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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 10 Apr 2014 (Thursday) 10:11
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Aperture and light gathering abilities

 
CanonVsNikon
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Apr 10, 2014 10:11 |  #1
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Is it a true statement that a shot at any set aperture from any lens has the same light gathering abilities?

So a 50mm shot at 2.8 vs a 200mm shot at same 2.8 will gather the same light?

But isn't aperture (f-stop) a ratio of the focal length? 200mm/2.8 and 50mm/2.8?




  
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jra
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Apr 10, 2014 10:16 |  #2

Yes....for all intents and purposes, that's true and so is the second part of your question. Aperture is a ratio of focal length. The longer the focal length, the physically larger the aperture must be to gather the same amount of light.....so f2.8 on a 400mm lens is physically larger than f2.8 on a 50mm lens although they will both provide the same exposure settings.




  
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Apr 10, 2014 10:29 |  #3

CanonVsNikon wrote in post #16823622 (external link)
Is it a true statement that a shot at any set aperture from any lens has the same light gathering abilities?

So a 50mm shot at 2.8 vs a 200mm shot at same 2.8 will gather the same light?

But isn't aperture (f-stop) a ratio of the focal length? 200mm/2.8 and 50mm/2.8?

Yes, the bold part answers your own question. The actual opening of the 200 mm lens is much greater than the one from the 50.




  
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CanonVsNikon
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Apr 10, 2014 11:38 |  #4
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gonzogolf wrote in post #16823671 (external link)
Yes, the bold part answers your own question. The actual opening of the 200 mm lens is much greater than the one from the 50.

Than why do they gather the same amount of light?

Not being argumentative, just trying to understand.




  
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bps
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Apr 10, 2014 12:16 |  #5

Actually, this is a pretty good question! My hypothesis: you're gathering the same amount of light/luminance from a distance that's further away, so you have to have a bigger opening to capture it.

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SkipD
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Apr 10, 2014 12:23 |  #6

CanonVsNikon wrote in post #16823835 (external link)
Than why do they gather the same amount of light?

The term "gather the same amount of light" is inappropriate.

Different focal length lenses, all set to the same f-stop, put the same light intensity on the film/sensor but that does not mean that they all actually collect the same number of photons.


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Apr 10, 2014 12:23 |  #7

bps wrote in post #16823920 (external link)
Actually, this is a pretty good question! My hypothesis: you're gathering the same amount of light/luminance from a distance that's further away, so you have to have a bigger opening to capture it.

Bryan

Sort of.

The source isn't necessarily farther away, it's simply a smaller part of the "overall" frame.


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20droger
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Apr 10, 2014 12:50 as a reply to  @ CanonVsNikon's post |  #8

Trying not to get too technical, the f/stop is, as you say, a ratio of the focal length. This ratio is the relative aperture of the lens and describes the physical aperture of the lens. For example, a 50mm lens at a relative aperture of f/2.8 has a phyisical aperture of 50/2.8 = 17.86mm, and a 200mm lens at a relative aperture of f/2.8 has a physical aperture of 200/2.8 = 71.43mm. For the same f/2.8 relative aperture, the 200mm lens has a physical aperture four times that of the 50mm lens (speaking in terms of aperture diameters).

Okay, if an f/2.8 200mm lens has an aperture diameter four times that of an f/2.8 50mm lens, why do they both admit the same amount of light?

This is because a lens' aperture is an area phenomenon, and the area captured by a lens—its angle of view— is determined by a lens' focal length and the size of the sensor/film.

Assuming the same sensor, when pointed at a blank, evenly lit sky, a 50mm lens sees 16× as much sky as a 200mm lens. The sky radiates a given amount of light per square meter. The 50mm lens therefore gathers 16× as much light as the 200mm lens. This means that to pass the same amount of light through the lens, so as to have the same effect on the film or sensor, the 200mm lens must have an aperture that is 16× the area (4 times the diameter) than the aperture of the 50mm lens. This is the relative aperture, measured in f/stops, and does exactly that (as shown above).




  
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20droger
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Apr 10, 2014 13:03 as a reply to  @ 20droger's post |  #9

Incidentally, it is because aperture is an area phenomenon that the relative apertures are what they are: 1.0; 1.4; 2.0; 2.8; 4.0; 5.6; 8.0; 11; 16; 22; 32; 45; 64; 90; and 128. The ratio between each value is (approximately) the square root of 2, but the light gathered by each value is 2× the light gathered by the next value.




