F58AM Overheat

Started Jun 23, 2012 | Discussions thread
Mark VB Senior Member • Posts: 2,912
Re: F58AM Overheat

Rexel99 wrote:

Oh, to add, I am still learning about how the flash actually works in regard to power and exposure. But as I understand, the output would change if you raised the iso, opened aperature using P/A/S modes, but in M it pretty much fires in full. Unless you overide the flash and set it to manual with 1/16 power for example. THis is great to reduce output and heat, but then wont adjust for the conditions... kids getting closer to you, snap and then they are over exposed.

Assuming the flash is in TTL mode, for a given aperture, as you raise the ISO the flash needs to put out less light to provide a "proper" exposure. For each doubling of the ISO the flash works half as hard. Thus, if you go from ISO 200 to 400, the flash needs to put out one-stop less light.

The way the flash controls its output is by the length of the flash burst. At full power (such as a manual, full power burst), the flash will have its longest duration (e.g., something like 1/800 or 1/1000 second). As the flash needs to work less hard, either in TTL mode or if you reduce the power in manual mode, the flash will use a shorter burst of light, down to whatever it's minimum power setting (fastest burst) is. For example, this is how high-speed flash photography of hummingbirds is done - setting the flashes used to reduced power in manual mode to assure a "high-speed" flash duration circa 1/20,000 second or faster).

As you use a faster aperture, the flash needs to work less hard and put out less light. The smaller the aperture, the more light needed and the harder the flash works.

Note that the shutter speed used is irrelevant to the required flash output, as long as you are within the camera's normal sync speed (e.g., for the A77 1/200 with SSS on or 1/250 with SSS off). Within the normal sync mode the flash uses a single burst to provide its lighting effect. If you are in HSS mode (high speed sync - when the shutter speed is faster than the normal maximum sync speed) the flash puts out a series of light bursts to provide the proper exposure. These bursts are very fast, meaning that the effective power level of the flash is greatly reduced, but the flash is working as hard, or nearly as hard, as if it was doing a full power burst because of the number of bursts it has to put out.

If a subject is changing its distance for the camera (or flash if the flash is off camera), then it is best to use TTL mode to adjust for the varying power required as the subject distance changes. However, if a subject is at a constant distance, then you can use manual flash mode, adjusting the power level to give you the desired results for a given aperture.

In either case you control the ambient light setting by the ISO and shutter speed used. Thus, if you can use a higher ISO to give you a faster shutter speed within the camera's normal sync speeds, you can make the flash work less hard for the same given aperture. But, once you go above the sync speed and the flash goes into HSS mode, you are back to operating the flash at higher power (just in a series of bursts rather than one longer burst).

This means that to provide maximum flexibility with your flash, and make the most effective use of the available flash power, you want to use shutter speeds within the normal sync mode and a higher ISO for any given aperture setting (and the faster the aperture the less power required). When shooting outdoors that may not provide much flexibility depending on the ambient light levels if you have to go above the maximum sync speed for a given aperture setting.

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