Acutancia and resolution, the recipe to create a good perception of sharpness

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Surely you will have friends who always tell you how good a clear picture looks. With this texture you feel so that you can touch it on a paper or screen, with the clarity that you can perceive every pore of your skin or even see the mountains in the iris of your eyes. This “sharpness” of the picture is a constant search for photographers. That’s why we’re looking for devices we buy to get a clearer picture.

Sharpness, however, is a subjective concept, for in its definition closest to our world, it would be the ability to see something that is well distinguished, not confusing. In photography, we need to come closer to two concepts: resolution and acutance. Well, between these two is the key to understanding our photographic system, to get as much detail as possible, or to manipulate it according to the needs we have. So let’s start by defining them.

Resolution in cubes

Perhaps, as a resolution, we most often associate the size of the image. This is because they sell us the idea that larger images can be taken with more megapixels. While a larger pixel density allows for larger display formats, size and resolution are independent.

Acutancia and resolution, the recipe to create a good perception of sharpness
More density of pixels in an area allows you to better notice the detail

Resolution is the ability of a registration system to distinguish between two nearby points. In the body, the resolution depends mainly on the sensor of the camera. With a fixed sensor size (full screen, APS-C, Micro 4/3), the higher the number of pixels in this area, the higher the resolution in the picture. And we will notice it between similar shots with cameras in different formats.

It is true that the camera does not make the photographer. However, it is also true that the photographer, if he wants a cleaner and more defined level of detail, should consider investing in cameras with a higher recording resolution.

Observing the acutancia

However, the camera is only part of the equation, the true ‘clarity’ comes from the optics we choose . If the objective is ‘soft’ it will give a soft image of focus independently if we capture it in a 400 megapixel camera like the Hasselblad H6D-400c or with a 10 megapixel camera.

Acutancia and resolution, the recipe to create a good perception of sharpness

In the picture above you can see the difference between the different optics. The camera with which the photos were taken is a Canon 7D. The test is performed so that the same area of ​​the sensor is covered with the Lego head. The picture on the left is that of an 18-55mm EFS Canon lens (from 2008) the cheap one that came with the camera at that time. In the middle is a Tamron 28-75mm aspherical lens. On the right side there is a Tamron macro-aspheric lens 90 mm. The photos were taken at F8.0 to work with the range of better optical performance. You can see in detail the difference that makes a higher quality lens, and even the difference between a fixed lens and a zoom lens.

The quality of the optics determines the sharpness in the picture. This refers to the contrast between details that differ in brightness or detail. A higher contrast is perceptibly interpreted as “greater clarity.” In order to determine the resolution and contrast of optics, it is therefore important to read the MTF diagrams on the pages when purchasing the optics. Do not panic with so many lines, the most important thing is to read the map in general.

Acutancia and resolution, the recipe to create a good perception of sharpness

First of all, it must be understood that the vertical axis defines the contrast and the horizontal axis defines a distance from the center of the lens. This second is important, because as you know, the lenses perform better in the center of the crystal (where the light falls directly on the sensor). In contrast, it is said that about 0.8 in the vertical axis is a good lens, between 0.8 and 0.6 is a normal power, and less of it are the lenses that we normally see with washed images, and it seems like you can never concentrate.

The tests are carried out with some primers having a diagonal of parallel lines running in diagonal proportions of one sensor to another diagonal of a rectangle. The lines in this diagonal have two standard densities of 10 LP / mm and 30 LP / mm.

The tests performed at 30 LP / mm are the higher, the higher they are. It is believed that the resolving power of the optics is high and therefore the visibility is much higher. The higher the contrast ratio in the lens, the higher the tests performed at 10 LP / mm.

Sometimes companies show performance with fully open membranes alongside a closed membrane like F8.0. As already mentioned, we work with the lens center when closing the aperture and should always recognize a better performance of the optics. Some companies apply the MTF with open membranes only and separate the tests with color to distinguish the LP / mm used.

Acutancia and resolution, the recipe to create a good perception of sharpness
Everything begins where the light comes.

So you know, start with good optics can be more fruitful in the search for images with a perception of sharpness much higher. If the level of resolution of the sensor with which you accompany it is high, it will help to obtain images with amazing level of detail and texture .

If reading the tables is difficult, there are also pages like DXoMark where you can see evidence of acutancia, contrast and color of many lenses. Compare how similar lenses behave . They will also be able to compare cameras and plan what is that equipment to acquire at the next birthday or Christmas.

Images | Sergio Fabara, Nikon USA

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