http://www.outdoorlanternsonsale.com/mode-flashlight/
Does the bulit-in flash on a DSLR, vary only in intensity ?
I am still rather confused about flash photography, since I owned a P&S camera for so much time and on that couldn't set anything regarding flash, except ON or OFF. So the question is this: I noticed that on Aperture priority for example, where I can change the f number, the shutter speed is adjusted by the camera to match a normal exposure. But if I pop up the flash, changing the aperture no longer seems to influence the shutter speed. Why ? Also, if I set an Exposure compensation of say +1 or -1 EV, the picture will look different, while the shutter speed stays the same. What is being tweaked by the camera in this case, the duration the flashlight illuminates the scene or the intensity of the flash ? Does the flash vary laso in duration or rather only in intensity. And if there are only 4 types of 'power' the flash can fire (as I can see in the manual flash modes - FULL, 1/4, 1/16, 1/64) why does the flash exposure compensation feature more steps ? Thanx.
Wow some question!
First the pop up flash has very limited power probably less than 10 feet. Within that range its output is monitored by the camera so whatever setting you select the output is varied (by both duration and/or intensity) to give a correctly exposed shot.
In Av mode altering the aperture in this situation will only alter the depth of field, if you go beyond the range of the camera flash for example f16 with the subject 10ft away you will get a severely under exposed picture. You can also go too close and over expose.
Normally once the flash is charged it will set the shutter speed to the fastest speed where the whole of the sensor is exposed, usually around 1/180th of a second.
You can select slow speed flash on most DSLR's here the shutter speed will follow the metering, it won't let you go faster than 1/180th of a second, but it will allow you to go slower. This is to use the ambient light for the exposure and the flash operates as a 'fill in' flash and is used primarily when photographing against a bright object such as the sky, normally your subject would be in shadow, slow speed sync adds the light to the front of your subject, but it can be used to balance the ambient light with flash to get into shadows. Done correctly the pictures look natural, without that flash photo look, even in bright sunlight.
Most of the time the flash fires on the first curtain (as soon as the sensor is fully exposed), but on higher end DSLR's you can get rear curtain flash where the flash fires just before the rear curtain closes.
It works in a similar way to slow speed flash, but it fires just before the shutter closes not when its just opened.
Imagine a skate boarder coming towards you, the light is low, aperture is small (high f number) and the shutter speed is around 1 second (or you use ND filters to cut down the light to get this kind of shutter speed) the camera is on a tripod. You will get a blurred picture of the skateboarder, but just before the shutter closes the flash fires and freezes his movement and fills the frame with light. Result a correctly exposed skateboarder with a blur behind him. Impressive stuff.
Adding a dedicated flashgun to your kit will enable all this automation to a range of several 10's of meters! Usually well over 100 feet.
Theres a lot more to it than this (people have written books about it), but a flash gun used creatively or just to get a good exposure in low light conditions is very, very useful.
Chris
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