Shutter speed controls the ambient exposure when shooting with flash

[Mare333]

Hello,
Marianne here with a question. I am learning how to balance ambient and flash exposure. In particular I learned that to change ambient exposure we change shutter speed. So if I have my ambient reading for example shutter speed 1/60 and aperture f/8 then I introduce the flash in manual mode for example at 1/16 power. So my aperture and my flash control how the subject is exposed while my shutter speed controls the ambient exposure. So if instead of shutter speed 1/60 I dial 1/200 my background i.e. the ambient will be darker and if instead of 1/60 I dial 1/15 my ambient will be overexposed while in both cases my [B]subject[B] exposure will remain the same (as long as we are NOT changing aperture) So I understand how this works in theory and watched several videos to see how it works (for example this video https://www.youtube.com/watch?v=vEnAhkL0i38)

However, when I try it, I am NOT getting this result. When I set my shutter speed to 1/200 (while the meter reading says I should set it at 1/60) and take a shot with flash firing my ambient i.e. my background does NOT go dark. Instead the background seems to be illuminated by the flash and the whole scene i.e. both subject and background are nicely exposed which is NOT even a bad thing, but I am wondering how to make it work so shutter speed will truly control the background i.e. the ambient exposure. Any thoughts and advice will be greatly appreciated, thank you very much.

[Manfred M]

Marianne - it would be very useful if you posted the images in question so that we can see what is happening.

Part of the issue could be the subject to background distance. Flash drop off is determined by the inverse square law, so if your background and subject are very close together, then the background will get a significant contribution from the flash.

Another possibility is that you are shooting in an automation mode, rather than in manual mode and the camera / flash are trying to create what the camera calculates as the correct exposure. You can also use a camera in an automation mode, but will have to set exposure compensation (EC) to give the background the correct exposure AND set the Flash Exposure Compensation (FEC) to correctly illuminate the subject. I find shooting manual is usually easier. The other issue is that different camera manufacturers have set up the integrated EC / FEC differently so it's hard to give a definitive answer here.

[dje]

Marianne the video you reference has the guy shooting outside in bright light and the flash is not going to have much influence on the background ambient light. If you are shooting indoors, the flash can still illuminate the background, and as Manfred says, it depends how far the background is behind the subject. Also it depends to some extent on the direction the flash is coming from. eg off camera flash from the side.

We need to know your test setup.

Dave

[dje]

Marianne I just had a closer look at that video you referenced and should point out that he is using shutter speeds of the order of 1/640s or faster which are in excess of the Flash Sync Speed (typically 1/200 or 1/250 sec). This means that his camera/flash must be in High Speed Sync mode. In this mode, shutter speed does affect flash exposure (because the flash signal lasts for the whole time the shutter curtains are moving) so this video is not a good example of the normal behaviour of flash.

This article might help.

Dave

[rpcrowe]

Here's another way of doing this. I will admit it is more for outdoor portraiture than for run and gun shooting...

Get your exposure for the background by using the camera's meter. I usually like to under expose the background by about 2/3 stop...

Once you have this exposure set, leave it that way. Don't adjust the shutter speed, f/stop or ISO...

Now with your flash in manual mode shoot an image. After a while you will get used to the manual power setting for any given distance, f/stop and ISO...

Check your result. That is where the capability to view the image in the EVF on a mirrorless camera rather than struggle to see the image with a DSLR LCD viewer in the bright sun. Quite likely starting at about 1/8 power might put you in the ballpark; although that is predicated on the power of your flash as well as the f/stop and ISO at which you are shooting.

Change the subject exposure by changing the manual power of the flash, don't mess with your camera's exposure settings unless... Unless the image is too dark at maximum power or too light at minimum power. Then, you might change the placement of the flash if you are using off camera flash (always remembering when using a modifier like an umbrella or softbox, the softest and most flattering light is when the modified is placed as close as possible to the subject) or increase or decrease the ISO or f/stop of your camera.

This technique works both with off camera flash (it is nice to be able to change the flash power using a controller like a Godox trigger or master flash) and with hotshoe mounted flash.

It is really extremely simple and requires no math like when using guide numbers...

However, you will (if using a wide aperture) most likely be shooting at a shutter speed higher than the max sync speed of your camera/flash combination and will probably need to use High Speed Sync (HSS).

