A Beginner s Guide To Exposure
What is exposure? A Beginner s Guide to Exposure
What is exposure? According to Wikipedia: In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film, as determined by shutter speed, lens aperture and scene luminance. In digital photography film is substituted with sensor. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance in a specified region. In photographic jargon, an exposure generally refers to a single shutter cycle. For example: a long exposure refers to a single, protracted shutter cycle to capture enough low-intensity light, whereas a multiple exposure involves a series of relatively brief shutter cycles; effectively layering a series of photographs in one image. For the same film speed, the accumulated photometric exposure (Hv) should be similar in both cases.
What is exposure? Simplified: Letting the right amount of light into the camera to get the result that you want.
The variables in exposure Available Light How much light is around you, bouncing off things or being created by lights ISO How sensitive your sensor is Shutter Speed How long your camera s shutter is open for Aperture How wide open the iris is.
ISO How sensitive your camera s sensor is to light Lower ISO values are less sensitive to light Produces less noise or graininess in the image Higher ISO values are more sensitive to light Produces more noise or graininess in the image Keep the ISO down For the best image quality and detail, keep the ISO to as low a level as possible for the conditions.
Shutter Speed How long your shutter is open for Faster shutter speeds let in less light It freezes action Slower shutter speeds let in more light Blurs motion Set for the style of photo that you require Depending on if you wish to freeze the action or not. The slower the shutter speed, the more camera shake is likely and a tripod may be required. Use a slow shutter speed and panning to give a moving subject a blurry background.
Aperture How wide open the lens iris is Larger f numbers let in less light Gives you a greater depth of field. More of the image is in focus. Smaller f numbers let in more light Gives you a shallow depth of field. Less of the image is in focus. Set for the style of photo that you require A shallow depth of field will make a subject pop out on a blurry background. A larger depth of field will make more of the image sharp. For instance, in a landscape photograph.
Your Camera All three variables (ISO, Aperture and Shutter speed) can be set on your camera and it is those variables that need to be balanced to get the right exposure for your image. In Auto or semi-auto modes, your camera will set one or more of those variables according to the others to attempt at getting the correct exposure.
Auto Mode or Scene Modes In Auto mode, you have no control over the setting of any of the variables. It will make a best guess on all of them and should give you a decently exposed image. In Scene modes, it will do the automatic adjustments prioritising one or more to get the picture appropriate to that scene mode. Landscape mode will give you a high f number for more depth of field. Sport mode will give you a fast shutter speed to freeze the action.
Semi Automatic Modes In Aperture priority, shutter priority or sensitivity priority (on some cameras), you can set two of the variables and the camera will calculate the remaining one. If the remaining variable is outside the possible range, your screen and viewfinder will flash to show that. Taking an image when that happens will either give you an over-exposed or under-exposed image.
Aperture Priority (A or Av) In Aperture priority, you set the desired Aperture value and the ISO and the camera sets the shutter speed accordingly. As the shutter speed on most cameras has the greater range, it is less likely to go out of bounds. Set the aperture to your desired effect and watch the shutter speed. If it is too slow to hand hold, or too fast for the camera, change the ISO to compensate.
Shutter Priority (S or Tv) In Shutter Priority, you set the desired shutter speed and the ISO and the camera sets the aperture accordingly. Most lenses have a relatively small range of apertures so it is possible that the camera will not be able to set the correct aperture. Set the shutter speed to your desired time and if the f number is incorrect, change the ISO to compensate.
Manual Mode (M) You are in full control of all the components for correct exposure. You will need to set the ISO, aperture and shutter speed yourself. Most cameras will indicate your exposure on a bar in the display and/or viewfinder. Manual mode is best used when you want full control over exposure. For instance, to eliminate the effect of changing or flashing lights.
Stops You will find that a lot of tutorials and photographers will talk about Stops. A stop is effectively the doubling or halving the amount of light entering the camera. Most cameras allow adjustment in ½ or ⅓ of a stop, ⅓ being most popular. Adjusting a value by a stop will mean clicking the adjustment wheel 3 times if your camera uses ⅓ stop.
Stop scale: Shutter Speed Each box on the scale going from left to right represents the halving of the amount of light entering the camera. The slowest speeds to the left will create blur, whilst those to the right will freeze action. The further from the right, the more likely you ll get camera shake.
