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Lens transmission


Alex Uzan

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Hi Filmmaker friends,

I was planning to buy some Sigma Art lenses in order to usethen on some video works.
But, I've read that despite to be F1.4, most of them have a 1.8 or 1.9 transmission (T-stop).

What does that mean ?
Less light ? Less Bokeh ? Less whatever ?

I tried to find information about that online, but so far no success.

Thanks for your help ?

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31 minutes ago, Alex Uzan said:

Hi Filmmaker friends,

I was planning to buy some Sigma Art lenses in order to usethen on some video works.
But, I've read that despite to be F1.4, most of them have a 1.8 or 1.9 transmission (T-stop).

What does that mean ?
Less light ? Less Bokeh ? Less whatever ?

I tried to find information about that online, but so far no success.

Thanks for your help ?

It means less light gets through.  Same bokeh though.

If you're wondering how less light can go through an aperture the same size then think about coatings reflecting or diffusing some light, the glass isn't perfectly clear (although it should be quite neutral) etc.  In todays world of great ISO performance I wouldn't worry about it.

T-stops were much more important when people were filming on film and needed to match exposures between lenses so they could swap lenses without having to adjust their whole lighting setups, so that's where it comes from.

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1 hour ago, Cinegain said:

Did you try the search engine YouTube? e.g.

Of course he didn't....   he typed a search query into the Search form (which is strangely called Start New Topic on these forums), hit Post and waited for a few hours for the hits to start showing up ???

It's common practice - everyone knows this!!

Searching for "t stop vs f stop" and hit "I'm feeling lucky" and you get this:  https://petapixel.com/2016/12/30/f-stops-vs-t-stops-difference-explained-plain-english/

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1 hour ago, kye said:

It means less light gets through.  Same bokeh though.

If you're wondering how less light can go through an aperture the same size then think about coatings reflecting or diffusing some light, the glass isn't perfectly clear (although it should be quite neutral) etc.  In todays world of great ISO performance I wouldn't worry about it.

T-stops were much more important when people were filming on film and needed to match exposures between lenses so they could swap lenses without having to adjust their whole lighting setups, so that's where it comes from.

Thanks for your answer.
It seems logical when you understand it.

1 hour ago, Cinegain said:

Did you try the search engine YouTube? e.g.

 

Oh Boy, I didn't see it.
And Gerald is ma favorite youtuber ?

10 minutes ago, kye said:

Of course he didn't....   he typed a search query into the Search form (which is strangely called Start New Topic on these forums), hit Post and waited for a few hours for the hits to start showing up ???

It's common practice - everyone knows this!!

Searching for "t stop vs f stop" and hit "I'm feeling lucky" and you get this:  https://petapixel.com/2016/12/30/f-stops-vs-t-stops-difference-explained-plain-english/

Not really, actually.

I've searched online, as usual, but with the bad words ; transmission instead of Tstop
English is not ma native language, so sometimes, it's just about what you think to understant and the words you use.

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3 hours ago, Alex Uzan said:

I was planning to buy some Sigma Art lenses in order to usethen on some video works.
But, I've read that despite to be F1.4, most of them have a 1.8 or 1.9 transmission (T-stop).

Also all lenses have some light loss, you won't find any f1.4 lenses that are also t1.4. (Unless its a pinhole lens but that won't be a happy solution for video lol.) So that doesn't mean they will have worse light transmission than any other given f1.4 lens, make sure to compare t stop to t stop when deciding on a lens.

4 minutes ago, IronFilm said:

Transmission value is the actual measured value

While F Stop is only the theoretical value. (a theoretical value which doesn't account for complex details like light loss due to reflections or thickness of glass elements, etc)

Thus why the T stop is always larger than the F stop. 

I have to quibble about semantics. :) f stop isnt theoretical, its focal length over the diameter of the aperture, a physical property of a lens you can measure. f stop simply is not a measure of light intensity, so its only theoretical if you use it as such.

Of course you are right in practical terms, its just wording.

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5 hours ago, Alex Uzan said:

Hi Filmmaker friends,

I was planning to buy some Sigma Art lenses in order to usethen on some video works.
But, I've read that despite to be F1.4, most of them have a 1.8 or 1.9 transmission (T-stop).

What does that mean ?
Less light ? Less Bokeh ? Less whatever ?

I tried to find information about that online, but so far no success.

Thanks for your help ?

Hi Alex,

Always best practice (IMHO) to do one's own homework when encountering unsupported claims on the internet. Case in point, the, um, observation that "despite to be F1.4, most of them have a 1.8 or 1.9 transmission (T-stop)". If one checks out Sigma's own web site where we find that their entire line of f/1.4 primes all turn out to have T1.5 (advertised) Cine variants...

FF High Speed Prime Line | Products | Cine Lenses | SIGMA GLOBAL VISION:

https://www.sigma-global.com/en/cine-lenses/products/ff-high-speed-prime/

...but is that advertising correct?

