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Also known as a threaded lens.The effect is better.It is often used in situations where the requirement for accuracy is not very high.But the cost is much lower than ordinary convex lenses
Fresnel lens USES the special optical principle of lens;An alternating "blind area" and "high sensitivity area" are generated in front of the detector, such as slide projector, film magnifier, infrared detector, etc.Body infrared is continuously alternating from "blind spots" into "high sensitive area", in order to improve its detection receiver sensitivity, strong and its range of energy, the second part is the detection region is divided into a number of bright and dark areas, thus make the received infrared signal input in the form of strong and weak pulse, Fresnel lens.
Fresnel lenses have two effects: one is focusing, which can act on the bandpass (reflection or refraction) of light over a specified spectral range when someone passes in front of the lens.
A typical example of this is the PIR, which is the refraction (reflection) of the pyroelectric infrared signal on the PIR, and if you look at one, the inside of the little hat is etched with teeth.The Fresnel lens can focus the light through the narrow-band interference filter on the photosensitive surface of the silicon photoelectric secondary detector.Fresnel lens can greatly reduce the cost, and then wipe with absorbent cotton.PIR is widely used in sirens and is out of focus.You'll notice that each PIR has a small plastic cap on it. The bandpass optical filter for traditional polishing optics is expensive to use for more precise focus, and the image of the subject in the center of the focus screen splits into two images.It shows that the focus is accurate.Fresnel lenses should not be wiped with any organic solution, such as alcohol.
The focal screen of Fresnel optical lens is also engraved with local metering or spot metering areas.With these teeth, it's hard to tell which point the camera is focusing on.Its advantage is bright and brightness uniform.Or there's a focus sound.The image on the focus screen is not clear.A focus screen with vertical and horizontal lines or scales is suitable for building photography and document remakes.It does not have the disadvantage of having a split image that is bright on one side and dark on the other, when the focus is off, when two split images merge into one.When dedusting, it can be rinsed with distilled water or ordinary clean water. The standard focusing screen of AF SLR is generally not equipped with image splitting device, but with a small rectangular box to represent AF area
Today's camera focus screens are frosted glass Fresnel lenses.In the early AF SLRS, it was often difficult to see the focus frame when focusing in a dark environment. This was the Fresnel lens.This Fresnel lens can limit the peak frequency of incident light to around 10 microns (the peak of human infrared radiation), making it easy to confirm the focus in a complex environment. Different types of focusing screens have different purposes.In general, the center of the focus screen is equipped with a split image and a micro-edge ring device. The middle part of the focus screen has no split image and only the micro-edge is suitable for a small aperture lens.To put it simply, there are equidistant tooth marks on one side of the lens. Many DSLR focus screens can be replaced by the user. Fresnel lens is equivalent to the convex lens of infrared and visible light in many cases.