Understanding couples in photography

Even if you are new to photography, I am sure you have already heard words like, exposure, depth of field, aperture, shutter speed, ISO speed or even light meter.

The purpose of this article is to give you an insight about how all of these work combined together.

First of all if you are serious about photography, you should start by forgetting any automatic mode of your camera and stick to the mode on which we have some or full control. On my camera (Canon EOS) they are noted by P Tv Av and M.

Second we should understand that the ISO speed is the sensitivity of the film to light. Which is a CONSTANT for a given ISO number, the smaller the number the more light is needed to give us a correctly exposed picture but the “neater” the picture will be. A higher number means less light is needed but the resultant picture will look “grainy” or “noisy”.

Now it is time to see how our camera works. When we point our camera towards a subject, the light meter that is built-in the camera, will calculate the amount of light reflected by the subject and, depending on the ISO speed of the film/sensor, will set a couple (aperture & shutter speed) to yield a correctly exposed picture.
Let’s say for example that the camera chose a couple of f/5.6 1/125 (with ISO speed set to 100).
The next step is to decide what you want to do with your picture, do you want a sallow DOF or in the contrary you need a deep one. So if you are trying to shoot a landscape you generally want a deep DOF to reach this you need an aperture in the f/16 range. If we go back to our main example the camera readings where f/5.6 1/125s and we need to move for f/5.6 till f/16 from the chart below we notice that we need to loose 3 f-stops that are going to be compensated by a longer exposure of 1/15 seconds instead of 1/125

f/1.0 f/1.4 f/2.0 f/2.8 f/4.0 f/5.6 f/8.0 f/11 f/16 f/22 f/32
1/4000 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8 ¼ ½ 1sec 2 4 8 15 30

What is important to understand from the above example is that since we have changed the couple in a symmetric way the exposure of the picture will remain the same and only the DOF is affected. (Please check the user manual of you camera to understand how to change the couple in P mode)

If the picture we want to take is a portrait and we want a shallow DOF we should move from f/5.6 to f/4.0 or even f/2.8 (if the lens supports it) this will result in a shorter exposure or faster shutter speed of 1/125 or 1/250 respectively.

In the above two stated examples, our main concern was the DOF that’s why we based our decisions on the aperture since wider aperture (small number) would yield a shallow DOF, in such cases, and instead of working in P mode than changing the couple, we can switch the camera to Av or (Aperture Priority). In the mode we fix the aperture and the light meter will decide on the shutter speed depending on the ISO speed and the amount of light.
In other cases shutter speed is our main concern and not DOF, such as shooting a water stream: you might want to give the impression of running water for this you need a slow shutter speed (at least ½ sec) because a high shutter speed will freeze the stream and the picture will lose interest. So let’s assume we pointed to camera to the stream and the readings where f/5.6 1/60, to move from 1/60 to ½ we are losing 5 stops that should be compensated by a 5 f-stops so the aperture should be set to f/32.

Or you need to shoot a horse while jumping over a fence, to freeze the action you need a shutter speed of 1/250 if originally speaking we got the same readings as above (f/5.6 1/60) to move from 1/60 to 1/250 will mean to set an aperture of f/2.8. Now let’s assume that our lens allow us a maximum aperture of f/4 does this mean it is impossible to reach 1/250?? Well we can sill play around with one factor and that is the sensitivity of the film/sensor if in the above example we use a film of sensitivity 200 instead of 100 (i.e. more sensitive to light) this will compensate for the extra stop that I am unable to reach because of my lens limitation. So f/2.8 1/250 ISO 100 is the same exposure than f/4 1/250 ISO 200

In the above two stated examples, our main concern was the shutter speed, in such cases, and instead of working in P mode than changing the couple, we can switch the camera to Tv or (Shutter Priority). In the mode we fix the shutter speed and the light meter will decide on the aperture depending on the ISO speed and the amount of light.

To have more deep understanding of the above, it is important for beginners to read the article more than once and to practice on their own cameras.

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