Low ISO vs. high ISO when using flash

Why is that when using flash inside at high ISO (e.g 3200) the background is fine but when taking the same photo using identical settings but at low ISO (e.g. 400) the background is perfectly black?

thanks in advance


ISO 400. Dark background


ISO 3200. Background is not dark.

MaxCh said:

Why is that when using flash inside at high ISO (e.g 3200) the background is fine but when taking the same photo using identical settings but at low ISO (e.g. 400) the background is perfectly black?

thanks in advance


ISO 400. Dark background


ISO 3200. Background is not dark.

Click to expand...

First of all, very pretty model. ;-)

OK, the reason is that more flash power is used at lower ISO settings than higher ISO settings. Thus, the light reflected from the model is relatively stronger than the light from the background at lower ISO settings (due to the model being closer to the camera than the background), hence the background is darker than the subject.

To put it another way, there is less light in the background. Your flash can only light the foreground, not the background. Your subject will be the same in each shot since she is in the foreground, thus illuminated by the flash. In one shot the background is at a higher ISO (3200), thus brighter and the flash fired at "X" power. In the other shot the background is at ISO 400 (thus darker) and the flash fired at 8X power (3200 > 1600 = 1-stop for 2x; 1600 > 800 = another stop, up to 4x; 800 > 400 = another stop totally 8x more power to illuminate the subject from the flash).

MaxCh said:

Why is that when using flash inside at high ISO (e.g 3200) the background is fine but when taking the same photo using identical settings but at low ISO (e.g. 400) the background is perfectly black?

thanks in advance


ISO 400. Dark background


ISO 3200. Background is not dark.

Click to expand...

And a third way to put it:

The light from the flash falls off as the square of the distance. The background is so far away your flash does nothing* to brighten the background.

Of course, the second photo taken at 3-stops higher ISO, has a brighter background.

The flash power has been adjusted up/down to make sure the model has just the right amount of flash to make her look nice.

*Flash photographers can use this property of light to their advantage:

Dragging the shutter

Background exposure

The Invisible Black Backdrop

--
Lance H

MaxCh said:

Why is that when using flash inside at high ISO (e.g 3200) the background is fine but when taking the same photo using identical settings but at low ISO (e.g. 400) the background is perfectly black?

thanks in advance


ISO 400. Dark background


ISO 3200. Background is not dark.

Click to expand...

A "flash" photo is essentially two separate (but combined) exposures, (ambient-background and flash-closer).

The ambient (background-brightness) is determined by (additive components of) Aperture & Shutter-Speed & ISO.

The flash exposure is determinned by (mainly/only) aperture & flash-power (controlled by flash or TTL). ISO and Shutter-Speed are less relevant than the "ambient" exposure/brightness.

So using a combination of "open" aperture + "long" shutter-speed + "high" ISO can indeed brighten the background, (and "auto" flash will lower-power to correctly "flash" expose).

Conversely using a "closed" aperture + "short/fast" shutter-speed + "low" ISO will result in a darker/black background, (but "auto" flash will INCREASE POWER to compensate and still provide a correct flash-exposure).

MaxCh said:

Why is that when using flash inside at high ISO (e.g 3200) the background is fine but when taking the same photo using identical settings but at low ISO (e.g. 400) the background is perfectly black?

Click to expand...

It's not perfectly black. It's just much darker than you would like and typical of many flash shots.

Your camera's auto exposure system is responsible for how these shots turned out because by asking this question I doubt you had everything in manual control. The concept that a flash shot is essentially a double exposure from two light sources, ambient and flash, as explained by others is going on behind the scenes when you use your camera's auto modes. Here is my take on what happened.

When the camera detects flash is being used it is programmed to give exposure priority to your foreground subject and to keep the shutter speed at reasonable hand-held levels. At 400 ISO this means it chose to let a dimly lit background be underexposed instead of dropping the shutter down to 1/13 sec or even slower resulting in motion blurring the background. At ISO 3200 the background is rendered lighter. At either ISO the foreground subject is correctly exposed since the TTL flash's light output is adjusted accordingly. Your camera should also have a custom setting for minimum shutter speed for P and A mode flash shots which limits the shutter going too slow.

On some cameras there is a "night scene flash mode" which would have produced a lighter background at 400 ISO. Because this mode will select longer shutter speeds it may also activate a "camera shake" warning for you to use a camera support (tripod) .

To have more control in situations like this will require you to go to manual exposure for at least the background. The foreground flash illuminated subject can remain with TTL flash or manual flash power. Manual flash will give more consistent results when flash to subject distance remains constant. In more fluid situations I find it convenient to let TTL flash adjust for distance changes.

