How does the autofocus work?

adminonJuly 21, 2022February 11, 2022Leave a Comment

here is a brief explanation of how the two common passive autofocus systems work. Passive means the camera does not send out a signal to measure the distance (ultrasound, infrared light). Instead, it adjusts the distance based on a phase comparison or contrast measurement.

phase comparison

At phase comparison (phase detection, phase shift, phase contrast, phase difference) parts of the subject are projected from the lens onto autofocus CCD image sensor pairs, a couple for each autofocus metering field. With the help of the signals of the partial image pairs, the camera calculates to which distance the lens should be set.

In a reflex camera, the mirror that reflects the image produced by the lens onto a matte screen lets some of the light through. Another mirror behind it (auxiliary mirror) redirects the transmitted light to the autofocus sensors in the camera bottom. On each autofocus sensor pair projects a small lens (microlens) the light from the same partial image coming from opposite sides of the lens.

Some mirrorless cameras also have a phase comparison autofocus. There, the autofocus sensors sit directly on the image sensor.

Based on the light intensity patterns of a pair of autofocus sensors, the camera processor / autofocus processor can calculate, how far and in which direction the autofocus motor must adjust the lens so that the image is sharp. Because (theoretically) the distance setting does not have to be determined by trial and error, but can fit at the first attempt, the phase comparison autofocus is quickly.

Figure: principle of the phase comparison autofocus for SLR cameras. The drawings are not precise in terms of distances, sizes and angles, they only illustrate the principle. we are looking at light beams that all pass from the same motif point through the lens via the auxiliary mirror through microlenses onto an autofocus sensor pair. each sensor of the pair receives light coming from opposite outer areas of the lens. Below is the intensity of the light ( y-axis, perpendicular) and its location ("x-axis", horizontal) of both sensors is sketched, the intensity increases upwards. In (1) the lens is correctly adjusted. The object point is a pixel on the sensor and the light is at the same location on both sensors. In (2) a too close distance is set. The light from the left sensor is on the left and the light from the right sensor is on the right: the "phases" (light signals) are shifted. The same is true for (3), only the lens is set to a too far distance. Now the light of the left sensor is on the right, that of the right sensor on the left. If the phases are out of phase, the autofocus processor knows that the lens is not properly focused. Based on the distance and direction of the shift, it can calculate to which distance exactly the lens should be set.

Figure: phase comparison-autofocus and partial images. Here a partial image a triangle viewed falling on a pair of autofocus sensors. On the left, the distance is set correctly. the triangle is projected on the same place on both sensors. If both sensors were superimposed, the images of the triangle would be congruent. This corresponds to (1) in the previous figure. In the center, the distance is set too close, which corresponds to (2) in the previous figure. The triangle is not in the same place on both sensors, it is shifted. Right corresponds to (3) in the figure above, the distance is set too wide. Again the triangles are not congruent.

Minimum aperture size

Phase comparison autofocus only works with lenses that do not fall below a certain light level. Usually this is 5.6, with some cameras also 8. If the lens has a lower light intensity, for example 8 or 11, the camera switches the autofocus off. This limiting light intensity is undershot if a low-light lens is combined with a teleconverter. A lens with a speed of 5.6 and a 2x teleconverter becomes one with a speed of 11.

Front focus, back focus (front focus, back focus)

phase comparison metering in SLR cameras is prone to errors: the autofocus sensors and the image sensor are not in the same plane, after the metering is done, time elapses until the mirror is tilted up, during which time the subject can move, and the lens may not correctly set the distance transmitted by the metering: a too close (front focus, front focus) or one too large (backfocus, back focus). This is most noticeable at short distances (large image scales) and large apertures, for example 2 or 1.4.

front- and backfocus can be determined with test charts like lensalign. With some cameras, front- and backfocus can be corrected by yourself with the help of a test chart, with most of them, only the camera service will be able to do this. With my nikon D300, I can set the distance via the camera menu under AF fine tuning set individual corrections for each lens. The camera recognizes which lens is attached to it and automatically adjusts the corrections made. I would make use of this, if i notice in a lens front- or backfocus in practice. I do not specifically test my lenses for front or back focus. For a professional photographer this could be useful.

Mirrorless cameras with phase-locked autofocus do not have these problems because the autofocus sensors are directly on the image plane.

extension of the structures

In SLR cameras there are sensors that can only focus on horizontal structures, sensors that can only focus on vertical structures and sensors that can focus on both structures, so-called cross sensors. Today’s cameras are likely to use all three types mixed together.

Contrast measurement

With the contrast measurement the camera measures the contrast, the brightness difference, of neighboring pixels on the image sensor. The image is focused at maximum contrast. When measuring contrast for the first time, the camera does not know in which direction to adjust the distance on the lens, it tries one out. If the contrast gets stronger, it can keep the direction, if it gets weaker, it uses the opposite direction. The contrast then becomes stronger and decreases again after a certain distance setting. If this is the case, the distance must again be adjusted slightly in the opposite direction, up to maximum contrast. The lens adjusts the distance in one direction and then in the other direction until the contrast deteriorates in both directions then the correct distance is set. Under certain circumstances (low light, low subject contrast, noisy autofocus motor), you can see on the display and hear on the lens how the distance is adjusted back and forth until the focus is "leveled" is.