[Mare333]

Marianne I just had a closer look at that video you referenced and should point out that he is using shutter speeds of the order of 1/640s or faster which are in excess of the Flash Sync Speed (typically 1/200 or 1/250 sec). This means that his camera/flash must be in High Speed Sync mode. In this mode, shutter speed does affect flash exposure (because the flash signal lasts for the whole time the shutter curtains are moving) so this video is not a good example of the normal behaviour of flash.

This article might help.

Dave

Thank you for taking the time to answer my question and looking at the video. I will definitely take a look at the article that you linked to.
Below are the sample images to illustrate my question and to show my setup.
1) ambient only. Shutter speed 1/20, f/3.5, ISO 100. The image is dark not enough light for the correct exposure
2) The same setup as in 1 above BUT adding flash in MANUAL mode at 1/32 power. So shutter speed 1/20, f/3.5, ISO 100, flash in manual mode 1/32 power. The image is exposed correctly BOTH subject and background.
3) The same setup as in 2 above BUT CHANGE SHUTTER SPEED to 1/200 (sync speed) So shutter speed 1/200, f/3.5, ISO 100, flash in MANUAL mode at 1/32 power. So the BACKGROUND should go DARKER than in scenario 2, but it does NOT. BOTH THE SUBJECT AND THE BACKGROUND are bright. There is virtually no change in BACKGROUND in scenario 2 and scenario 3.

[Mare333]

Marianne - it would be very useful if you posted the images in question so that we can see what is happening.

Part of the issue could be the subject to background distance. Flash drop off is determined by the inverse square law, so if your background and subject are very close together, then the background will get a significant contribution from the flash.

Another possibility is that you are shooting in an automation mode, rather than in manual mode and the camera / flash are trying to create what the camera calculates as the correct exposure. You can also use a camera in an automation mode, but will have to set exposure compensation (EC) to give the background the correct exposure AND set the Flash Exposure Compensation (FEC) to correctly illuminate the subject. I find shooting manual is usually easier. The other issue is that different camera manufacturers have set up the integrated EC / FEC differently so it's hard to give a definitive answer here.

Thank you for your insight. All the points you bring up make sense. I have posted the sample images and their setup in my reply to dje. So this way you can take a look and let me know what you think on the subject. Thank you very much for your advice,
Marianne

[Mare333]

Here's another way of doing this. I will admit it is more for outdoor portraiture than for run and gun shooting...

Get your exposure for the background by using the camera's meter. I usually like to under expose the background by about 2/3 stop...

Once you have this exposure set, leave it that way. Don't adjust the shutter speed, f/stop or ISO...

Now with your flash in manual mode shoot an image. After a while you will get used to the manual power setting for any given distance, f/stop and ISO...

Check your result. That is where the capability to view the image in the EVF on a mirrorless camera rather than struggle to see the image with a DSLR LCD viewer in the bright sun. Quite likely starting at about 1/8 power might put you in the ballpark; although that is predicated on the power of your flash as well as the f/stop and ISO at which you are shooting.

Change the subject exposure by changing the manual power of the flash, don't mess with your camera's exposure settings unless... Unless the image is too dark at maximum power or too light at minimum power. Then, you might change the placement of the flash if you are using off camera flash (always remembering when using a modifier like an umbrella or softbox, the softest and most flattering light is when the modified is placed as close as possible to the subject) or increase or decrease the ISO or f/stop of your camera.

This technique works both with off camera flash (it is nice to be able to change the flash power using a controller like a Godox trigger or master flash) and with hotshoe mounted flash.

It is really extremely simple and requires no math like when using guide numbers...

However, you will (if using a wide aperture) most likely be shooting at a shutter speed higher than the max sync speed of your camera/flash combination and will probably need to use High Speed Sync (HSS).

Thank you very much for such a detailed answer. I will definitely keep this in mind.

[dje]

Thank you for taking the time to answer my question and looking at the video. I will definitely take a look at the article that you linked to.
Below are the sample images to illustrate my question and to show my setup.
1) ambient only. Shutter speed 1/20, f/3.5, ISO 100. The image is dark not enough light for the correct exposure
2) The same setup as in 1 above BUT adding flash in MANUAL mode at 1/32 power. So shutter speed 1/20, f/3.5, ISO 100, flash in manual mode 1/32 power. The image is exposed correctly BOTH subject and background.
3) The same setup as in 2 above BUT CHANGE SHUTTER SPEED to 1/200 (sync speed) So shutter speed 1/200, f/3.5, ISO 100, flash in MANUAL mode at 1/32 power. So the BACKGROUND should go DARKER than in scenario 2, but it does NOT. BOTH THE SUBJECT AND THE BACKGROUND are bright. There is virtually no change in BACKGROUND in scenario 2 and scenario 3.

