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why do stops double in one direction and half in the other? (3 replies and 4 comments)

Gustavo Perez
3 months ago
Gustavo Perez 3 months ago

I have had some conversations and read a little more on the subject, however, if anyone could explain why exactly a T/4 admits half as much light as a 2.8 but a 2.8 admits double the amount of light as a 4?

my mind is having a little bit of trouble understanding this relationship (half closing the aperture/ double when opening). Below is an excerpt from The Camera Assistants Manual, and a link to an article I read on the subject to help me understand. 

Each f-stop admits half as much light through the lens as the f-stop before it. In other words, an f-stop of 4 admits through the lens half as much light as an f-stop of 2.8. Conversely, each f-stop admits twice as much light through the lens as the f-stop after it. In other words, an f-stop of 5.6 admits through the lens twice as much light as an f-stop of 8.

https://www.outdoorphotographyschool.com/aperture-and-f-stops-explained/»

Leander S
3 months ago
Leander S 3 months ago

To put it simple... when you go a stop brighter, it is always double the amount of light. Therefore a stop darker is always HALF the amount of light  (stop up= light * 2, stop down= light /2).

How the f-or t stop is "named " (2,2.8,4,5.6,8,11,16...) is due to some math with the circular shape I guess. You always have to times by the root of 2 to get the next number but you can see this just as a naming convention. Dont get distracted by the numbrs of the stops. Between 2 and 2.8 is just as much of a dfference as between 11 and 16.  

Hope it answered your question.. Best! 

MiguelMas
3 months ago

Gustavo The f-stop of a lens is determined by dividing the focal length of the lens by the diameter of the size of entrance pupil, i.e., the lens opening
In short, a f stop is a fraction. What does something like “f/8” even mean? Actually, this is one of the most important parts about aperture: it’s written as a fraction.
In short, You can think of an aperture of f/8 as the fraction 1/8 (one-eighth). An aperture of f/2 is equivalent to 1/2 (one-half). An aperture of f/16 is 1/16 (one-sixteenth). And so on.
.1/2 cup of sugar is much more than 1/16 cup of sugar. A 1/4 pound burger is larger than a 1/10 pound slider.
By that same logic, an aperture of f/2 is much larger than an aperture of f/16.
This link explaining f stops in detail is from B&H Photo...
https://www.bhphotovideo.com/explora/photography/tips-and-solutions/what-are-fstops-and-how-do-you-define-fast-lens»

Gustavo Perez
3 months ago

I think where I am getting lost is where they are deriving the actual numbers from. I'm sorry if this is redundant to the original post, however, I still don't really understand why you gain 2 stops of light when you open up the iris, but lose only half a stop when closing down. I feel like that relationship should be linear. I know I'm missing a piece of the puzzle as the book is meant for an AC and not for a lense engineer or cinematographer, so I think it's reasonable that they never went into the most fundamental aspects of a lens up to every last detail like a science textbook would... but that also means that I'm missing something that may be crucial to my understanding this.

thank you for the replies !!!

dmullenasc
3 months ago

I think you're confusing "losing half the light" with "losing half a stop". It's the same loss or gain in each direction -- opening up one-stop increases the light entering by twice (100%), closing down one-stop decreases the light entering by half (50%), not by a half-stop. If you saw the meter reading in foot-candles it would be more clear. If you cut 100 foot-candles of light to 50 foot-candles, you'd lose one-stop of exposure. If you increased 100 foot-candles to 200 foot-candles, you'd gain one-stop of exposure.

EvanKrik
3 months ago

full stops only ?

Blainewestropp
3 months ago
Blainewestropp 3 months ago

first of all, 10 is twice as much as 5. 5 is half as much as 10. 

if your lens is set to T2.8 and you open it up one stop, to T2.0, twice as much light passes. If your lens is set to T2.8 and you close it one stop to T4, half as much light passes. 

dmullenasc
3 months ago
dmullenasc 3 months ago

"I still don't really understand why you gain 2 stops of light when you open up the iris, but lose only half a stop when closing down."

Again, the issue here is language, you're confusing "half a stop" with "half the light" and "two-stops of light" with "twice as much light".

 

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