I often receive private messages from photography enthusiasts, and I have been trying to help you solve more common problems in photography. Of all the questions I've received. One of the most troublesome questions is "Why does my image look blurry?" ”。
This problem is something that almost everyone has encountered, and the reasons for this can be said to be diverse and relatively complex. At first, I always thought that it was caused by the focus problem, and there were many related articles such as "post-press focus" and "focus mode analysis" that I had written before, but recently I suddenly found that our focus seemed to have a certain deviation, because there was a more complex problem waiting for us to solve, that is, "running focus"!
<h1>An in-depth analysis of the focus problem</h1>
After a long day of shooting, returning home to find yourself facing a blurry image is the most frustrating thing. Often, blurry images are a clear sign of "focus technical problems". But if this happens on most images, then it's by no means as simple as a technical flaw, and there's a good chance we're running out of focus. Regarding out-of-focus, we can divide it into two causes, one is the focus accuracy of the camera itself, and the other is the optical aberration from the lens.
Let's first talk about the focus accuracy of the camera itself. As camera manufacturers continue to compete for pixels, our camera sensors are increasingly able to bring us better image quality, and of course, this is also more and more testing of the focus accuracy of our cameras themselves, because in clear images, a small deviation in focus will be magnified under the high pixels of the image.
On the other hand, we all thought our lenses were perfect from the date of purchase, but they weren't. Any light passing through the spherical mirror will cause a certain aberration problem, and we need a lot of optics to adjust to achieve the effect we want. Therefore, under the combination of a large number of optical devices, the lens has a certain tolerance in the manufacturing process, so the focus of each lens is not perfect, and the long-term use of the lens will gradually increase the error.
<h1>What is camera out-of-focus? </h1>
In modern high-pixel cameras, even slight aft focus between front and rear can have an irreversible effect in the image. If you're a landscape photographer who's used to shooting all scenes at an aperture of F/16, you probably won't notice this problem too much. However, if you are a portrait photographer, you should pay attention to the bokeh effect brought by the long-term preference for large apertures, and be sure to ensure that the focus is absolutely accurate. Because the combination of large aperture and high pixels to distinguish whether the focus is on the subject's eyelashes or eyes is a big problem.
Focus out-of-focus issues with the autofocus system itself:
When it comes to autofocus performance, the camera and lens need to work closely and accurately together. For this reason, some cameras often have two autofocus systems. For example, most SLR cameras use contrast autofocus in live view, while phase detection autofocus is used when viewing in the viewfinder. The drawbacks of the irrational use of these two focus systems often cause the vast majority of out-of-focus problems.
Analytical Phase Detection Autofocus System:
The phase detection autofocus system enables the SLR camera to accurately track the fast and irregularly moving subject, in which the reflected light is split into two separate images by the prism and microlens in a separate phase focus sensor unit, each aligned with two lines corresponding to the AF point on the sensor.
If the image hits these two lines precisely, it indicates that the subject is clear. The distance between the two images tells the camera how far the lens is out of focus. If the two images are closer together, the lens focuses on the front of the subject. If the two images are far apart, the lens focuses behind the subject. The autofocus unit calculates how far to move the lens to focus the subject, and then in which direction to drive the lens to that position. Fast and precise, ideal for tracking fast-moving objects.
Phase detection autofocus also has significant limitations, with phase detection focus not working well in low-light mode. Cameras need light to focus, and the less light there is, the harder it is to focus accurately. And most cameras have an AF point that is not covered enough in the viewfinder to allow you to autofocuse near the edges of the frame.
Focusing with a wide aperture is error-prone. This is because the phase detection part is a mechanical process, and the length of the optical path from the lens to the sensor is theoretically the same as the length of the optical path from the lens to the sensor. However, in the real world, cameras and lenses are set to tolerances. Such accuracy is too expensive and time-consuming. If your particular camera and lens are within the limits of their tolerances, the lens may be slightly focused in front of or behind the position the camera considers focused. When using the lens at a large aperture (narrow depth of field), this can lead to a focus error. Most mid-range and high-end DSLRs allow you to calibrate the lens to eliminate focus errors.
