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11-04-2014, 07:59 PM   #16
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QuoteOriginally posted by shamer Quote
This is why a 50mm f/1.4 gets darker and darker when you add macro tubes and why a fast telephoto lens needs to be so large.

That's where t stops come in. A "t' measurement of a lens is actually a measurement of the light transmitting properties of a lens. T
I think you are getting the issue of T-stops and close-up corrections mixed up there.

close up corrections ( macro extension tubes or bellows draw) increase the f/- based on the number of focal lengths (s) the object is distant.
For example s = 3 when the object is 300mm distant from a 100mm macro lens
Then f/- is increased by s/(s-1) example 3 /(3-1) = 3/2
So the click f/4 would be actually 4 * 3/2 = 6; the aperture is f/6

This is quite separate from the transmission of light through the glass elements(t-stops)

11-04-2014, 09:30 PM   #17
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QuoteOriginally posted by wombat2go Quote
I think you are getting the issue of T-stops and close-up corrections mixed up there.
Yep...the nominal f-number is based on the focal length at infinity. For close focus the effective f-number increases proportional to the lens extension. This is a well-known consideration for large format photography where even moderately close focus may require significant exposure compensation.


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11-06-2014, 12:56 PM   #18
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My impression was that T-stop for complex zooms differed more from the F-stop than for primes. However I realized that I don't have any data on this. I went to DXO to see what I could find and honestly had no idea how to read the data. They only list a single T-Stop for each lens and they don't clarify on the zooms what the F-Stop was, what the focal length of the test was etc. So I'm lost. I saw most primes had a net increase in T-stop of a linear 0.2 to 0.4 so for example an F2.8 lens might show 3.2 T stops. If this is very linear and does not increase in the zooms substantially then I would think we can ignore T-stops - but I'd love some real data - are there any video sites out there with more info?
11-06-2014, 01:13 PM   #19
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QuoteOriginally posted by bradshea Quote
My impression was that T-stop for complex zooms differed more from the F-stop than for primes. However I realized that I don't have any data on this. I went to DXO to see what I could find and honestly had no idea how to read the data. They only list a single T-Stop for each lens and they don't clarify on the zooms what the F-Stop was, what the focal length of the test was etc. So I'm lost. I saw most primes had a net increase in T-stop of a linear 0.2 to 0.4 so for example an F2.8 lens might show 3.2 T stops. If this is very linear and does not increase in the zooms substantially then I would think we can ignore T-stops - but I'd love some real data - are there any video sites out there with more info?
It's really about matching exposure across different lenses and from different perspectivves. you need the accuracy of a t stop for that (because it does vary from lens to lens within a size even (ie a zeiss 28 will be different than a samyang etc)

We can pretty much ignore it for stills because we meter for the shot not for the ongoing scene and series of shots

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