Sensibility versus definition

Started Jan 7, 2015 | Questions
PeterBM New Member • Posts: 4
Sensibility versus definition

Why, for the the same camera, have i the same sensibility for different image definitions?

For example for a given scene:

- full definition 4000x3000 pixels at 200 ISO F4.0 good time is 1/100s

- 1/4 definition 2000x1500 pixels at 200 ISO F4.0 good time is still 1/100s. Why isn't it 1/400s ?

ANSWER:
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MysteryLemon Regular Member • Posts: 344
Re: Sensibility versus definition

PeterBM wrote:

Why, for the the same camera, have i the same sensibility for different image definitions?

For example for a given scene:

- full definition 4000x3000 pixels at 200 ISO F4.0 good time is 1/100s

- 1/4 definition 2000x1500 pixels at 200 ISO F4.0 good time is still 1/100s. Why isn't it 1/400s ?

Not sure I get what your getting at but from what I think you mean, the output resolution of the file has no effect on the exposure, hence why you see no difference in settings.

You're camera will still use the whole sensor to capture an image whether it be outputting 4000x3000 or 2000x1500.  The sensor hardware will dump the information it captures to the image processor that will then output the file.  Image output resolution has no direct relation to exposure.

Tom Axford Veteran Member • Posts: 9,828
Re: Sensibility versus definition

PeterBM wrote:

Why, for the the same camera, have i the same sensibility for different image definitions?

For example for a given scene:

- full definition 4000x3000 pixels at 200 ISO F4.0 good time is 1/100s

- 1/4 definition 2000x1500 pixels at 200 ISO F4.0 good time is still 1/100s. Why isn't it 1/400s ?

The second case of 2000x1500 is 1/2 definition, not 1/4, and one would expect the lowest shutter speed to drop from 1/100 to 1/50. However, determining the slowest shutter speed depends on how steady you hold the camera and that can easily vary hugely from shot to shot, e.g. if you jerk the camera as you release the shutter.

Don't worry about it, unless you reduce the definition a great deal more (e.g. down to 400x300), you'll probably not notice the difference in minimum shutter speed.

OP PeterBM New Member • Posts: 4
Re: Sensibility versus definition

Thanks to try to explain.

You are right: resolution is the good word, not definition.

My question. With a given camera, say with a 4000x 3000 sensor resolution i can choose (most cameras have this kind of choice in their menu) to get a photo with a 4000x3000 photo resolution or a 2000x1500 photo resolution (same ratio, notice).

In first case a sensor pixel provides data for one photo pixel. In the second case, there are ( is it true ?) 4 sensor pixels for one photo pixels, so I could expect four times less amount of light going through the lens for the same scene. Why is it false ?

Mark S Abeln
Mark S Abeln Forum Pro • Posts: 19,441
Re: Sensibility versus definition

PeterBM wrote:

Why, for the the same camera, have i the same sensibility for different image definitions?

For example for a given scene:

- full definition 4000x3000 pixels at 200 ISO F4.0 good time is 1/100s

- 1/4 definition 2000x1500 pixels at 200 ISO F4.0 good time is still 1/100s. Why isn't it 1/400s ?

Exposure is defined relative to unit area. That’s why the same exposure will basically work for all cameras — without regard to sensor or film size — as long as they have the same ISO rating. Handheld light meters works with any camera.

“In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film, as determined by shutter speed, lens aperture and scene luminance. In digital photography "film" is substituted with "sensor". Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance in a specified region.”

http://en.wikipedia.org/wiki/Exposure_(photography)

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