Stop scale: ISO sensitivity Each box on the scale going from left to right represents the halving of the amount of light entering the camera. The highest ISOs to the left will introduce more noise. Always try to keep the ISO as low as possible to get the best image quality.
Stop scale: Aperture Each box on the scale going from left to right represents the halving of the amount of light entering the camera. The lowest f numbers will give you a shallow depth of field whilst those to the right will give you greater areas of focus. Each lens has a sweet spot, normally around f5.6 to f8. This gives you the best image quality without colour fringing.
Combined Stop scale These are the combined boxes which go to make up your exposure. These boxes will need to be filled according to available light.
Available Light This is the one part of exposure that you really have no control over. You can partially add light using flashes, torches or floodlights but your ambient light is fixed. Your available light is measured in EV and has a scale running from -12 (shots of the Milky Way) right through to +16 on a bright sunny day in snow. Your camera will measure the EV averaging to that 18% grey and that is what it uses to base the exposure on.
Available Light Let s think of light as tokens which can be spent, ranging from 0 to 30. Each location will give you tokens to spend on ISO, Aperture and Shutter speeds to create your image. You have to spend all the tokens to get a properly exposed image. If you have tokens left over, it will be over-exposed and if you don t have enough tokens, it will be under-exposed. For simplicity, each token represents one stop. In reality, these tokens can be split into thirds or halves depending on your camera s setting.
Scenario one It s a sunny day with a bit of cloud and you re shooting a landscape. You want a relatively large depth of field. Seeing as the most important part of your composition is depth of field, your Aperture is of highest priority. Set your camera to Aperture Priority (A or Av) and dial in the desired f number, say f11 and set your ISO to 100.
The light has given you 26 tokens. A Beginner s Guide to Exposure
You ve chosen an ISO of 100 which uses 7 tokens. A Beginner s Guide to Exposure
and an f number of f11 which uses 8 tokens, leaving 11 tokens.
The camera works out the exposure based on the remaining 11 tokens, giving you a shutter speed of 1/30 of a second.
You re using a wide angle lens at 18mm on a crop sensor (28mm on full frame), so this is getting close to the hand held limit. To be sure of getting no camera shake, you need to find another token to increase the shutter speed to 1/60s.
You have two choices. Increase the ISO to 200 and sacrifice some image quality to a bit of noise. Decrease the f number to f8 and sacrifice a bit of depth of field.
Scenario two It s a very clear and sunny day and you re shooting a statue and want a blurry background (shallow depth of field). Once again, you want control of your Aperture as the shallow depth of field is of primary importance. Set your camera to Aperture Priority (A or Av) and dial in an f number of f2.8 and set your ISO to 100.
The light has given you 29 tokens. A Beginner s Guide to Exposure
You ve chosen an ISO of 100 which uses 7 tokens. A Beginner s Guide to Exposure
and an f number of f2.8 which uses 4 tokens, leaving 18 tokens.
The camera works out the exposure based on the remaining 18 tokens, giving you a shutter speed of 1/4000 of a second.
That s about as fast as your camera can go. Any brighter and you d have to sacrifice some of the depth of field by upping the f number. If you re lucky, your camera might be able to go down to ISO 50.
Scenario three It s getting dark and you are wanting to take some images around a village and you don t want to use a tripod. In this case, you re looking to keep the shutter speed up to a decent level to hand hold. The lens is 18mm (28mm) so you ll need a fast enough shutter speed to avoid camera shake. Set your camera to Shutter Priority (S or Tv) and dial in a speed of 1/30 of a second and set your ISO to 400 to give a fighting chance.
The light has given you 18 tokens. A Beginner s Guide to Exposure
You ve chosen an ISO of 400 which uses 5 tokens. A Beginner s Guide to Exposure
and a shutter speed of 1/30 which uses 11 tokens, leaving 2 tokens.
The camera works out the aperture based on the remaining 2 tokens, giving you an f number of f1.4.
x x This is way below your lenses smallest f number of f2.8 so if you took an image now, it would be under exposed. You need to find two tokens to get a correct exposure.
x x x x The shutter speed is about as low as you can go so you can t take the tokens from there.
x x The only option is to take them from the ISO, increasing that to 1600 and introducing noise. It s not desirable but at least you will be able to get a blur free image.