  
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xchangx
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Apr 10, 2014 13:19 |  #10

CanonVsNikon wrote in post #16823622 (external link)
Is it a true statement that a shot at any set aperture from any lens has the same light gathering abilities?

So a 50mm shot at 2.8 vs a 200mm shot at same 2.8 will gather the same light?

But isn't aperture (f-stop) a ratio of the focal length? 200mm/2.8 and 50mm/2.8?

Not knowing anything about the 3rd question, I have found out that not all lenses let in the same amount of light at the same aperture.

I have a 400 2.8 AF-ED II and had a loaner 400 2.8 VR while mine was getting repaired. Realized at a HS football game (same stadium, same body a week later) the settings I had the week before was under exposing.

After a little bit of research, come to find out that the VR in the lens actually caused the lens to lose some light. Something about glass surface/air.

If you look at cinema lenses, they are measured in T-Stops which is the light measurements.


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CanonVsNikon
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Apr 10, 2014 13:23 |  #11
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20droger wrote in post #16824009 (external link)
Trying not to get too technical, the f/stop is, as you say, a ratio of the focal length. This ratio is the relative aperture of the lens and describes the physical aperture of the lens. For example, a 50mm lens at a relative aperture of f/2.8 has a phyisical aperture of 50/2.8 = 17.86mm, and a 200mm lens at a relative aperture of f/2.8 has a physical aperture of 200/2.8 = 71.43mm. For the same f/2.8 relative aperture, the 200mm lens has a physical aperture four times that of the 50mm lens (speaking in terms of aperture diameters).

Okay, if an f/2.8 200mm lens has an aperture diameter four times that of an f/2.8 50mm lens, why do they both admit the same amount of light?

This is because a lens' aperture is an area phenomenon, and the area captured by a lens—its angle of view— is determined by a lens' focal length and the size of the sensor/film.

Assuming the same sensor, when pointed at a blank, evenly lit sky, a 50mm lens sees 16× as much sky as a 200mm lens. The sky radiates a given amount of light per square meter. The 50mm lens therefore gathers 16× as much light as the 200mm lens. This means that to pass the same amount of light through the lens, so as to have the same effect on the film or sensor, the 200mm lens must have an aperture that is 16× the area (4 times the diameter) than the aperture of the 50mm lens. This is the relative aperture, measured in f/stops, and does exactly that (as shown above).

Excellent explanation. I get it now.




  
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archer1960
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Apr 10, 2014 13:30 |  #12

xchangx wrote in post #16824076 (external link)
Not knowing anything about the 3rd question, I have found out that not all lenses let in the same amount of light at the same aperture.

I have a 400 2.8 AF-ED II and had a loaner 400 2.8 VR while mine was getting repaired. Realized at a HS football game (same stadium, same body a week later) the settings I had the week before was under exposing.

After a little bit of research, come to find out that the VR in the lens actually caused the lens to lose some light. Something about glass surface/air.

If you look at cinema lenses, they are measured in T-Stops which is the light measurements.

Yes, that is true. f/stops don't take into account the transmittance (essentially the transparency) of the lens elements like T-stops do. The numbers will generally be close to each other, but the T-stop will always be a larger number (lower light transmission) than an f-stop for the same physical lens.


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Apr 10, 2014 13:53 as a reply to  @ archer1960's post |  #13

All I know is that when I use my Sekonic handheld doodad to take a reading using that white bulb, the device gives me the proper exposure without asking for any information about what focal length I’m using.


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Windsun33
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Apr 10, 2014 14:09 |  #14

The F-stop refers to the amount of light, not the size of the lens opening. The ratio you speak of is derived ("reverse engineered", so to speak) from that. f1 would be that the film or sensor gets the same amount of light as the scene reflects.


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Apr 10, 2014 14:40 |  #15

Windsun33 wrote in post #16824200 (external link)
The F-stop refers to the amount of light, not the size of the lens opening. The ratio you speak of is derived ("reverse engineered", so to speak) from that. f1 would be that the film or sensor gets the same amount of light as the scene reflects.

Nope, it's the ratio of the focal length to the aperture. If it was your method, you could never have a lens with an f-stop less than 1.0, or the lens would be gathering more light than the scene reflects, which is obviously impossible. And lenses with f/stops less than 1.0 do actually exist.


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Aperture and light gathering abilities
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