Advertisement
  1. Photo & Video
  2. Night Photography
Photography

Exposure Explained: ISO, Shutter Speed and Aperture for Night Photography

by
Difficulty:IntermediateLength:LongLanguages:
This post is part of a series called Night Photography.
How to Set Up Your Digital SLR for Night Photography
How to Photograph an Eclipse

Before you can move out of your comfort zone and begin manually controlling your camera, you need to understand the relationship between the three variables that influence exposure.

ISO, shutter speed and aperture settings not only control the amount of light entering your camera, but also give you the ability to inject some of your own personality into the images you create. 

Once you possess the technical skills required for exposure control, you can begin to exploit many of the creative aspects these settings influence, giving you the freedom to express the visions in your mind's eye with clarity while adding your own unique interpretation to your subject matter of choice. 

With practice and experience, controlling the many variables these settings affect will become second nature, and I encourage you to take the plunge and throw yourself in the deep end in a quest to improve your knowledge and add to your photographic skill set.

Exposure Basics

Before I begin explaining how you manually control exposure, it is worth defining terms of reference for the three parameters controlling the amount of light in your images.

ISO

ISO is an acronym for International Standards Organisation. This term was traditionally used to describe the light sensitivity of a specific film emulsion, and this standard has carried over into the digital realm, where it describes the signal amplification from the sensor's base ISO setting. 

Shutter Speed

This setting controls the amount of time the shutter mechanism is open, with longer times allowing more light to enter the camera. It also directly influences how much motion blur is present in an image. 

Aperture

This setting controls the size of the iris diaphragm within the lens in much the same way the pupil controls the amount of light entering our eyes. This setting also allows you to control the depth of field or the area of focus you see in your pictures. 

The Art of Compromise

Each of these three exposure parameters can be adjusted in standardised increments called "stops". Modern cameras usually allow for adjustments in smaller 1/3 stop increments for any of these three exposure variables.

To compensate for any change in one of these settings and maintain correct exposure, you will need to compensate by adjusting one of the other exposure settings by the same amount. 

For example, if you increase the ISO setting by one stop, you will need to compensate for this by "stopping down" (decreasing by one stop) either the aperture or shutter speed settings to maintain correct exposure. You could alternatively adjust both aperture and shutter speed by half a stop each... it's all about equilibrium and the art of compromise.

The standard stops scale
The relationship between the three exposure variables displayed in single stop increments. As you increase exposure by one stop, you get a doubling of the amount of light being exposed in your image. Conversely, decreasing exposure by one stop results in a halving of the amount of light being exposed.

To get correct exposure, you need to know how to interpret the metered exposure reading from your camera and select the correct type of metering for the particular subject you intend to shoot. This is a crucial part of getting consistently well-exposed images.

Modern digital cameras also provide you with a plethora of tools like a histogram display which assist in getting correct exposure, and I find these exposure aids very useful. The real payoff to understanding exposure control, however, is the ability to select optimised settings in a way that complements the subject matter you are shooting. 

Night photography is no different, and I generally stick to a set of guidelines that I know work well for the majority of images I shoot within this discipline. 

I will delve into using your camera's exposure meter and some of the other exposure tools and camera settings in my forthcoming article on shooting night photography in manual exposure mode.

Aperture Shutter ISO triangle
It helps to think of exposure as a three-way tussle between ISO, shutter speed and aperture. It is important to balance these three competing elements to get correctly exposed pictures, but you also need to use settings that will complement your subject matter.

Aperture

The initial stage of the light-gathering process occurs when light enters the camera through the lens, and as such aperture is a good starting point for discussion. 

Aperture can either be a fixed size or a variable sized opening within the lens, and this setting allows you to vary the amount of light hitting the film plane or digital image sensor. It can be adjusted by using an aperture ring on the lens itself, or by using a command dial or menu setting in the camera. 

A lens's "speed" is determined by its maximum aperture. Fast lenses have a larger diameter hole, which allows more light in, and these are a good option for the low-light conditions prevalent in night photography. A lens is considered fast if it has a maximum aperture of f2.8 or below.

