how your camera controls exposure - robert auckland

how your camera controls exposure

Exposure is simply how light or dark your photograph looks. There is no right or wrong exposure , just what looks right. Some pictures look great light and bright....


.....and other photos look best dark and moody.

I am always striving to make different photos at a weddings. This one is so far away from a high key white wedding dress. I love the sunlight on Vicki's Auburn hair. Vicki and Dave Mace Wedding Reception

The majority of photos look best when they are not too dark and not too light..... and if you want to get your exposure just right you need to think about the journey of the light from your subject to the back of your camera. On this journey the light passes through three things: the aperture in the lens, then the shutter and finally it arrives at the CCD at the back of the camera. All cameras have an aperture, a shutter and a light sensitive CCD (or a film). These three elements make up the exposure triangle


                                                                  ISO/light sensitivity  

             shutter                                                                                          aperture            

Check out the two cameras below, a modern digital camera and an old fashioned film camera. They expose in the same way but it's the film camera I'm going to take apart as it's much easier to see the aperture,shutter and light sensitive CCD/film. 


Twisting the lens off the Nikon film camera and looking at the back of the removed lens reveals the aperture. The aperture is a hole whose size you can change by twisting the lens barrel. An aperture works just like the pupil/iris of your eye. When it's bright  your pupil shrinks and   reduces the amount of light falling on your retina. When its dark  the iris opens up to allow more light to hit the retina. You vary the size of the aperture by twisting the lens barrel on a film camera or twisting a thumbwheel on the digital camera. 

Inside in dark conditions you should use a wide open aperture as shown in the photo below. This corresponds to a LOW aperture number like  f2.8 - this is the number next to the black spot on the lens barrel. The aperture number is usually called the f-number.

Remember : low f-number of f2.8  gives a wide open aperture for shooting in dark conditions.

OK, so lets say you step outside into bright conditions, the pupil of your eye shrinks to reduce the light hitting your retina. You need to reduce the size of your aperture so less light hits the film/CCD.  Look closely at the photos below to work out what f-number you should set.

Thats right, you need a large f-number of f22 in bright conditions! Hopefully you are starting to get a feel for the topsy turvy relationship between small apertures and large f-numbers. Here is a visual summary..



After passing through the lens aperture the light arrives at the shutter. The pictures below are taken with the camera back open. The film moves left to right in the direction of the white rails and winds onto a sprocketed drum just visible on the right. You can see the shutter wide open and exposing the film in the last shot. In the middle shot you might be able to see a narrow slit. You can think of the shutter as an eyelid. In the sunshine  we tend to narrow our eyes and blink quickly. In bright conditions you need a fast shutter speed of 1/1000th of a second. In dark conditions a slow shutter speed of 1 second would be needed ( the analogy with an eyelid mechanism is not perfect but it does help everyone to use shutter speeds in the right way). 

Here is a quick visual summary......


ISO/light sensitivity

The CCD or sensor is the place right at the back of the camera where the image is recorded

The ISO controls the sensitivity of the CCD to light. Reactolite sunglasses are dark in bright sunny conditions. Use a low ISO e.g. ISO 200 in bright conditions. When you head inside into the dark your glasses become clearer and so you should also increase your ISO to e.g.3200 ISO.

Trying to handle 3 variables : ISO, shutter and aperture is VERY difficult and 95% of people give up. SO get rid of the ISO variable by ALWAYS setting it manually to ISO200 outside and ISO 3200 when you head indoors. You MUST practice this first and then understanding the link between shutter and aperture will become easy. Again- a quick summary diagram..


Why not use a high ISO of 3200 all the time and never bother changing the ISO?

Well, although modern digital cameras are far better at high ISOs than film cameras there is a big drawback to high ISO's. Noise. Noise looks like mottling or grain in your photo; the grain is particularly obvious  in the shadow areas of your photo and it does degrade the image. This is why you should always work at a low ISO e.g. 200 when there is lots of light. Low ISO means higher image quality;less grain and better colours. 

Next we will look at how we can use aperture priority to set the aperture ourselves and let the camera set the shutter speed. Aperture also controls depth of field. This unlocks possibilities for blurring distracting backgrounds and for getting lovely crisp landscapes from from to back. Click here to find out how to blur backgrounds

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