combining

Here I will try to explain the basics in exposure and how exposure effects the finished image... I will start off by telling you that there is no such thing as a universally perfectly exposed image... The way one person likes an image to look is not necessarily how someone else would like it to look, therefore it can never be perfect... A technically correctly exposed image should have details in both the highlights and shadows with a good tonal range and be capable of producing a clean white and a crisp black, if there are a white and a black in the image...

Basically, exposure is the term used for the amount of light which is allowed to fall on the recording medium, Slide film, Negative Film or Digital sensor... The amount of light is controlled by two elements, the shutter speed and the lens aperture... It´s the combination of these two elements which are permanently linked and must be adjusted in unison to maintain the correct exposure in normal lighting conditions... There are however certain lighting conditions where a little compensation is required when using a reflected light reading like those from your camera to obtain the correct exposure...
Hopefully you will have read my pages on shutter speeds and aperture values, or already have a basic knowledge of them both before proceeding... Let´s overview the ratio between shutter speeds and aperture values from a correct exposure reading of 1/125 @ f/5.6... • Every time you open the aperture by 1 full f/stop you are letting in twice as much light as the previous aperture, so you must decrease the amount of time the shutter is open by half to compensate for it...

• Every time you increase the time the shutter is open by 1 full stop you are allowing in twice as much light as the previous shutter speed, so you must stop down the lens by 1 full f-stop to compensate for it...

In both cases one action counteracts the other so the exposure is still correct... All the alternate combinations marked in green in the table on the left would produce the same correct exposure as our starting point of 1/125 @ f/5.6, only the depth of field and the ability to freeze action will have changed... If you chose the 1/2000 @ f/1.4 option the action would be frozen in most situations but very little apart from the point of focus would be sharp in the image... If you chose the 1/8 @ f/22 option the image would probably be sharp from front to back if you focused about 1/3 of the way into the scene but any movement would create a visible blur in the image...

Above you will see some examples of aperture sizes in relation to each other... Below you will see an example of how long a shutter must stay open to compensate for a change of aperture of four stops in 2 stop increments if the original exposure was 1 second @ f/2... The reason for this being the same amount of light has to pass through a smaller aperture taking longer, the smaller the aperture the longer the shutter needs to stay open... At f/2.8 it would take 2 seconds, at f/5.6 it would take 8 seconds and at f/11 it would take 32 seconds...

If the water in the tanks shown below were the amount of light needed to expose an image correctly you would somehow need to get that water on to the sensor, or film, to create that correct exposure...

If you made a large hole in the tank like f/2.8 it would not take as long to transfer the water to the sensor or film compared to putting smaller holes in the tanks like f/4 and f/8 as shown...

It would take 1 second at f/2 to empty the water... 2 seconds at f/2.8, 4 seconds at f/4, 8 seconds at f/5.6 and 16 seconds at f/8...

Each stop down of the lens will make the water take twice as long to empty as the previous one...

BASIC EXPOSURE CORRECTION... Here I will try to explain the basics in exposure and how exposure effects the finished image... I will start off by telling you that there is no such thing as a universally perfectly exposed image... The way one person likes an image to look is not necessarily how someone else would like it to look, therefore it can never be perfect... A technically correctly exposed image should have details in both the highlights and shadows with a good tonal range and be capable of producing a clean white and a crisp black, if there are a white and a black in the image...
Basically, exposure is the term used for the amount of light which is allowed to fall on the recording medium, Slide film, Negative Film or Digital sensor... The amount of light is controlled by two elements, the shutter speed and the lens aperture... It´s the combination of these two elements which are permanently linked and must be adjusted in unison to maintain the correct exposure in normal lighting conditions... There are however certain lighting conditions where a little compensation is required when using a reflected light reading like those from your camera to obtain the correct exposure... Hopefully you will have read my pages on shutter speeds and aperture values, or already have a basic knowledge of them both before proceeding... Let´s overview the ratio between shutter speeds and aperture values from a correct exposure reading of 1/125 @ f/5.6... • Every time you open the aperture by 1 full f/stop you are letting in twice as much light as the previous aperture, so you must decrease the amount of time the shutter is open by half to compensate for it... • Every time you increase the time the shutter is open by 1 full stop you are allowing in twice as much light as the previous shutter speed, so you must stop down the lens by 1 full f-stop to compensate for it...
In both cases one action counteracts the other so the exposure is still correct... All the alternate combinations marked in green in the table on the left would produce the same correct exposure as our starting point of 1/125 @ f/5.6, only the depth of field and the ability to freeze action will have changed... If you chose the 1/2000 @ f/1.4 option the action would be frozen in most situations but very little apart from the point of focus would be sharp in the image... If you chose the 1/8 @ f/22 option the image would probably be sharp from front to back if you focused about 1/3 of the way into the scene but any movement would create a visible blur in the image...
Above you will see some examples of aperture sizes in relation to each other... Below you will see an example of how long a shutter must stay open to compensate for a change of aperture of four stops in 2 stop increments if the original exposure was 1 second @ f/2... The reason for this being the same amount of light has to pass through a smaller aperture taking longer, the smaller the aperture the longer the shutter needs to stay open... At f/2.8 it would take 2 seconds, at f/5.6 it would take 8 seconds and at f/11 it would take 32 seconds... THIS MIGHT EXPLAIN IT BETTER... If the water in the tanks shown below were the amount of light needed to expose an image correctly you would somehow need to get that water on to the sensor, or film, to create that correct exposure... If you made a large hole in the tank like f/2.8 it would not take as long to transfer the water to the sensor or film compared to putting smaller holes in the tanks like f/4 and f/8 as shown... It would take 1 second at f/2 to empty the water... 2 seconds at f/2.8, 4 seconds at f/4, 8 seconds at f/5.6 and 16 seconds at f/8... Each stop down of the lens will make the water take twice as long to empty as the previous one...
2 seconds @ f/2.8	4 seconds @ f/4	16 seconds @ f/8
Basics Menu - Aperture - Exposure Correction - Combining - Digital Exposure - Depth of Field - Resizing - Rule of Thirds - Shutter Speeds - Crop Factor

KEN McDONALD

P H O T O G R A P H Y

BASIC EXPOSURE CORRECTION...