The contrast measurement is therefore not (yet) as fast as the phase comparison. On the other hand, it is free of front and back focus errors and I am not aware of any limiting light intensity (nevertheless, fast lenses are an advantage). Since the measurement is made directly in the sensor plane, the contrast autofocus can also be more accurate than the phase comparison autofocus in SLR cameras.

The position and number of the "autofocus sensors" is not limited in principle as with phase comparison measurement and the size of a (virtual) autofocus sensor can be adjusted more or less continuously.

Size and number of the autofocus measuring fields and coverage of the image area

Modern cameras have 30 and more autofocus measuring fields. If the camera has many metering fields, it is more likely to find a field that captures the desired subject location without having to tilt the camera for metering and reposition it for shooting. This saves time and is more precise.

If a measuring field is small, small objects can be captured more precisely and not accidentally objects in front of it. A larger metering field can be advantageous for fast moving subjects, as it takes longer for the subject to be out of the metering field area. The maximum size of the measuring fields is indirectly related to the number of measuring fields: the more measuring fields there are, the smaller they become. In phase comparison metering, the size of the fields is fixed; in contrast metering, it can be changed.

The ideal situation is when the measuring fields cover the entire image area. With contrast metering this is possible (although not necessarily realized). Phase comparison measurement usually covers less image area than contrast measurement.

If the sum of the measuring field areas is smaller than the covered image area, there are gaps between the measuring fields, there the autofocus does not capture the motive.

Several adjacent measuring fields can be combined to form a group. Each measuring field of the group is active. This means that small objects can no longer be captured precisely, but fast-moving subjects can be captured more reliably: if the subject leaves one measuring field, another one in the group can still cover it.

A camera should have many small measuring fields that cover as much of the image as possible. It should be possible to control (activate) the measuring fields individually and combine them into groups.

Figure: autofocus points of the nikon D300 (1. image) and olympus OM-D E-M5 (2. Image). The nikon D300 has 51 measuring fields, whose size and spacing are fixed. Less image area is covered upwards and downwards than to the left and right. the olympus OM-D E-M5 has 35 measuring fields, which cover more image area than the nikon . The size of the measuring field is variable: the measuring fields can be enlarged to such an extent that there is no longer a gap between them.

Autofocus in principle

Contrast is required in the motif (differences in brightness) for the autofocus to work. A completely blue sky or a smooth white surface will not be able to be focused by the autofocus as a rule.

Which autofocus to use?

As a rule, all SLR cameras have a phase comparison autofocus and mirrorless cameras have contrast metering. There are mirrorless cameras with both autofocus metering systems, where the phase comparison metering is done on the image sensor.

If the mirror is tilted up and the SLR camera shows the image in the live view on the display, it also uses the contrast metering for the autofocus, because then the phase comparison metering no longer works.

For fast-moving subjects that are constantly moving toward and away from the camera, such as athletes and animals, it is (still) best to use SLR cameras with phase comparison autofocus. Otherwise nothing speaks against the contrast measurement.

If you want to sell a used car, the first thing you should do is to determine the value of the vehicle. On the one hand, you don’t want to give away any money, but on the other hand, no potential buyer will contact you if you have overestimated the selling price. You can get an initial overview by determining the vehicle value on mobile or another online car exchange with a vehicle of the same value.

If you have a selling a used car if you want to buy a car, you should first determine the value of the vehicle. On the one hand, you don’t want to give away any money, but on the other hand, no potential buyer will contact you if you have overstated the sales price. You can get an initial overview by calculating the vehicle value on mobile or another online car exchange with a vehicle of the same value.

If you are selling your car privately, the car price is usually the most important criterion for most private buyers. Therefore, you should make absolutely sure that you find out the most correct value possible when calculating the value of a used car.

This is why the used car valuation is so important

You must take into account the condition of the vehicle and its equipment. For a quick sale, the price should be lower than for comparable vehicles. How to make the car interesting for the buyer. In addition, so your car appears at the top of the search lists and is thus seen first.

How to calculate the value free of charge?

For the calculation of the value there are different possibilities. First of all, you can calculate the average price of similar cars for free at mobile de. Look for similar cars with similar equipment. The following features are important:

Car make
Initial registration
Model
Mileage
Performance
Transmission type
Extras

then compare similar cars from the database. These are always prices that the seller would like to have and not necessarily the selling price that can be obtained. You should not consider individual cars that are offered at unrealistic prices.

In the next step, add up the desired sales prices and divide this value by the number of vehicles. How to get the average value of the vehicle. With this value calculation you can quickly and easily determine an approximate value.

What other options are there for the vehicle valuation?

You can of course also carry out a paid valuation by obtaining a professional valuation report. This is accurate because it also takes into account any damage your vehicle has, such as paint damage, scratches and dents. On the internet and in trade directories you can find an appraiser who is close to you.

