What you say is true, but then from a practical standpointThank you all for great responses!
I do understand most of this, and I get that the f/stop number is related to the area/geometric property and has a log(2) relationship.
I guess what I'm getting out of this is as follows:
Full f/stop values go in the sequence: f/1.0 f/1.4, f/2, f2.8. This scale is standard because it begins at 1.
However my lens *clicks* at f/1.8, f/2.8, f4 ... and so on. This means that the difference between the first and second click is in excess of one f/stop
This means that if I go from the first to the second *click* I'm closing the aperture slightly more than a stop, and slowing the shutter by one stop will not result in the exact same exposure, so the rule of clicking the aperture and shutter in opposite directions but keeping exposure identical is not a correct assumption between the first two clicks on my lens because they are more than a stop. (Though probably not enough to get upset about)
All of your listed value are within the value of 1 decimal point except the f/3.29 for f/3.5.What you say is true, but then from a practical standpoint
For instance in various tests by Popular Photograph
- often the engraved f/stop is not the true f/stop, as magazine tests from decades past prove
- unless you are shooting color transparency, variances in exposure are masked by enlarging and test prints made to determine right number of seconds of enlarger light
- we may see variances in the accuracy of shutter speeds, which add to or compensate for variances in f/stop
and Modern Photography found in several examples of OM 50mm f/1.8 lens
- an OM f/2 lens was truly f/2.06
- an OM f/2.8 lens was truly f/2.73
- an OM f/3.5 lens was truly f/3.29
IOW, you worry about exact difference when reality is that few lenses are really exactly as engraved, even at the engraved usual 'full f/stop values', and shutter speeds are often not exactly as engraved either!
- two samples were really f/1.86
- one sample was really f/1.87
"Polishing a turd" comes to mind.
This means that if I go from the first to the second *click* I'm closing the aperture slightly more than a stop, and slowing the shutter by one stop will not result in the exact same exposure, so the rule of clicking the aperture and shutter in opposite directions but keeping exposure identical is not a correct assumption between the first two clicks on my lens because they are more than a stop. (Though probably not enough to get upset about)
My point was the engraved value typically is not the actual value...even when it is within manufacturing tolerance.All of your listed value are within the value of 1 decimal point except the f/3.29 for f/3.5.
However my lens *clicks* at f/1.8, f/2.8, f4 ... and so on. This means that the difference between the first and second click is in excess of one f/stop
That is interesting. I assumed that your lens, as for instance my FD lens, still got an unmarked half stop at F2.4 (as I described in post #14).
Which is how you get the f/stop progression for whole f/stops (some rounding error embedded in the numbers) 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22I understand what you guys are saying, but i am not able to follow the math.
A lens aperture is the area of the lens that is exposed by the blades...isn't it.?
To double the area of a circle... a lens aperture.. you do not multiply by 2...you multiply by the square root of 2, is that correct.?
Yes...Thank You.Which is how you get the f/stop progression for whole f/stops (some rounding error embedded in the numbers) 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22
The area = pi x the radius of the aperture squared.I understand what you guys are saying, but i am not able to follow the math.
A lens aperture is the area of the lens that is exposed by the blades...isn't it.?
To double the area of a circle... a lens aperture.. you do not multiply by 2...you multiply by the square root of 2, is that correct.?
Right.The area = pi x the radius of the aperture squared.
To double the area, yes you multiply the radius of the aperture by the square root of 2, because then when you square the new radius the area becomes twice what the area was first.
To add complexity, the f/stop is the ratio of the diameter of the aperture and the focal length. And of course the diameter is twice the radius.
I am NO Math Whiz. But i do think that the ratio of focal length to the diameter of the aperture is important to learn, at least once.
I understand what you guys are saying, but i am not able to follow the math.
A lens aperture is the area of the lens that is exposed by the blades...isn't it.?
To double the area of a circle... a lens aperture.. you do not multiply by 2...you multiply by the square root of 2, is that correct.?
Yes...Thank You.
Although f/stops are weird because they are fractions so i guess the opposite math would apply.?
To show half the area, you would multiple by sq2 f8 to f11
To double the area you would divide by sq2 f11 to f8
Correct answer. Making faster lenses is a marketing gimmick. Difference between whole stops is meaningful, fractional stops less so. So my camera has a lens with f2.0, competitor comes out with a lens marked f1.8, next competitor ups the ante with a f1.7 lens. Don;t overthink it.A simple answer with reference to the lens in question: the lens does offer 1.8, but then not enough room to inscribe f2 between 1.8 and 2.8. The 1.8 probably is a promotional ploy. Many of my lenses marked f2 actually open up slightly more...probably to 1.8, but who would ever need the tiny difference? As others have commented, just consider the 1.8 to be read as 2. Most high end lens makers wouldn’t bother.
Note however that you probably don't want to label your f/1.8 lens as an f/2.0 lens, because that will screw up some things related to slide film.Correct answer. Making faster lenses is a marketing gimmick. Difference between whole stops is meaningful, fractional stops less so. So my camera has a lens with f2.0, competitor comes out with a lens marked f1.8, next competitor ups the ante with a f1.7 lens. Don;t overthink it.
Correct answer. Making faster lenses is a marketing gimmick. Difference between whole stops is meaningful, fractional stops less so. So my camera has a lens with f2.0, competitor comes out with a lens marked f1.8, next competitor ups the ante with a f1.7 lens. Don;t overthink it.
Thank you for raising this topic. I was aware of the irregularity seen at some lenses between the first two clickstops, but not to this extent.
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