Why use flash? Flash assists with the following situations: Insufficient light Incorrect lighting
Incorrect colour balance Very fast subject movement Special Effects
Insufficient light Night shots Dimly lit indoor shots etc There is less need for flash since the digital era as
camera sensors are much more sensitive to light than film. Modern digital cameras can use ISO numbers of several thousand (though the images are noisy)! However - Flash produces harsh unflattering lighting. Furthermore it is only effective up to a few meters from the gun.
Incorrect lighting Examples Sun behind subject. We can use ‘fill in’ flash to
light the front area. – provided we are close enough! Very useful for backlit portraits. Light comes in from one side – we can use offcamera flash to light the other side.
Incorrect colour balance Taking photos indoors without correcting colour balance gives a yellow hue. Flash has a very white light that will help in correcting this.
Very fast subject movement If you have a very fast-moving object you can
freeze the motion using a very fast shutter speed.
However this restricts the amount of light entering the
camera, necessitating a wide aperture. This in turn creates a narrow depth of field leading to focussing problems... Increasing the ISO leads to a noisier picture...
Flash has a very short duration and can freeze
very fast motion, at the same time providing plenty of light. Problem solved? – but see below!
Shutter Speed What shutter speed should I use? The flash itself has a very short duration (down to
approx 1/50,000 sec) so at first it does not appear to matter. But consider.... Flash has a limited range so.. We may want to use ambient light as well. (distant backgrounds – fill in flash etc). This may require a slower shutter speed so we must think about subject and camera movement. We may want to freeze fast motion using the ambient light as well as flash. (sports, wildlife). This will require a very fast shutter speed. Unfortunately there is a limit..
Shutter Speed continued What is the upper limit?
The upper limit for most DSLRs is 1/200 sec. (A few go up to 1/250 sec) Why? To answer this we need to delve into how shutters work. There are two ‘curtains’ in front of the sensor front and rear- (Imagine them as roller blinds). The front curtain rolls up from below and the rear rolls down from above. The normal positions prior to the shot are – front curtain rolled up to hide the sensor. Rear curtain up waiting. To get an exposure the front curtain opens (rolls down), then after a delay, the rear curtain closes (rolls down). THE FASTEST that either curtain can traverse the sensor is 1/200th to 1/250th sec. We will take the slower speed as example. During the time when both curtains are open the flash fires. The normal flash trigger setting is therefore just after the first curtain has finished rolling down.
Long shutter speed:
The blue is the front curtain that comes up from below. Red is rear curtain
falling from above. Before exposure: blue is up, red is up so sensor is covered. During exposure: blue falls – delay – red falls. So exposure is complete with red covering sensor. After exposure. Blue rises then red rises so sensor is not exposed but is ready for next shot.
The flash is triggered when blue is fully down. The main exposure is caused by the flash with little ambient light.
Shorter shutter speed: During exposure: blue falls – shorter delay – red falls. So exposure is shorter but still has a short time when shutter is fully open (yellow region).
The flash being very short in duration still has time to work.
Very short shutter speed above 1/250 sec: During exposure: blue falls – red starts to fall before blue has reached the bottom!!. The exposure now occurs as a band of light travelling down the sensor. The reason is that there is a limit of about 1/250 sec for the front curtain to fall completely due to its mechanical nature. This technique enables very fast shutter speeds of a few thousandths of a second.
However the flash has no opportunity to light the whole sensor in its duration. THE TOP IS CUT OFF as seen. This appears as black band at the bottom of the inverted image on the sensor.
This is not the end!!!!
Very short shutter speed above 1/250 sec: During exposure: blue falls – red starts to fall before blue has reached the bottom!!. The exposure now occurs as a band of light travelling down the sensor. The reason is that there is a limit of about 1/250 sec for the front curtain to fall completely due to its mechanical nature. This technique enables very fast shutter speeds of a few thousandths of a second.
To prevent this effect being seen all modern DSLR cameras prevent a shutter speed above or slightly below the 1/250 sec duration during flash. (Put your camera into shutter priority, employ flash, and see what is the highest speed you can dial in!) I can now show you this.. For the purpose of demonstration I have used a non-TTL flash on my
camera that the camera doesn’t know about. This is what happened: However the flash has no opportunity to light the whole sensor in its duration. THE TOP IS CUT OFF as seen. This appears as black band at the bottom of the inverted image on the sensor.
Picture taken with non-TTL flash. Shutter speed : 1/160 sec No problem.
Shutter speed : 1/200 sec No problem.
Shutter speed : 1/250 sec Still no problem so Nikon are playing safe by preventing this normally.
Shutter speed : 1/320 sec. Bottom of picture starting to cut off.
