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Fractional F-Stops compared, or, how fast is F1.2?

Author/Copyright:

John Jovic

We all know that an F1.2 lens is fast and certainly faster than an F1.4. That's not rocket science. But exactly HOW MUCH FASTER IS IT? For that matter, is an F0.95 lens THAT much faster than an F1? If these are the questions that keep you awake at night then read on.

The F-Stop is a simple mathematical progression which can be calculated easily. The table below shows this progression in 1/3rd, 1/8th and 1/10th stop increments and to a resolution of 4 decimal places. These values can then be used to compare commonly used aperture values and to show their relative differences in fractional terms.

In practice apertures or F-stops are typically rounded to one decimal place which makes their values a little vague at times. This is the exact reason that these tables are at a much higher precision, so that the actual mathematical values or 'fractional F-stops' can be clearly defined. Why in 1/8th stops? Because 2/8th stops is a 1/4 and 4/8th is 1/2, etc.
 
 
F-Stop EV deviation in 1/3rd stop increments
F 0.7071 -1 EV
F 0.7937 -1 2/3
F 0.8909 -1 1/3
F 1.0 0 EV
F 1.1225 1/3
F 1.2599 2/3
F 1.4142 1 EV
F 1.5874 1/3
F 1.7818 2/3
F 2 2 EV
F 2.2449 1/3
F 2.5198 2/3
F 2.8284 3 EV
F 3.1748 1/3
F 3.5636 2/3
F 4 4 EV
F 4.4898 1/3
F 5.0397 2/3
F 5.6569 5 EV
F-Stop EV deviation in 1/8th stop increments
F 0.7071 -1 EV
F 0.7384 -7/8
F 0.7711 -6/8
F 0.8052 -5/8
F 0.8409 -4/8
F 0.8781 -3/8
F 0.9170 -2/8
F 0.9576 -1/8
F 1.0 0 EV
F 1.0443 1/8
F 1.0905 2/8
F 1.1388 3/8
F 1.1892 4/8
F 1.2419 5/8
F 1.2968 6/8
F 1.3543 7/8
F 1.4142 1 EV
F 1.4768 1/8
F 1.5422 2/8
F 1.6105 3/8
F 1.6818 4/8
F 1.7563 5/8
F 1.8340 6/8
F 1.9152 7/8
F 2 2 EV
F 2.0885 1/8
F 2.1810 2/8
F 2.2776 3/8
F 2.3784 4/8
F 2.4837 5/8
F 2.5937 6/8
F 2.7085 7/8
F 2.8284 3 EV
F 2.9537 1/8
F 3.0844 2/8
F 3.2210 3/8
F 3.3636 4/8
F 3.5125 5/8
F 3.6680 6/8
F 3.8304 7/8
F 4 4 EV
F 4.1771 1/8
F 4.3620 2/8
F 4.5552 3/8
F 4.7568 4/8
F 4.9674 5/8
F 5.1874 6/8
F 5.4170 7/8
F 5.6569 5 EV
F-Stop EV deviation in 1/10th stop increments
F 0.7071 -1 EV
F 0.7320 -0.9
F 0.7579 -0.8
F 0.7846 -0.7
F 0.8123 -0.6
F 0.8409 -0.5
F 0.8706 -0.4
F 0.9013 -0.3
F 0.9330 -0.2
F 0.9659 -0.1
F 1.0 0 EV
F 1.0353 0.1
F 1.0718 0.2
F 1.096 0.3
F 1.1487 0.4
F 1.1892 0.5
F 1.2311 0.6
F 1.2746 0.7
F 1.3195 0.8
F 1.3660 0.9
F 1.4142 1 EV
F 1.4641 0.1
F 1.5157 0.2
F 1.5692 0.3
F 1.6245 0.4
F 1.6818 0.5
F 1.7411 0.6
F 1.8025 0.7
F 1.8661 0.8
F 1.9319 0.9
F 2 2 EV
F 2.0705 0.1
F 2.1435 0.2
F 2.2191 0.3
F 2.2974 0.4
F 2.3784 0.5
F 2.4623 0.6
F 2.5491 0.7
F 2.6390 0.8
F 2.7321 0.9
F 2.8284 3 EV
F 2.9282 0.1
F 3.0314 0.2
F 3.1383 0.3
F 3.2490 0.4
F 3.3636 0.5
F 3.4822 0.6
F 3.6050 0.7
F 3.7321 0.8
F 3.8637 0.9
F 4 4 EV
F 4.1411 0.1
F 4.2871 0.2
F 4.4383 0.3
F 4.5948 0.4
F 4.7568 0.5
F 4.9246 0.6
F 5.0982 0.7
F 5.2780 0.8
F 5.4642 0.9
F 5.6569 5 EV
F 5.8563 0.1
F 6.0629 0.2
F 6.2767 0.3
F 6.4980 0.4
F 6.7272 0.5
F 6.9644 0.6
F 7.2100 0.7
F 7.4643 0.8
F 7.7275 0.9
F 8 6 EV
F 8.2821 0.1
F 8.5742 0.2
F 8.8766 0.3
F 9.1896 0.4
F 9.5137 0.5
F 9.8492 0.6
F 10.1965 0.7
F 10.5561 0.8
F 10.9283 0.9
F 11.3137 7 EV
F 11.7127 0.1
F 12.1257 0.2
F 12.5533 0.3
F 12.9960 0.4
F 13.4543 0.5
F 13.9288 0.6
F 14.4200 0.7
F 14.9285 0.8
F 15.4550 0.9
F 16 8 EV
F 16.5642 0.1
F 17.1484 0.2
F 17.7531 0.3
F 18.3792 0.4
F 19.0273 0.5
F 19.6983 0.6
F 20.3930 0.7
F 21.1121 0.8
F 21.8566 0.9
F 22.6274 9 EV
F 23.4254 0.1
F 24.2515 0.2
F 25.1067 0.3
F 25.9921 0.4
F 26.9087 0.5
F 27.8576 0.6
F 28.8400 0.7
F 29.8571 0.8
F 30.9100 0.9
F 32 10EV
 

So to answer the original question, F1.2 can be defined as either a 1/3rd or 1/2 stop increment depending on the resolution being used. The aperture which is half a stop faster than F1.4 (actually F 1.4142) is F1.1892 which is conventionally rounded to F1.2.

Oh, and about that F0.95 lens. F0.95 is a little more than an 1/8th of a stop faster than F1, so it's not really very much faster at all.

 

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