1" sensor versus APS-C

Started Aug 15, 2013 | Discussions thread
nigelht
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Re: 1" sensor versus APS-C
In reply to ultimitsu, Aug 21, 2013

ultimitsu wrote:.

On the issue of light amount of light falling onto the sensor. lets do this thought experiment. Imagine we have two cameras, Camera A is a 135 FF, Camera B is a 3x crop (slightly smaller than Nikon 1).

Lets not do a thought experiment because your thought experiment is wrong.  T

his is why a reference is usually much better.

First, lets imagine we shoot a white wall using a 18mm lens for both cameras, same F-ratio (lets say F/2.8), same shutter speed. The image from the Sensor A is much wider than sensor B because 18mm on FF is UWA, but a normal, 54mm equivalent FOV on a 3x crop sensor.

Now lets imagine you cut the image from Camera A by 3 by 3, you end up with 9 equal crops. The centre crop should be exactly the same as the image from Camera B. they were shot using the same FL, same F-ratio, same shutter speed after all, and now they are the same, 3x crop. They received the same amount of light.

Yes.  The center crop in A receives the same amount of light as the sensor in B.

Now think about this, there are 8 other crops from the Camera A image, they look look identical the the centre crop, which means they each received the same amount of light as the centre crop. It must mean that the full image from camera A had received 9 times as much light as the image from Camera B. Have I lost you so far?

Yes.  For the FULL image and a FOV 3 times as wide and 9 times the surface area.

As we both agree, amount of light fall onto the sensor remain constant for all FL for the same F-ratio and same sensor size (we only differ in that you think it holds true even for different sensor size). This means, for Camera A, it receives the same amount of light at 18mm/2.8 as it would at 54mm/2.8.

The amount of light hitting the sensor is the same.  The amount of light in the scene is different.

Therefore we can conclude that Camera A at 54mm/2.8 received 9 times as much light as Camera B at 18mm/2.8.

Yes but that light has to cover 9 times the surface area in Camera A than in Camera B.  You aren't just lighting up the center crop of the Camera A sensor anymore.  There are 9 sensor squares to light up instead of 1 square.

Ooops.  This is where your thought experiment falls over.  Hard.

If we reduce aperture diameter of the 54mm lens down to one third, we would get a f-ratio of 8.4 and diameter of 6.43mm. The amount of light that comes through the 54mm/8.4 is only 1/9 of the 54mm/2.8. The same amount as Camera B with 18mm/2.8.

Except that you need to spread that light across the much larger sensor area of Camera A making every square 1/9th as bright as the sensor on Camera B requiring 9 times the sensor sensitivity to generate the same output.

the same diameter as the 18mm/2.8 lens for Camera B is 6.43mm, exactly the same as the diameter of the 54mm/8.4 lens.

A much slower 54mm/8.4 lens.

Magic, isnt it?

Anything is magical when you make a mistake and increase your results by a factor of 9.

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