Shutter speed : 1/400 sec. Bigger cut off.
Shutter speed : 1/500 sec.
Shutter speed : 1/640 sec.
Shutter speed : 1/800 sec.
Shutter speed : 1/1000 sec.
Shutter speed : 1/1250 sec.
Shutter speed : 1/1600 sec.
Shutter speed : 1/2000 sec.
High speed sync There is a work around employed by many high
end cameras, so that sports pros for example can use flash and ambient light with high shutter speeds. This uses special flash triggering called high speed sync that gives a continuous series of flashes throughout the exposure. It would have to start the moment the front curtain starts to fall.
High speed sync Slow shutter speed
Fast shutter speed
Special Effects Slow sync Flash A combination of slow shutter speed and fast flash
duration enables you to use an interesting technique called slow sync flash. The shutter is opened recording the background – this could be a moving object such as a runner or cyclist or moving the camera to say twist the background. At a certain point the flash goes off, freezing a sharp image of the runner against a blurred multiple image of the runner, giving an effective impression of movement. Short of a runner I will use a lantern!
Slow sync flash continued There are three options here: Front curtain sync – the flash goes off when the front
curtain finishes. Not a good option for the runner shot. It would show the runner sharp at the start with her blurred image in front of her. –sort of forward time travel! Rear Curtain sync – the flash goes off at the end. This must be set by delving into your camera’s flash menu. Flash goes off in the middle. This can be done by triggering an off-camera flash gun manually or remotely. Most cameras would not have this option built in. This would freeze the object in the middle of its motion.
Slow sync flash – front curtain sync
In this case I moved the camera to the right. Normal Front curtain flash. Exposure .5 sec at f/9. The lantern was still but gives the impression of movement!
Rear curtain sync. The flash gun was triggered at the end of the exposure. A slow shutter speed (1 sec) captured only the blurred candle light and faint outlines of the lantern. The off camera flash caught a sharp image of the lantern at the centre of its swing along with a nice shadow. The blue background occurred because of the camera’s white balance being set to indoor light. The flash appears blue in comparison!
Placing flash gun or guns. On camera flash gives harsh flat lighting that does not do justice to most subjects. The background can be dark. With built-in guns we have no choice! Built in guns have TTL – the flash power is controlled by the camera’s exposure reading. Expensive guns also have this.
Placing flash gun or guns. Off-Camera flash: One or two guns placed at the sides of the object
for modelling and creating wanted shadows. One can experiment with heights and direction. Bouncing flash off ceilings or white walls, especially when back lighting objects. Using reflectors, diffusers and soft boxes all help to soften shadows. Some flash guns have slide out reflectors and diffusers. This brings up the problem of firing the off camera gun(s)
Triggering off-camera guns. Three main methods: Cable connection – either a hot shoe or plug in connector each end to suit camera and gun design. – cumbersome and fiddly to set up. Gets in the way. Optical triggering. A gun with an optical sensor that triggers in response to the flash from another gun. This requires at least one gun that is triggered another way. One can use the built in camera gun. The triggered gun needs its sensor pointing roughly towards the triggering light or its reflection. Most flash guns have a rotating head for this purpose. Wireless triggering. A transmitter with a hot shoe connector sends a radio signal to one or more receivers. These are mounted under the flash guns by suitable connectors. This is the method I use and is very versatile.
Adjusting output. With built-in guns choices are more limited! See later.
Add on Flash guns mounted on the camera: The power level of the flash gun can be adjusted manually or controlled by Through-the-lens metering (TTL) (see later)
Off camera guns TTL is possible but one would usually adjust the power manually.
Low powers with correct exposure give more
bias to ambient light and vice versa.
Exposure TTL – through the lens exposure When the shutter is pressed the camera sends out pre-
flashes before the shutter actually opens. The camera then measures this light coming through the camera lens by its built in meter. The real flash during actual exposure is set accordingly. A TTL flash gun is controlled by this light measurement to adjust the amount of light the gun emits! The overall exposure is then correct. You can even control the proportion of flash and ambient light using flash exposure compensation! Ouch! Built in guns can normally be TTL controlled. The more expensive external guns can also be TTL controlled with the correct triggering. Cheaper guns (like mine!) require other methods:
Exposure 2 Off camera flash meter. This is a specially designed meter that can
respond to the flash light. A test flash is carried out and reading taken. The gun output can then be adjusted. These are not cheap! And no I haven’t got one! Manual adjustment Since it costs nothing to take a shot you do a trial and error, adjusting flash gun(s) output and camera exposure settings for the best result of exposure of subject, background and shadow depth. Time consuming but in the end probably the best method.
Holly!
Thats it folks!!
I will set up my flash
equipment for a practical portrait session after the break.
