Lens focus shift

Lens focus shift

Lens focus shift. Focus shift may be a problem that many friends often encounter. It is clear that the focus is in focus but the photos are blurry. Maybe there is some minor problem with the camera. So what's the problem? The following is the reference for lens focus shift. Lens focus shift 1

1. Focus forward/backward shift

SLR may cause damage due to material wear or original factory design error. The distance between the lens focus, the AF module in the body and the photosensitive element deviates, resulting in focus shift. Because it is related to the focus of the lens, some camera bodies will also produce focus shift when used with different lenses.

When a general SLR camera operates, after light enters the lens, it can generally be simplified into three routes. In the picture, L1 passes through the reflector, reflects through the focusing screen, and is refracted in the pentaprism into the person behind the viewing window. In the eyes; L2 is the light path that directly leads to the photosensitive element when the mirror is turned up when the shutter is pressed to expose; L3 is that when the shutter is pressed halfway, part of the light is guided by the secondary mirror to the autofocus module for focusing. Theoretically, the optimal state is when the path lengths of L1, L2, and L3 are the same. The focus determined when the shutter is pressed halfway to focus does look clear in the viewing window, so after pressing the shutter and exposing, the result It's also clear.

In the case of focus shifting, the three paths are of different lengths. When the length of L1 is different from the other two, even if the camera is indeed in focus, the viewfinder window will look blurry; when L2 is not equal in length, it will appear that you can see in the viewfinder window if you press the shutter halfway. The focus has been achieved, but the shot is blurry. This is a common situation when the focus is moved; when the L3 is different from others, it is also impossible to confirm the focus status in the viewfinder. The path lengths of L1, L2, and L3 are different. This may be caused by slight errors in the design of the camera itself, or may be caused by wear and tear of the camera after being used for a long time, especially the reflector.

The wear of the reflector may also cause focus shift problems.

In response to this common problem, some camera manufacturers have developed an "auto-focus fine-tuning" function, which is mainly found on some mid-to-high-end camera bodies. It can realize auto-focus adjustment within a certain range. Try to eliminate focus shifts caused by tolerance errors in the design of the camera body and lens. The automatic fine-tuning function on some cameras can also be used to fine-tune the zoom lens at the wide-angle end and telephoto end to ensure the accuracy of auto-focus within the zoom focal length, and save adjustment data for different lens models. Such automatic fine-tuning function can also be combined with some lens correctors to judge the camera for adjustment, making the operation easier.

Focus adjustment Some high-end cameras have an automatic focus fine-tuning function, which can solve some of the focus shift problems.

2. Focus shift

Excluding problems with the camera body, focus shift may also occur due to the optical design of the lens itself. It most commonly occurs with large aperture lenses. , especially the F1.0-1.8 type. Since the spherical lens itself has the problem of spherical aberration, when the aperture is larger, although it can absorb more light, the light entering from the center and periphery of the lens will cause uneven focus due to different refraction angles, resulting in an image. The blurry condition is called spherical aberration. Although most current lenses use aspherical lenses to correct aberrations in the lens structure, depending on the factory positioning of each lens, there will be differences in design and materials, which will affect its response to spherical aberration. Inhibition.

When the aperture of a large aperture lens is fully opened, since a lot of light enters through the periphery of the lens, there will be an error in focus with the light entering from the middle of the lens, resulting in unclear images, that is, "aberration" occurs. .

When a lens with a smaller aperture is fully open, most light enters through the middle of the lens, making it less prone to "aberration".

After understanding the causes of spherical aberration, let’s take a look at the relationship between spherical aberration and autofocus. Generally, when a camera autofocuses, it will focus at its maximum aperture, and then quickly zoom down to the set aperture value for shooting. A lens with a larger aperture, even if it has suppressed aberrations, will still produce blurry images. Due to the precision of the structural design and the materials used in the lens, the focus judged when the aperture is open and the actual focus after shooting with the aperture are offset, resulting in a situation where point A is clearly in focus but point B is clear.

The focus is obviously on the left eye, but the actual focus point is shifted to the mouth, and due to the shallow depth of field of the large aperture, the eye is completely outside the depth of field.

If you encounter such a situation, many seniors will tell you how to reduce the aperture to see if the situation improves.

Why can shooting with a reduced aperture improve the problem? Doesn’t it mean that cameras automatically focus based on the open aperture of the lens? Even if you shoot with the aperture narrowed, it still uses the maximum aperture to determine focus! Yes, the point of judging focus by the maximum aperture will indeed not change. Even if you narrow the aperture, the focus shift will still exist. If you originally focus on point A, it will still go to point B. But don’t forget that after narrowing the aperture, the depth of field will inevitably become deeper. , the wider the clear range in the picture, the greater the tolerance for focus shift. That is to say, even if the focus shifts, it is still within the depth of field range, which will give the user the desired focus effect. The comparison has no impact.

After narrowing down the aperture to shoot, the depth of field becomes deeper. Even if the left eye is originally focused, the actual focus point shifts to the mouth, but due to the deeper depth of field, the eye part is still within the depth of field.

Of course, the focus shift will be different depending on different lenses, shooting distances and other conditions. Generally, it is difficult to detect in the viewing window when shooting. It is often discovered after zooming in and checking. Therefore, if you are using a large aperture lens such as F1.0-1.8, you should pay special attention to the focus shift. If the camera has a depth of field preview or LiveView function, you can also make good use of it to increase the success rate of shooting. Lens focus shift 2

1. What is focus shift

Focus shift refers to the phenomenon of focus shift, which will bring a lot of trouble to our shooting.

When you shoot with a large aperture or shoot subjects with a very shallow depth of field, the negative effects caused by out-of-focus will be more serious. The most typical one is macro theme.

