Understanding Shutter Speed, Aperture, ISO and their Interdependence

One might have often pondered or had the inclination to use the Auto settings in a camera and let the camera select the parameters on its own. But does it always end up giving how we wanted the image to be portrayed? Quite contrary. The power of modes such as the full manual mode, Shutter Priority and Aperture Priority mode help us select the parameters ie Shutter Speed, Aperture and ISO as we require them. Therefore it is essential to understand how they behave and interact.

Let us try to fathom what each means and represents…

Shutter Speed

Shutter speed refers to the amount of time for which a camera sensor is exposed to the light coming in from the lens ie the image. The shutter is actuated when the shutter button is pressed. The light coming from the lens is initially deflected by a mirror in front of the sensor to the viewfinder prism. When the shutter is actuated, the mirror lifts upwards and the sensor is exposed to the light from the lens for the preset duration of time. The longer the sensor is exposed, more is the exposure and similarly the faster the shutter clicks, lower is the amount of light falling on the sensor, hence exposure is lower.

Shutter speed is an important variable to obtain sharp images. The primary reason is that, it opens the window to the sensor for a time period and whatever signals it receives is what we see in the image. Therefore, if the shutter speed is high, then the time frame is low. Therefore if there is subject movement, it would not be as visible as with a slower shutter speed where we would tend to get blurs. Shutter speed relates to the time for which the sensor is exposed like I said before. However, the amount of light falling on the sensor is decided by the aperture.

Therefore the rule of thumb here is “Use a higher shutter speed when you want the image to be sharp and devoid of motion blurs and a slower shutter speed when you want motion to be captured as a blur in the image. A higher shutter speed corresponds to a lesser exposure and vice versa given the same value of aperture and ISO”

Aperture

Aperture defines the amount of light that enters a lens. A lens consists of diaphragms which help to vary the aperture of the lens. Usually 8 diaphragm lenses are common. The shape of a diaphragm defines the shape of the bokeh (the out of focus highlight shapes in the background). The aperture is represented by the F number. A higher F number such as F4 or F2.8 means that the opening of the lens is more in comparison to F8 or F11 say. The minimum and maximum values of the F number are dependent on the Lens. F2.8 lenses have larger apertures and hence larger glass is used. Therefore they are bigger in diameter and are heavier. The F number is calculated by the following formula.

F number = Focal Length/Diameter of Aperture

A larger aperture will allow more light to pass through the lens, but the depth of field or the region in front and behind the subject in focus would be shallow. A lower aperture on the other hand constricts the amount of light which enters the lens but the depth of field or the DOF is much more. Therefore a greater region around the subject will be in focus. A rule of thumb for landscape images is to use narrower apertures or higher F numbers. This facilitates a greater region in the image to retain focus. To see how much the DOF is, one can use the DOF preview button in front of the camera. It stops down the lens to the desired aperture and the corresponding DOF can be seen in the viewfinder.

The Aperture values of Lenses available in the market are usually in the multiples of Square Root of 2 (1.414) ie one ‘Full Stop’. The F numbers are F - 2, 2.8, 4, 5.6, 8, 11, 16 etc. The intermediate F stops are however more numerous. Usually upto 1/3 stop least counts are used.

The various combinations can be seen from this table.

Typical one-third-stop f-number scale
f/#
1.0, 1.1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.5, 2.8, 3.2, 3.5, 4, 4.5, 5.0, 5.6, 6.3, 7.1, 8, 9, 10, 11, 13, 14, 16, 18, 20, 22

Therefore the rule of thumb here is “Use a Lower F number when you want more light to enter the lens and have a shallow depth of field and a Higher F number when you want a higher Depth of Field to be covered in the image”

ISO

ISO refers to the sensitivity of the Sensor to light. The light signals falling on the sensor (CCD type or CMOS type) are analog and need to be amplified before they can be converted into digital signals for digital imaging. So the amount of amplification is given by the ISO. A low value of ISO say ISO 100 would give lesser amplification to the signals. Amplification is required when the strength of the signal is low ie in case of low light the light signals are low. Hence they need to be amplified, so a higher ISO is used. However with increase in amplification the signals reproduced is not a faithful reproduction and noise is introduced. Nowadays various noise reduction algorithms are integrated into the cameras to take care of the same.

Therefore the rule of thumb here is “Use Low ISO when light is adequate, and High ISO when light is inadequate keeping in mind the trade off between noise and exposure”

Use of Shutter Speed and Aperture in Tandem

I shall begin by stating one basic concept.

“With increase in one stop of Aperture and one stop decrease in Shutter Speed simultaneously, the exposure remains the same, given the same ISO level.”

If we are without a tripod, there are limits to how slow our shutter speed can be before body movements blur the photo, so there are some constraints. But the point remains, all these combinations yield the same amount of light on the sensor and an identical picture in terms of brightness. What does vary is the ability of the camera to stop action and the depth of field, or how much is in focus in front of and behind the subject.

For example, if I’m shooting at F4 and 1/1000 sec, then that is equivalent to an exposure of F5.6 and 1/500 sec given the same ISO. So if one wants to increase the exposure one can either decrease the shutter speed or increase the aperture ie decrease the F number. But the decision criterion majorly rests with the required DOF. So, if the exposure is to be increased by one stop but DOF too is to be increased. Then one would stop down the aperture (increase F number) by one stop and decrease the shutter speed by 2 stops so that the equivalent exposure is increased by 1 stop. In case an image still gets underexposed or if one wants a higher shutter speed, then the ISO may be bumped up, the trade off however being increased noise.

Use of Shooting Modes

Manual Mode

In case of the Manual Mode, one can set the ISO, Shutter Speed as well as the Aperture for an image. However, here there is a possibility of getting the exposure wrong. Trial and Error and experience helps here.

Aperture Priority Mode

A better option is the Aperture Priority mode. In the Aperture Priority mode, the user selects the Aperture value and the ISO and the camera automatically selects the shutter speed required based on the metering used. This is useful as the photographer often doesn’t have much time to select all values and experiment. An important concept which comes with the Aperture Priority mode is that of Exposure Compensation.

Exposure Compensation: Exposure as read by the camera metering isn’t always correct or as per requirement. Often when contrasty light is present, it is often fooled. So Exposure Compensation (EC) is used. It helps to either manually increase or decrease the exposure. Positive EC increases the exposure and similarly negative EC decreases the exposure. Usually the increase of decrease is done in steps of 1/3 or 0.3 stops. So +1/3 EC would increase the exposure by 1/3 stops keeping the same value of Aperture and ISO. It works similarly with negative compensation too. Positive EC is used when the subject appears dark compared to the background, ie when there is a bright background causing the subject to get underexposed. Similarly when the image gets overexposed, negative EC is dialed to reduce the exposure. When there are highlights in the image, negative EC may be used to tone them down.

Shutter Priority Mode

In the Shutter Priority mode, the shutter speed is set by the user and the camera automatically selects the Aperture based on the exposure metering and the set ISO. This is usually not preferred as it puts the DOF issue to the discretion of the camera AI. It is usually used in case of high shutter requirement say freezing a moving subject, where a high shutter speed is of more importance than the DOF.

These are a few of the basics that are essential to understand how the various parameters may be varied to get that perfect image. They help us to get the image the way we want it and these modes aid us immensely in achieving the same. The concepts have been shown pictorially below.

Click on the images to view them in larger size

Hope the above few basics help in understanding the basics of using the Digital SLR and the various concepts involved. In case you have any queries do mail me at bibhav.behera@gmail.com and I would be glad to respond.