shutter speed and f/stop numbers

[glen_h]

<p>I have wondered about this for a while, and decided to ask here. Maybe there is a better forum.</p>

<p>It seems to me that shutter speeds aren't speeds. Speeds are something where faster is a higher number, but shutter speeds are actually times. Except that the number on the camera dial is the inverse of the shutter speed, so actually does increase with speed. Except again, that we still write 1/125s instead if 125/s. (That is, a speed should have time in the denominator.)</p>

<p>There is a similar complication with the aperture. The numbers on the aperture, f/stops, decrease as the aperture (size of the hole) increases. When photographers say "increase the aperture" they mean "decrease the f/number", but then again, the f/number is in the denominator.</p>

<p>https://en.wikipedia.org/wiki/F-number</p>

<p>makes a big point of the fact that the f-number gets smaller as the hole gets larger. </p>

<p>It seems that in optics, it is usual to discuss optical systems in terms of their f-number, that is, the number that goes in the denominator of f/#, but this is rare in photography. We say "increase the aperture" not "decrease the f-number". That is, increase f/#, not decrease #. And it is usual in photography to write the f/# form.</p>

[User_6502147]

<p>Ugh, semantics! Open or close the F-stop....without convoluting it. IMO.</p>

<p>Les</p>

[bebu_lamar]

<p>In photography as long as you understand what it is it's OK! If you seek for language correctness you will find photographers used a lot of wrong terminology.</p>

[Dieter Schaefer]

<p>There are a lot of speeds in photography - all of them misnomers: shutter speed, lens speed, film speed. Shutter speed just happens to be the conventional misnomer for exposure time, measured in seconds (or fractions thereof).</p>

<p>Lenses are said to be fast or slow - even though they generally travel at most at the same speed as the photographer; fast and slow here refers to "large or small" aperture, allowing for faster shutter speeds or making the use of slower shutter speeds necessary. Same goes for "fast" or "slow" film.</p>

<p>Confusion arises when one tries to apply strict definitions to those commonly (mis-)used terms. Then it turns out that the shutter speed isn't a speed at all - at least not the one that is being used (the shutter curtains certainly travel at a certain speed, but that speed has nothing to do with the exposure time). 1/125 of a second is a time, not a speed (since there is no distance involved). And aren't we glad that no one has come up with calling it a "rate" yet?</p>

<p>Aperture: f/N (= D) is the correct way to state the f-stop number N; the result is the aperture diameter D (size of the entrance pupil). Dividing the focal length f by some small number N results in a large aperture diameter; dividing by a larger number N gives a smaller aperture diameter. Using "decreasing" or "increasing" in conjunction with the f-stop number N can indeed be confusing - decreasing N does indeed increased aperture diameter. The advantage of using a dimensionless number N instead of giving the actual aperture diameter should be clear to everyone.</p>

[glen_h]

 

 

<blockquote>

<p>Aperture: f/N (= D) is the correct way to state the f-stop number N; the result is the aperture diameter D (size of the entrance pupil). Dividing the focal length f by some small number N results in a large aperture diameter; dividing by a larger number N gives a smaller aperture diameter. </p>

</blockquote>

 

 

That is what I always thought, but it seems that in optics they use N instead of f/N,

or as sometimes in photography 1:N. I had thought about trying to make an argument that the wikipedia page was wrong, but figure that I would lose.

 

As with not putting the 1/ on the shutter speed dial, no-one puts the f/ on the aperture ring. But when written out, they usually have the 1/ and f/.

 

 

 

 

[Dieter Schaefer]

<blockquote>

<p>As with not putting the 1/ on the shutter speed dial, no-one puts the f/ on the aperture ring. But when written out, they usually have the 1/ and f/.</p>

</blockquote>

<p>Glen, you can either call it sloppiness or accept it as a commonly accepted convention. In both cases, there's an inverse relationship - so as long as everyone knows that 1000 on the 'shutter speed dial" really is 1/1000 s and the 16 on the aperture ring (or scale) is the dimensionless number N in f/N, things shouldn't go wrong. I am so used to writing f/16 that it wouldn't occur to me to omit the "/"; but you will never hear me say "f over 16 but always f 16). By contrast, I always say 1000th of a second, and would never omit the "1/" in writing.</p>

<blockquote>

<p>but it seems that in optics they use N instead of f/N</p>

</blockquote>

<p>AFAIK, in optics, the much more useful numerical aperture NA is often used instead of the f-number N that is used in photography; the fact that one is (in air) almost exactly twice the other would certainly add a layer of complexity if one were to use NA in photography as well (just think of the "mysterious light losses" when doing high-magnification photography that N can't account for but NA can).</p>

 

[SCL]

