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Why are 1/3 stop apertures uneven numbers apart?

For f/stops, there is a precise multiplied difference of 1.122462 X intervals (cube root of √2) between all third stops. The precise third stops are actually numbers like 8.98 or 10.08. My meaning of ...
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20 votes

Why are DSLR lenses measured in F stops instead of T stops?

Because in cinema, it's common to change lenses within a shoot while preserving identical exposure. This is rarely important in still photography (and even less so with the flexibility of digital). ...
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17 votes

Why are DSLR lenses measured in F stops instead of T stops?

To add to mattdm's great answer: In addition to the added exposure precision, which is not important to photography, T-Stops are LESS precise in other ways which ARE important to still photography. ...
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16 votes

Why are DSLR lenses measured in F stops instead of T stops?

F-Stops matter most when you care about knowing your composition and depth of field, T-Stops matter most when you care about knowing your exposure. Photographers want to control composition first and ...
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  • 611
13 votes

Why are f-stops not linear?

It's not linear because it's based on area rather than lines. Or, to put it another way, the scale isn't based on square roots. It's based on the actual math that represents the physical properties ...
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13 votes
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Why does Rosco's filter page show that 33% transmission is 1 stop rather than 50%?

If you look at filter #209 you'll see "Reduces light 1 stop (Transmission = 51%)". That is what you expect to see. Filter #210 reduces light 2 stops and has transmission of 24%. Again, what you expect....
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13 votes
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What motivated lens makers to now use 1/3 f-stops rather than say 1/4 or 1/2 stops?

Historically, the unit of exposure was a doubling or halving of the exposing energy. This is the origin of the f/stop. Initially, this adjustment was made by inserting a thin metal plate with a ...
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13 votes

Confused about T-stop number. Does it account for crop factor?

Crop factor has nothing to do with T-stop. T-stop is strictly about light transmission which affects exposure. If a lens could be 100% transmissive the T-stop and f-number of the lens would be the ...
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12 votes

Why are 1/3 stop apertures uneven numbers apart?

Whole f-numbers are an expression of the powers of the square root of two (√2). Every odd-numbered or fractional power of the square root of two is a non-integer with an endless number of places to ...
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10 votes
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Are the aperture, ISO and shutter speed stops perfectly interchangeable?

In a theoretical sense, these things are perfectly interchangeable. See the second half of my answer to What is the "exposure triangle"? (after I get done ranting about the terminology). ...
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10 votes

Are the aperture, ISO and shutter speed stops perfectly interchangeable?

The intention is that the actual exposure should be exactly the same for equivalent exposure settings, but there are small deviations. There are also some other differences to the images other than ...
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10 votes

How many extra stops do monopods offer for tele photographs?

The monopod removes three degrees of liberty: distance from ground is fixed, and roll and pitch are linked to position in space. But you are overlooking that you are no longer lifting the (...
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9 votes
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Can a lens exist with an f-stop of less than 1?

Yes, there have been numerous lenses made that are faster then f/1.0 One of the most famous is the Zeiss_Planar_50mm_f/0.7 which was was designed for the NASA and used by Stanley Kubrick in Barry ...
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8 votes

F-stop vs T-Stop - how do I quickly grasp a videographers view of the world?

There's nothing here to grasp. T-stop is the same thing as f-stop, except the manufacturer or a third party has physically tested the lens to find how much light it transmits, rather than making a ...
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7 votes
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Why isn't glass transparency part of the exposure calculation?

It IS part of the exposure calculation in fully professional cinematography; these folks prefer lenses marked in T-stops. A full F-stop is actually a factor of two (or one half) in light transmitted, ...
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7 votes

How many extra stops do monopods offer for tele photographs?

It's instructive to look where monopods are most often used: sporting events, and shooting wildlife. In all of these cases, it's not a matter of "how many stops" a monopod can provide. It's simply a ...
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7 votes
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What's the difference between aperture value and f-number?

The Exif standard says that: ApertureValue=2*log₂(FNumber) which is also: ApertureValue=2*log(FNumber)/log(2) There is even a nice conversion table: However, looking at my own photos (EOS 70D), I ...
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6 votes
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Why are f-stops not linear?

The f number is the inverse square root of the light collection efficiency. Why such a crazy relationship? First - let me define "light collection efficiency". You build an image by collecting ...
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6 votes
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Does crop factor affect exposure time?

No. You can imagine the micro4/3 to simply cut out the middle of the image. If you had a 25mm lens on a full frame camera and exposed it properly, the middle part of the frame is part of what's ...
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6 votes
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Why does Photoshop say the f-stop is f1.6 in the summary but 1.4 in the details at the bottom?

The value (f/1.6) at the top is the aperture used when the picture was taken. The aperture value under Camera Data (Exif) (f/1.4) is the maximum aperture value to which the lens can be set.
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6 votes
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Why don't the Shutter Speed, Exposure Time, Lens Aperture, and F-Number metadata values from my camera make sense?

That is quite common in EXIF data but the values are usually closer. It is possible the EXIF data for that photo is telling us a lens with a maximum aperture of f/1.0 was used at f/2.8 and the shutter ...
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6 votes

Confused about T-stop number. Does it account for crop factor?

The f-stop is a simple ratio. It is the focal length divided by the working diameter. We use this value to compare the image brightness of one lens vs. another. The f-stop is often in error because it ...
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6 votes
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How does sensor size affect minimum front element size for a given f-stop and focal length?

How does sensor size affect minimum front element size for a given f-stop and focal length? It affects the width of the angle from which the entrance pupil should be visible when looking through the ...
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5 votes

How does physical aperture size relate to f/stop?

Let's consider a simple pinhole camera, taking a picture of a uniformly illuminated wall. Because this is a uniformly illuminated wall, each of these rays represents the same amount of light. There ...
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5 votes

Does using an extender actually change the aperture of the lens?

Yes: Remember that the f-stop is a relation between your aperture and the focal length. In theory if you have a f/1.0 50mm lens, that means that you have an aperture of 50mm. If you double the focal ...
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5 votes
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How does sensor size affect the brightness of an image given the same field of view and aperture?

The image brightness will be the same. The entrance pupil size does not matter for image brightness - what matters is only the F number (or more accurately, T number which is equal to the true f ...
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5 votes
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Flash guide number f/stop calculation

However, 25 is NOT 2/3 of the way between 16 and 32. You just have to remember that the scale is exponential, not linear. 4 2/3 stops allows 24.66 more light, and 24.66 = 25.28132..., or roughly 25. ...
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5 votes

Why does illuminance stay the same for a given f-stop even when focal length changes?

Illuminance is "the amount of luminous flux per unit area." Suppose the 100mm lens was aimed at a large wall, and suppose distance was so that the lens sees a 10 x 10 foot area of that wall, ...
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5 votes
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Why do lower f-stop numbers mean larger apertures?

/ is division. A number divided by 22 (f/22) is smaller than the same number divided by 2 (f/2). As simple as that. Now, WHY? f is the focal length. Describing the aperture as a fraction of the ...
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