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【CK's Theory Related Post】PART III: If the upper portion of the lens is covered, is the formed image the lower portion of the scene?

CK Shene

Finally, this is PART III, the last part.


Below is a Nikkor-Q 135mm f/3.5 , in this (actually Sonnar ) design, the left focal point F and the two nodes H and H ' are some distance in front of the lens, this is a standard telephoto ( telephoto ) design, with to shorten the length of the lens barrel.


Generally speaking, the left focal point and left principal point of SLR/DSLR wide-angle lenses are usually inside the lens barrel. As the focal length increases, the two gradually move forward, and both may be outside the lens barrel when the telephoto lens is reached. Although we have only used three examples to illustrate, this is a general trend, although there will always be exceptions.

Real Answer

Armed with this knowledge, we can go back to covering half of the shot. Please see the picture below, the L in the picture is equivalent to the part between the two main planes, I simplified it into a lens. If the blocking object is at B 1 in front of the lens , then B 1 is a virtual image under the lens L, and it is not focused on the film, so no image can be taken. This is the same as the reason why objects too close to the lens cannot be imaged; in fact , As long as B 1 is placed between the lens and the left focal point F , a virtual image will be obtained and will not be imaged on the film. The image of B2 at the left focal point F is at infinity and will not appear on the film. Once the blocking object moves beyond the focal point F (for example, at B 3 ), a real image will be obtained, so it will be recorded on the film; if the focus is at O , and B 3 is within the depth of field, there will be a clear B 3 on the film If B3 is outside the depth of field, what is recorded on the film is the blurred image of B3 .


Consider wide-angle lenses first (see previous image). If the blocking object is in front of the lens, because the left focus of most SLR/DSLR wide-angle lenses is in the lens barrel, the blocking object is out of focus, so there will be a real image of it on the film, so you can see the black shadow part.

For a standard lens, if the left focal point is inside the lens barrel, the result will be an obstructed image as described above. If the left focal point is outside the lens barrel, but the obstruction is to the right of the left focal point, then a virtual image will be produced and will not be imaged on the negative, but because half of the light is blocked, the amount of light reaching the negative is reduced.

If it is a telephoto lens, the blocking object is very likely to be on the right side of the left focal point, so the conclusion is as above. Take the above Nikkor-Q 135mm f/3.5 as an example, no matter whether the blocking object is between two nodes or on the right side of the right node, it only blocks half of the light used for imaging, and there is still an image of the subject on the film , while the obstruction is still a virtual image and will not appear on the film.

In Conclusion

Therefore, the "covering the upper part of the lens" in the original question is the problem. In the case of a single lens, the cover in front of the lens is to block the light. Under the premise of the photographic lens, placing the black card in front of the SLR/DSLR wide-angle lens will capture the blocking object (so the answer is true); but under the telescopic lens, it is likely to only reduce the amount of light entering the lens without blocking the image ( So the answer is false); standard lenses are hard to say, it may be true or false, depending on the design of the lens (that is, the position of the left focus). In short, unless the key parameters of each experiment are really mastered, sometimes "pictures" may not necessarily have "truth". As far as this question is concerned, no matter what the original intention of the questioner is, I think he (or she) is thinking about single-lens theory rather than photographic optics.