop and are not measuring the flash (even if you fire the flash). The measurement is made at the time you press the trigger button.

Oh, my word, then I totally misremembered my last session then - will set up the lights and check it out.

Much appreciated, glad to know it isn't some aberration of logic, but just an aberration on my part.

OK, so I tested it, and you are right, it zeros when I press the buttons.

But the confusing part remains in that, in Flash Mode, you set your shutter speed before you take your reading and what you set it as affects the aperture that it returns. But if shutter does not affect non-ambient flash, why would this give a different result?

 

[Ellis Vener]

The meter has no idea of what the lighting conditions are.

 

[PeterB66]

The meter has no idea of what the lighting conditions are.

OK, but then does that mean I could simply disregard the shutter speed, and focus on aperture only? (Unless metering for ambient, which with my studio lights, I am not.)

 

[john isaacs]

Hi, everyone - to clarify, if you take a reading on a light meter, and then click the buttons on the light meter to stop up and down, it adjusts both the aperture and the shutter speed.

Perhaps I neglected to mention it is an incident meter: the camera is not involved at all.

So what I don't understand is, when I take a flash reading, and click the buttons afterwards, the light meter still adjusts both. Yet the shutter speed should not affect a flash-only exposure (unless it is outside of sync limits), so I was curious why it would do that in this instance.

Similarly, if I set a 'preferred' shutter speed on my light meter before measuring the strobe, if affects the aperture that it outputs.

I have a Polaris meter, and it doesn't work like that in flash mode. And this is verified by the user manual.

In Ambient Mode f/stop, once you take a reading it will change aperture and shutter speed as you press the up/down buttons.

In Ambient Mode EV, it only reports the EV and the up/down buttons have no effect.

In Flash Mode, you set the shutter speed and it reports the aperture for the combined ambient and flash exposure. The up/down buttons reset aperture to zero.

In Multiple Flash Mode, you have to hold the MULTI button down while pressing the buttons, and then press up/down buttons to change the number of flashes and it displays the aperture required for those flashes.

If aperture is changing as you press the up/down buttons, you are in Ambient Mode f/stop and are not measuring the flash (even if you fire the flash). The measurement is made at the time you press the trigger button.

Oh, my word, then I totally misremembered my last session then - will set up the lights and check it out.

Much appreciated, glad to know it isn't some aberration of logic, but just an aberration on my part.

OK, so I tested it, and you are right, it zeros when I press the buttons.

But the confusing part remains in that, in Flash Mode, you set your shutter speed before you take your reading and what you set it as affects the aperture that it returns. But if shutter does not affect non-ambient flash, why would this give a different result?

Becase the flash meter measures ambient light as well as flash, and produces a recommended aperture for the combined exposure. If the shutter speed is fast, and the flash is bright, there will be little effect. But you can set ISO and shutter speed so there is enough light to affect the image (ambient light), and the flash can be set to complement or overpower the ambient light, and the meter will simply tell you what aperture to use to get a proper exposure in either case.

 

[Ellis Vener]

The meter has no idea of what the lighting conditions are.

OK, but then does that mean I could simply disregard the shutter speed, and focus on aperture only? (Unless metering for ambient, which with my studio lights, I am not.)

Yes.

 

[fotowbert]

OK, so I tested it, and you are right, it zeros when I press the buttons.

But the confusing part remains in that, in Flash Mode, you set your shutter speed before you take your reading and what you set it as affects the aperture that it returns. But if shutter does not affect non-ambient flash, why would this give a different result?

If you are seeing different reported apertures when you change shutter speed that means you have sufficient ambient light present to influence the flash meter reading.

A quick way to confirm this is to take a test shot with strobes off and see how much of an image you get. Otherwise take an ambient only reading and compare that to the flash reading. Note how many stops apart these two f values are.

I am assuming here that your Polaris meter is accounting for both ambient and flash when in flash mode but your linked instructions at B&H doesn't actually say. A couple of test shots with strong ambient in one while holding flash power constant would confirm this.

John