Calculating Lens Position for Photographing Objects at Different Distances

In summary, the conversation discusses the use of a 70 mm focal-length lens to focus an image on the film of a camera. The maximum distance allowed between the lens and the film plane is 120 mm. The question is how far ahead of the film the lens should be for objects at different distances. The formula 1/f=1/di+1/do is used to calculate the position of the lens, with f being 70 mm and do being the object distance. The film plane is significant because it determines the maximum distance between the lens and the film. To solve for the image distance, the formula is substituted with the given values. The lens is moved back by a distance x to focus the image on the film.
  • #1
mustang1988
27
0
1. A 70 mm focal-length lens is used to focus an image on the film of a camera. The maximum distance allowed between the lens and the film plane is 120 mm. How far ahead of the film should the lens be if the object to be photographed is at the following distance from the lens?When it is 13.2m away, 3.2m away, and 1.9m away



2. 1/f=1/di+1/d0



3. I am not sure which numbers to plug in. Is the film plane significant?
 
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  • #2
di is the image (ie film) to lens distance, do is the object to lens distance.

Hint - make sure that f,di and do are the same units
 
  • #3
Ok so I know f is 70mm and do is the 13.2,3.2, and 1.9 and I am supposed to solve for di but I don't understand what the film plane of 120mm is for.
 
  • #4
The object distances are given from the lens when it is at 120 mm from the film. To focus the image on the film, you have to move lens back. Let x be the distance moved by the lens to focus the image.
Now in this position what is the object distance and the image distance.
Substitute these values in the formula and find the value of x.
 
  • #5
Ok, that makes a lot more sense. Thanks
 

FAQ: Calculating Lens Position for Photographing Objects at Different Distances

How do I calculate the lens position for photographing objects at different distances?

The lens position can be calculated using the thin lens equation: 1/f = 1/u + 1/v, where f is the focal length of the lens, u is the distance from the lens to the object, and v is the distance from the lens to the image. Rearranging the equation gives v = (uf)/(u-f). Therefore, to photograph an object at a specific distance, you can calculate the lens position by plugging in the known values for f and u.

What is the relationship between lens position and distance of an object in a photograph?

The lens position and the distance of an object in a photograph are inversely proportional. This means that as the lens position increases, the distance of the object decreases, and vice versa. This relationship is described by the thin lens equation, which takes into account the focal length of the lens as well.

Can I use the same lens position for objects at different distances?

No, the lens position needs to be adjusted for objects at different distances. This is because the distance of the object affects the focal length of the lens, which in turn affects the lens position. Therefore, to get a clear and focused image of an object at a specific distance, the lens position needs to be calculated accordingly.

How can I determine the focal length of a lens for calculating lens position?

The focal length of a lens can be found by looking at the specifications of the lens or by measuring it using a lensometer. It is usually expressed in millimeters and can range from a few millimeters for a macro lens to hundreds of millimeters for a telephoto lens. Once the focal length is known, it can be used in the thin lens equation to calculate the lens position for photographing objects at different distances.

Is there a specific unit of measurement for lens position?

The unit of measurement for lens position is usually millimeters (mm). This is because the lens position is measured from the lens to the image plane, and the distance is commonly expressed in millimeters. However, some cameras may use other units, such as inches or centimeters, so it is important to check the specifications or settings of your camera to ensure consistency in calculations.

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