<p>And just to comment on shutter speeds as well, they aren't as simple as they seem, depending on the type of shutter. Think for a moment of a leaf shutter, where the aperture is selected in advance of exposure; the shutter is the leaves of the diaphragm - beginning the exposure as a fully closed diaphragm, opening the leaves to the preselected aperture, and then closing again. The question to ask is since the diaphragm opens from the center...is exposure (or the shutter time) greater in the center of the negative/sensor...than the edges, or is there some controlling mechanism in which the blades move faster when opening and closing...thereby evening out the exposure. If so, exactly what does the shutter time mean...is it the average exposure time, is it the time of full cycle from closing to opening to reclosing? Outside of scientific anaylsis, do we care (not usually, as long as the exposure conforms to meeting standard criteria) But what if you are photographing something moving...well the answer is that you can get some special effects.....which is why you often see in movies, moving cars with the wheels not being entirely circular or seeming to be turning backwards. And when you turn to focal plane shutters, they accelerate and brake at the beginning and ends of their cycles as well as having a slit widen and narrow as it moves across the plane of the film/sensor. Again...is the exposure the average at any point of its travel or is it the totality, or do you care? Personally, I don't care...I just use the numbers as rough indications of speed and to calculate proper exposure times. But it can be fascinating to see what is really behind those numbers.</p>

[Dieter Schaefer]

<blockquote>

<p>the shutter is the leaves of the diaphragm</p>

</blockquote>

<p>Can be, but AFAIK, in most cases aperture and leaf shutter are separate entities.</p>

<blockquote>

<p>The question to ask is since the diaphragm opens from the center...is exposure (or the shutter time) greater in the center of the negative/sensor...than the edges,</p>

</blockquote>

<p>You are right that with a leaf shutter the length of time that the shutter is open is greater in the center of the shutter than it is at the edge. So more light does pass through the center of the lens than at the edge of the lens.<br /> However, it is not correct to deduce from this that the center of the image will receive a longer exposure than the edge of the image. Light passes through the center of the lens in all possible directions and goes to every part of the image.</p>

<blockquote>

<p>And when you turn to focal plane shutters, they accelerate and brake at the beginning and ends of their cycles as well as having a slit widen and narrow as it moves across the plane of the film/sensor.</p>

</blockquote>

<p>Why would the slit widen and narrow as the shutter curtains move? They actually shouldn't do by any appreciable amount; maintaining a constant slit width is quite important for an even exposure across the sensor/negative.</p>

[ellis_vener_photography]

"Why would the slit widen and narrow as the shutter curtains move?"

At the beginning of the exposure the gap between the curtains widens until the scond curtain starts moving and at the end

of the exposure the gap narrows as soon as the first curtain stops moving up to the point where the second curtain ends

its travel.

 

The shutter curtains of a vertically travelling shutter cover the height of the gate in almost

precisely 1/300th second. The width between the two curtains determines the length of the exposure (AKA "shutter

speed".)

[Ed_Ingold]

<p>In simple terms, one click is one stop change in exposure (intensity x time), whether on an aperture ring or shutter speed dial. In practice, process gets "interesting," at least to the nerdier types like me.</p>

<p>A focal plane shutter may continue to accelerate as it crosses the film gate. If the gap between curtains were constant, the exposure would decrease from one side to the other in the direction of travel. In the interest of accurate exposures, some focal plane shutters retard the trailing curtain. The gap increases just enough to compensate for this acceleration, so the exposure for any point on the film is constant.</p>

<p>I believe Leica cloth shutters operate in this manner. It is a design feature to minimize the travel time (1/50 or so for a cloth shutter), and minimize the lag time from shutter release (< 20 msec for a film Leica).</p>

<p>Focal plane shutters in digital cameras can be tricky to characterize. In some cameras, the mechanical shutter speed increases until the maximum full opening is reached, the maximum "flash sync" speed. Beyond that point an electronic shutter turns the sensor off. A Nikon D2x works in this fashion, so you can actually use an electronic flash at any speed setting - the shutter is never closed into a slit.</p>

<p>A leaf shutter takes a certain amount of time to reach full opening, and a roughly equal amount of time to close. At the maximum shutter speed, the shutter begins to close the moment it reaches full opening. The shutter is regulated and marked so that the effective shutter speed is proportional to the area under the time v opening curve, which is approximately trapezioidal. The area of a trapezoid is the height x the width at half-height. At maximum speed, the "curve" is an isosceles triangle (same formula for area).</p>

<p>A leaf shutter, like the diaphragm, is placed near one of the nodes in a compound lens. If you look through the back of the lens, the opening for the shutter or diaphragm is ideally constant and circular. In practice the apparent opening is round in the center and elliptical when viewed off center. This causes vignetting, which is most pronounced when fully open and becomes less as the diaphragm is closed. Since the diaphragm is never larger than the shutter opening, vignetting by the leaf shutter is moot except at full aperture.</p>