These are the limitations of the phase detection autofocus, which are part of the reason for the out-of-focus.
Analytical Contrast AF System:
Contrastive focus is achieved by analyzing pixels on the camera sensor. It works by saying that when the contrast is highest, the subject is focused. To find that point, it must push the focus point of the mirror group back and forth. Therefore, the speed of contrast focus is slower than the speed of phase focus, but it is much more accurate at focusing on stationary objects. There is no need to calibrate the lens, and there is no mechanical focus error, which reduces focus running.
Contrastive focusing is somewhat similar to the operation of manual focus, which is to determine the focus in constant attempts, but the difference is that the responsibility for determining the accuracy of the focus is finally left to the camera itself. Contrastive focusing also has great limitations, and while this relative lack of speed isn't important when shooting stationary objects, it makes a big difference when tracking moving objects, and even directly leads to complete out-of-focus.
Autofocus is quite complex, and to fully understand it requires an in-depth understanding of the technology, which most photographers don't have. Here we can only simplify the basic principles, which will help you understand how the different autofocus systems work, as well as the advantages and disadvantages of each type. In short, we can use contrast focus when shooting portraits and still life themes, as well as for low-light environments, and use phase focus relative to sports subjects, which helps us prevent out-of-focus and out-of-focus, and get a higher quality image as much as possible.
<h1>What is lens out-of-focus? </h1>
As we age or use our eyes excessively, our vision may begin to decline. Often we need some external forces to correct blurred vision and focus light correctly. Camera lenses also need such help, either just getting started or after constant use. We need lens focus accuracy to a tolerance standard to ensure that the image is in focus, that's as simple as that.
Below we will look for the fastest and easiest way to test your lens for out-of-focus issues and how to perform simple and effective correction.
Interpretive lens calibration:
Calibrating a lens is not difficult, the process is actually to calibrate the autofocus on the camera to the desired lens without changing the focus of any lens other than it. If your lens is corrected, the process doesn't make any modifications when you focus manually.
The calibration process involves only a specific camera setting that allows fine-tuning the lens's autofocus operation, which means we don't change any of the hardware on the lens, so this operation is absolutely safe. The physical calibration of the lenses can only be performed by the manufacturer, as the lenses must be disassembled, adjusted and reassembled. I would never recommend doing this at home, as it will not only make the merchant refuse the warranty, but also easily cause many more serious problems such as optical axis shifting.
Why does the lens run out of focus?
When a SLR camera uses phase detection autofocus, it requires the sensor to accurately pass the command to the lens so that the target object can be focused. While the focus system works well, it can happen that the focus is not on the image plane, but on the area in front of or behind it. The light passing through the sensor can be seen as a separate path, but the lens may have a problem with the shift in focus before and after. Another reason may be that the autofocus motor of the old lens cannot precisely adjust the focal length, which is a sign that the focus shift will alternate between before and after.
Lens testing with test cards:
We can check at home for a front-to-back focus shift (out-of-focus) in our camera lens combination. For this test, a test card is required with which to detect focus shifts. Test cards of different sizes can be purchased online, of course, if you have a printer at home, you can also DIY yourself. There are also many templates on the Internet for us to download, but most of them are not clear, here I also provide a high-quality version for everyone to download (get the way: after paying attention, reply to the "focus card" in the private message, you can get).
Note: In order to ensure the use of the test card, try to use A4 size photo paper for printing.
This test method can be explained by depth of field. In order to be able to read out the focal area, the depth of field should use the lightest depth of field, which is important because only in this way can a front and rear focus be found. Depending on the depth of field, the scale of the test card should cover the corresponding scale range. Of course, you don't need to use a different test card for each lens, one focus test card is enough, because the photographer can also control the depth of field.
