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10-21-2016, 09:07 PM   #1
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T stops vs sensor? ??

I saw the new sigma cina lenses price was posted. The 18-35 T-2 blah blah. I was thinking my sigma 18-35 was T2 also. Checked DXO and Nikon on 3300 is t2. Checked on Canon and its 1.8. Why differences? Same lens. Is it sensor size, registration distance, pixel density? What is the t-stop on Pentax?
I also wonder if at higher f-stops if the T-stop evens out to the f-stop. Say my f2.8 lens has a t-stop of 4, at f4 and beyond are they the same?
Thanks in advance.

10-21-2016, 09:48 PM   #2
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T stops are a measure of transmission. F stops are a mathematical measurement that don't take into account how much light is lost by reflection off glass surfaces or is absorbed by the glass. That lens apparently loses 1/3 f/stop of light. Not too bad. Years ago Popular Photography measured the T-stop of various 50mm lenses. The Pentax 50mm f/1.4 transmitted slightly more light than an Olympus 50mm f/1.2. A lens that loses 1/3 f/stop of light at f/2 will still lose 1/3 of a stop of light at any aperture. The f/stop is a mathematical measurement no matter what the aperture is. Through the lens light meters compensate so that exposure is correct. T-stops are traditionally used by cinematographers because professional movie cameras don't have through the lens meters and they want exposure to be consistent between wide and close up shots that get edited next to each other.
10-21-2016, 10:17 PM   #3
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Ok so a lens with tstop 2 vs lens with tstop 2.8 will still have a stop of difference of light when both are at f8?
10-21-2016, 11:24 PM   #4
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Yes.Years ago I had a Tamron 70-220 f/4 zoom lens. It was made before multicoating. I replaced it with a Super Multi-Coated Takumar 85-205 f/4.5. My stop down metering Spotmatic showed that the Takumar was "faster" at all apertures.

10-21-2016, 11:32 PM   #5
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That clears up that part. Now to find out why on different cameras it measures differently.
10-21-2016, 11:35 PM   #6
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With a 50mm lens, f/2 means that the diaphragm opening is 25mm and f/4 is 12.5mm (focal length/4 or 50 divided by 4 is 12.5mm ). An f/stop doesn't account for transmission, only a T stop does. If the lens for Canon is marked in f/stop and the others in t-stop that would be the difference. Sensor size, sensor type or camera brand can't affect this.

Last edited by pentaxus; 10-21-2016 at 11:41 PM.
10-21-2016, 11:49 PM   #7
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Dxomark measures tstop. They have measurements on different bodies. The sigma 18-35 on nikon tstop was 2 on all Nikon I looked at. On most canon it was 1.8. it was 1.9 on one and 2.1 on one. They didn't try it on Pentax.

10-22-2016, 04:05 AM   #8
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QuoteOriginally posted by swanlefitte Quote
I was thinking my sigma 18-35 was T2 also. Checked DXO and Nikon on 3300 is t2. Checked on Canon and its 1.8. Why differences? Same lens. Is it sensor size, registration distance, pixel density? What is the t-stop on Pentax?
That's really odd. T stop should be fairly objective. I guess it comes down to methodology and measurement errors. Just goes to show once more that DXo is not some objective, perfect tool, especially for comparisons between brands.
10-22-2016, 04:11 AM   #9
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QuoteOriginally posted by swanlefitte Quote
I saw the new sigma cina lenses price was posted. The 18-35 T-2 blah blah. I was thinking my sigma 18-35 was T2 also. Checked DXO and Nikon on 3300 is t2. Checked on Canon and its 1.8. Why differences? Same lens. Is it sensor size, registration distance, pixel density? What is the t-stop on Pentax?
I also wonder if at higher f-stops if the T-stop evens out to the f-stop. Say my f2.8 lens has a t-stop of 4, at f4 and beyond are they the same?
Thanks in advance.
read this : F-stop blues for explanation.
10-22-2016, 05:46 AM   #10
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I did some more dxomark score comparisons. Pentax 55 had a t of 1.8 on k3 but 1.7 on k50. The 200 had a t of 3.1 on k3 and 3.2 on k50.
10-22-2016, 07:26 AM   #11
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T stops should not depend on camera model. The data is odd. Lens design differences between mounts I can buy into. But the lens on k-50 vs another Pentax showing differences isn't making any sense.
10-22-2016, 08:21 AM   #12
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The data makes less sense when the k50 produces both a lower and higher score.
10-22-2016, 08:30 AM   #13
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The thing to take away is that light transmission isn't lossless in a lens. T stop should always be lower than or equal to f stop and almost never equal.
10-22-2016, 09:07 AM   #14
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different sensor= different T stop, see here. excerpt: ''When you look at the structure of CMOS sensors, each pixel as basically a tube with the sensing element at the bottom. If a light ray that is not parallel to the tube hits the photo site, chances are the light ray will not get to the bottom of the tube and will not hit the sensing element. Therefore, the light coming from that light ray will be lost. It appears from this graph that when using large aperture lenses on Canon cameras, there is a substantial amount of light loss at the sensor due to this effect. In other words, the “marginal” light rays coming in at a large angle from near the edges of the large aperture are completely lost.''
10-22-2016, 09:10 AM - 1 Like   #15
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QuoteOriginally posted by UncleVanya Quote
T stops should not depend on camera model. The data is odd. Lens design differences between mounts I can buy into. But the lens on k-50 vs another Pentax showing differences isn't making any sense.
QuoteOriginally posted by swanlefitte Quote
The data makes less sense when the k50 produces both a lower and higher score.
QuoteOriginally posted by UncleVanya Quote
The thing to take away is that light transmission isn't lossless in a lens. T stop should always be lower than or equal to f stop and almost never equal.
What he said ^ ^ ^

For the most part, T-stop vs. f-stop should not be a consideration when using TTL metering, is not an indication of lens quality and is definitely not something that varies across bodies.* DXOMark is know for playing loose and easy with established terminology and this may well be another situation where they have applied sensor response metrics to an existing term. Remember, these are the guys that equate sensor S/N ratio to dynamic range.


Steve

* T-stop is determined on an optical bench with a calibrated light source and is a measure of actual transmittance.
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