Exposure – The trinity of Speed, Depth of Field and ISO

When we think of the art of photography and the craft of photography, the first important feature of our images is their exposure. Exposure is a critical part of taking a photograph, determining what exactly is captured on the film or image sensor. To take a well-exposed picture, three parameters need to be set correctly: ISO (sensitivity), aperture and shutter speed.

What controls exposure?

The ISO values give the sensitivity of the image sensor, each value class indicating the amount of light captured. An increase or decrease in the ISO value causes the light sensitivity of the sensor to double or halve.

The aperture, also known as the lens aperture, determines the amount of light that passes through the lens to the plane of the film. The size of the aperture is indicated by the f-number, each f value characterising the amount of light captured.

The shutter speed is an indication of the length of time the sensor is recording and the amount of light reaching the sensor. This factor is measured in fractions of a second.

When you combine these three factors, you get a certain exposure value (EV) as a setting. Changing one (any) of these three values has a measurable, specific effect on the result produced by the other two, i.e. the way the image will eventually appear. For example, increasing the f-stop value will reduce the aperture of the lenses, so less light reaches the image sensor, but it will also increase the DOF (depth of field) of the final image.

Reducing the shutter speed will affect the capture of movement, blurring or blurring the background or subject in the final image. However, it will also increase the amount of light reaching the image sensor, making everything brighter.

Increasing the ISO value will allow you to shoot in less bright settings and situations, but it will also reduce the noise that comes with it. It’s impossible to change one of the three values without affecting the photo independently, without changing the EV, or without having the opposite effect on the image of the other two values.

ISO Sensitivity

ISO is actually an acronym that stands for International Standards Organization. ISO has a minimum value of 25, but can go up to 3200 (or more) and is an indicator of light sensitivity. The lower the ISO value, the less sensitive the image sensor, and the smoother the image, because the noise level will be lower. The higher the ISO value (more sensitive), the more the image sensor has to work to produce an effective image, which will result in more image noise (think of the tiny colour spots in the shadows and midtones).

Well, what do we call a picture then?

Image noise is a light signal that does not originate from the subject and thus causes random patches of colour in the image. Digital camera engineers have designed the image sensor to perform best at the lowest possible ISO (just like film). In most digital cameras this is ISO 100, but some professional DSLR cameras have a feature that allows you to bring this down to 50 or even 25.


The lens aperture controls the amount of light that passes through the lens. At a low f-stop, say f/2, a huge amount of light passes through the lens in a short fraction of a second; but at f/22, when the aperture is probably as small as possible, only a very small amount of light passes through (even at longer shutter speeds).

It is interesting to note about blend and f-values that the focal point of the lens does not matter as long as the f-number remains constant. That’s because the arithmetic equation that determines the f-value indicates that the same amount of light passes through a 35mm lens and a 100mm version at a shutter speed of 1/125 second. The aperture size is of course different, but the amount of light passing through is the same.

Shutter speed

Shutter speed is measured in fractions of a second, and is a measure of how quickly the curtains open and close on the film. The shutter speed determines how long light is allowed to pass through the lens and be recorded on the image sensor or film.

The shutter speed also determines how we get a moving object. If the shutter speed is faster than the subject or background, the captured image will be pin sharp. If the shutter speed is slower, you get a blurred image. Just think of rain in a storm, how fast do raindrops fall? Well, at 1/30 the raindrops are no more than indistinguishable white streaks, while at 1/250 they float in the air, each raindrop perfectly distinct and visible.

Over-exposure and under-exposure

How can we define overexposure and underexposure if we have argued that “correct” exposure is subjective? Simply and concisely, overexposure is when the information in the highlights is definitely illegible. In the case of this type of overexposure, there is no way to “recover” the missing information in the digital darkroom.

The concept of underexposure is based on much the same concept, with the difference that in this case there is no image information at all in the shadow part. This non-existent information also cannot be “recovered” by post-production. In digital photography, once a piece of image information is lost, there is no way to recover it. This is not always the case in the world of photochemistry in film photography. In film processing (as opposed to digital), it is possible to “find” the “lost” image information of overexposed images and, in printing, the missing image information of overexposed photos.

You may also like to read one of these too: