# How Does Aperture Size Affect Exposure Time in Photography?

• sghaussi
In summary, if the aperture diameter of the large lens is 22.8mm, and the aperture diameter of the small lens is 8.5mm, then the exposure time for the large lens should be 3.33×10−2 seconds, and the exposure time for the small lens should be 1.33×10−2 seconds.
sghaussi
Hello! I'm having difficulty answering the following question:

Camera A has a lens with an aperture diameter of 8.50 mm. It photographs an object using the correct exposure time of 3.33×10−2 s.

What exposure time should be used with camera B in photographing the same object with the same film if this camera has a lens with an aperture diameter of 22.8 mm?

I have no idea how to approach this problem. It's the only problem of the chapter that deals with time and aperture diamter - the only formula I can use is : f/D (focal lenght/aperture diameter). However I don't understand where time fits in there. I hope someone can help me start this problem. Thank you.

sghaussi said:
Hello! I'm having difficulty answering the following question:

Camera A has a lens with an aperture diameter of 8.50 mm. It photographs an object using the correct exposure time of 3.33×10−2 s.

What exposure time should be used with camera B in photographing the same object with the same film if this camera has a lens with an aperture diameter of 22.8 mm?

I have no idea how to approach this problem. It's the only problem of the chapter that deals with time and aperture diamter - the only formula I can use is : f/D (focal lenght/aperture diameter). However I don't understand where time fits in there. I hope someone can help me start this problem. Thank you.

Exposure time must be regulated to limit the amount of light energy captured by the camera and delivered to the film. If the rate of light energy is constant (it is), then the amount of energy captured depends on how big the aperture is, and how long it is left open. Think about how increasing the size of the aperture will affect the amount of light energy that gets into the camera.

So what you are saying is if the aperture diameter increases, then less exposure time is needed since larger apertures allow more light to come in. However, I still don't understand how to approach this problem. =(

You are on the right track. Light energy is spread uniformly over a surface. The energy of the light that you capture with a lens depends on the how much of that surface you capture. If you increase the aperture from 8.5mm to 22.8mm, you will be capturing a larger surface. How much larger? How much SURFACE is associated with each diameter? How much more surface does the larger lens have, or better yet, what is the ratio of the two surfaces? You have to shorten the exposure time of the large lens so that the same amount of energy gets captured by both lenses.

## 1. What is the definition of geometric optics?

Geometric optics is a branch of physics that studies the behavior of light as it travels in a straight line and interacts with different materials and objects.

## 2. What are the basic principles of geometric optics?

The basic principles of geometric optics include the law of reflection, which states that the angle of incidence is equal to the angle of reflection, and the law of refraction, which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the speeds of light in the two media.

## 3. How do lenses and mirrors affect light in geometric optics?

Lenses and mirrors are essential components in geometric optics. Lenses can bend and focus light, while mirrors can reflect and redirect light. These properties are used in various optical devices, such as cameras and telescopes.

## 4. What is the difference between convex and concave lenses?

Convex lenses are thicker in the middle and thinner at the edges, and they converge light to a focal point. Concave lenses are thinner in the middle and thicker at the edges, and they diverge light.

## 5. How is geometric optics used in everyday life?

Geometric optics plays a significant role in our daily lives, from corrective lenses in glasses to the design of traffic lights. It is also used in various industries, such as medicine, telecommunications, and photography.

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