The relevance of the focus on thermography and how to avoid this mistake
How important is focus in thermography? Following the series of publications on the 7 most common mistakes, today we will review the influence of focus on thermography. It is part of the main thermography errors, so it is necessary to take it into account so that our data collection are of the best possible quality.
If you have not seen our first post about the most frequent errors in thermography, we recommend that you take a look. This will help you get a better perspective on common thermography errors, how to address them, and most of all, how to fix them.
This is why, first of all, we need to understand what is focus in thermography and why it is relevant. Focus while performing a thermography data collection consists of the process of adjusting the lens of a camera in order to achieve maximum detail and sharpness in the final image. That is, to focus is to make the subject at a specific distance sharp and defined. In figure 1 we see an example of a subject in focus (a) and one out of focus (b).
Another way of looking at it is that when focusing on a person, what we actually do is blur or blur all the other elements that appear in the image. Therefore, if we focus on a patient or an athlete, in reality, that person and also everything that is at the same distance will be in focus. Or what is the same, everything that is at any other distance (closer or further from the lens), will be out of focus and, therefore, less sharp.
Understanding this concept is really important to keep moving forward, because when we focus on an athlete at a specific distance (about 2 or 3 meters, usually), if we zoom in or out before the shot, the focus will be wrong, since the distance will be different and we will have to refocus.
In addition, each camera has different characteristics and parameters, which means that the way of focusing may be different depending on the model that is being used:
These cameras have a specific button that allows you to focus automatically at the distance from what is in the center of the image. To focus it is necessary, just press the button that is indicated with a red arrow in figure 2a.
Usually, a box appears on the screen indicating where you are going to focus (figure 2b). So we must place that box on the surface of the subject we want to analyze.
This type of camera has a wheel near the lens (figure 3a). This allows us to modify the focus manually when rotating it. Therefore, in doing so, we have to seek to reduce the outline of the subject as much as possible. In figure 3, we can see the difference in the definition of the contour of a leg, between an unfocused image (b) and a focused one (c).
These cameras work with a fixed focal distance, that is, it is constant. This implies that the only thing we can do to focus correctly is to adjust the ideal distance so that the outline of the subject is as defined as possible, as in the example in figures 1a and 3b. Usually, we will set this distance between lens and subject at about 2 or 3 meters, depending on the angle of the lens.
Some cameras have the peculiarity of having a laser that measures the distance to the object they are pointing at. In this way, this allows us to program an automatic focus with the camera or with moving subjects, since the laser allows us to measure the distance and adjust the focus accordingly. It is true that this laser can be uncomfortable for the patient or athlete, so we do not use it too much. In video 1, we have a brief summary of the ability of this functionality to improve the focus when using thermography to analyze moving objects.
Tips to focus perfectly in thermography
In almost all protocols, before focusing it is advisable to first make a correct framing (if you don’t know how, take a look here) and in a second time perform the focus.
If we have an automatic focus thermography camera, the frame on the screen will indicate what is being focused on in the center of the image, which will be the subject and not the background. Hence we have done the framing first. This happens in almost all protocols, except those in which the subject is not in the center of the image, but the background, as in the legs or feet protocols. For this reason, for these protocols we will first focus on one of the legs and, later, the frame including both legs, as shown in figure 4.
If we have a manual focus camera, we will have to adjust the focus wheel so that the outline of the subject is as regular and defined as possible. In this way, we will have to try a close approach and a further one until we find the ideal focus point, as shown in figure 5.
When we use a fixed-focus camera, we must modify the physical distance with the subject until the outline of the subject is as defined as possible.
Finally, with a laser focus camera, we must point the pointer to the surface of the subject’s skin so that the camera measures the distance and focuses automatically, as we see in figure 6.
How to know if your focus in thermography is incorrect
Regardless of the type of focus, the final image reveals whether it was done correctly or not. To do this, we must look at certain details, since from an image with a good focus, quality conclusions can be drawn. Or what is the same, with a poor quality image, the results, both clinical and sports and both qualitative and quantitative, will not be appropriate.
In figure 7, we can see the analysis carried out with the ThermoHuman © software with three different approaches:
We can see a completely wrong approach in figure 7a, where we can see the very thick outline and without any definition. This is an image that gives us very little qualitative information and no quantitative information.
Image 7b shows the most common situation. In it, the focus is on the background of the image and not on the subject’s own skin. This, as we explained previously in figure 4, happens in leg or foot protocols, when the background is in the center of the image and not the subject. Both qualitative and quantitative information are of limited quality. ThermoHuman © software is able to read and analyze these images, but the conclusions are really conditioned, so it is not ideal.
Finally, in figure 7c we find an image with the correct focus, which allows us to carry out a qualitative analysis of the best quality. Furthermore, the quantitative analysis is obviously much more reliable, as we can see in the differences between avatars 7b (incorrect) and 7c (correct).
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