Photography Basics: Aperture II

In the first post about aperture we saw what aperture is, how we describe it and how we can use it in a technical way. Now it’s time for a little bit of composition and practice. If you have already read the last post, you will remember that we ended with a question:

Why would I want to reduce the aperture of my lens and let less light to reach the sensor?

If we stop to think about it, at first glance it doesn’t make much sense. If you ask any experienced photographer about their opinion about light, there is one single concept in which every one of them will completely agree: There is never enough of it. Of course, you can have enough to take a good picture, but if you just had a slightly little tiny bit more… you could try to increase the quality a little bit more. And of course, even if you just had that tiny bit more, you might always wish for more and more. This is because photographing consists in painting with light and after that capturing it. Having more light is like, for example, having more colors when painting with oleum.

To understand why we would want to reduce the aperture of the lens, first we need to understand how focusing works. When we focus on the target of our picture, in fact we are focusing only on a plane located at the focusing distance chosen. Any point just in front or behind the plane will be out of focus, and the further away the point is of the plane, the more blurred will appear. Here, the circle of confusion appears in scene. The circle of confusion is the maximum size a spot can have with our brain interpreting it as a point and not as a spot. It is a complex variable that depends on many parameters, but the important concept is that if a spot is smaller than the circle of confusion our brain will assume it is a single point. So, when we focus on something, not only the plane, but all the area in front and behind it that is less blurred than the circle of confusion will appear as if it was focused to us. This area is what we call the depth of field.

The key point in this explanation is simple now: the more wide open we have our lens to take our pictures, the less depth of field we will have. And this explains why we would like to increase the f number in our picture. If we want, for example, to take a picture of a landscape, containing perhaps a few trees close to us, a river in the middle distances and a mountain near the horizon, we probably want to be everything on focus. If we try to take that picture with a diaphragm fully opened the depth of field won’t be enough to cover all the distance and something will appear out of focus. Closing the diaphragm will eventually increase the depth of field up to a point when it covers all the distance intended.

But this is not always the desired effect. For example, if we are making a portrait, having a very sharp background might confuse the viewer, whose sight might wonder around the background instead of focusing on the face as they both catch the eye attraction. In this case what we desire is to isolate the model from the background, blurring the background into soft shapes while keeping the face sharp. For this purpose we will use a low f value.

Narrow aperture

This little fella wants to spend his money in a portrait. A narrow aperture gives too much importance to the background (the depth of field extends too much behind our target), and the leprechaun doesn’t feel special. The photographer won’t see his money for such a poor work. [100 mm; 13 s.; f/13; ISO 200]

Wide aperture

The photographer read about the depth of field and understood that he had to use a wide aperture. Now the leprechaun is isolated from the background, and the good fella is eager to spend his money on more photographs. Everybody is happy! [100 mm; 1/4 s.; f/2.8; ISO 200]

Aperture comparison

In this case, the photographer wanted to show not only the leprechaun, but also the danger behind him. Choosing a wide aperture fails to show the desired details, which are important in this case. The poor leprechaun on the left side doesn’t see the zombie coming and dies eaten by him. Nobody sees his money again. That’s why in this situation the narrow aperture is more important. [Left: 100 mm; 1/4 s.; f/2.8; ISO 200. Right: 100 mm; 25 s.; f/25; ISO 200]

Closing the aperture has also other effects. As it limits the amount of light, we will need to decrease the shutter speed, and if low enough speeds are used we might capture the trail of a moving element. On the contrary, opening the aperture will increase the amount of light and we will be forced to use higher shutter speeds, which leads to freezing the action. As you can see, both of them are valid situation and is the composition and the style you have in your mind who will dictate which configuration is better in each situation (if not both).

Holy Week III

In this picture, taken at night, the illumination is very dim. Although the lens can open up to f/1.4 an aperture of f/3.2 is used instead. Increasing the depth of field makes focusing easier in a very dynamic scenario where the point of interest lasts for very few seconds and requires a fast reaction.  Closing the aperture implies using a slow shutter speed (1/40 s) which is not fast enough to freeze de censer that the girl is moving, becoming blurred and capturing its movement. [50 mm; 1/40 s.; f/3.2; ISO 2000]

Another effect of aperture, in this case at high apertures, is called bokeh. Bokeh happens when in a situation of moderate lightning we have punctual sources of light in our background. As they are out of focus they will appear as very big and soft points, and as they emit a lot of light compared to the ambient lightning, they will highlight compared to the surroundings. The effect is visually and aesthetically very pleasing. Also, a punctual source of light doesn’t have to be a small light, for example, the direct sunlight filtered through the leaves of a tree behind the face of a model under the shadow of the tree is a perfect source of lightning for a bokeh (just go outside and try it!). Also, at night, the street lights, traffic lights, car lights and even some other sources may form a colorful bokeh full of applications.

Finally, a few words about diffraction are worth mentioning. As stated before, the more we close our lens, the best sharpness we will get, as increasing the depth of field makes the focusing be more tolerant with little errors and wide areas will be most likely covered. But this happens only up to a point, different in every lens. Reached that point, the light will start to bend when it passes through the tiny hole, and this will star to blur the picture, even in the best focused part. This is an inevitable effect, caused by the own nature of the light. This doesn’t mean that we shouldn’t use tiny apertures, it is good to use them as long as you have a good reason to use them (technically or artistically), as it is there to be used, but having in mind that it means a little trade with quality, which sometimes might be important, and sometimes might be unnoticed.

Effect of diffraction at low apertures

Detail of an incense box. The one on the left is taken at the sweet spot of the lens (the point of best quality trade between depth of field and diffraction), the one on the right is taken with the lowest possible aperture. The details on the left are sharp and clear while the details in the right picture, although correctly focused, show a blurred contour, more noticeable on the text (not fully visible on the reduced view. Right click and “show image” to see it larger. [Left: 100 mm; 0.8 s.; f/5.6; ISO 200. Right: 100 mm; 25 s.; f/32; ISO 200]

After all this explanation, now you are ready to go out there and try everything on your own. Why not trying to photograph the same subject with different angles and apertures looking for the best result? Or perhaps searching for a good place to try bokeh? Try, practice, and comment if you find any question or suggestion you might think about, or just to share a couple of pictures you have taken after reading and trying all of this. That’s the key for getting better and better pictures every day.


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