From one of my recent articles for someone...
Frequently wondered points about flash photography (basics and advanced):
1. They say pop-up flash ruins photos. It is true?
Yes and no. Pop-up flash eliminates all shadows as seen from the camera as it is located right above the lens. It can flatten up texture of a food image and ruin it. But it can provide a vivid and proper image when used on a non-reflecting surface with no texture. For example, shooting images of watercolor artwork on paper.
2. What is fill-in flash and flash as main light source? No such setting on my camera!
A flash is said to be used as fill-in flash when the flash illuminates the subject slightly to lighten up shadows. Example, a person in the midday sun has shadows under the nose/eyes. Use fill-in flash to light up the shadowed part.
Flash is said to be the main source when its intensity is quite brighter than ambient lighting. Example, shooting ancient paintings in a dimly lit cave with flash.
3.Why a separate EV (exposure compensation value) for flash and camera?
Flash exposure and normal exposures are two completely different things. The camera EV controls exposure of ambient light. Flash EV controls flash output with TTL meter value as reference. Confusing? Read on.
4. Flash overexposed my photo. I increased shutter speed. Still no effect, why?
Flash isn't continuous lighting. It usually lasts for around 1/10,000th of a second which is faster than any shutter speed you'd ever use. That means the shutter speed cannot affect exposure by flash. The shutter opens and the flash fires then shutter closes. Decrease the flash EV (flash power output) to decrease exposure. ISO and aperture values DO affect flash exposure because they control sensitivity and light input only and are independent of light duration.
5. How do I use fill-in flash?
Just use a flash EV of -1 or -1/3, something usually lower than EV 0 to achieve the fill-in effect. Sometimes you may need to make the EV positive, but such cases are rare and only when ambient light is very strong.
6. I used bounce flash and my image is underexposed.
The camera calculates exposure based on what it sees through the lens. The camera assumes the flash light would fall directly on the subject. But the actual light that falls is lower in intensity because of scattering and absorption by the bouncing surface. So just increase flash power by making flash EV +1 or +2/3, whatever gets you a proper exposure.
7. Half my image is dark and half is properly exposed. Whatever happened?
The shutter mechanism of the camera has two curtains. At higher shutter speeds (usually above 1/200s), the mechanism exposes the image sequentially from top to bottom instead of exposing the entire sensor at a time. If your flash has no "high speed-sync mode", it cannot give you proper exposure at above 1/200s.
8. What is 1st curtain and 2nd curtain sync?
With 1st curtain sync, the flash fires at the beginning of the exposure and exposes that moment's image correctly.
With 2nd curtain sync, the flash fires at the end of exposure duration and exposes that moment right.
For natural motion blurs and to stop motion while using longer shutter speeds, 2nd curtain sync is recommended.
9. What is guide number?
It is just a measure of flash output power/range at a particular ISO and focal length. For example, 33@ISO 100, 35mm is stronger than 18@ISO 100, 35mm
I'd not go into the details of this parameter.
10. My photos have unnatural color, yellowish or bluish tints.
Your in-camera meters are unable to calculate the proper white balance. Better use a grey card or custom white balance setting on your camera. This problem arises when there are lights of different color temperatures. For example, using fill-in flash in a room lit with tungsten bulbs.
Such extreme difference in white balance cannot be resolved in post processing properly and may need you to use modifiers on your flash.
A simple trick would be to just step up the flash power and make it bright enough to suppress all other light sources. Then use a small aperture and low ISO to capture the scene. This would make influence of tungsten, etc sources negligible. But you'd lose the ambient light and mood of the scene.
I hope you found this article useful. Discussions about any issue welcome in comments. :)