<p>Why write 1000 on a shutter dial when you mean 1/1000? To save space and make the numbers more readable. Times longer than 1 second are usually in a different color or labeled as such (e.g., a double hash mark, "). The same is true for aperture dials, using "16" instead of "f/16", or 1/16th the focal length.</p>

<p>Each higher aperture value admits half the light as the previous setting. The ratio between stops is 1.4 (sq rt of 2), because light is proportional to area, and area is proportional to the square of the diameter.</p>

<p>Shutter speeds are also equivalent to 1 stop apart. However liberties are taken with the numerical values for legibility. The next step from 1/60 is usually 1/125, at least in cameras made in the last 50 years or so. The maximum speed of a leaf shutter is what it is. It might be a tidy 1/500, but 1/300 or 1/800 are common values too.</p>

[Landrum Kelly]

<blockquote>

<p>Except that the number on the camera dial is the inverse of the shutter speed, so actually does increase with speed. Except again, that we still write 1/125s instead if 125/s. (That is, a speed should have time in the denominator.)</p>

</blockquote>

<p>Do you want to be able to use the dial in actual shooting, or do you want to use it to teach a course on the mathematics of exposure values? If the value for T (or S) says 125, do you really think that means "one hundred twenty-five seconds"?</p>

<p>"Dry fire" an old film camera for a few seconds. You'll get it.</p>

<p>--Lannie</p>

[Landrum Kelly]

<blockquote>

<p>When photographers say "increase the aperture" they mean "decrease the f/number", but then again, the f/number is in the denominator.</p>

</blockquote>

<p>Again, these numbers are not tutorials on the mathematics of exposure values and associated conventions. An f-stop refers to the focal length divided by the aperture (diameter of the opening). There are a lot of interesting implications of this. When one is trying to set the f-stop during an actual shoot is probably not the time to be thinking about all that for the first time. Some people never think about this but manage somehow to become pretty good photographers.</p>

<p>--Lannie</p>

[Landrum Kelly]

<p>What?! Nothing on ISO?</p>

<p>There's quite a bit to be said. For example, if one were to drop the "double zeros" convention on ISO, one would have numbers like 1, 2, 4, 8, 16, 32, 64, 128, etc. (rather than 100, 200, 400, 800, 1600, 3200, 6400, 12800), with those simple integers being viewed simply as the values in a sequence beginning with 2 to the 0 power (1), 2 to the 1 (or first) power (2), 2 to the 2 (second) power (4) etc. If that were the convention, an ISO of 800 would really be simply 8, which would be "2 to the 3rd power." "Three stops" would then mean, for example, going from "2 to the 0 power" (1--> ISO 100) to "2 to the 3 (third) power" (8 -->ISO 800). That is in fact how I think of such things: as <em><strong>powers of two</strong></em>. ("ISO" is not really a power of two, though. ISO values are <em><strong>multiples of two</strong></em>, which most people find easier to work with.)</p>

<p>Do most people think that way or need to think that way? No, they just need to know how to deal with too little (or too much) light. They do understand (some of them) that they are doubling something. Some pretty good photographers probably could not, however, begin to define an "exponent," much less understand log scales of base anything.</p>

<p>I don't blame you for asking the questions, Glen. It's good to see someone who actually thinks about these numbers that we use, but most shooters don't think too much about them. I would like to think that the best photographers have thought about them in great depth, but I don't actually know that to be the case. In fact, I rather doubt that it is.</p>

<p>--Lannie</p>

[charles_stobbs3]

The usual system allows equivalent terminology for cameras of different sizes. An exposure of say 1/200 at

f 11 will yield the same result for an 8"X10" as 21/4X31/4 or any other side. You don't need an exposure

meter calibrated for a particular camera.

[Ed_Ingold]

<p>"Reciprocity" is another term used by photographers without giving much thought. The total amount of light reaching the film is intensity (aperture) x time (shutter). Reciprocity means if you admit twice the light for half the time, the exposure doesn't change. This relationship is valid for all cameras and, to a broad degree, various film types.</p>

<p>This leads to another phenomena which isn't used as much any more. "Reciprocity Failure" occurs when the foregoing relationship fails. It occurs if the exposure is extremely long (time exposures) or short (e.g. flash), or the light intensity is extremely low. Astronomical photography is subject to both low light and long exposure "failure."</p>