Nikkor 85mm lens with large aperture
Fast lenses are great for night photography! Having a large aperture means the image you see in the viewfinder is brighter, making focussing manually easier in low-light conditions.

As you stop down from the lens's maximum aperture, the size of the aperture hole decreases, allowing less light to enter through the lens and pass on to the image sensor or film plane. As a consequence, the depth of field (area in focus) within the image increases. 

By varying aperture, you control the depth of field in your images. Different styles of photography require different aperture settings to complement the subject matter, and for the majority of night photography subjects you will need a large depth of field to keep all parts of the image in sharp focus.

To aid in maximising depth of field and achieve critical focus, some lenses have hyperfocal markings on them. Many manual focus primes have this along with a hard infinity stop, and these are good for night photography subjects, where focus can be difficult in low light. 

Image quality is directly influenced by aperture, and when lenses are used at extreme settings, problems such as diffraction and astigmatism can lead to a loss of resolution and contrast. Most lenses perform at their best when set to middle aperture settings like f8 or f11. As a general rule of thumb, use aperture settings at least two stops in from the minimum or maximum settings.

See my two lens articles for more info on this subject with special reference to night photography:

There is one other way you can control exposure and decrease the amount of light entering the lens, and that is to use a neutral density filter. By placing an ND filter in front of the lens, you restrict the amount of light the camera receives, and ND filters with up to 10 stops of light reduction or more are available. 

These can be handy in situations where there is too much light or you wish to use very long exposure times.

Shutter Speed

The shutter mechanism is the second link in the imaging chain, and by learning to control shutter settings you become a master of time. 

Shutter speed settings alter the length of time the shutter remains open, and therefore how long light is captured. These settings also control the amount of motion blur within your images, and when used creatively they can add a surreal feeling or give emotional context to a photo, for example by increasing the sense of speed or motion.

Image of motorbike in tunnel with motion blur
Motion blur can intensify and exaggerate the feeling of speed, giving your images a dynamic presence.

Motion blur can be introduced in two ways: by the motion of the camera or by the movement of the subject you are shooting. As you increase shutter speeds, you lessen the effect of motion blur, essentially "freezing" the action.

Camera motion blur is introduced when the camera itself moves while the shutter is open. 

It is more problematic when the camera is hand-held and results in blurry or smeared images, which are especially visible when using a slow shutter-speed setting. 

You can minimise camera motion blur by mounting the camera on a stable platform like a tripod, and for night photography this is essential. Camera motion blur shouldn't be a problem unless the camera moves due to wind or some other force bumping it, or when placing the tripod on a moving or unstable surface.

Subject motion blur occurs when the subject you are shooting is moving through the frame whilst the shutter is open. 

The shutter speed you choose influences the amount of motion blur in your photos, but this blur is also dependent on the focal length of your lens and the speed and direction of subject movement relative to all three dimensions.

Subjects that move horizontally or vertically across the frame in the x and y directions will have more motion blur present compared to subjects that move towards or away from the camera in the z axis. You need to compensate for this by increasing or decreasing shutter speeds for different subjects accordingly.

Focal length also has a bearing on motion blur, especially for hand-held shots, making wide-angle lenses easier to use hand-held than telephoto lenses. 

A general rule of thumb to prevent any motion blur while hand-holding cameras is to match the lens's 35mm equivalent focal length to a reciprocal shutter speed. A lens with a focal length of 100mm would therefore require a shutter speed of at least 1/100th of a second to prevent any blur when hand-held. 

Image stabilisation can also help prevent camera motion blur. If the lens or camera has image stabilisation, you can decrease the shutter speed by up to four stops for hand-held shots. For night photography, using a tripod or camera support device is a much better option for preventing camera motion blur.

One thing to keep in mind is that extended shutter speeds can cause issues for both film and digital cameras.

Most films are designed to work within a range of shutter speeds between one second and 1/10,000th of a second, and shutter settings outside of these speeds can cause reciprocity failure, leading to exposure errors and colour shift. The effects of reciprocity failure can be minimised by the use of colour correction filters or exposure compensation.

