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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

 
SkipD
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Apr 10, 2014 14:45 |  #16

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.

Not correct....

archer1960 wrote in post #16824283 (external link)
Nope, it's the ratio of the focal length to the aperture.

Correct....

The "f-stop" number is actually a fraction. f/8, for example, literally means "focal length divided by 8" and it describes the "entrance pupil" of the lens which is the effective opening through the lens.


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Windsun33
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Apr 10, 2014 14:50 |  #17

archer1960 wrote in post #16824283 (external link)
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.

Yes, and they do that by essentially "compressing" the amount of light from a larger source down to the actual image size. It could be argued either way - like chicken and egg I suppose. See the 1874 historical note below:

From Wiki: "It must be observed, however, that in order to find the real intensity ratio, the diameter of the actual working aperture must be ascertained. This is easily accomplished in the case of single lenses, or for double combination lenses used with the full opening, these merely requiring the application of a pair of compasses or rule; but when double or triple-combination lenses are used, with stops inserted between the combinations, it is somewhat more troublesome; for it is obvious that in this case the diameter of the stop employed is not the measure of the actual pencil of light transmitted by the front combination..."


My first real camera was a Canon F1. That was a long time ago.

  
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archer1960
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Apr 10, 2014 14:58 |  #18

Windsun33 wrote in post #16824308 (external link)
Yes, and they do that by essentially "compressing" the amount of light from a larger source down to the actual image size. It could be argued either way - like chicken and egg I suppose. See the 1874 historical note below:

From Wiki: "It must be observed, however, that in order to find the real intensity ratio, the diameter of the actual working aperture must be ascertained. This is easily accomplished in the case of single lenses, or for double combination lenses used with the full opening, these merely requiring the application of a pair of compasses or rule; but when double or triple-combination lenses are used, with stops inserted between the combinations, it is somewhat more troublesome; for it is obvious that in this case the diameter of the stop employed is not the measure of the actual pencil of light transmitted by the front combination..."

Ah, but there you're not measuring the light reflected from the subject, but rather the light that reaches the front element. Big difference...


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edge100
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Apr 10, 2014 15:12 |  #19

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

Not being argumentative, just trying to understand.

http://mfphotography.c​a …n-fstops-and-trigonometry (external link)


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20droger
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Apr 10, 2014 21:34 |  #20

archer1960 wrote in post #16824283 (external link)
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.

Correct. And the two whole-f/stops larger than 1.0 are 0.5 and 0.7.




  
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20droger
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Apr 10, 2014 21:34 |  #21

I said all this in post #8, without the trigonometry.




  
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edge100
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Apr 10, 2014 22:42 |  #22

20droger wrote in post #16825106 (external link)
I said all this in post #8, without the trigonometry.

Good for you. I wrote that 2 years ago.


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tonylong
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Apr 11, 2014 02:49 |  #23

Well, it's fun stuff, especially when you consider the various complications involved! For example, just consider, for example, the discussion of a 50mm lens at f/2.8 and a 200mm lens at f/2.8 -- the aperture opening with the 50mm is smaller than a 35mm (full frame) sensor, so the light can just shine on in with no noticeable distortion. There's a reason why the 50mm f/1.8 lens is cheap and yet delivers great IQ at f/2-f/2.8. it's because the lens delivers light that doesn't need to be "messed with", no engineering optics to make it more expensive!

But then move up to the 200mm f/2.8 and you will see the challenge: the aperture of the 200mm has to be wide enough to deliver all that light, but not to the sensor, the camera frame is not that large, not to mention the sensor! So for a telephoto lens, optical engineering and elements have to go into place to compress all that light, and then if you want a good lens, that work has to be of premium quality, to avoid distortion from all that compression. In fact, even very good lenses can occasionally show up with softness around the edges and in the corners...possibly a result of that compression, I don't know, 'cause I'm not an optical engineer!

And then, go even farther, to say the 300mm f/2.8 or the 400mm f/3.8, those things are huge beasts! And a big reason for this is that aperture keeps getting physically wider, and so the body of the lens has to be wider, and then the internal optics have to go to work to take the light delivered by that aperture and to "boil it down" so that the signal passes through the lens mount and onto the sensor!

