Aperture, exposure time, ISO – understanding and reading them

I called the three above "faithful companions" for good reason:

• They have always existed in photography, even before digital photography. If you wanted to turn a simple wooden box with a lens into a camera, you would need to at least
• somehow insert a film or mount an image sensor and know how sensitive it is to the incident light
• drill a hole = an aperture behind the lens and
• decide how long you will open the hole = set an exposure time.
• They are as relevant as ever in digital photography. Look at the displays of professional or beginner cameras; you will always see these three. Even if you're a beginner shooting on full auto and can't influence them, your camera will probably still display the three.
• They have a visible influence on your images in many shooting situations.

Once you go beyond full auto, they will always be around you. It's worth making friends with them.

One rule applies to all the numerical values that follow on this page about aperture, exposure time and ISO and is fundamental to understanding exposure:

Exposure changes occur by repeatedly doubling or halving the amount of light.

So an increase to 2, 4, 8 times etc. or reduction to ¹⁄₂, ¹⁄₄, ¹⁄₈ etc. We will later call such a doubling or halving a change by one exposure value or one stop. For this page, the logic behind the numbers – repeated doubling or halving – is what matters.

With this rule in mind, you will find it easier to become familiar with the following series of numbers and later understand how aperture, exposure time and ISO interact.

Explanation | The aperture is the opening in the lens through which light falls onto the image sensor to expose a photo.

With lenses that do not control the aperture electronically, there is a small metal pin that moves the aperture mechanism. Have a look on the picture how the camera uses it to open and close the aperture.

Numbers | The size of the aperture is indicated by the aperture number; a plain number without a unit. It is usually decorated with the letter f and also called f-number or f-stop.

The usual aperture numbers are in the chart and take some practice for beginners:

Smaller aperture numbers mean a larger aperture = more light and vice versa.

You will find a doubling of the aperture number like 2 – 4 – 8 etc. with every second value. This is because the aperture is roughly circular and to double the light passing through it, the diameter only needs to increase to about 1.4 times. A 2-fold diameter lets 4 times the amount of light through, so you need an intermediate value to double or halve the amount of light.

The series shows full f-stops however cameras use a finer division, mostly 1/3 f-stops, i.e. two additional values between each of the f-stops shown.

Setting range | The smallest achievable aperture number depends on your lens; the smaller it is, the better. 1 is very rare, 1.8 or 2.8 is already considered fast, long telephoto and zoom lenses often do not go below 4 or 5.6.

The largest achievable aperture is also limited, cameras with a small sensor can have a smallest achievable aperture of only 11, for example.

Smartphones almost always have a fixed aperture of mostly 1.8 to 2.8.

Impact on the image look | The most important effect of the aperture is its influence on the depth of field.

In addition, there are two weaker effects for which you need to look very closely:

• Lens aberrations decrease as the aperture becomes smaller.
• At very small apertures, the achievable sharpness decreases due to diffraction effects.

Explanation | The exposure time or shutter speed is the time for which light falls on the sensor of a digital camera to expose your photo.

What exactly happens is a little more complicated. For starters, it is sufficient to imagine that a small flap opens instantly, remains open for the duration of the exposure time to let light onto the sensor and closes again just as abruptly.

Numbers | Starting from one second, the exposure time is always doubled or halved and rounded a little. This results in the series shown in the graphic.

Cameras use additional values for the exposure time between the full exposure steps shown; as for the aperture two intermediate values are common.

Setting range | The exposure time has the greatest leeway, with good cameras; ¹⁄₈₀₀₀ s is common as the shortest and 30 s as the longest exposure time, and with the right setting, exposures of any length are possible.

Smartphones can keep up well with short exposure times, ¹⁄₂₀₀₀ s or shorter is no problem, but they limit the longest exposure time. I have observed several times that ¹⁄₄ s is the end.

Impact on the image look | The exposure time determines whether motion blur occurs with moving subjects and how great the risk of blur due to camera movements is.

Explanation | The ISO sensitivity controls how sensitively the camera reacts to incoming light. Technically, it is a measure of the amplification of the signals recorded by the image sensor. Think of it as a kind of additional volume control for brightness.

Numbers | The common ISO values result from the standard value 100 by doubling, the graph shows the usual values. It is common to put the abbreviation ISO in front of the number, please say e.g. “ISO 100”. With a statement like “100 ISO” you reveal yourself to old hands as a beginner.

Higher ISO values mean a greater amplification of the signals recorded by the image sensor, they act like a higher light sensitivity.

Setting range | 100 is the lowest ISO value for most cameras, but it can also be lower at ISO 50 or 64.

The highest ISO value is again camera-dependent; cameras with larger sensors allow higher ISO values and over the years the highest possible ISO sensitivity has increased. For smartphones or cameras with small sensors, max ISO 3200 or 6400 seems to be common. Full frame cameras can go well up to ISO 51200 or 102400, with the top of the range models going a few notches further.

Keep in mind, however, that because of the continuous doubling between, say, ISO 6400 and 102400, there is only a 4-fold doubling despite the large difference in the numbers. A change from ISO 6400 to ISO 102400 acts like opening the aperture 4 stops on the scale shown above, or like increasing ISO 100 to ISO 1600.

It is also common for cameras to use two intermediate values each for finer gradation in ISO sensitivity.

Impact on image look | As ISO sensitivity increases, so does image noise.

Here are two typical readouts from different cameras for aperture, exposure time and ISO.

Some notes on how to interpret what you see:

• If you spot an f or F, the number next to it is the aperture number; remember it is also called f-number.
• The exposure time is sometimes given without a fraction, e.g. a simple ”125“ stands for ¹⁄₁₂₅ second.
• If your camera displays the exposure time without a fraction bar, the second sign ″ is added to whole seconds, e.g. 2″ for 2 seconds.
• The ISO value is (hopefully) easy to recognise by the label ”ISO“. However, it may be that there is no numerical value but ”AUTO“ – for an ISO value automatically selected by the camera.

If your camera does not display any numerical values for aperture, exposure time, ISO, you will have to change the information displayed. There may be a ”DISP“ or ”DISPLAY“ button for this purpose, perhaps also labelled i or info... if in doubt, please consult the manual of your camera.

Don't expect such displays on a smartphone; they may have a PRO mode for somewhat advanced photography, but usually don't display aperture, exposure time and ISO as in the examples.

Hier sind zwei typische Anzeigen von verschiedenen Kameras für Blende, Belichtungszeit und ISO.