Photometer or Exposure Meter (What It Is and What It Is Used For)

You have already used it a few thousand times, but have you considered how a photometer really works? Do you know how to get the most out of it? Do you know that there is life beyond the one integrated into your camera? Well, if you have never asked yourself these questions or if you did and did not find the answer, I hope that today’s article will help you 🙂

What is a photometer?

To begin with, if we want to be perfectionists, when we refer to the photometerwe should talk about exposure meter, because not all photometers are designed to measure light, there are also designed to measure color temperature (colorimeter) and not to offer us exposure values. On the other hand, exposure meters, as their name suggests, are designed to offer exposure values.

light meter (or photometer for less perfectionists) is designed for measure light intensity of the scene and based on it, gives us some exposure values certain (shutter speed and/or diaphragm depending on the shooting mode in which we are working).

Once the terminology has been clarified, we will continue as we have been doing all our lives, with the term photometer that, for those of us who had not thought of using colorimeters, is already doing well 😉

Incident light external photometer

What types of photometer are there?

There are basically 3 types of photometers: those that measure reflected light, those that measure incident light, and those that can measure both types of light.

Reflected light photometer (or exposure meter)

The photometer or exposure meter of reflected light measures the light reflected by people or objects what we are photographing This means that the exposure values ​​that the reflected light photometer will offer us, will depend on the tones that the objects havesince objects or people with shades Lighter colors reflect more light than objects or people with darker hues..

That is, a person with white skin will give different exposure values ​​than a person with darker skin.

The photometer that integrates your camera is one of these 🙂.

Advantages of the reflected light photometer

It comes integrated in your camera, you can’t forget it, you don’t have to buy it, you don’t have to carry it and, if you get to know its shortcomings well, you can work perfectly with it in most situations.

Disadvantages of the reflected light photometer

Since it doesn’t reflect real light, but light that things reflect, it’s very easy for get lost in situations where we have multiple light inputsor in situations where light tones abound (thinking that there is more light than there really is and consequently underexposing the scene) or where dark tones abound (thinking that there is less light than there actually is and consequently overexposing the scene ).

That is, the built-in light meter will work fine when the tones in the scene are close to the medium gray, which is the reflectance tone that is considered standard for a scene, and the one used to work. The more it differs from that tone, the more chances you have of having a wrong exposure.

Do you remember that we have ever talked about how snowy landscapes trick the photometer into thinking that there is more light than there really is? And how the value it gives us makes the snow gray instead of white when try to compensate for what you consider to be an excess of light? well that’s it fault of the reflected light photometer.

Very light or very dark scenes confuse the reflected light meter that gives us wrong exposures if we do not know how to compensate for the scene

Incident light photometer (or exposure meter)

The incident light photometer or exposure meter measures the intensity of light real of the scene that affects the subject regardless of the object or person we are photographing and offers us exposure values ​​accordingly. That is, whether the object is white, black or brown, the exposure value it offers us will be the same.

Advantages of Incident Light Photometer

It is the one that allows us to really know the light of the scene regardless of its color or shades. I don’t know pervert thus the exposure values ​​and not deceives to the photometer.

Likewise, it allows us to know the value of the different lights of a scenewhat exact difference of diaphragms there is between some lights and others (contrast of lights), and what intensity of real light the scene has.

Consequently, it is the more accurate of the two.

Disadvantages of the Incident Light Photometer

You have to buy this photographic accessory, load it, learn to use it, and not forget it 😉

The incident light photometer allows to know the real light of the scene

How do photometers work?

We have already commented that photometers measure light (reflected or incident) and provide us with exposure values ​​that are, in principle, correct for the light of the scene in front of us.

How the reflected light photometer works

In the integrated exposure meter of reflected light that our camera has, it is very important to choose correctly the type of measurement necessary for each scene and knowing how to interpret the result, since the correct or unexposure of the shot will depend on it.

  • Matrix or evaluative: Takes the exposure references of different points distributed in the frame and proposes an average exposure based on the different lights. It works well in most situations where there is not much contrast between light and shadow.
  • Punctual: Measures exactly at the point that we indicate in the scene, focusing on it and ignoring the rest of the lights. It works well when we have a lot of difference in light between our center of interest and the rest of the scene.
  • Partial: Similar to spot but covers a little more area in the image to assess the exposure.
  • Center Weighted: It performs the measurement in the central area of ​​the image to which it gives priority, although it takes into account the rest of the lights in the scene.

Likewise, we must choose a shooting mode. If we choose the manual mode, we are the ones who must adapt the values ​​of the exposure triangle (ISO, shutter speed and diaphragm opening) based on what the camera’s built-in exposure meter tells us.

If we choose a priority mode, the photometer or exposure meter will offer us the value of the other variable of the exposure triangle. On the other hand, if we choose the automatic mode, the camera will make the necessary adjustments to shutter speed, aperture and ISO that it deems appropriate.

Once the measurement and shooting modes have been chosen, we must interpret the measurement offered by the exposure meter, and we do this through the histogram. The histogram helps us know if the exposure is correct. That is to say, not only if we have made the correct adjustments so that the measurement in the photometer remains at ¨0¨, but to know if we have been able to correctly interpret the exposure that it offers us, finally obtaining an image according to the scene.

How does the incident light photometer work?

As we have already mentioned before, the incident light photometer is the one that measures the light intensity of the real scene, regardless of the tone of everything that appears in it.

  1. We must choose an ISO manually, the same for the camera as for the external photometer.
  2. Then we must choose a shutter speed and indicate it to both the camera and the external light meter.
  3. Once we have the previous settings, we will measure the light for which we want to know the exposure values. The photometer will give us a result in the form of a diaphragm opening. If we vary the ISO or the shutter speed, the diaphragm will vary to give us a correct exposure value.

Do I need an external photometer?

Although the incident light meter (external) is better than the internal reflected light meter of our camera, the truth is that the latter is usually more than enough to get by in most situations. Before throwing it away, the main thing is to learn what are the limitations of our integrated photometer, when it fails and whyand the tricks with which we can solve them.

Now, if your thing is portrait or product photography, then I wouldn’t hesitate, I’d go for one with incident light without thinking too much about it 😉

I hope it has been useful to you, the key to everything, as always, a lot of practice, many situations, many mistakes and only some theory 😉 . The best school is out there waiting for you to go out and photograph it ;-). Ah, but before you go all out for it, remember to share this article with everyone you think might be interested. Thank you and see you next time.