Digital image sensors don't exhibit reciprocity failure, but shutter speeds of more than a few seconds can create other issues like amp glow and hot pixels. This is due to the camera's image sensor overheating, but this is far less of a problem with newer sensor designs.

Using a flash can also help to lessen motion blur, as it effectively simulates a fast shutter speed of about 1/2500th of a second (the flash's duration), even though the camera's flash sync speed may only be 1/60th of a second. If you want to use flash, you must set the shutter speed at or below the camera's flash sync speed so that synchronisation occurs. 

For night photography, the flash sync speed normally isn't a problem as shutter speeds are generally slow, and flash can be an effective way to illuminate a particular subject or light paint areas within the frame. Rear curtain flash sync is a good trick to use with slower shutter speeds, as it fires the flash at the end of the exposure, freezing any subject motion more effectively.

Image with rear curtain flash sync
I used rear-curtain flash sync to freeze the main subject while the camera is hand-held and rotated during this slow-shutter-speed photograph. It isn't always about getting stable camera shots, and you can use blur to your advantage to get creative with your photography.

ISO

ISO is perhaps the main factor influencing the image quality of your photos—as you increase ISO settings, the image quality of your photos will decline. 

In film cameras, the ISO rating points to the size of the film grains within a particular emulsion. These individual grains are like "film pixels", and their size increases as you increase the ISO value, leading to a loss of resolution. Slower films like Fujichrome Velvia have a finer grain characteristic, exhibiting more intense colours and resolving more detail.

In digital cameras, the ISO rating refers to the amplification of the signal from the image sensor. As you increase ISO from the base setting (usually around ISO-100), you also electronically amplify the amount of noise present in the digital data captured by the sensor. This leads to artifacts like hot pixels and a loss of contrast, dynamic range, resolution and colour. 

Depending on the size of the image sensor in your camera, image quality at settings of ISO-400 or above is often compromised, with colour saturation, resolution and dynamic range all suffering compared to lower ISO settings. 

In certain low-light situations, however, it can be impossible to get the required shot using a lower ISO setting, and you may need to compromise image quality under these circumstances.

Comparison of the same shot using two different exposure settings
The same scene shot using two different exposure settings shows how increasing ISO value affects the quality of your images. The image on the left was taken using a base ISO setting of ISO-200, whilst the image on the right used a setting of ISO-3200. You can see in the 100% crop images that there is a loss of resolution, colour and contrast, with high ISO noise particularly visible in the area of sky above the towers.

For the low-light conditions prevalent in night photography, it is best to use a combination of fast lenses with large image sensors, like the 35mm size full-frame versions used in some Canon, Sony and Nikon cameras.

These large-sensor cameras have superior low-light performance, and you can push them to extreme ISO settings without compromising image quality too much. 

The ISO rating on newer digital cameras like the Sony A7s has reached stratospheric levels, with this model offering a setting of ISO-409,600. The low pixel count (12 Megapixels) combined with the full-frame sensor used in this camera, allows for good image quality at levels up to ISO-6400.

Having a camera with good IQ at higher ISO values gives you the luxury of optimising the two other variables that control exposure. Shutter speed and aperture settings allow for more artistic control over your images than ISO, which really only influences the image quality within your photos.

Bottom line: Higher ISO settings degrade image quality, so keeping ISO settings low is the goal. A combination of high resolution (high megapixel) and small sensor size leads to decreased image quality at high ISO values, so choose fast lenses and large camera sensors to combat these problems.

Manual Override

Whilst modern digital cameras do a pretty good job of taking photos in various automatic exposure modes that require very little user input, it is far more important to understand the basic principles of how to use a camera manually, and this article hopefully goes some way to increasing your understanding and knowledge of these important principles.

You'll get a great sense of satisfaction from knowing when, how and why you need to compromise these exposure variables to achieve a desired end result, and also from knowing that you are taking control over all aspects of your photography. 

Learning to optimise exposure settings for different photographic situations takes time, effort and practice... so get out there and get shooting!

In my next article I will describe what settings to use and how to operate a digital camera in manual mode for night photography. See you then...

Advertisement
Advertisement
Looking for something to help kick start your next project?
Envato Market has a range of items for sale to help get you started.