IMO the key to understanding the balancing act between the two exposures that make up a flash shot is to recognize that (1) the flash exposure is independent of shutter speed (stay within sync limits) and (2) your flash illumination is adjustable and falls off with distance (Inverse Square Law). You can therefore change the ambient/background brightness with shutter speed and then control the relative subject brightness with flash power/distance.

Aperture and ISO apply to both ambient and flash and as such can't be used to shift the brightness balance between subject and background. They are selected for depth of field and image quality considerations.

• John

???

Well, think about it. Take the model out of the photo. The flash really has nothing to do with this. Take a shot. Let’s say it’s 1/60 @ f/8 ISO 800. Now take the same shot. Same Shutter and aperture but you give it 1600 ISO. Isnt ISO part of the exposure triangle. Yes, it is. It will make all the ambient light brighter. By one stop.

The model means nothing. She IS also getting more ambient light but depending on where she is, it may not affect her that much. She is being more lit by the flash than anything else.

Basically you are controlling the ambient light with your ISO in this case. This would also happen if you opened the aperture or used a slower shutter speed. And one reason I encourage people to USE higher ISOs or slower shutter speeds inside. I’m usually at 1/60 when shooting candids at events indoors. It helps lighten up the background. As does a higher ISO.

But know this. If you were at ISO 3200 and shot at say 1/60. Your background will go dark again if you use a faster shutter like 1/125 - 1/250. Or whatever you camera can sync at.

photolando said:

???

Well, think about it. Take the model out of the photo. The flash really has nothing to do with this. Take a shot. Let’s say it’s 1/60 @ f/8 ISO 800. Now take the same shot. Same Shutter and aperture but you give it 1600 ISO. Isnt ISO part of the exposure triangle. Yes, it is. It will make all the ambient light brighter. By one stop.

The model means nothing. She IS also getting more ambient light but depending on where she is, it may not affect her that much. She is being more lit by the flash than anything else.

Basically you are controlling the ambient light with your ISO in this case. This would also happen if you opened the aperture or used a slower shutter speed. And one reason I encourage people to USE higher ISOs or slower shutter speeds inside. I’m usually at 1/60 when shooting candids at events indoors. It helps lighten up the background. As does a higher ISO.

But know this. If you were at ISO 3200 and shot at say 1/60. Your background will go dark again if you use a faster shutter like 1/125 - 1/250. Or whatever you camera can sync at.

Click to expand...

I'd like to thank you everybody for taking the time to write these explanations. There is a lot of information here, I did not realize this issue was complex.

MaxCh said:

photolando said:

???

Well, think about it. Take the model out of the photo. The flash really has nothing to do with this. Take a shot. Let’s say it’s 1/60 @ f/8 ISO 800. Now take the same shot. Same Shutter and aperture but you give it 1600 ISO. Isnt ISO part of the exposure triangle. Yes, it is. It will make all the ambient light brighter. By one stop.

The model means nothing. She IS also getting more ambient light but depending on where she is, it may not affect her that much. She is being more lit by the flash than anything else.

Basically you are controlling the ambient light with your ISO in this case. This would also happen if you opened the aperture or used a slower shutter speed. And one reason I encourage people to USE higher ISOs or slower shutter speeds inside. I’m usually at 1/60 when shooting candids at events indoors. It helps lighten up the background. As does a higher ISO.

But know this. If you were at ISO 3200 and shot at say 1/60. Your background will go dark again if you use a faster shutter like 1/125 - 1/250. Or whatever you camera can sync at.

Click to expand...

I'd like to thank you everybody for taking the time to write these explanations. There is a lot of information here, I did not realize this issue was complex.

Click to expand...

As mentioned, you are taking two separate photos, one with ambient light, the other with flash. If you use an automatic mode, the camera will decide what the balance between the two should be. When you set ISO, that modifies how the automatic mode balances between flash and ambient. You can either 'psych' out your auto mode (I am not offering any hints in that area as I am not a Canon shooter) or you can use manual exposure mode for flash shots. Manual probably sounds scary - but it's actually easier than trying to understand all the complexities of what your auto system is doing.

Other than manual exposure and iso, your camera should let you separately control flash power Light drops off proportionately to the square of the distance between flash and light - so how far away your subject is from the flash makes a huge difference.

I'd suggest Bernie Raffe's course on flash photography over at udemy.com for a better understanding of how all this works.

bobkoure said:

MaxCh said:

photolando said:

???

Well, think about it. Take the model out of the photo. The flash really has nothing to do with this. Take a shot. Let’s say it’s 1/60 @ f/8 ISO 800. Now take the same shot. Same Shutter and aperture but you give it 1600 ISO. Isnt ISO part of the exposure triangle. Yes, it is. It will make all the ambient light brighter. By one stop.

