What’s the difference between optical zooming and cropping later?

Asked 5/9/2014

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If I frame a subject by zooming in with the lens, versus taking the shot wider and cropping afterward, what practical differences should I expect? I understand that cropping reduces resolution, but are there other differences in image quality, distortion, depth of field, or exposure?

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Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

12y ago

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Adjusting the focal length of your lens (ie, optical "zooming") will impact the depth of field of your image. This will change how much of the scene is in focus.

It will also subtly change aspect of the distortion of the image in order to project it on to a rectangular surface as lenses don't quite perfectly project their image and the exact variations change as the lenses are reconfigured for the different focal length.

It may also result in an increase in motion related blur indirectly as many consumer lenses may reduce their maximum aperture as you move towards the higher focal lengths. If you were shooting with the aperture wide open, this reduction in aperture may result in a longer shutter speed needing to be used to expose the scene.

Originally by user11392. Source · Licensed CC BY-SA 4.0

user11392

12y ago

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If you crop from the exact center of a wider shot and compare it to a zoomed shot at the same final display size, the main difference is usually resolution: optical zoom uses more of the sensor on your subject, while cropping throws pixels away.

Other practical differences can appear:

Depth of field: Changing focal length affects how much of the scene appears in focus.
Distortion/rendering: Zooming changes the lens configuration, so distortion and edge rendering can differ. If your crop is not from the center, this can be more noticeable, especially with wider lenses.
Exposure/aperture: Many zoom lenses have a smaller maximum aperture at longer focal lengths. If you zoom in while shooting wide open, you may need a slower shutter speed or higher ISO, which can increase motion blur or noise.

So, cropping is essentially a composition trim after capture, while optical zoom changes how the lens records the scene. If all else is equal and you crop from the center, they can look very similar apart from lost resolution, but in real use zooming can also affect focus rendering, distortion, and exposure.

What’s the difference between optical zooming and cropping later?

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