One can try and do a calculation, as suggested in Gerald Undone's above linked video, where we can determine (roughly) the light transmission of a given Cine lens where it has a photographic counterpart (such as the Sigma Primes)... f-Stop/T-Stop = squareroot Transmission %. Therefore 1.4 ÷ 1.5 = 9.3333, therefore 9.3333 x 9.3333 = (roughly) 87.1% light transmission. (Similarly, the Canon Cine f/1.4 Photographic Primes equated into 1.5 T-Stop variants.)

But those numbers are based on advertised claims.

Now, the folks at DxO have been publishing their own Lens Transmission measuments and they can be sifted through here...

Lenses Database - DxOMark:

https://www.dxomark.com/Lenses/

...using filters for f/1.4 and 24mm we generate a chart like this...

1930533843_DxO24mmf1dot4.thumb.jpg.0b2c0e3d8bb83a3b95e8efbb4f9c41de.jpg

...where their results show results closer to what you asked about.

Again using the above fstop-to-tstop math we find with the EF Sigma 24mm f/1.4... 1.4 ÷ 1.8 = 0.7777, square that and we see the DxO claim works out to 60.49% Light Transmission?! Can this be so? Any the, um, "lowly" Samyang has an impossible-to-achieve f/1.4/T1.4 100% measured transmission? Hrmm. :/ Let's read as to how they perform this measurement and what that method tells us...

DxOMark lens testing protocol and scores - DXOMARK:

https://www.dxomark.com/dxomark-lens-camera-sensor-testing-protocol/

...where they state...

<<

We chose the light source for its remarkable stability. It is exactly the same source as for our ISO speed measurement: A halogen lamp filtered to achieve a daylight color temperature of 5500K. This is worth noting because we use ISO sensitivity values in the T-stop calculation. We measure the luminance of the diffusing surface (about 140 cd/m²) with a certified luminance-meter. Knowing the entrance light flux, the sensor response, and the shutter speed, we can then calculate the T-stop of the lens for a given focusing distance.

We place the camera at a distance equal to 40 times the focal length of the lens (for example 2 meters for a 50mm lens). We take one picture for each aperture of the lens using full-stop increments.

To compute the transmission score, we measure the T-stop at the largest possible aperture iat each focal length. We then average those values over the range of focal lengths to calculate the final score.

>>

...so, in a nutshell, a 5500K f/1.4-16 average, which doesn't explain the anomolous Samyang test result.

But, their results being what they are, as cinematographers/videographers is a single-color-temperature test helpful to what we need to know about any given lens? What about color shift, character, MTF, etc? Seems more homework needs doing (well, for my needs). Turns out the folks over at LensRentals have an ongoing blog about their lens testing methods and I found this article (with some results) about testing Cine lenses using full-spectrum testing very informative and helpful...

Lens Rentals | Blog: Looking at Cine Lens Color Shifts Using Spectrometry

https://www.lensrentals.com/blog/2018/04/looking-at-cine-lens-color-shifts-using-spectrometry/

...there we find actual Cine (vs photographic variant) Lens tests and the Sigma Cine lenses weigh in at >80% transmission from ~430–720nm (and greater than 90% in mid-ranges)....right up ther with Canon, Veydra, Schneider, Zeiss and, yes, Samyang, too. Also, a couple of comparative color swatches showing color tones for several of the lenses.

Anyhoo, just some food for thought as you consider and do more homework on lenses.

To your other questions, since both the Photographic and Cine versions of these Sigma lenses incorporate the same optical designs the light transmission and bokeh should be indestiguishable...barring any optical coating differences.

IME, there's always a lot to learn, hope this was helpful, :)

Jimmy G

P.S. Not sure if I did reverse squareroot correctly?

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Of course, after all this, does anyone actually pay attention to this figure anymore?

I mean, in most situations you're using an ND anyway, so throwing away the vast majority of the light.  Then in those rare cases when there's no ND because it's low light (scene lit by candles, outdoor night scenes, etc) we simply adjust aperture / ISO settings (according to the priorities of the production, equipment performance, and artistic direction of crew) and then adjust the NR processing in post to match amounts of grain.

Even in cine-lenses, I'm only looking at the T-stop and thinking "the F-stop is at least that large" to try and understand what DoF the lens will have when wide open.

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9 hours ago, KnightsFan said:

I have to quibble about semantics. :) f stop isnt theoretical, its focal length over the diameter of the aperture, a physical property of a lens you can measure. f stop simply is not a measure of light intensity, so its only theoretical if you use it as such.

Of course you are right in practical terms, its just wording.


Maybe we're just coming from different backgrounds of what words mean, but my background is studying optics at university. So I feel I'm using the right terminology here. 

Theoretical value means what you calculate using values you know from the specs (such as looking up what is the focal length and the diameter), while the T Stop of course is different because it is taking a specific lens and putting it on a test bench to measure the damn thing when you shine light in vs what you get out at the other end. 

Thus theoretical vs measured values. 

 

8 hours ago, Jimmy G said:

...using filters for f/1.4 and 24mm we generate a chart like this...

