Find Largest Focal Length Lens for Photo of Poster - 2.79mm

In summary, to determine the largest focal length lens that can be used for a poster measuring 0.8 m by 1.2 m to fit in a 24 x 36 mm frame, the ratio of the sides of the poster and film should be used. Using the equations 1/f = 1/do + 1/di and M = -di/do, the focal length was calculated to be -2.79 mm.
  • #1
deenuh20
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Homework Statement



You take a picture of a poster on the wall of a room and maximum distance you can get from the picture is 3.1 m. The poster is 0.8 m wide and 1.2 m tall and you want it to fit in the 24 x 36 mm frame of the film in your camera.

What is the largest focal length lens that you can use? ____mm

Homework Equations



1/f = 1/do + 1/di
M=-di/do



The Attempt at a Solution



I have absolutely no idea how to go about this. What I tried, for starters, was find the ratio of the areas of the poster (0.96m^2) and the film (8.64E-4 m^2). I thought that by finding the ratio of the film to the poster would give me magnification which was equal to 9E-4. Then, using this number, I used the equation M=-di/do, plugged in 9E-4 for M and 3.1 for do and tried solving di and got -0.00279m. Then, I plugged this in the equation 1/f=1/do +1/di to solve for f and I got: 1/f=(1/3.1)+ -(1/.00279) = (-358.1 m) and took the inverse to be -0.00279 m. Then, converting to mm, it was -2.79mm.
 
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  • #2
Don't use the ratios of the areas. Use the ratios of the sides.
 
  • #3
Thank you very much. I finally got it!
 

1. What is the purpose of finding the largest focal length lens for a photo of a poster?

The purpose of finding the largest focal length lens for a photo of a poster is to ensure that the poster is captured in its entirety without any distortion or blur. Using a lens with a larger focal length allows for a wider field of view, which is necessary for capturing larger objects like a poster.

2. How do I determine the focal length of a lens?

The focal length of a lens can be determined by looking at the specifications of the lens or by physically measuring the distance between the lens and the image sensor when the lens is focused on infinity. This distance is equal to the focal length of the lens.

3. What is the significance of the number 2.79mm in the question?

The number 2.79mm represents the size of the poster that needs to be captured in the photo. It is the minimum focal length required to capture the entire poster without any distortion or cropping. Using a lens with a focal length smaller than 2.79mm would result in parts of the poster being cut off in the photo.

4. Can I use any lens with a focal length of 2.79mm or larger to capture the poster?

Yes, any lens with a focal length of 2.79mm or larger can be used to capture the poster. However, it is important to note that using a lens with a larger focal length will result in a wider field of view and may capture more of the surrounding area than just the poster itself.

5. Are there any other factors to consider when selecting a lens for this specific task?

Yes, there are a few other factors to consider when selecting a lens for this task. These include the aperture of the lens, which affects the amount of light that enters the camera and can impact the overall image quality, and the distance between the camera and the poster, which can also affect the focal length needed to capture the poster without distortion.

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