Flash and ambient exposure

In the previous post about front-curtain and rear-curtain sync modes, I used a two-exposure model to describe the final image created by the contribution of both ambient and flash lights. In this post, I will go into details about the shooting parameters that affect both exposures and the most effective techniques of balancing flash and ambient exposure. I will mostly speak in Nikon terms since that’s the only brand of DSLR camera I have access to.

Four parameters

The four camera parameters that affect the final exposure are: ISO, aperture, shutter speed, and flash output amount. If the camera and flash are in full manual mode, all four parameters can be freely adjusted by the photographer. In the chart below, a check mark is used to indicate that the particular camera or flash parameter affects ambient or flash exposure. “–” means it does’t. An important assumption is made here: Flash duration is so short compared to shutter speed that shutter speed does not affect flash exposure. This excludes the FP high speed sync flash mode from our discussion to avoid complications.

Clearly, if you want to adjust the amount of the ambient exposure and flash exposure independently, the straight forward approach is to adjust the ambient exposure using shutter speed and adjust flash exposure using flash output amount. However the adjustments need to be within the confinement of the total exposure, which should be the proper exposure for the photo.

Total exposure = Ambient Exposure + Flash Exposure

Obviously, if either one of the ambient exposure or flash exposure equals or exceeds the amount of proper exposure, an overexposed photo will be produced.

If you are after a certain ratio between ambient and flash exposure, you may hit limiting factors such as maximum output amount from your flash or maximum sync speed of the camera. In these cases, the ISO and aperture settings need to be adjusted to bring shutter speed and flash output amount to a workable range. These two parameters affect both the ambient and flash exposure to the same degree so the ratio of ambient and flash exposures is maintained.

The all-manual mode is quite easy to understand. It offers the most degrees of freedom for exposure control. However not all shooting situations permit extensive adjustments that do need time to complete. The most frequently used settings are auto exposure modes and auto flash output via TTL flash system. The chart below outlines the four common exposure modes and who (user or camera) decides ISO, aperture, shutter speed, and flash output amount.

When camera is in control of a camera setting, it follows a pre-programmed logic and parameter range for the setting. The following discussions look at how Nikon DSLR camera behaves in P/S/A/M exposure modes and TTL flash mode.

P: Programmed Auto

In the P  exposure mode, the camera picks both aperture and shutter speed automatically to obtain a proper level of exposure.

For example, one reading in a dim light gives me 1/13s f/1.8 at ISO 400. When a Speedlight flash is mounted on the hotshoe or the built-in flash is popped up, the shutter speed is automatically changed to the flash shutter speed (typically 1/60s but can be changed via Custom Settings Menu). What happens to the aperture? It goes from f/1.8 to f/4. From 1/13s  to 1/60s is 2 and 1/3 stop under exposure and from f/1.8 to f/4 is another 2 and 1/3 reduction in exposure. In total, the camera decided to underexpose the ambient by 4 and 2/3 stops. The flash is expected to make up the difference to bring a proper exposure to the main subject.

In another example, the camera meter reading was 1/125s f/5.6 at ISO400. When the built-in flash is popped up, the exposure changes to 1/250s f/4. There is no change to ambient exposure.

S: Shutter-priority Auto

In the S exposure mode, the user selects the shutter speed and the camera picks an aperture based on meter reading. What happens to the aperture when an external flash is mounted on the hotshoe or the built-in flash is popped up? Nothing, unless the shutter speed is too high (but still slower than the sync speed) that a proper exposure could not be obtained (you will see “lo” in viewfinder), in which cases the largest aperture of the lens will be used. This leads to under exposed ambient. If the user selected shutter speed exceeds the sync speed of the camera, the camera will jump to the sync speed and the aperture will be increased automatically to compensate so the ambient exposure is maintained. Therefore in the shutter-priority exposure mode, the camera does not intentionally under-expose the ambient but the user can choose to under expose the ambient by using a faster shutter speed when the aperture is at its maximum.

A: Aperture-priority Auto

In the A (aperture priority) exposure mode, the user selects the aperture and the camera selects the shutter speed.

For example, the camera meter decides 1/4s f/4 at ISO 400 is the correct exposure. When the flash is added, the shutter speed jumps to 1/60s (flash shutter speed, changeable via Custom Settings). In this case, the ambient is under-exposed by 4 stops. In another example, the meter shows 1/160s f/5 at ISO400 is the correct exposure for the ambient.  Adding the flash doesn’t change it.

M: Manual

In the M (manual) exposure mode, both aperture and shutter speed are selected by user. User has complete control of the ambient exposure. Mounting a Speedlight flash or enabling on the built-in flash does not cause the camera to change either aperture or shutter speed.

Summary

In P/S/A exposure modes, it is not straight forward to control the ambient exposure without affecting the flash exposure. You have no direct control of at least one of aperture and shutter speed.

In P or A modes, if the ambient exposure reading indicates a shutter speed slower than the Flash Shutter Speed, the camera underexposes the ambient automatically with the underexposure amount depending on the difference between the metered ambient shutter speed and the Flash Shutter Speed. If the ambient exposure reading indicates a shutter speed faster than the Flash Shutter Speed, the ambient will be exposed as is, no ambient underexposure happens.

In S mode, no such intentional underexposure will occur but user has the option to underexposed the ambient by using a faster shutter speed after pushing the aperture to the largest possible on the lens.

One very important factor to consider: On Nikon DLSR cameras, Exposure Compensation dial affects both the ambient and flash exposure. It is convoluted.

In M exposure mode, it is easy to control both ambient and flash exposure without affecting each other because the user has complete control over the ambient exposure via aperture and shutter speed adjustments and flash exposure via Flash Exposure Compensation dial.


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