Scenario four Same spot, same village but you want to get some crisp stars around the lights and some light trails from the traffic. In this case, you re looking to get a nice long shutter speed and a nice high f number. Set your camera to Shutter Priority (S or Tv) and dial in a speed of 15 seconds and set your ISO to 100 to give the best chance of a clean image. You could also use Manual mode (M) and use the figures from the previous reading to set up.
These are your previous settings from the hand held shot.
x x x x x x x x x x x x x Dial in a shutter speed of 15 seconds which will give you back 9 tokens plus the 4 from the Aperture making a total of 13.
Set your ISO to 100 which will cost 4 tokens leaving you with 9 tokens.
The camera will work out the Aperture value using the 9 tokens which will be f/16.
ND Filters ND filters, or neutral density filters, screw onto the front of your lens and effectively act like sunglasses for your camera. They reduce the amount of light entering the lens. They come in varying strengths and are labelled in different ways and this can sometimes be confusing. You can also get a variable ND filter which can be changed by rotating the filter to give differing strengths.
ND Filters By far the common way of describing the strength of a filter is using the NDx format. An ND4 filter will give you two stops of light reduction. An ND32 filter will give you five stops. There is also a filter called a big stopper. It is labelled as an ND100 or ND110 but really, using the above formula, it should be an ND1024 or more. It will give you 10 stops of light reduction.
ND Filters If you use an ND filter, you are effectively using some of your light tokens on it. The number of tokens depends on the strength of the filter. A two stop ND4 will cost 2 tokens, A five stop ND32 will cost 5 tokens.
Scenario five It s a very clear and sunny day and you re shooting a statue and want a blurry background (shallow depth of field). Once again, you want control of your Aperture as the shallow depth of field is of primary importance. Set your camera to Aperture Priority (A or Av) and dial in and f number of f2.8 and set your ISO to 100.
Based on scenario three, these are the settings that your camera has given you. Remember that 1/4000s is very close to your limit.
x x x x x An ND32 is a five stop filter, reducing the Shutter speed by 5 tokens bringing it down to 1/125 of a second. This will allow for proper exposure if it gets brighter.
High number ND Filters As soon as you start getting into the high number ND filters say ND32 and above, it becomes increasingly difficult to see through them. That means composition and focusing is difficult for both you and the camera. You will need to do all of this before fitting the ND filter.
High number ND Filters Set your camera up, focus on your subject and make a note of the settings on the rear of the camera. Switch to Manual (M) mode and dial those settings in so you don t forget them. Set all three exactly as the camera measured them. You will then need to take your tokens from those values according to how many stops your ND filter has.
Scenario six It s a very clear and sunny day and you re at a beach with the water lapping up onto the sand. You will need a very slow shutter speed to blur that motion, somewhere around 10 seconds. You have your 10 stop ND filter with you. Set your camera on Shutter priority, and set it at 1/1000s, ISO 100 and let the camera do it s work.
The light has given you 29 tokens. A Beginner s Guide to Exposure
You set ISO to 100 costing seven tokens. A Beginner s Guide to Exposure
You set shutter speed to 1/1000 of a second costing 16 tokens.
The camera uses the over 6 tokens to set the aperture at f5.6.
x x x x x x x x x x x x You know that your ND filter is using 10 stops and the primary concern is getting that shutter speed as low as possible. Start by removing the 10 tokens from the shutter speed.
Still not low enough. It s giving you a 1 second exposure.
x x x x Let s fill the Aperture section up to f22, using four tokens from the shutter speed.
x x x x That should do the trick. You ve now got a 15 second delay in bright sunshine. Due to the nature of ND filters blocking out light, both you and the camera would be unable to do this automatically.
Exposure compensation Cameras are dumb. They are computer controlled and it is that computer that evaluates exposure. Your camera will expose the image averaging to 18% grey. If your subject is a bright object, it is likely to underexpose the image. If your subject is dark, it is likely to overexpose.
Exposure compensation You can over-rule your camera by using the exposure compensation button. By holding the button and using your control wheel, you can either increase or decrease the exposure that the camera has evaluated. Each click of the wheel will give you ⅓ of a stop. This setting is normally retained, so make sure that you reset it to zero when you re finished with it.