Also the ADAC can give appropriate assistance and has a list with car appraisers. Another possibility is the so called schwacke list. The used car prices for dealers are given here. However, these are the purchase prices and not the sales prices.

You can check the price online, or alternatively you can purchase the matching schwacke book. This is regularly updated and almost all car models are listed here with their value. The online query is also not free of charge, it is subject to a fee.

Conclusion:

There are several ways to determine the value of a car. The fastest and easiest way is certainly the calculation of the average value on corresponding internet portals, where vehicles are sold.

A nice car is something special. The typical vehicle from the assembly line is tuned to a real unique, which not only attracts attention, but often drives much better.

But in terms of cost, tuning is the rule and not the exception. Like all other work on and in the car, tuning measures cost a lot of money, especially since not all of them can be done by yourself. But how can the work be financed? Is there any savings potential or ways to borrow money at short notice for a special bargain?? This article looks under the hood at tuning costs.

What do tuning measures cost in detail??

There are, of course, various tuning options – and they all come at different prices. roughly speaking, the painting of special patterns costs much more than the mere application of appropriate tattoo foils. In principle, work that requires real intervention in the vehicle and even has to be carried out by certified workshops is particularly expensive.

These measures usually have to be approved by a test center, which means that the test fee and the costs for the corresponding entries in the vehicle documents are added to the costs. An overview:

Chiptuning – the engine performance is directly affected here . Car manufacturers only run most cars at about 80 percent of their potential, so of course there really is more power to be had here. However, chiptuning is not equal to chiptuning. In the so-called 10-cent-tuning, the fuel fine is increased, and at the same time a resistance device is placed between the temperature element and the engine control unit. This tuning is partly already available for less than 100 euro. It becomes more expensive if the engine control unit, or. Whose chip is completely reprogrammed. But this variant has the advantage that the car can be set back to "factory settings" almost effortlessly. Depending on the vehicle this tuning costs up to 1000,00 euro. The last variant is the exchange of the chip. Here the price depends on the vehicle, the desired chip and the effort.

Turbocharger – the turbocharger consists of a housing with compressor and turbine housings at the ends. the turbocharger is mounted on the exhaust manifold. These parts can be tampered with to extend the life of the component, but also to produce a general optimization. there are different tuning options, so the price also varies. Under 350,00 euro tuning friends do not get away however.

Tuning the exhaust system – it’s not just about performance, it’s also about looks. The tuning of the exhaust system can consist of the installation of a new muffler, but also of a completely new system. How expensive the tuning is, depends on the extent and the used materials.

Air filter – it also belongs to the engine tuning and causes a performance increase. The sport air filters also increase the engine noise. At this point – as with the tuning of the exhaust system – it must be remembered that the car can quickly become a moving nuisance. If the noise level exceeds 80 decibels, a fine is due.

Those who not only want to increase the performance of the car, but also to adapt it visually to the new engine power, often have to dig really deep into their pockets. This applies in particular to those who would like to have the car repainted and have sports seats fitted.

Sound machine – the exhaust system is also a sought-after tuning object

How can this be financed?

The best way to pay for tuning is, of course, to save and top up whenever the money is available. But in many cases this is not so easy. If a chiptuning is carried out, a new air filter is also recommended, so that the engine lacks nothing. Basically, tuning enthusiasts should adhere to the following points before starting:

Consult experts – at this point, it is first of all only a question of which work is worthwhile on the respective vehicle and which of these can be carried out first. At the same time, it can be considered whether some work can be replaced by other work. If you want an eye-catching paint job, you can often reduce the costs with adhesive films.

Get quotes – Many workshops offer tuning measures, but the costs often vary. For some work, a DIY workshop is also worthwhile . In it all working tools are available, there are always specialists on site and it is helped mutually. You have to buy the parts yourself, and there is also a usage charge.

Inquire about legal issues – in addition to the pure tuning costs, a new registration and the acceptance of the vehicle at the TuV or dekra are often required. This also involves costs that have to be taken into account.

Now it is also possible to borrow money for tuning. So-called mini-loans are particularly suitable here. This is especially the case when a workshop or a supplier has a special offer that only runs for this month. Mini-loans amount to small sums and run only a maximum of 90 days.

Nevertheless, one thing is true: no one should go into debt for tuning. The money for the repayment must therefore be available. Mini-credits usually fit when a fixed higher cash inflow is expected in the next month anyway, which is not yet planned. Christmas or vacation money, for example, is the best way to repay a loan.

How to save on tuning?

If you visit several garages and get quotes, if you order the necessary parts yourself on the internet or – for seats and parts – look for them at scrap yards, you can save money . The same applies to those who are confident enough to visit a DIY workshop.

The important thing is not to put looks and performance before your own safety. Especially work on the chassis, the brakes, but also the engine must be carried out professionally, because the best tuning is of no use if the chassis suddenly breaks down while driving.

Conclusion – tuning costs

Just before getting their driver’s license and their first car, everyone probably thought that their parents were never right about the car being a piggy bank without a spending hole. A year later, everyone knew that the saying was true. The costs for tuning measures are included in the wisdom, because here, too, massive costs are incurred. Nevertheless, there are ways to save at least something.