The model means nothing. She IS also getting more ambient light but depending on where she is, it may not affect her that much. She is being more lit by the flash than anything else.

Basically you are controlling the ambient light with your ISO in this case. This would also happen if you opened the aperture or used a slower shutter speed. And one reason I encourage people to USE higher ISOs or slower shutter speeds inside. I’m usually at 1/60 when shooting candids at events indoors. It helps lighten up the background. As does a higher ISO.

But know this. If you were at ISO 3200 and shot at say 1/60. Your background will go dark again if you use a faster shutter like 1/125 - 1/250. Or whatever you camera can sync at.

Click to expand...

I'd like to thank you everybody for taking the time to write these explanations. There is a lot of information here, I did not realize this issue was complex.

Click to expand...

As mentioned, you are taking two separate photos, one with ambient light, the other with flash. If you use an automatic mode, the camera will decide what the balance between the two should be. When you set ISO, that modifies how the automatic mode balances between flash and ambient. You can either 'psych' out your auto mode (I am not offering any hints in that area as I am not a Canon shooter) or you can use manual exposure mode for flash shots. Manual probably sounds scary - but it's actually easier than trying to understand all the complexities of what your auto system is doing.

Other than manual exposure and iso, your camera should let you separately control flash power Light drops off proportionately to the square of the distance between flash and light - so how far away your subject is from the flash makes a huge difference.

I'd suggest Bernie Raffe's course on flash photography over at udemy.com for a better understanding of how all this works.

Click to expand...

Thank you for the advice. Both photos were taken using manual mode: ISO, shutter speed and aperture were set by me, not the camera. I

MaxCh said:

bobkoure said:

MaxCh said:

photolando said:

???

Well, think about it. Take the model out of the photo. The flash really has nothing to do with this. Take a shot. Let’s say it’s 1/60 @ f/8 ISO 800. Now take the same shot. Same Shutter and aperture but you give it 1600 ISO. Isnt ISO part of the exposure triangle. Yes, it is. It will make all the ambient light brighter. By one stop.

The model means nothing. She IS also getting more ambient light but depending on where she is, it may not affect her that much. She is being more lit by the flash than anything else.

Basically you are controlling the ambient light with your ISO in this case. This would also happen if you opened the aperture or used a slower shutter speed. And one reason I encourage people to USE higher ISOs or slower shutter speeds inside. I’m usually at 1/60 when shooting candids at events indoors. It helps lighten up the background. As does a higher ISO.

But know this. If you were at ISO 3200 and shot at say 1/60. Your background will go dark again if you use a faster shutter like 1/125 - 1/250. Or whatever you camera can sync at.

Click to expand...

I'd like to thank you everybody for taking the time to write these explanations. There is a lot of information here, I did not realize this issue was complex.

Click to expand...

As mentioned, you are taking two separate photos, one with ambient light, the other with flash. If you use an automatic mode, the camera will decide what the balance between the two should be. When you set ISO, that modifies how the automatic mode balances between flash and ambient. You can either 'psych' out your auto mode (I am not offering any hints in that area as I am not a Canon shooter) or you can use manual exposure mode for flash shots. Manual probably sounds scary - but it's actually easier than trying to understand all the complexities of what your auto system is doing.

Other than manual exposure and iso, your camera should let you separately control flash power Light drops off proportionately to the square of the distance between flash and light - so how far away your subject is from the flash makes a huge difference.

I'd suggest Bernie Raffe's course on flash photography over at udemy.com for a better understanding of how all this works.

Click to expand...

Thank you for the advice. Both photos were taken using manual mode: ISO, shutter speed and aperture were set by me, not the camera. I

Click to expand...

OK, then. To re-quote your question

MaxCh said:

[in manual mode] Why is that when using flash inside at high ISO (e.g 3200) the background is fine but when taking the same photo using identical settings but at low ISO (e.g. 400) the background is perfectly black?

Click to expand...

A flash photo is two photos together. You're paying attention to the background. Never mind what you're doing with flash. In manual mode, raising the ISO brightens the shot (at the expense of dynamic range and possibly blowing out some highlights). Get your background right, then worry about getting the flash part right. The only setting shared by both ambient and flash parts is your aperture. My own tendency (I'm very much no expert here) is to use as low an ISO as I can, and to adjust flash strength downwards as necessary. If the flash isn't bright enough, I have to get the flash closer or raise ISO.

Depending on your camera, you may find a dark background perfectly fixable in post (just raise shadows and blacks). The one thing that's un-fixable is blown highlights. A background that looks perfectly black might not be as black as you think (assuming you're not relying on JPGs out of the camera)