Fascinating when you see the Samyang lenses (which should be all identical, aside from the different lens mounts) return results varying from T1.4 to T1.8

That indicates quite a large sample variation from Samyang (assuming dxomark is doing their measurements with precision), but perhaps not so radically surprising when you consider that Samyang is a low budget lens making? Precision quality control isn't their #1 priority. But still, I'm surprised at the variance. I wonder what the variance would be if you took half a dozen identical (but each from different production runs, so the serial numbers can't be close together at all) Sigma or Canikon lenses. 

8 hours ago, Jimmy G said:

ny the, um, "lowly" Samyang has an impossible-to-achieve f/1.4/T1.4 100% measured transmission?

My guess is Samyang is making lenses which are say perhaps f1.36 but because they know their quality control is sloppy they realize it is smarter to advertise them as lenses rounded up to f1.4 than to exaggerate it rounded down to f1.3

Then dxomark perhaps measured a T1.44, which gets expressed in their limited space as a "T1.4", thus you then got a drop (in this example I'm making up, the reality could quite possibly be even less extreme) from F1.36 to F1.44 (a drop of 0.8) which seems quite reasonable in the stituation of them getting a really good quality sample. 

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3 hours ago, IronFilm said:

Maybe we're just coming from different backgrounds of what words mean, but my background is studying optics at university. So I feel I'm using the right terminology here. 

Theoretical value means what you calculate using values you know from the specs (such as looking up what is the focal length and the diameter), while the T Stop of course is different because it is taking a specific lens and putting it on a test bench to measure the damn thing when you shine light in vs what you get out at the other end. 

Thus theoretical vs measured values. 

That's pretty much what I was saying terminology-wise: f stop is not just a theoretical number from a spec sheet, it's a value you can measure on a physical copy of a lens. I didn't mean to imply you were incorrect at all, just thought what you said could be misunderstood to mean that f-stop is some sort of voodoo number a manufacturer pulls out of a hat.

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1 hour ago, KnightsFan said:

f stop is not just a theoretical number from a spec sheet, it's a value you can measure on a physical copy of a lens.

Where do you think a theoretical number comes from? From the figures on a spec sheet / the design spec drawings. 

 

1 hour ago, KnightsFan said:

I didn't mean to imply you were incorrect at all, just thought what you said could be misunderstood to mean that f-stop is some sort of voodoo number a manufacturer pulls out of a hat.


I see, but hopefully nobody thinks a theoretical number is "just made up!!"

That would be a sad state of affairs. I guess for me, it is simply kinda obvious there is always some calculations behind a theoretical result. 

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On 1/10/2020 at 3:01 AM, Jimmy G said:

Hi Alex,

Always best practice (IMHO) to do one's own homework when encountering unsupported claims on the internet. Case in point, the, um, observation that "despite to be F1.4, most of them have a 1.8 or 1.9 transmission (T-stop)". If one checks out Sigma's own web site where we find that their entire line of f/1.4 primes all turn out to have T1.5 (advertised) Cine variants...

FF High Speed Prime Line | Products | Cine Lenses | SIGMA GLOBAL VISION:

https://www.sigma-global.com/en/cine-lenses/products/ff-high-speed-prime/

...but is that advertising correct?

One can try and do a calculation, as suggested in Gerald Undone's above linked video, where we can determine (roughly) the light transmission of a given Cine lens where it has a photographic counterpart (such as the Sigma Primes)... f-Stop/T-Stop = squareroot Transmission %. Therefore 1.4 ÷ 1.5 = 9.3333, therefore 9.3333 x 9.3333 = (roughly) 87.1% light transmission. (Similarly, the Canon Cine f/1.4 Photographic Primes equated into 1.5 T-Stop variants.)

But those numbers are based on advertised claims.

Now, the folks at DxO have been publishing their own Lens Transmission measuments and they can be sifted through here...

Lenses Database - DxOMark:

https://www.dxomark.com/Lenses/

...using filters for f/1.4 and 24mm we generate a chart like this...

1930533843_DxO24mmf1dot4.thumb.jpg.0b2c0e3d8bb83a3b95e8efbb4f9c41de.jpg

...where their results show results closer to what you asked about.

Again using the above fstop-to-tstop math we find with the EF Sigma 24mm f/1.4... 1.4 ÷ 1.8 = 0.7777, square that and we see the DxO claim works out to 60.49% Light Transmission?! Can this be so? Any the, um, "lowly" Samyang has an impossible-to-achieve f/1.4/T1.4 100% measured transmission? Hrmm. ? Let's read as to how they perform this measurement and what that method tells us...

 

 

 

 

Regards DXO, also note that the transmission for those lenses varies with the camera format used.       They also actually measure a few lenses with the SAME F stop as T stop (probably down to correcting in camera or maybe just so little loss as to not matter...maybe both).      Some Sony lenses in particular when used on FF cameras are rated by DXO as being the same (55 1.8 for example).

 

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