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Exposure
Setting manual exposure
Too much light
Zebras
Gain
AE shift
How to get the exposure right

Sony PD 150 camera workbookPART 3: EXPOSURE AND GAIN

by Christina Fox

There are lots of ways to control the exposure of your picture. We look at all of them and show you how to be sure you're getting the exposure just right.

We have to be able to control the amount of light that passes through the lens and goes into the camera. If there is too much light the picture is said to be over exposed - too little light and it will be under exposed.

We can control the amount of light coming through the lens by the use of an IRIS or APERTURE. This works in the same way as the iris (or pupil) of your eye: when you are in bright light the iris is made as small as possible (it 'stops down') to prevent too much light entering your eye, when you are in a darkened room your iris becomes as wide as possible ('opens up') to let in as much light as possible. Of course your brain does all this for you and is constantly adjusting your iris depending upon the prevailing light levels. Domestic and professional cameras allow you to operate the camera iris in manual or in automatic. However, professional users tend to operate in manual only, using the auto iris only to do an occasional 'spot check'.

The AUTO EXPOSURE works by looking at the whole scene, averaging out the bright areas and dark areas and setting the exposure for this average. The problems start when something bright or dark comes into frame. e.g. If a white car drives across the frame the average light level of the scene will go up. The camera over compensates by stopping down the iris causing the surrounding picture to be under exposed. If a black car now drives past the average light level will go down and the iris will open up. Now the surrounding scene will be over exposed. This exposure "hunting" looks amateurish, is a dead give away that you're in auto AND is totally unnecessary.

There are a selection of apertures to choose from when the camera is in manual exposure mode. Apertures are referred to as f-stops (or sometimes just "stops") - the smallest aperture is f11on most cameras (although some lens are able to offer even smaller apertures) which lets the least amount of light into the camera. The largest aperture is f1.6 which allows the maximum amount of light into the camera. The aperture sizes (f stops) available are:

OPEN 1.6      f2      f2.4      f2.8       f3.4      f4      f4.8      f5.6      f6.8      f8      f9.6      f11 CLOSED

Largest aperture <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>Smallest aperture

 

 (not to scale)

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SETTING THE MANUAL EXPOSURE

  1. Set the AUTO LOCK switch into the middle position
  2. Press the IRIS button on the barrel of the lens - the letter F and a number will appear in the viewfinder/LCD.
  3. You can now select the aperture size manually by using the silver IRIS dial.

To return to AUTO mode just press the IRIS button again.

NB if the gain is in AUTO mode when you manually select iris you'll find the gain increase as you stop down in low light conditions. If you do not want gain to rise make sure you switch it to manual and set it to 0dB until you need it.

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TOO MUCH LIGHT

Daylight on a clear sunny day can be around 35,000 lux. In very bright sunlight or in highly reflective places (i.e. where there is sand or snow) it is necessary to reduce the amount of light entering the camera so that the pictures do not become over exposed. There are two ways you can reduce the amount of light entering the camera:

  1. IRIS - higher f number = less light. f11 is the highest.
  2. NEUTRAL DENSITY (ND) FILTERS - reduce the amount of light entering the camera. The ND1 filter reduces light to (= 2 stops) while ND2 reduces light to 1/32 (=4 stops) , but does not affect the white balance.
  • When the subject has a bright background behind them use the BACK LIGHT button
  • when the subject is under a spotlight i.e. more brightly lit than the background try the SPOT LIGHT button.
  • However, rather than use the back light and spot light button you may find the AE SHIFT function more flexible - or even better - why not try manual exposure.

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HINTS

  • If the camera is on AUTO exposure it will automatically reduce the iris to a minimum to reduce the amount of light entering the camera.
  • In manual you can do the same by selecting IRIS and reducing the aperture.
  • The camera will also automatically inform you of the need use the ND FILTER by flashing ND1 or ND2 in the view finder.
  • The NEUTRAL DENSITY (ND) filter switch is to the right of the manual zoom ring.
  • When the light level drops ND OFF will flash in the viewfinder and the ND FILTER should be turned off.
  • The shutter will also reduce the amount of light entering the camera. A shutter speed of 1/120th of a second will reduce the amount of light entering the camera by about a half.
  • Another problem with very bright light is that it causes harsh shadows which will look almost black to the camera and unflattering to your subject. One way of reducing these shadows is to use a reflector (either a professional one made by Lastolite or a home-made version such as a piece of white paper,notebook or a newspaper). The trick is to get the light source to bounce off the reflector and into the shadows.

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ZEBRAS

The ZEBRA switch is in the panel protected by the LCD screen.

  1. ZEBRA OFF no zebra stripes
  2. ZEBRA 70% - zebra stripes appear on any part of the picture that is about 70% bright.
  3. ZEBRA 100% - zebra stripes appear on any part of the picture that is 100% bright or above.

Zebra stripes are a device that puts diagonal lines into highlights in the viewfinder (not on to the tape! Just in the viewfinder). They act as an indication of exposure levels and so can be used as an aid to getting the exposure right.

Think of the 100% zebra level as an audio meter it tells you when things are too loud (in this case, too bright). When you record sound too loud, it distorts and becomes unusable. Well in video terms if large parts of your picture are 100% bright and above the detail in those areas will becomes crushed (and so lost ).