And so, those who have delved into the "big lenses", whether it be the f/2.8 Canon L lenses or the high-end Nikon lenses have an answer as to why 1) those lenses are so freakin' huge, and 2) why they can be so freakin' expensive (which only factors in the optical engineering at the back of the lens, not that that goes into the light-collecting qualities at the front of the lens!


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Apr 11, 2014 07:10 |  #24

20droger wrote in post #16825103 (external link)
Correct. And the two whole-f/stops larger than 1.0 are 0.5 and 0.7.

Yep. And there have been 50mm f/0.7 lenses built, though they're not currently for sale, and a 50mm f/0.95 lens is in regular production.


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CanonVsNikon
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Apr 11, 2014 08:28 |  #25
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To throw a wrench into this discussion, what about smaller sensor lenses like m4/3 such as Panasonic 25mm 1.4

I am sure everything still applies but with a smaller sensor, smaller physical lens and I guess smaller physical lens opening, it still has the same "light gathering" abilities as a full frame lens at same 1.4 aperture




  
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archer1960
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Apr 11, 2014 08:35 |  #26

CanonVsNikon wrote in post #16825947 (external link)
To throw a wrench into this discussion, what about smaller sensor lenses like m4/3 such as Panasonic 25mm 1.4

I am sure everything still applies but with a smaller sensor, smaller physical lens and I guess smaller physical lens opening, it still has the same "light gathering" abilities as a full frame lens at same 1.4 aperture

Correct. You just need to be sure you're using the lens' actual focal length, not the ff equivalent when you're doing the calculations.


Gripped 7D, gripped, full-spectrum modfied T1i (500D), SX50HS, A2E film body, Tamzooka (150-600), Tamron 90mm/2.8 VC (ver 2), Tamron 18-270 VC, Canon FD 100 f/4.0 macro, Canon 24-105 f/4L,Canon EF 200 f/2.8LII, Canon 85 f/1.8, Tamron Adaptall 2 90mmf/2.5 Macro, Tokina 11-16, Canon EX-430 flash, Vivitar DF-383 flash, Astro-Tech AT6RC and Celestron NexStar 102 GT telescopes, various other semi-crappy manual lenses and stuff.

  
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edge100
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Apr 11, 2014 09:26 |  #27

CanonVsNikon wrote in post #16825947 (external link)
To throw a wrench into this discussion, what about smaller sensor lenses like m4/3 such as Panasonic 25mm 1.4

I am sure everything still applies but with a smaller sensor, smaller physical lens and I guess smaller physical lens opening, it still has the same "light gathering" abilities as a full frame lens at same 1.4 aperture

"1.4" is not an 'aperture'; it's an f/stop. That may seem like pedantry, but it's crucial to grasp the difference.


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Film: Leica MP | Leica M2 | CV Nokton 35/1.4 | CV Nokton 40 f/1.4 | Leitz Summitar 50 f/2 | Canon 50 f/1.2 LTM | Mamiya 7 | Mamiya 80 f/4.0 | Mamiya 150 f/4.5 | Mamiya 43 f/4.5
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tonylong
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Apr 11, 2014 21:08 |  #28

CanonVsNikon wrote in post #16825947 (external link)
To throw a wrench into this discussion, what about smaller sensor lenses like m4/3 such as Panasonic 25mm 1.4

I am sure everything still applies but with a smaller sensor, smaller physical lens and I guess smaller physical lens opening, it still has the same "light gathering" abilities as a full frame lens at same 1.4 aperture

Well, the small lens would gather less light than the bigger ones but it would concentrate that light into a smaller area, and so would give the equivalent exposure.


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skilsaw
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Apr 12, 2014 01:58 |  #29

The squiggy little thingamajig does it all by magic.




  
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Dan ­ Marchant
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Apr 12, 2014 23:12 |  #30

skilsaw wrote in post #16827788 (external link)
The squiggy little thingamajig does it all by magic.

At last, a sensible answer I can understand :lol:


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Aperture and light gathering abilities
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