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PostPosted: Sun Sep 02, 2012 5:25 am 
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From This site i found that

http://www.ronmartblog.com/2012/05/comp ... -mark.html

“”"”"” The Nikkor 70-200mm f/2.8G ED VR II is a rear-focusing lens so it behaves differently than the Canon 70-200mm f/2.8L IS II USM despite being on a tripod and full-frame camera. Thom Hogan describes issues surrounding rear focusing lenses well in his review, so despite the tripod and camera not moving the view of 100mm from both systems was not identical. To get roughly the same data captured I had to adjust the Canon lens to 85mm as shown below.

Despite both lenses being an f/2.8 lens, the Canon is a brighter lens that allows you to use faster shutter speeds. If this is a combo you use often, this is something you might want to factor into your decision if you are considering switching camps.”””””

My question here,, how canon become a brighter lens?
and what about the frame difference, when we r using the same format and same focal length?
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PostPosted: Sun Sep 02, 2012 7:55 am 
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Trivial difference probably - very much doubt it would be more that 1/3rd stop difference. Hardly worth changing systems and brands over


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PostPosted: Sun Sep 02, 2012 9:52 am 
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There is always the difference between f-stop and t-stop. The first is the geometric calculation of how "fast" the lens is. That is also the value that photographic lenses are designated with. So both Canon and Nikon have the same f2.8 f-stop.
t-stop talks about he "transmission" of a lens which is also influenced by how much light each single glass-element in the lens lets through. That in turn depends on the coating of each element and the type of material the element is made of. As far as I remember uncoated elements might reflect something between 5 and 10% of light back, so that light is lost for transmission/exposure. Modern lens-elements should have a light-loss of 1-2%. But with around 20 lens-elements in a modern zoom that can still add up to around 30% loss in the complete lens.
Now the Nikon zoom has 21 lens-elements and the Canon sports 23 lens-elements. If we assume that Nikon has 2% loss per lens-element and Canon has cut this loss in halve down to 1% the Nikon zoom would lose 35% of light and the Canon zoom would lose 21%. The difference is less than 1/3 of a stop.

By the way: film-lenses are marked in t-stops so that changing lenses during shoots never changes the exposure of the film/video.

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PostPosted: Sun Sep 02, 2012 10:07 am 
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How are these lens `coatings` measured? Can each manufacturers coating be compared against another for efficiency for their intended purpose?
Can we then assume the Canon coating is more efficient......or just that they use less of it somehow to gain the extra T-stop rating over rivals?

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PostPosted: Sun Sep 02, 2012 1:42 pm 
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Hi everyone,

I have a question regarding this "t-stop" that Bjorn mentioned. How or where can I obtain the "t-stop" for any particular lens?

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PostPosted: Sun Sep 02, 2012 2:44 pm 
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I figured I'd might chime in. I own a 70-200mm 2.8 L IS II zoom. I've never won a Cameralabs assignment contest. So all the Nikon users can rest easy. Technique + effort > gear. =)


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PostPosted: Sun Sep 02, 2012 5:40 pm 
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@oldCarlos: It's not the question of "how much" coating you use - at least in this case less does not equal better. The coating is applied to suppress reflection from the glass-surface. As this is mostly specific to a wave-length (or color) of the light most manufacturers apply multiple coating for different optimized for different wave-length/colors. More on this topic can be found in this article.

@pierovera: I think the absolute t-stop is hard to obtain. But you might experience differences in transmission between two lenses by photographing a constantly lit surface with the same exposure value (i.e. combination of aperture and shutter-speed) and measure the differences in brightness between them.
This is best been done at max open aperture and measured from a small crop in the middle of the lens. Why that?
- open aperture: because differences in the aperture-closing mechanism can lead to observed differences that have nothing to do with transmission of a lens but with the precision of the aperture-lever mechanism (see my latest experience with the Tamron 24-70/2.8 VC).
- middle crop: because different light-fall-off in the corners of each lens might influence the brightness of the image much more than differences in transmission.

@lagnificent: Yeah right, I forgot: Canon lenses s*ck and Nikon lenses rule :wink:

@all: I personally wouldn't bother about transmission differences that are around or below 1/3 EV, honestly!

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PostPosted: Wed Sep 05, 2012 2:33 am 
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thanks
then why dont they claiming their t- values.. i hope,, i didnt found those numbers anywhere in their adds or other reviews, may i missed it?

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PostPosted: Wed Sep 05, 2012 7:52 am 
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I've never seen any camera-lens-manufacturer claim t-values. That is a pity but spares them measurements and discussions.
But as it was more important in the film-industry to avoid even small changes in exposure I think it is mostly irrelevant in the photographic industry especially now that it's very easy to correct those small differences (supposedly <1/3 EV) in post-processing.
On another note: dof and background blur are influenced by the f-stop and not the t-stop. Proof: mount a neutral density filter on your lens and it won't change dof or bg-blur.

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