First, you need to place the camera and the selected lens at the appropriate distance from the test card. This distance depends on the focal length of the lens you want to calibrate, usually the lens is 25 times the focal length of the test card. For example, if the focal length of the lens we tested is 50mm, the distance from the lens to the test card should be set to 1250mm, which is 1.25 meters. To calibrate the lens, the camera needs to be placed on a flat surface such as a tripod or table, and the test card also needs to be placed on the same horizontal plane as the ruler plane of the test card is placed at 45°.
Now we need to set the aperture to the maximum aperture allowed by the lens. For example, in this case, I'm calibrating the lens of F/2.8, so I set the aperture of the camera to F2.8. Adjust the ISO to the lowest possible level, and then, I adjust the shutter speed to accommodate other settings to create accurate exposures. We need to make sure you have enough ambient light and shoot at a fast shutter speed to capture a sharp enough image and avoid slight movements that de-focus. Here's a tip where we can set the shutter to a 2-second delay in shooting or use the shutter line to avoid artificially caused jitter.
When you're done, focus the center focus point in the center of the ruler and shoot. Again, my advice is to take at least three photos here so that you can get a broader picture of autofocus. Defocus the image each time and then refocus again to properly test autofocus.
How do I interpret the test?
Open the image on your computer, zoom in, and use the ruler in the image to see if the lens you are using is shifting in front or after focus. But sometimes it's hard to tell what's in focus and what's not, and one of the best tricks to use is to use a color picker to see what colors appear within focus. For example, most lenses will have aberrations that are consistent in digital images. Areas outside the postfocus tend to be green, and areas outside the prefocus tend to be magenta.
According to this approach, we can also understand that if 0 is green (focus after focus), 3 is magenta (prefocus focus), and 1 or number 2 is black gray (current focus), this means that the focus of your lens is shifted backwards and needs to be corrected forward. In the same way, and vice versa, this will give us a rough idea of the lens's out-of-focus range.
<h1>How do I correct lens out-of-focus? </h1>
Many SLR cameras offer a feature similar to "autofocus fine-tuning" if a lens combination has a slight forward/rear focus shift. Calibration with this function is still very necessary. Different cameras and models have different adjustment methods, and below we will briefly analyze the setting menus of Nikon and Canon.
Nikon's adjustment settings:
For Nikon, it's labeled AF Fine-Tuning under the settings menu. When you select this setting, all adjustable settings that you can change are displayed. Make sure that the autofocus is fine-tuned on, there will be a chart showing the position of the moved focus, you just need to change the value you want to "save". Every time you place the lens on the camera, the camera remembers these changes. So, you only need to calibrate each lens once.
Canon's adjustment settings:
For Canon cameras, start by entering the menu, scroll to the functional area, and autofocus. Make sure the autofocus AF micro-adjustment function is enabled. There are two setting options in Canon, which are adjusting all lenses uniformly and adjusting by lens, here we choose the latter, make visual changes according to the prompts, and the operation is basically the same as that of Nikon.
Considerations for lens correction:
Calibrating a lens is a very patient job, and we often need several calibration operations to get the lens back to tolerances. Try not to touch the tripod or move the camera during lens correction, we are best to use SD to read the photo to shoot here. Although it may seem complicated, in practice you will find that it is a very simple process. Before deciding to calibrate a lens, it is only necessary to know and understand all the attentions and details in the process. In the end, you are sure to get a satisfactory result from the calibration.
<h1>epilogue</h1>
Whether you pay special attention or not, the autofocus accuracy of the camera and lens changes over time. There are many different factors that affect accuracy, and the purpose of testing and calibration is simply to make sure that the lenses I use are more reliable. Here we need to understand that if you find that the autofocus behavior has a huge error, and the autofocus adjustment is not working properly, please contact the after-sales service and send the equipment for repair, which is most likely caused by hardware problems.
Here is Giant Photography, the "Harvard" in the hearts of photography lovers, thank you for your attention