Optical Instruments-Cameras

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In summary, when taking pictures of the beach at sunset, a shutter speed of 1/125 s at f/11 will produce a properly exposed picture before the Sun sets. After the Sun sets, the light meter indicates that the scene is only one third as bright as before. To maintain the same exposure, the shutter speed would need to be tripled to 1/375 s. Alternatively, if the shutter speed is kept at 1/125 s, the aperture would need to be opened to three times its original size.
  • #1
kmj9k
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You are taking pictures of the beach at sunset. Before the Sun sets, a shutter speed of 1/125 s at f/11 produces a properly exposed picture. Shortly after the Sun sets, however, your light meter indicates that the scene is only one third as bright as before.
(a) If you don't change the aperture, what exact shutter speed will produce a picture with the same exposure?

(b) If, instead, you keep the shutter speed at 1/125 s, what exact f-number will produce a picture with the same exposure?


2. f-number = focal length / Diameter of aperture



3. I'm not sure what the significance of the picture being 1/3 as bright after the sun sets is. But, I do know that when you change the shutter speed, you need to change the area of the aperture to compensate if you want to maintain the same exposure. Other than that, I don't know where to start on this problem. I would appreciate any help!
 
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  • #2
hmmm. To maintain same exposure level I would think require having same number of photons strike the film in each case. Assuming "brighness" here is a measure of the number of photons that strikes the film, how much time to capture the same number as before. Remember shutter speed is a usually a small fraction of a second, 1/125 is 80 millseconds open time.

Same principle applies for next question. How much larger would opening of aperture have to be to collect the same number of photons as in original case.
 
  • #3
Yes, I understand that part...you want the same amount of light / photons. I thought the answer to (a) might be 1/375, to make the shutter speed 3x faster so that less light is able to get in and the photo is 1/3 as bright. But, I know that's wrong...
 
  • #4
yea, that's why I inverted the "shutter speed" to msec open time. Triple the open time, then take reciprocal to get shutter speed..
 
  • #5
OK, inverting the shutter speed makes it much more easy to understand! I get the problem now and got the right answers. Thank you very much for your help!
 
  • #6
No problem, happy to be of help.
 

1. How do cameras capture images?

Cameras use lenses to focus light onto a photosensitive material, such as film or a digital sensor. The light creates an image on the material, which is then processed and recorded to create a photograph.

2. What is the difference between a digital camera and a film camera?

A digital camera captures images using a digital sensor, which records the image as a series of pixels. A film camera, on the other hand, uses a roll of film coated with light-sensitive chemicals to capture the image.

3. What is the purpose of different lens sizes on a camera?

Lens sizes, or focal lengths, determine the angle of view and magnification of the image. Shorter focal lengths result in a wider angle of view and smaller magnification, while longer focal lengths result in a narrower angle of view and larger magnification.

4. How do optical zoom and digital zoom differ?

Optical zoom physically adjusts the lens to magnify the image, resulting in a higher quality zoom. Digital zoom, on the other hand, simply enlarges and crops the image, resulting in a lower quality zoom as it can lead to pixelation.

5. How do cameras focus on a subject?

Cameras use a variety of methods to focus on a subject, including manual focus, autofocus, and face detection. These methods use sensors and algorithms to determine the distance and adjust the lens accordingly to achieve a sharp image.

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