Flash Tutorial by Ross McKelvey

The DSLR camera can be used in Automatic (P) mode, Aperture Priority, Shutter Priority or Manual mode.
When using flash indoors, it is easier to get the best results by using the automatic or P mode. Many wedding photographers use P mode. P stands for Professional, right? ;-) When using P or automatic mode, the camera automatically sets a shutter speed which is (a) fast enough to allow hand held picture, and (b) not too fast in terms of the camera’s flash sync speed (normally in the region of 1/200 sec). The aperture is also set automatically according to the camera’s auto metering. The flash will then either act as fill-in flash in bright conditions, or as the main light for the subject in dim lighting conditions.

AV or Aperture priority

If you use flash with your camera in AV mode, you set whatever aperture you choose. The camera will then set a shutter speed based on the ambient light level. The difficulty here is that in very bright conditions the correct shutter speed may need to be faster than the camera’s flash sync speed, but the camera will automatically default to the x-sync shutter speed. (This can only be avoided if your flash has a high speed sync (or FP) mode).
In dark conditions (think in terms of dimly lit interiors), the camera will set a shutter speed again based on the ambient light level, which means it will set a shutter speed which is too slow for handheld results. Anything slower than 1/60 sec is normally too slow for handheld, although image stabilisation and good technique can often produce reasonable results at slower shutter speeds

TV or shutter speed priority

You set the desired shutter speed – but if you set a shutter speed which is faster than your camera’s flash sync speed, the camera will default back to x-sync shutter speed.
The camera then sets an aperture value according to ambient light level and your chosen shutter speed.
In low light situations the aperture setting may ‘blink’ in the lcd panel if the widest aperture setting (eg f2.8) is not wide enough to allow a proper exposure.

Manual setting

Manual exposure mode lets you control both the shutter speed and the aperture. Keep in mind that manual exposure mode on the camera can be combined with fully automatic flash exposure, since the camera’s metering systems for flash and existing light are independently controlled.
I recommend that indoors you think in terms of 1/60 sec for shutter speed, raise your ISO to at least 400, and choose an aperture no smaller than f5.6 or f8. The smaller your aperture, the more the power you will need from your flash, so keeping a wider aperture allows your flash to recycle faster and uses less power. The flash still works automatically, even though your camera is in full manual mode.

EXPOSURE COMPENSATION & FLASH EXPOSURE COMPENSATION

More advanced camera will have both EC and FEC dials. Exposure Compensation affects only the background or ambient exposure, and NOT the flash exposure. Flash Exposure Compensation only affects flash exposure. Thus you can independently control the exposure of the background and the exposure of the subject, by using EC and/or FEC.

FLASH METERING DIFFICULTIES

The flash fires out an extremely quick burst of pre-flash which is reflected back to the camera, and used to determine how much power is needed to light the subject. The difficulty is that the camera does not know what the subject is, and for instance whereas white is extremely reflective, black is not. If your subject is wearing a white shirt, the flash will throw out very different power compared to a subject wearing a black shirt. However, regardless of whether your subject is wearing white or black, the amount of power required to create a proper exposure (according to your camera settings) is exactly the same. You can balance this by using the FEC dial to compensate for a subject wearing white or black.

THE RELATIONSHIP BETWEEN FLASH AND SHUTTER SPEED

Shutter speed has no bearing on the flash exposure, so long as your shutter speed is within your camera’s sync speed limit, ie say 1/125 or slower. In other words, if for instance your subject is properly exposed at 1/125 sec and f8, if you change your shutter speed to 1/60 sec, the exposure of your subject will be exactly the same. All that the slightly longer shutter speed will do, is allow the background to become brighter, by letting more ambient light in.
This is a useful exercise in trying to balance a flash lit subject in an indoors environment. If your flash correctly exposes the subject at say f8, but the background is too dark for your liking, then all you have to do is keep your aperture and flash settings the same, but use a longer shutter speed in order to allow more of the background light to record in the photograph. Again, be careful of going much slower than say 1/30 sec unless your technique is good and/or you have the benefit of image stabilization. Alternatively of course, you could use a tripod. :-)