Understanding Reciprocity in Photography

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Perfect exposure settings: Reciprocity may be the law with the relationship between shutter and aperture. It stipulates any particular one stop increases in aperture is the same as the shutter duration doubling. Both increase light by one stop.

reciprocity in photography

“Jump inside the water” captured by Ornela Pagani (Click Image to See More From Ornela Pagani)

Thus, once you have the correct level of light for perfect exposure you are able to choose to increase aperture by one-stop and trade this off with a doubling from the shutter speed (halving with the shutter duration). Twice and far light arriving for the rest = same amount of light.

This allows the photographer to support the same exposure but change either aperture or shutter for artistic or logical reasons.

In addition to this particular the photographer may wish to over expose or under expose. Understanding what the law states of reciprocity allows them to do this in the controlled and intuitive manner, comprehending the exposure differences on the image PLUS the different changes on the depth of field or any motion blur increase or decrease.

Sometimes however, the aperture you need and the shutter speed required do not give an acceptable exposure with the available light (either natural or including flash). Fortunately there's another variable which effects exposure using the same level of effect and working inside same measurements of stops of light.This is ISO and refers merely towards the cameras sensor’s sensitivity to light.

You therefore have THREE variables to use. Each however modify the image beyond just the brightness of the final image.


The Law of Reciprocity

In summary image brightness is dependent upon the amount of light that contacts using the light sensitive sensor and how sensitive that sensor is.

This is controlled through the duration in the exposure (the shutter speed), the intensity of the light (aperture) as well as the sensitivity of the sensor or film (ISO).

shutter speed reciprocity

“tree” captured by Aaron Bauer (Click Image to See More From Aaron Bauer)

When setting these 4 elements you take into consideration the law of reciprocity which states an inverse relationship relating to the intensity and duration the camera is confronted with in order to shoot at the correct exposure. ISO merely acts just as one additional technique of control.

Exposure = intensity x time so equivalent exposures could be gained by decreasing one variant and improving the other proportionally. So if duration is halved, intensity should be doubled.


Aperture = How FAST does light get in

Aperture could be the diameter in the lens opening controlled by an iris. The larger the iris the greater light gets in over the given time period.

Aperture is discussed in f-stops’ e.g. f/4 or f/2.8 (the length from the focal lens divided with the diameter) but like all things photographic this refers to stops of light. The smaller the f number the larger the aperture.

Apertures may be remembered with a simple concept, the numbers 1 and 1.4 doubling alternately. So 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 with each being a growth by 1 stop. Therefore f/1.4 is one stop higher than f/1… f/8 is three stops greater than f/2.8.

aperture and reciprocity

“Inside the Pyramid” captured by Richard Crowe (Click Image to See More From Richard Crowe)

Larger apertures are preferable in low light allowing more in the light in. Every lens includes a maximum aperture or aperture range (for a lot of telephotos the location where the aperture closes when zoomed in)

The larger your aperture nevertheless the shallower your depth of field.


Shutter Speed = How LONG the lens lets light in for

A cameras shutter normally remains closed. When the button is pressed your camera’s shutter opens for the given stretch of time. (either set by you or cameras auto settings). The ‘Shutter speed’ is the term for how long the shutter stays open for.

A shutter speed of say 1/100 sec is open for twice as long as say 1/200 second, letting learn how to for twice as long.

Rather conveniently shutter speeds are discussed in seconds and fractions of seconds and will also be compared in stops of light with doubling of duration meaning a boost of 1 stop.

reciprocity and shutter speed

“Subway” captured by Joey (Click Image to See More From Joey)

1/ 100 second is 1 stop of light brighter than 1/200. 3 seconds is 1 stop less than 6 seconds.

Therefore, along with the aperture setting, shutter is controlling the sunshine reaching the sensor or film. How sensitive that sensor or film is obviously a different matter.


ISO = Sensitivity may be the camera’s sensor or film?

ISO (International Standards Organization) is an older concept in the days of film. It identifies sensitivity to light and brilliantly at all times it works on the basis of again, stops of light.

The higher the ISO greater sensitive as well as the brighter the ultimate image. ISO can therefore be familiar with effect shutter and aperture combinations.

If you discover the shutter is simply too slow for the fast action you are shooting and also you cannot achieve a wider aperture allowing light in faster to counteract the issue, you can then select a higher ISO making the camera more sensitive meaning you can speed up the shutter speed.

Standard ISO is measured as 100, 200, 400, 800…. which doubling numbers represent 1 stop of light each time it doubles. 100~ISO is 1 stop of light darker than 200~ISO…. etc….

iso sensitivity

“A Mariners Guide” captured by Tony Taffinder (Click Image to See More From Tony Taffinder)

In summary, once you understand the partnership between shutter speed and aperture, after which how you can utilize ISO to increase hone your settings you're safe to use an electronic digital slr in its’ manual mode and not have to rely on manufacturers program modes for lazy photographers. As these program modes make assumptions that might not be what you’re searching for.

About the Author
Hi, i'm michael duivis Keith Trigwell. I’m a live music photographer and I also have a passionate fascination with most other types of photography, particularly portraiture and Fine Art. My live music shots may be seen at http://www.flickr.com/bigmojo

To pass enough time I also write a few articles on photography and technical photographic matters.

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