Do I really "Gain a stop?" Locked

Started 1 month ago | Questions thread
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A Marcus Junior Member • Posts: 49
Re: Do I really "Gain a stop?"

An f-stop is a 2x (double or half) change to the light energy of the exposure. The f-stop number set is: 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 - 22. Note each number going left is an aperture setting that gains 2x more exposing energy. Each number going right is an aperture setting that decreases the exposing energy by half.

The f-number set is cryptic (puzzling) to most. Each number going left is its neighbor on the right divided by 1.4. Conversely, each number going right is its neighbor on the left multiplied by 1.4. The aperture is a circle. The amount of light that this lens opening passes can be doubled if you could somehow double the area (number of square millimeters). This act doubles the light passing ability of the lens. The secret is: Multiply the diameter of any circle by 1.4 and you double its surface area. Conversely, divide the diameter of any circle by 1.4, you have calculated a revised circle with half the surface area.

Also: The f-numbers are universal. They are valid for all lenses regardless of size. No need to worry about equivalent crop factor math, the f-number remains valid regardless of format size.

If you somehow did something to your 24mm f/2.8 lens to give it a speed boost of 1 f-stop, it would then operate at f/2. We find the f-number by dividing the focal length by the diameter of the lens opening. Your 24mm, operating at f/2.8 has a working diameter of 24 ÷ 2.8 = 8.57mm. If you mount a 0.71 speed booster, the 24mm focal length changes to 24 x 0.71 = 17mm. In other words, the speed booster shortens the focal length which grants this camera lens combination a wider field of view. The working aperture diameter remains unchanged. We now compute the f-number this lash-up grants. The math is now 17 ÷ 8.57 = 1.98. We can round this to f/2, a gain of 1 f-stop as to light gathering ability.

The crop factor you are worried about is not applicable, so we ignore.

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tko
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tko
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