Explanation of some difficult problems in camera lens optics

1. What is the difference between zoom and focus?

Zooming is changing the focal length of the lens (exactly the image distance) to change the camera's viewing angle, which is what is usually said to bring the subject closer or farther. For example, 18-55mm and 70-200mm lenses are typical zoom lenses. the longer the focal length, the narrower the viewing angle .

Focus usually refers to the adjustment of the distance between the lens group and the negative (sensor plane) so that the subject becomes clear on the Ccd/cmos.

The "focus" we usually call "focus" is generally referred to as "focusing". Some people think that the fixed-focus lens can not focus the argument is wrong.

2, why the maximum aperture of the lens is usually poorly imaged, or "imaging comparison of meat"?

The pursuit of image sharpness should be so the lens of the pursuit. The aperture value of the lens generally ranges from f1.2-f32, such as F1.8-F16, for ordinary SLR lenses, usually the sharpest aperture value of the image is F5.6 or F8, why? This involves 2 concepts, one is spherical aberration and the other is diffraction.

Diffraction: The factors that influence sharpness in ultra-small aperture circles.

Diffraction (diffraction) is a physical phenomenon that deviates from the original straight line propagation when the wave encounters an obstacle. In classical physics, waves spread in different degrees after crossing obstacles such as slits, small holes, or discs.

Associated with the concept of love spot, Rayleigh criterion, interested in self-search, no longer unfold.

Spherical aberration: The factor that affects sharpness at large aperture.

In the simplified picture of lens imaging we usually see, the parallel light is converging at a point through the lens, which is called the focus, which is actually not the case. As shown in the following:

Since light does not converge perfectly, it is impossible to produce sharp images. When the aperture is large (aperture f value is small), the lens receives a large circle of light, so the light convergence will be very scattered, resulting in the image is not sharp. When the aperture is narrowed, the lens receives a small circle of light, and when the light passes through the lens, the convergence is more concentrated, so the image is sharper.

Of course, the focus will not be very accurate, the aperture is small when the depth of field, will also appear sharper, but this is something.

Of course, for the actual lens, lens is very complex, there may be several or even more than 10 lenses, some lenses are more expensive materials, such as fluorite, their main role is to improve the optical quality of the lens, correction of various ball aberration, coma, dispersion, distortion and so on.

For example EF 200mm f/2l IS USM Lens:

3, why some macro lens can not be in the distance to the upper Coke?

This is going to start with the most basic formula for imaging.

U is the object distance, V is the image distance, F is the focal length.

(in Figure U = U,v = V,of = F)

Everyone has a mobile phone, the mobile phone to take pictures of all know that the mobile phone can be in infinity (far away) on the Coke, for example, you can be half-meter of the cup focus, you can also 50 meters outside the high-rise on the coke.

But some fixed macro lens can not, for example, it can only focus on objects 10 centimeters, at other distances can not be on the upper focus. What is this for?

We look at the formula above: when the material distance (u) is very large, the 1/u is close to 0, at which point the focal length is approximately equal to the image distance. This means that the focus and the image face (negative) coincide.

The focal length is defined as the distance from the lens light heart to the focal point of the light gathering when the parallel light is incident.

We generally think that the focal length of a fixed lens is constant, which is of course no problem, but it is worth noting that the focal length is defined in the case of a parallel light incident. Another accurate argument is that the fixed-focus lens does not change in the same distance as the viewing angle.

The general fixed focus lens can be considered as the focal length unchanged. For example 50mm F1.8, its closest focus distance is 450mm, the object distance is a little bit smaller than 450mm. According to the above formula, at this time 1/u is very small, can be ignored, at this time v≈f, that is, the focus falls on the image surface. It is considered that the focal length is basically unchanged.

But for canon "hundred Micro" and Nikon "105 micro" such macro lens, the maximum magnification is 1 time times, when the focus distance is very close, at this time relative to the focal length, 1/u can not be ignored, then the actual lens focus to the distance of the light center is not equal to the focal length.

For the "Hundred Micro" and "105 micro" lens, because of its very special optical structure, can be in the infinite distance of Coke, but also in a very close "different" position Coke. This is achieved by the multi-group lens set inside the moving lens.

Having said so much, there is still no explanation why some macro lenses can only focus on a definite position. In order to simplify the problem and easy to understand, I take the SLR to install close to the ring as an example, many of the macro enthusiasts to play SLR because the bag is shy should have bought a close-up ring for macro creation, if the 50mm fixed focus lens plus close to the camera, then the lens of the focal distance is a definite distance. In the case of a 18-55mm zoom lens and a close-up lap, the focal range of the lens is a small distance. Why is it?

What is the lap? An adapter is a pipe with a thread or bayonet at the interface. Like you want to reach the height of things, so you step on a stool above the foot of the ground to the distance forcibly pulled up. The lap is the equivalent of a stool.

When it comes to the lap, it involves a concept-the flange distance. The distance between the lens mount and the camera negative is the flange distance. Then you take a lap that is equivalent to forcing the flange to lengthen the distance.

