Exposure with aperture, shutter speed and ISO

Tuesday 17 August 2021, 20:20 by Marco de Groot | 4,932 views | 2 comments

This article was translated from Dutch with the help of AI. Read the original.

Photography literally means 'writing with light'. Three factors that affect exposure are aperture, shutter speed and ISO. Yet exposure does not determine the choice of these three elements. It is a creative choice.

The sensor

The light reflected by the subject enters the camera body through the lens. There it is projected onto the sensor. This consists of millions of photosites, or pixels (12 megapixels means 12 million pixels).

You can think of a photosite as a bucket that fills with light. An empty bucket is translated into black and a full bucket into white.

When a bucket is full, it will overflow into the neighbouring photosites. Several full photosites create a bleached-out white area, meaning the photo is overexposed.

When the amount of light in several photosites does not reach a certain threshold, this creates a solid black area, meaning the photo is underexposed. The amount of light let through must therefore not be too much, but not too little either.
 

3 factors that determine the amount of light let through

• Aperture
The lens contains an aperture made up of blades. These blades can be used to make the aperture opening larger or smaller. A large opening lets in a lot of light, which is a low f-number. A small opening lets in little light, which is a high f-number.

• Shutter speed
The sensor is covered by a shutter curtain. When the photo is taken, the first curtain opens, after which the second curtain closes.

Shutter speed is the length of time the shutter remains open and determines the amount of light let through. If the shutter remains open for a long time, a lot of light is let through. If the shutter remains open for a short time, little light is let through.

Shutter speed is shown as fractions of a second. 1/60 of a second lets in less light than 1/30 of a second.

• ISO
ISO indicates the light sensitivity of the sensor. At a higher ISO setting, the current is increased, meaning the sensor needs less light. Increasing the ISO setting causes noise. Noise looks like stray image points and is rarely attractive.

The creative effect of these 3 factors

It should be clear that aperture, shutter speed and ISO determine the amount of light let through. Yet this is only a side effect.

These three have a much more important function, which shapes the creative effect of the photo. Let's look at all three again.

• Aperture
A large aperture opening, which means a low number, gives a shallow depth of field. The focused subject is sharp, while the foreground and background are blurred.

A small aperture opening, which means a high number, gives a large depth of field. The foreground, subject and background are sharp.

A useful way to remember this is 'low is blurry'.

• Shutter speed
The photographer holding the camera is shaking. When this movement is too great in relation to the shutter speed, motion blur occurs.

A rule of thumb is 1/60 when shooting handheld. At a shutter speed slower than 1/60 of a second, the camera should be placed somewhere stable, such as on a tripod.

If you deliberately choose a slow shutter speed with the camera on a tripod, stationary objects will be sharp, while the moving subject will be shown as a streak. This adds dynamism to the photo.

If you want to photograph a fast-moving subject such as a car without motion blur, freezing it, choose a fast shutter speed such as 1/1,000 of a second.

• ISO
A high ISO setting causes noise, and noise is always ugly. Start with the lowest possible ISO setting for the least noise.

Important: Exposure is a side issue

The combination of aperture, shutter speed and ISO is determined by the photographer's creative choice, not by the amount of light. So what is the order of operations:

• Set the lowest possible ISO setting for the least noise.
• The photographer decides whether priority lies with depth of field or motion blur.
• When depth of field is the priority, the photographer starts by setting the correct aperture. The amount of light let through is compensated for with the shutter speed to achieve the correct exposure.
• When motion blur is the priority, the photographer starts by setting the correct shutter speed and the amount of light let through is corrected with the aperture.
• In A-(V), T-(V) or S mode, the camera looks for the correct combination itself. In M mode, the photographer sets the combination based on the light reading.
• Only if the desired combination of shutter speed and aperture is not possible can the photographer choose a higher ISO setting. The increased noise is accepted.
• Focus, compose correctly and press the shutter.

Don't forget that aperture, shutter speed and ISO determine the sharpness in the photo. Exposure is a side issue.

Manual exposure

I believe the photographer should be in charge of the camera. In a semi-automatic mode, there is a risk that the camera becomes in charge of the photographer, meaning they no longer understand what is happening.

So set the camera to M mode. Take ISO off automatic and start with the lowest ISO setting. The camera is now fully manual and will no longer set anything itself. The photographer is in charge.

For a precise light reading, set the light meter to spot metering. The light meter now measures the light reflected by the subject in the centre of the viewfinder.

Note: The camera is being operated manually. The light meter itself does not set a combination of shutter speed, aperture and ISO. But the combination of aperture, shutter speed and ISO set by the photographer affects the position of the light meter.

The biggest misconception

'When the light meter is at 0, it produces a correctly exposed photo'. This is a false assumption and is not necessarily true.

When the light meter is at 0, this means that the subject visible in the centre of the viewfinder will have average brightness in the photo.

It will not be black or white, but will have a brightness exactly in between. A face has average brightness by itself and is a good object on which to base the light reading.

When the camera makes a different composition and a black object is visible in the centre of the viewfinder, the light meter moves towards the - sign.

 

The histogram replaces the light meter

Most cameras show a live histogram on the LCD screen while taking photos. The histogram is a graphic representation of the pixel brightness in the photo.

The horizontal axis shows brightness. The dark areas are on the left and the light areas on the right.

The middle section is not very interesting. It can take any shape and is always fine. The vertical axis shows the relative number of pixels representing a particular brightness.

Because the photographer does not want bleached-out whites or solid blacks, the base of an ideal histogram ends precisely in the bottom corners on both the left and right.

When the histogram touches the right edge, the highlights are blown out. When the histogram touches the left edge, the shadows are blocked up.

When the subject contrast is too great, the histogram does not fit within the graph. Expose so that the histogram ends in the bottom-right corner.

It can touch the edge on the left, because blocked-up dark areas can still be reasonably recovered in photo-editing software. Blown-out light areas cannot be saved.

Just like the light meter, the histogram does not set anything. It responds to the combination of ISO, aperture and shutter speed set by the photographer. The histogram replaces the light meter but provides much more information.

This is how you use technique to add creative value to the photo. Good luck with it.

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