<p>Some electronic flash units "quench" the flash tube before it completely discharges. While the normal duration is 1/1000 (1/250 for some studio units), it can be as brief as 1/80,000 for a Nikon flash unit. Expect reciprocity failure occurs at 1/10,000 seconds, more or less. Flash times of a millionth of a second or less will "freeze" a high speed bullet, and are accomplished using oil-filled capacitors and low impedance circuitry.</p>

<p>At one time, we would expose film to ammonia fumes, which improves sensitivity for exposure times of minutes or hours for astronomical photography. Most observations are now made with digital sensors, which have a linear response. The lower end is limited by thermal noise, which is reduced by cooling the sensor with liquid nitrogen. Most digital medium format cameras use a cooling fan or a thermoelectric array for cooling. (You get more than pixels for the big bucks.)</p>

[Ed_Ingold]

<p>The European standard for film speed is/was a geometric series, where an increase of one number (degree) represented a doubling of film speed. The U.S. system was ANSI (American National Standards Institute), a linear (non-geometric) series which we now know as ISO. Evolution does not always move in a forward direction.</p>

[JDMvW]

<blockquote>

<p>The tradition of all dead generations weighs like a nightmare on the brains of the living.</p>

</blockquote>

<p>Eighteenth Brumaire of Louis Bonaparte. Karl Marx 1852<br>

Even if the standard usage were wrong, there's little point to trying to "correct" or alter something that is so well established and has a long historical tradition behind it. <br>

It's best to "suck it up" and live with it.</p>

[bebu_lamar]

<blockquote>

<p>The European standard for film speed is/was a geometric series, where an increase of one number (degree) represented a doubling of film speed. The U.S. system was ANSI (American National Standards Institute), a linear (non-geometric) series which we now know as ISO. Evolution does not always move in a forward direction.</p>

</blockquote>

<p>Not so! If you meant the DIN number increasing 1 number represents 1/3 stop or about 1.26 times the previous one. ISO combined both the American ASA and the European DIN into one. To be correct one should say ISO 100/21 and not just 100.</p>

[bob_bill]

<p>Glenn, for shutter speed, rarely do most of us shoot slower than 1/2 second shutter speed so using 125th is understood to mean 1/125 of a second. It's just a common shorthand. Also, the recognized main shutter speeds like 1/60, 1/125, 1/250 are common and are understood among photographers without the numerator. Aperture is expressed as a fraction, 1/the aperture. 1/2, f/2, is a larger opening than 1/4, f/4. Again, the numerator is often not mentioned or printed as a form of shorthand. Where I see real confusion is in describing lighting ratios. Probably from the 2 different methods of measuring ratio, either from the difference between the intensity of the lights, the source method vs the intensity of the main plus fill where they overlap against the fill alone, the additive method. </p>

[glen_h]

<blockquote>

<p>Do you want to be able to use the dial in actual shooting, or do you want to use it to teach a course on the mathematics of exposure values? If the value for T (or S) says 125, do you really think that means "one hundred twenty-five seconds"?</p>

</blockquote>

 

 

No, I was thinking of it as 125/s. That is, the inverse of the time, and increases as the shutter speed gets faster. There is the small complication that it almost looks like a frequency, but not too much.

 

 

I have wondered about both shutter speed and aperture for some time, but it was reading the wikipedia page on f-number

 

https://en.wikipedia.org/wiki/F-number

 

that I started to wonder more about it. The problem mostly comes up when trying to explain to a beginner, which wikipedia likes to try to do. (Though many pages note in the talk section that they are only understandable to those who already know.)

[Ed_Ingold]

<p>I forget the details, but the point I attempted to make is ASA was linear and DIN a geometric series, representing film speed (both "consensus" standards per the American Standards Association, later American National Standards Institute, and Deutsches Industrie Normalle, later Deutsches Institut für Normumg).</p>

[rashedahmed]

<p>Glen, actually every camera is a precision made built-in watch when it comes to aperture-shutter relation.</p>

[donald_miller5]

<p>It is not a matter of semantics. Just like in any other field or practice these are technical terms with specific definition. If you read the definitions and understand what they mean they are not counter intuitive, ambiguous or inappropriately applied. </p>

[Uhooru]

<p>Speeds are meaningless when you think about it when it comes to shutters, though you can convert time over distance to speed if you want to obsess over it. They use times, because what the shutter "speed" is telling you is how long your film is being exposed. So historically the exposure time has been labeled shutter speed. Its a term of art and knowing the time of exposure is much more important than knowing the speed and much better in terms of understanding exposure. In terms of f stops, it isn't a "similar complication". It seems so because you maybe don't understand what the number represents. You would have seen in Wiki that the f number represents a ratio where the number (or f stop) equals the focal length of the lens divided by the diameter of the opening. The ratio is of F2 is a much larger ratio than f 22. </p>