Or look at the previous formula:

As you forcibly lengthen the flange distance, the distance from V must be much larger. The distance from the focus to the center of the light is basically small, so the image distance from U is much smaller. Assuming that the ring is added, like the distance v = 2F, then U = 2F, that is, only the object distance is equal to 2F when the perfect Coke. This explains why some macro lenses cannot focus at infinity. It also explains why the zoom lens has a small distance, rather than a fixed distance, after the focus range is added to the lap. (Because the zoom lens has a larger range of focal lengths)

4. What is a finite conjugate lens and what is an infinite conjugate lens?

We learn in mathematics the conjugate complex, the conjugate root, so that the conjugate refers to some of the corresponding 2 points or two elements. The dots and the dots are a set of conjugate point. Also involves two the concept of chaos called into the pupil and out of the pupil, this does not unfold.

The infinite conjugate lens means that the point corresponding to the point is at infinity (which can be understood as far away).

For example this:

Finite conjugate lens: the point corresponding to the point can only be within a limited distance. As shown below:

Obviously, mobile phone camera, ordinary lens are infinite conjugate lens, and industrial macro lens is mostly limited conjugate lens, such as "Hundred Micro", "105 micro" Such a wonderful, can be infinite conjugate lens, but also can make the limited conjugate lens.

5. What is the difference between the use of red and blue in industry?

This needs to be divided in two ways.

The solution: blue light due to shorter wavelength, the diffraction effect is weaker, so the ability to portray details more powerful, shooting small objects, Blu-ray is the first choice. And red light, black and white CCD is more sensitive to red, but in fact, this advantage is not large, perhaps in need to minimize environmental light interference, a bit of a role. (as you can see, the advantage of the red light source is that the price is cheaper than the blue light source)

Shooting colored objects: Many people think that it is necessary to distinguish color objects from color cameras.

For an RGB color map, the following formula is usually used when converting to a grayscale map with a light and dark change:

Result luminance Grayscale value = 30% Red + 59% green + 11% Blue

In other words, the brightness is not the same when the objects of different colors turn gray. Even more magical, when you give the object a certain color of light, and then with a black and white camera, the resulting grayscale image is not the same. As shown in the following:

Some people say, you spend so much effort, do not use the color camera to shoot it? First of all, different camera price difference factor is on the one hand, more importantly, black and white images of data only 1/3 of color pictures, this difference will be reflected in the speed of image processing. The speed of production is extremely important for industrial production.

The rules can be summed up as follows: use red light to color objects, and then shoot with a black and white camera, the body of the red part of the object into a bright white, the white part of the object into a light gray, with the red difference between the color becomes dark black, black or black. (the specific conversion formula is unknown)

It is also important to note that the red light source is usually cheaper than the blue light source. Red LED light source made simple, long ago invented, and blue LED development is much later, the 2014 Nobel Prize in Physics awarded 3 have been in the Blu-ray led research and development has made remarkable achievements in the scientists, this shows.

6, why does the industrial field not mention ISO this parameter?

Engineers who do industrial machine vision generally know the aperture, shutter (exposure time), but you have to ask him what is the ISO, he probably does not know. Is there an ISO parameter in the field of industrial machine vision? Some, just changed a name, called gain (gain), this parameter is generally no one tune it, generally do not tune.

Aperture, shutter time, ISO is the three carriages to obtain the appropriate exposure, the three co-operation and mutual restraint. ISO usually has a value of 100-3200.

Usually the higher the ISO, the higher the camera's ability to sense light, and the more noise it has, the worse the image quality. For every photographer, in the vast majority of cases, he wants to fix the ISO at 100, because it can get the purest picture, but it's almost impossible. Because in the dark light environment, the ISO is too low, can only forcibly adjust the aperture, but the aperture is limited, so can only increase the shutter time, but the shutter time is long, handheld shooting inevitable paste.

But in the industrial field does not exist this problem, one is because industrial cameras generally do not move, exposure time is very long and will not paste, in addition, industrial vision equipment light source, almost no dark light environment.

7, why is the light source brightness higher the better?

High-brightness light source at least these benefits: high light source brightness, shutter time can be lowered down, it is possible to improve the speed of the drawing, the next light source brightness, can greatly reduce the impact of ambient light, in addition, the light source is high brightness, you can reduce the aperture, and usually reduce the aperture to get sharper picture and greater depth of field, These are extremely important for machine vision systems. So when choosing a light source, the higher the brightness, the better.

8, what is a single pixel quality?

A single pixel refers to a pixel on the sensor, and the quality of a single pixel is usually directly related to the size of the pixel particles. For example, some camera pixel particles are 5umx5um, and many cell phone pixel particle size is 1.12umx1.12um, or 1.3umx1.3um, the larger the value, the better the quality of a single pixel.

The main reason is that there is telecommunications interference between the pixels, the smaller the pixel particles, then the smaller the spacing of neighboring pixels, the more likely to produce mutual interference, resulting in a decline in the quality of the image purity.

Like a small square has 1000 aunt in Jump Square dance, everyone only points to 1㎡ space, so crowded aunt is likely to dry rack, but if only 10 aunt in Jump Square dance, then more spacious comfortable, perhaps after the square dance can also set 3 table fight the landlord, of course, the remaining one can wait for rotation.

Explanation of some difficult problems in camera lens optics