# I have an astronomy question?

If you try to take a photograph of something just a few centimeters in front of the camera lens, it will be out of focus. Based on your data, explain why a camera cannot focus on an object which is very close to the lens (within 7 or 8 centimeters, for example).

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• Alan
Lv 6
1 month ago

The camera lens projects an image of objects onto film or digital sensor. To accomplish this feat, light from the object traverses the camera lens. As this light passes through the lens, its direction of travel is altered. The lens has caused the light rays to trace out the shape of an ice-cream cone. We now focus the image of the object by adjusting the distance, lens to apex of this cone of light. Focus is achieved when the apex of this cone of light just kisses the light sensitive surface.

The distance lens-to-apex is a variable. If the object being imaged is far from the camera, the length of this image cone of light will be short. We call a measure of this distance the focal length. If the object is quite close to the camera, the image cone of light will be quit long. Because the distance image cone to lens is a variable, we must focus the camera lens by adjusting its distance from the light sensitive media. Suppose the camera lens has a focal length of 50mm (about 2 inches). Such a lens will form an image 50mm down stream from the lens but only if the object being image is at a far distance. If the object being imaged is placed 2 inches (50mm) from the front of the camera lens, the focused image will form 100mm downstream. In a simple camera, often called a box camera, focusing the lens is not a user setting. Such a simple camera is pre-focused to handle objects spaced from about 3 feet (1 meter) to infinity (as far as the eye can see). To accomplish the camera lens is restricted to a tiny circular opening. This lash-up can only take pictures in bright light. If we want a camera that can take pictures in subdued light, we are forced to make the working diameter of the lens adjustable. Such a lash-up must also allow the user to focus the image by adjusting lens to image sensor distance.

Additionally: Camera lenses are optimized to image distant objects and slightly compromised when tasked to image objects at close range.

The f-number engraved on the lens or reported by software is the focal length divided by the working diameter. As you image objects that close, the image rays elongate and we change the name from focal length to “back focus distance”. As the back focus distance increase image brightness decreases. At “unity” = magnification 1 or 1:1 the back focus is 2X the focal length. Do the math, the f-numbers are now in error by 2 f-stops. In other words is set to f/8, the actual f-number is f/16.

As a rule of thumb, camera manufactures limit close focusing when the f-number error exceeds 1/3 of an f-stop. In modern times, with through the lens metering, this point is moot so many cameras allow super close focusing.

However, close work done mainly on flat or nearly flat subjects. A macro lens is optimized to do close-up work and slightly compromised when tasked to do distance work. Thus macro lenses are optimized to work a flat field, ordinary lenses are optimized to work subjects with different camera-to-lens distances.

• qrk
Lv 7
1 month ago