What’s even more terrible is that whether the lens is out of focus will not be marked on the product information. You need to get it and test it to get the result - and cases of out of focus are very common, and many people will do it. "Win the jackpot".

Different manufacturers and brands have the possibility of out-of-focus phenomena, and this has nothing to do with the grade of the equipment - whether it is a high-end machine or a low-end machine, it is possible. Focus outage occurs; some brands are even nicknamed the "focus outage king".

In addition, during the repair process of the lens, the original internal structure position of the lens may be changed due to technical reasons, causing serious focus loss. Be extremely careful when trading used lenses.

Lens maintenance is one of the important causes of focus loss

In addition to the above situations, there is also the problem of matching between the lens and the body - especially when the lens and body belong to When using different manufacturers, the phenomenon of out-of-focus will become very common.

2. What should I do if the focus is out of focus?

It can be said that in the digital age, out of focus is a very common phenomenon. This problem only causes harm if "you don't realize it exists." Once you know how to debug and correct out-of-focus, the negative impact it brings will be completely eliminated, and your lens will still be a good lens.

The following are the specific steps for correcting out-of-focus lenses:

Make a test card support plate

You need to find a flat slope with a 45° angle to place the test card . The production is very simple, just cut a corner of the carton and make the two right-angled sides equal in length.

Leveling the Camera

Most cameras today have built-in electronic levels. Turn on live view and enable the electronic level to ensure the camera is perfectly level.

If there is no electronic level, you can use an external bubble level to assist with leveling.

Make the center height of the camera and the test card consistent

You need to use a tape measure to measure the height of the center of the test card (that is, the length from the ground to the center of the test card), and then measure the height of the center of the camera. Make sure they are equal.

Determine the minimum focusing distance

First, observe the minimum focusing distance marked on the lens.

For example, today we are testing the Tamron 35/1.4, and its shortest focusing distance is 0.3 meters, or 30 centimeters.

Note that this distance does not refer to the distance from the lens to the subject, but the distance from the photosensitive element to the subject.

Use a tape measure to ensure that the distance from the center of the test card to the photosensitive element is 30 cm for the most accurate test results.

Start the test

Adjust the aperture of the lens to the maximum and start the test.

Taking the lens I have on hand as an example, when I shoot at the center of the test card, the results are as follows:

It can be seen that the lens is out of focus and shifted 8 mm backward. .

Adjustment and calibration

After measuring the focusing distance, it is time to adjust and calibrate.

This time we are using a Nikon camera. Next, we will take the Nikon camera as an example to introduce how to correct out-of-focus.

You need to find the AF fine-tuning setting in the camera menu settings:

In the default option, we can see the focus adjustment ruler.

Through the test in the previous step, we know that the focus of this lens is shifted back by 8 mm, so in this interface we move the focus forward by 8 mm, then save and exit.

Retest

After the calibration is completed, follow the method just now, use the maximum aperture to aim at the center of the test card and take another picture to see if the focus is still out of focus.

If such a result occurs, the focus is exactly on the 0 scale, congratulations, your lens is no longer out of focus: Lens focus shift 3

Why are large-aperture zoom lenses The topic of easy focus offset has actually been mentioned in the previous blog post "123 Tips to Know the Performance Characteristics of Camera Lenses". Because this topic is very important, it is necessary to mention it again in order to It has attracted great attention from photography enthusiasts. Why large-aperture zoom lenses are prone to focus shift is described in the previous blog post: "Under normal circumstances, the maximum aperture of a camera's zoom lens should be F4, but it is usually acceptable to use F3.5. It is questionable for zoom lenses with larger apertures (such as F2.8). It must be remembered that modern single-lens reflex cameras always adjust the focus when the aperture is fully opened. Therefore, if the aperture is narrowed to the actual shooting needs. A slight shift in the focus will cause the image to be unclear. Usually, a zoom lens with a large diameter will cause the focus to shift, resulting in a unclear image. "In fact, its subtitle has indirectly highlighted its key points. It’s very clear – it’s better to have a smaller aperture (lens).

I wonder if you have this feeling. When taking landscape photos, especially on cloudy days or under poor lighting conditions, when using the camera's autofocus function, the focus is clearly adjusted very "correctly". ", but the subject of the photos taken is always unclear. Sometimes even if hundreds of photos are taken continuously, the results are basically the same, which is really puzzling. Is there something wrong with the camera's autofocus function? Or is there something wrong with my eyes? Or maybe the camera was shaking at the moment of shooting...

Actually it's not the case, so where is the culprit?

Because when the camera is composing the picture and focusing on the subject, the camera is in a fully open aperture state, but once the shutter is pressed to take a picture, the aperture of the camera is at the predetermined aperture you set. Usually, when shooting The actual aperture is much smaller than the full aperture. Once the aperture is narrowed, the actual focus point will shift slightly. This is a characteristic of large aperture zoom lenses. Especially when the light is not ideal, the probability of occurrence will be greatly increased. Sometimes It can almost reach "100%".

So how to overcome this problem, the countermeasures are:

1. Use full-time manual (lens with this function) to shoot Fine-tuning is required to correct possible focus shifts, but the failure rate of this method is relatively high;

2. When using the camera autofocus mode, after completing the focus on the subject, turn the camera lens Turn off the auto focus mode before shooting, but this method is more troublesome;

3. The best way is to use the full manual focus mode, which is the most reliable way.

Since large-diameter zoom lenses basically have this kind of accuracy that cannot be "adjusted", in order to make the photos (subjects) taken clear, it is necessary to know its characteristics and use them during the shooting process. Try to avoid it. As a photographer, this is not only a theoretical issue, but also a shooting skill issue. The most important thing is to combine the two organically and not get confused. You might as well give it a try.