November 25, 2014, 11:24:43 PM

Author Topic: Aperture and Light  (Read 1832 times)

gferdinandsen

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Aperture and Light
« on: April 25, 2013, 07:48:42 AM »
I just got the new 24-70 to replace my Mark I version of the lens.  Here is my question, given that the new lens has a diameter of 5mm more, does that mean that more light enters the lens?  I presume yes.  So, following that presumption, shooting the same subject in the same lighting at the same aperture and ISO speed, does the 82mm lens need less time to properly expose the subject?  If it does, it follows that the new lens is better in low light, correct?

Unfortunately, I sold the Mark I lens the day before the Mark II arrived in the mail so I was unable to test the theory.
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Aperture and Light
« on: April 25, 2013, 07:48:42 AM »

neuroanatomist

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Re: Aperture and Light
« Reply #1 on: April 25, 2013, 07:55:45 AM »
I just got the new 24-70 to replace my Mark I version of the lens.  Here is my question, given that the new lens has a diameter of 5mm more, does that mean that more light enters the lens?  I presume yes.  So, following that presumption, shooting the same subject in the same lighting at the same aperture and ISO speed, does the 82mm lens need less time to properly expose the subject?  If it does, it follows that the new lens is better in low light, correct?

Unfortunately, I sold the Mark I lens the day before the Mark II arrived in the mail so I was unable to test the theory.

More light might enter the front of the lens, but since the aperture controls the amount of light reaching the center, the 'extra' light won't change the exposure.

The bigger front element means less vignetting (and can help with other aberrations). 
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TrumpetPower!

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Re: Aperture and Light
« Reply #2 on: April 25, 2013, 09:49:41 AM »
I just got the new 24-70 to replace my Mark I version of the lens.  Here is my question, given that the new lens has a diameter of 5mm more, does that mean that more light enters the lens?

Any differences there may be will result in negligible changes to exposure. Indeed, if you look at the actual specifications of pretty much any lens, you'll see that the numbers on the barrel are all rounded. A 200 mm lens might actually be a 193 mm lens; an f/4 lens might actually be an f/3.87 lens; and so on.

In addition, glass (and fluorite and the other materials used to make lenses) doesn't transmit 100% of the light coming through. Each lens element absorbs some of the light. A lens might have a focal length : aperture ratio of 1 : 1.4, but the amount of light that makes it through to the camera might only be as much as an impossible theoretically-perfect f/1.8 lens.

Since most people shooting SLRs use the camera's through-the-lens metering and because modern DSLRs have so much dynamic range and therefore exposure flexibility in post-production, those minor variations are insignificant in the real photographic stills world.

However, they are of concern in the motion picture world, where you've got multiple cameras shooting the same scene from multiple angles and whose resulting images will be composited together in such a way that even minor variations in exposure will be apparent and distracting. And that's why cinematography lenses are calibrated in T-stops instead of F-stops. The T-stop is a measured number of the actual light that makes it through the lens. You might have a lens that, as in my previous example, has an F-stop of 1.4 but a T-stop of 1.8.

It would be interesting to compare the T-stops of the two versions of the 24-70, but really only useful if you're a cinematographer. And I'd be surprised if the two of them differed by more than a third of a stop.

Cheers,

b&

The Bad Duck

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Re: Aperture and Light
« Reply #3 on: April 25, 2013, 09:50:39 AM »
The II-version has a lower T-value, measured by dxo.

http://www.dxomark.com/index.php/Lenses/Compare-Camera-Lenses/Compare-lenses/(lens1)/886/(brand)/Canon/(camera1)/0/(lens2)/165/(brand2)/Canon/(camera2)/0#tabs-2

Look at "transmission".

It can be that the glass is more transparent compared to previous version, or whatever other reason. I don´t know and frankly I don´t care about what causes the T value to be lower on the new version. However it is interesting to know the T-value of ones lenses when every bit of light counts, for instance when reaching for a lens at about 100mm I´d rather use my "in reality faster" 70-200 /4 L IS than my "quite a bit slower" 24-105 /4 L IS even though they have the same f/4 aperture.

TrumpetPower!

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Re: Aperture and Light
« Reply #4 on: April 25, 2013, 10:45:40 AM »
The II-version has a lower T-value, measured by dxo.

http://www.dxomark.com/index.php/Lenses/Compare-Camera-Lenses/Compare-lenses/(lens1)/886/(brand)/Canon/(camera1)/0/(lens2)/165/(brand2)/Canon/(camera2)/0#tabs-2

Look at "transmission".

Interesting.

The old version has perfectly constant transmission across the entire focal length range. The new version is a lot brighter at the wide end but winds up almost as dim as the new version at the telephoto end.

Seems to me that the old version is going to remain the favorite of the cine crowd....

b&

ishdakuteb

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Re: Aperture and Light
« Reply #5 on: April 25, 2013, 12:01:42 PM »
well...  you do not need another 24-70mm mark i to test.  you can use your 70-200mm to test at focal range of 70mm (prefer tripod and correct distance since both lengh are different in length).  like neuro already said, "exposure would not be changed"... and i would say the same thing.

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Re: Aperture and Light
« Reply #5 on: April 25, 2013, 12:01:42 PM »