The 70% zebra is probably more important, because it help you get faces correctly exposed. White Caucasian faces - when correctly exposed are between 60-70% bright. So if a face has a little bit of 70% zebra on it then it will be about right. There will always be a few hot spots such as noses, foreheads and the shiny bald head - it's the reason we use makeup in TV to reduce the hot spots.

Remember a good way to check exposure is to ask the camera. Zoom in to the face - go to auto iris and let the camera decide the face's exposure - go back to manual iris - and zoom out to frame your shot.

There really is no real guideline as to how much Zebra you'll see in the picture. It entirely depends upon what you are shooting and how much there is of it in the frame. Eg a correctly exposed shot of the sky could have 70% zebras all over it - but little or no 100% zebras

PLEASE NOTE -

  1. Working in the manual exposure mode you would be well advised to use the zebra stripes.
  2. Professionals tend to set their Zebras to between 90-95%. Because 70% is thought to be too low and 100% to high.
  3. Zebras only appear in the viewfinder and LCD screen - not on the output. Think about it - when did you last see zebra stripes on the telly (except on wildlife programmes!).

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GAIN

When shooting in low light conditions (e.g. at night) the iris is will be fully OPEN at f1.6 but, the camera may still needs more light to produce a picture. In this case the EXPOSURE control can be used to switch in GAIN (up to 18dB) until the correct exposure is achieved

The GAIN is an electronic method of increasing the brightness of your picture. There are seven gain levels to chose from:

DARKER PICTURES<<< 0 dB    +3 dB    +6 dB    +9 dB    +12 dB    +15 dB    +18 dB >>> BRIGHTER PICTURES

0dB of Gain    9dB of gain added   18dB of gain added

(If you want to see this image across the full range of gain see our Gain page)

DECREASED PICTURE NOISE <<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>INCREASED PICTURE NOISE

SETTING THE GAIN MANUALLY

  1. Set the AUTO LOCK switch down to the middle position
  2. Press the GAIN button at the back of the camera (beside the battery)- A number will appear in the viewfinder/LCD between 0dB and 18dB.
  3. You can now select the gain level you want by turning the SEL/PUSH EXEC dial (below the AE SHIFT button).

To return to AUTO mode just press the GAIN button again.

As you dial in more and more gain you will notice that the picture becomes grainy or noisy.

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AE SHIFT

You can adjust the automatic exposure levels to give you brighter or darker pictures compared to the normal default setting.

  1. Set the AUTO LOCK switch down to the middle position
  2. Press the AE SHIFT button. AS 0 will appear in the viewfinder/LCD
  3. Turn the SEL/PUSH EXEC dial (below the AE SHIFT button) to adjust the auto exposure levels.

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HOW DO YOU EXPOSE CORRECTLY?

I reckon there are three ways of setting exposure.

METHOD 1: Does it look right!
Unfortunately this method has two downsides. If you're a beginner you probably not sure what "right" is anyway. Plus, you need to set up your viewfinder/LCD screen correctly. If your screen is too bright or too dark you could get false positives. But as you get more experienced with your kit and after shooting in a range of conditions - you'll just sorta know when it is right.
METHOD 2: Ask the camera.
Zoom in to the subject - go to auto iris and let the camera decide the subjects's exposure - go back to manual iris - now zoom out to frame your shot.
You must zoom in to the subject to prevent the auto function under or over compensating for the surrounding light levels. Remember there will be times when the subject will be correct but the background is too bright or too dark. That's the nature of video ie poor contrast ratios (about 50:1).
 
METHOD 3: Use the zebras.
see above

The trick is to use all three techniques together. eg start by asking the camera (method 2). it says F5.6, but you've a little more zebra (method3) than you think is necessary - then iris down until it looks right (method 1).

BEWARE...
The shutter, gain and iris all effect exposure. So, if you leave either shutter, iris or gain in automatic - the camera will be changing them behind your back (or should that be under your nose!) which is not a good idea.

My advice would be to set the shutter manually to 50 (PAL) or 60(NTSC). Only use the shutter for effect.

Gain should be set to 0dB. That said you can usually get away with 3 and 6dB without people noticing. At 9dB and above you'll start to get grainy pictures.

Expose using manual iris (or in Auto to start with until you get used to the camera). Zebras on 70% with the occasional flick to 100%.

..and if the sun comes out and it all gets a bit too bright - flick in one of the ND filters.

© 2000 - 2010

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Move on to...
Part 4: WHITE BALANCE
Or back to...
Part 2: FOCUS
Also check out our images showing a night scene recorded at 0dB up to 18dB of gain.
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Christina Fox

Reader's Comments

>>>>How do I use the Zebras?<<<<<

Zebras are one of the tools for getting exposure right. Think of the 100% zebra level as an audio meter it tells you when things are too loud (in this case, too bright). When the sound is recorded too loud, it distorts and becomes unusable. Well in video terms if large parts of your picture are 100% bright and above the detail in those areas will becomes crushed (and so lost ).

The 70% zebra is probably more important, because it help you get faces correctly exposed. White Caucasian faces - when correctly exposed are between 60-70% bright. So if a face has a little bit of 70% zebra on it then it will be about right. There will always be a few hot spots such as noses, foreheads and the shiny bald head - it's the reason we use makeup in TV to reduce the hot spots.