Correct the autofocus of your digital camera; how it works?

Correct the autofocus of your digital camera; how it works?

Correct the autofocus of your digital camera; how it works?

One of many reasons for blurred images is focusing on the wrong area in the picture and it was not properly focused. The main motive (eg face in portraiture) is out of focus, while the background is clear. This source of error for unfocused images you can prevent. I want to give you in this article some basics and some tips along the way, the better to focus you allow.

Autofocus - Function and Procedure
In virtually any digital camera from compact to SLR makes today the autofocus to focus on it. Once you half press the shutter button, the camera calculates on the basis of reflected light or using an ultrasonic signal how far the subject is from the lens and adjusted by a motor, the lenses so that the correct area is brought into focus.

For this purpose, the camera uses different pixels, the so-called AF points. Depending on the camera, these are different identifiable and usually in the viewfinder by small rectangles or circles, but also on the camera screen. Once the camera has successfully faced sharply after the half press the shutter button, "beep!" Blink one or more of these measurement fields (in the viewfinder) in red and the camera is also a short signals the successful focusing. Now, the trigger can be pressed all the way. The properly focused photo's in the box!

"Unfortunately it does not always work smoothly. Autofocus encounters in some motivational as light situations often reaches its limits. The most common problems and questions I would like to describe a solution-oriented below".

Although the autofocus adjusts easily sharply, however, on the wrong area in the picture
This can happen especially when several pixels in the image are located and, moreover, are at different distances from the camera under certain circumstances. The camera is now looking out the picture element, which you want to focus their opinion. But you can only do so on the basis of technical information such as differences in brightness fix and not on the basis of the image content itself. Still less can know the camera, which image statement is to convey the picture and therefore, what exactly do you want to ask ultimately sharp. So it seems like in the example that the wrong subject is focused. The autofocus will work technically, but not content.

Correct the autofocus of your digital camera; how it works?
You want the plant have brought into focus, the lens does not? Without manual intervention (individual selection of the measuring field) Camera decide independently and in this example has the "wrong" motif, namely the lens (red marked areas) focused, while the plant already again blurred background blur.
The problem can be but quite simple to get a grip. To tell the camera what exactly they want to focus, you enabled only the AF point, which is located on the motive. All others are ignored and the camera focuses exclusively focuses on the selected measurement field. The measuring fields can be selectively filter On most bridge and SLR. In compact cameras, however, these individual settings are mostly, if at all, very limited available. Here you should look carefully when buying, if you want to have this flexibility even with compact cameras.

Manual selection of the measurement fields takes too long and is not flexible enough
Now it is according to experience so that this approach of constant manual selection of the AF point in some hectic situations quickly comes up against its limits. Especially with lively motif situations often changes the position of the main subject, you want to focus on that. Every time to go to the menu to select another measuring field, can not only be annoying, but it also runs the risk of missing out on beautiful thematic situations and occasions, because no time the correct setting has been chosen. Moreover, it occurs naturally, that the main motive just not reflected in any of the AF points.

So what to do if you now want to continue to use the auto focus? A proven technology to work very flexible, yet very precisely by means of auto-focus, is the following:

  • You can activate only one AF point (the best in the middle).
  • You select the image so that this one point is at the design that you have brought into focus like.
  • You ask sharp for this picture by looking semi durchdrückst the shutter
  • You position the camera so that your desired image section can be seen in the viewfinder, but hold the shutter button down halfway (the motive remains thus focused)
  • You press the shutter button completely - the photo with the right focus is in the box!

This can be a little confusing and complicated. With a little practice you have this rotation but out quickly and can react very flexibly to rapidly changing shooting situations. One thing though has to be considered: it may be indeed change the picture while you let the shutter button halfway, but not the distance to the subject. This can happen, where one looks at the camera from your subject / on the subject approaching or even changed his / her position to the front rear. When that happens, must be refocused again to get the focus on the shot.

Correct the autofocus of your digital camera; how it works?
Focusing with the central AF point: preferred motive focusing (left picture), select preferred image section (right picture) and trigger.
The autofocus fails and can not focus - you can not tripping
The motor in the lens buzzing desperately to herself and the focus range changes in the stroke of the engine, but can not find a point at which he would like to focus. Triggering? It does not work! If you have not already experienced this situation, they will meet you at the latest on high-contrast non-subjects or in low-light conditions. With the former, for example, white walls are meant. If you want now provide a white, pale, barely lit Vase fumbled in front of white wall, the sensors are used in the camera quickly reach their limits, since they measure the distance to the subject based on differences in brightness. Even ultrasound or an AF-assist beam, if any, are not a panacea.

The first measure that should be taken in such cases: enable the center AF point and make so sharp. This is a cross-type sensor with most camera models. Cross-type sensors are more precise because they measure both vertically and horizontally as opposed to the other sensors. Especially in low light, the autofocus works with this measuring field still quite reliable.

If you focus the main subject does not work, because the contrast and / or light are missing, you can dodge to other elements in the image for focusing. Choose a higher contrast element in the image (edges, corners, color differences, etc. work well), which are equidistant from the camera has as your main subject, place sharp and advance before raising your main subject again to the desired location in the image.

Correct the autofocus of your digital camera; how it works?
Rockabilly-evening in a bar in Leipzig - in low light conditions is worth the choice of the central AF point and the focusing on high-contrast subjects.
With these tricks can make quite good sharp even in low light situations with the autofocus. But at some point gets the best system to its limits. In such situations, the examiner has only manual focusing. This can go depends on focal length and focusing screen used more or less difficult by equip. The theme of manual focusing is here but not be viewed in detail. I'll deal with it at times even in their own contribution.

Even when focusing with only one measurement field-point leads to focussing errors
First make sure you have the distance to the subject when focusing and triggering the meantime does not change or your motive has not changed the distance. For short distances and open aperture settings (= very shallow depth of field) can have the smallest front and back mean that your main subject is in the background blur.

If this reason can be excluded, there is another, quite annoying cause in question: a front or back focus problem. The autofocus does not control the focused target. Depending on the quality of the lens and quality of the camera, the autofocus is sometimes more and sometimes a less precise. No autofocus can operate correctly at 100%. At the latest in low light conditions, he may be next often times, even though the photographer has done everything right. A major problem is the matter, however, if the autofocus is very often deviated and often the area in front of the focused Scene (= front focus), or behind the subject (= back focus) is sharp. How reliable Your camera system works in this regard can be determined at with this test.

What to do in this case? Some SLRs offer the setting option to correct inside the camera manually to front or back focus of the lens, an EOS 5D Mark II can, for example. However, this comes at a price. Entry-level SLRs offer this option but usually not. Then, this problem can unfortunately only be removed by an adjustment or repair of the lens. When buying a new lens, I recommend, therefore, the above-linked test conscientiously carry out and to send the lens in an eye-catching fehlfokus behavior back and test another specimen.
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