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What are ISO, aperture and exposure time and how do they relate to each other?

ISO, aperture and exposure time are the three factors that influence how much light reaches the camera sensor. Each parameter can individually allow more or less light to reach the sensor and thus determine the brightness of the photograph. But it's not quite that simple, because each of the three parameters has different properties with which it influences the image individually. However, these three parameters can cancel each other out in their effect on the overall brightness of the image. These individual properties of the three parameters are what make each image individual in its effect, as every photographer uses these properties differently and thus gives the image its own touch and expression. These effects can be both desirable and undesirable; It is therefore up to the photographer to know their effects, weigh them up and use them in a targeted manner. In order to be able to control these effects yourself, you should take photos in manual mode, otherwise the camera's automatic system makes this decision for us and therefore gives us no freedom to decide how the image should appear. In order to better understand the individual parameters, we will take a closer look at each of them below. I'll start here in the order that's easiest for the beginner to understand. Since the ISO value usually has the same value, at least for daytime shots, this is the simplest parameter, so I start with the ISO.


1. The ISO


The ISO value influences the light sensitivity of the camera's image sensor. Nowadays, with high-quality cameras you can choose the ISO value between 50 and 6,400 or sometimes even higher. The ISO values ​​always double (50, 100, 200, 400, 800, 1600,…), with each doubling always resulting in a doubling of the light sensitivity and thus the brightness of the entire image. Basically, you should choose the ISO value as low as possible, because as the ISO value increases due to the electronic amplifier in the camera, the noise increases and the images appear grainier or more pixelated. During the day you should leave the ISO value at 100 and choose the brightness based on the exposure time. So, for example, if you halve the ISO value and double the exposure time, you get the same overall brightness of the image. There is another problem with the exposure time, namely that the risk of camera shake increases with increasing exposure time if you take photos handheld without a tripod.

2. The aperture


The second parameter is probably the most challenging for beginners, but after setting the ISO, I choose the aperture. The aperture has a very large influence on the effect of the overall image and at first glance probably offers the greatest scope for design. The aperture number is specified with “f”, i.e. f1.8, f2.8, f5.6, etc… Here the “f” in front of the number means, as in mathematics, the same as 1/1.8 or 1/2.8, 1/5.6, etc. The larger the number under the fraction line becomes, the lower its value of light incidence is on the sensor. An aperture with a value of f1.8 allows more light to hit the sensor than a value of f5.6. Unfortunately, this is not linear like the ISO or exposure time, so that, for example, the f4.0 aperture allows twice as much light to hit the sensor as the f5.6 aperture. Explaining this in more detail at this point would become too technical and would not offer any advantage when choosing the aperture. The light falling on the sensor is always halved between the following aperture levels: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16. The smaller the f-number, the larger the aperture opening/diameter, the more light enters. This is also known as wide aperture or open aperture. Conversely, the larger the aperture number, the smaller the aperture diameter and the less light enters. This is also known as a small aperture or closed aperture. The open aperture is the complete opening of the aperture, i.e. the largest aperture diameter in a lens. In summary, it can be said that a lot of light comes into the lens with a small aperture number, while very little light comes in with a large aperture number.

You can also influence the overall brightness of the image by choosing the aperture number. Lenses with an aperture number of f2.8 or smaller are referred to as fast lenses; these are correspondingly more expensive because they allow more light to reach the sensor. A great effect with fast lenses is that you can “freeze” well or create beautiful bokeh; an effect that many photographers would like to achieve because it makes the image look much more interesting. Bokeh means that an object is sharp while everything around it appears blurred. This has to do with the depth of field. The smaller the f-number, the larger the aperture opening and, as a result, the shallower the depth of field and the blurrier the background. When taking landscape photos, you want both the foreground and the background to be in focus, so you choose a narrower aperture (larger f-number) because the depth of field increases as the aperture number increases. The aperture therefore has the greatest influence on the image composition.

3. The exposure time


The exposure time is the time that an image is exposed, i.e. the time that the camera shutter is open and light falls on the sensor. For most cameras, the exposure time is between 1/8000 second and 30 seconds or the “Bulb” mode, where you can also expose manually. As already mentioned with ISO, the amount of light hitting the sensor doubles when the exposure time is doubled. If you expose the image for 1/40 of a second instead of 1/80 of a second, it is twice as bright. Freehand images with exposure times of at least 1/30 of a second can still be taken without blurring with good practice; for longer exposure times you will need a tripod , so that there are no unwanted camera shakes. With the tripod you can also use longer exposure times, for example you can show particularly beautiful running water in a waterfall by exposing the course of the water for longer. With a longer exposure time, the running water appears “blurred,” but the surroundings remain sharp.

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