Marianne I can't be certain but as far as I can see the second and third images have the same file name and are the same image?

Dave

[Manfred M]

Thanks for posting. This is exactly the case that Dave (dje) wrote about, the technique simply will not work in a small indoor environment like the one shown in this shot. The light is not controlled and will bounce everywhere and will light up the background and will totally overpower the ambient light. It is very much a technique used outdoors (as demonstrated in the video you provided the link to)

Indoors, we tend to not use this type of approach. In a small room, like the one in your image, the light spill from small flash is so significant that it overpowers any ambient light. In larger environments, for instance, in a studio, we set up the light to ensure that the light only falls where it is needed and the background can be lit separately.

[george013]

Marianne,
It's a matter of how you approach the method, using flash.
If you use flash you make two pictures at once.: 1 with ambient light and one with flash light.
Ambient light is made with the aperture and shutter speed of the camera, flash is made with the aperture of the camera and the flash time of the flash. Flash time is about 1/1000s, that explains why shutter speed doesn't have any influence on the flash picture. Flash intensity is controlled by the flash power. The light leaving the flash is leaving it as a an expanding beam: the further away of the flash the more surface it has to illuminate, the weaker it gets. What you see in the image is the result of the strongest of the two. That's why you probably didn't see much difference between the last two pictures. Beside the fact that you posted twice the same as Dave mentioned.

Just trying to show you a way of thinking that explains what's going on. It was and is helpful for me.

George

[Manfred M]

A quick test on how much ambient light is impacting the image is to set up your exposure that you will be using the flash for and then take a picture at those settings (ISO, shutter speed and aperture) and look at the resulting image. That will show you how much or how little the ambient light is contributing to the image.

[Mare333]

Marianne I can't be certain but as far as I can see the second and third images have the same file name and are the same image?

Dave

Hi,
I am not sure why they have the same file name, but it's definitely NOT the same image.

[Mare333]

Thanks for posting. This is exactly the case that Dave (dje) wrote about, the technique simply will not work in a small indoor environment like the one shown in this shot. The light is not controlled and will bounce everywhere and will light up the background and will totally overpower the ambient light. It is very much a technique used outdoors (as demonstrated in the video you provided the link to)

Indoors, we tend to not use this type of approach. In a small room, like the one in your image, the light spill from small flash is so significant that it overpowers any ambient light. In larger environments, for instance, in a studio, we set up the light to ensure that the light only falls where it is needed and the background can be lit separately.

Got it, thanks.

[Mare333]

Marianne,
It's a matter of how you approach the method, using flash.
If you use flash you make two pictures at once.: 1 with ambient light and one with flash light.
Ambient light is made with the aperture and shutter speed of the camera, flash is made with the aperture of the camera and the flash time of the flash. Flash time is about 1/1000s, that explains why shutter speed doesn't have any influence on the flash picture. Flash intensity is controlled by the flash power. The light leaving the flash is leaving it as a an expanding beam: the further away of the flash the more surface it has to illuminate, the weaker it gets. What you see in the image is the result of the strongest of the two. That's why you probably didn't see much difference between the last two pictures. Beside the fact that you posted twice the same as Dave mentioned.

Just trying to show you a way of thinking that explains what's going on. It was and is helpful for me.

George

Thank you, much appreciated.

[Mare333]

A quick test on how much ambient light is impacting the image is to set up your exposure that you will be using the flash for and then take a picture at those settings (ISO, shutter speed and aperture) and look at the resulting image. That will show you how much or how little the ambient light is contributing to the image.

Makes sense, thanks

[Manfred M]

Hi,
I am not sure why they have the same file name, but it's definitely NOT the same image.

You might want to check that. When I download them and overlay them in Photoshop, they are IDENTICAL pixel for pixel.

[Dave Humphries]

I am not sure why they have the same file name, but it's definitely NOT the same image.

You might well have uploaded 3 different images Marianne, but as Dave and Manfred have said, the files names linked from TinyPic are the same for the 2nd and 3rd above. You can confirm this yourself by hovering a mouse pointer (if not using a tablet!) over them and moving down the strip of 3 photos, the file name changes between 1st and 2nd, but stays the same for 2nd and 3rd. So what we see are two of the same.

However, that's not really relevant to your issue in those test shots, which has been adequately explained.

BR,
Dave