Exposure compensation If you subject is dark, for instance a shaded woodland, the camera will tend to make the image too bright and turn the blacks to a dark grey. By using a negative exposure compensation, you can bring that exposure down to make it more natural. A setting of -⅔ will normally do the trick. A night scene may require more depending on the light in the image, maybe -1 or as much as -1⅔
Exposure compensation If you subject is light, for instance a brightly lit white room it will make those whites appear a light grey. By using a positive exposure compensation, you can bring that exposure up. A setting of +⅓ would bring back those whites. A high white content, like a snow scene might require +1 or +1⅓. A misty or foggy day might need +⅔.
Exposure compensation Sometimes, your lens may not report exposure accurately or may differ slightly from lens to lens. You may want to use exposure compensation all the time if this happens. Most lenses that do this tend to over expose so a little negative exposure compensation might need to be dialled in. This will need to be added on to the previous light and dark compensation.
Vibration compensation A lot of lenses and some camera bodies have built in vibration compensation (often known as shake reduction). This moves either the sensor or the lenses to reduce camera shake. In general terms, it will give you a 2 or 3 stop advantage on shutter speed meaning that you ll need to spend less in the shutter speed section of the stop chart.
Vibration compensation If you calculate that you need a shutter speed of 1/30 of a second (the normal 18mm/28mm wide angle lens) in order to hand hold the camera, vibration compensation will allow you to reduce the number of tokens spent in the shutter column. If you have a 2 stop advantage, that will bring the speed down to 1/8 of a second, a 3 stop would bring it down to 1/4 of a second. Although this will still give you an image free from camera shake, reducing the shutter speed will introduce motion blur.
Metering modes Most cameras have a way to determine the way that exposure is calculated and these are known as metering modes. By default, the camera will use the whole of the image to determine exposure and this is known as Multi Segment. Your camera will run its own calculations. Some cameras are better than others at doing this and may be able to work out what you re taking a photo of and adjust accordingly.
Metering modes A second mode often found is Centre weighted, which does the same as the Multi Segment but gives greater value to the shades of grey in the centre of the image. This mode may solve problems with very bright or dark backgrounds and give a better overall exposure.
Metering modes Another option is to use Spot Metering which takes a very small area around the focus point (or some cameras around the centre point) and calculates the averagre grey from that. It s particularly useful for tricky situations where you need a specific subject to be correctly exposed. For instance, you could use spot metering on a person and get the camera to meter for the face.
Metering modes In most cases, the Multi Segment mode is your go to place. Understanding what your camera is going to do with a specific scene and adjusting the Exposure Compensation is a lot easier than changing metering modes. If you do change the metering mode, remember to set it back to the default or you ll end up getting a lot of wrongly exposed images.
Exposure bracketing A lot of cameras allow you to take multiple exposures, normally in odd sets (ie 3,5,7). You can choose how far apart the exposures are in stops. This will allow you to have a correctly exposed image, and a number of under and over exposed images. The first image taken is normally the correct exposure including any exposure compensation that you have dialled in.
Exposure bracketing For instance, a 3 set of bracketed image with a 1 stop value will be based on your exposure compensation and the images taken will be 0, -1 and +1 stops. These images can be combined to make an HDR image or used separately to ensure that you get the correct exposure you are looking for.
Exposure bracketing If you have bracketing turned on, you will need to take those 3 individual shots to complete the cycle. If you only take one or two, it will be out of sync and the exposures will be all over the place. Make sure that you turn off Exposure Bracketing after you have used it otherwise you will end up with over and under exposed images being shot for the rest of your trip.
f/2.8, 1/125s, ISO 100
f/5.6, 1/30s, ISO 100
f/11, 1/8s, ISO 100
f/22, 1/2s, ISO 100
f/22, 1/8s, ISO 400
f/22, 1/30s, ISO 1600
f/22, 1/125s, ISO 6400
f/11, 1/125s, ISO 1600
f/5.6, 1/125s, ISO 400
f/2.8, 1/125s, ISO 100
Final thoughts That is basically the idea behind exposure. You are given a certain amount of light that has to be balanced between the three settings, Aperture, Shutter speed and ISO sensitivity. If you take from one, you will need to give to another to keep the balance and a correctly exposed image. Cameras sometimes mis-calculate exposure and need a bit of help using exposure compensation. After a while, you will get to know what your camera is likely to do and will know to adjust the compensation value before you take your image.