# Does All F2.8 Lenses Let in the same amount of light?



## donny1963 (Mar 31, 2017)

Here is something i wanted to point out, that many of you may not know Specially if you use a light meter..

Ok so you got this lens you paid $1,500.00 for it, and lets say for argument sake, a Sony Stm  F2.8
Then you have another lens The sony 70-200 Gmaster F2,8 And so you would think you would get the same exposure using them at F2.8 on the same camera same settings Right?
Wrong!!!

First off let me say Not all lenses that are F2.8 or any other Aperture are really what they say.
It may say your lens is a F2.8 but that doesn't always mean your getting F2.8..

The real Spec (number) that you want to Look at for true accurate F-Stops is the T-stops spec
The difference between the F-stops number and the T-stops Number is basically this..

The F-stops is how much light that is allowed into the lens, the T-Stop number is what your actually getting in light out the end of the lens into the sensor, this makes a huge difference

So for example the 2 lenses i mention the Sony STM f2.8 is Rated at T-stops 5.6 which is what your getting for light on the sensor, The sony Gmaster F2.8 is rated at T-stops 2.9 which is what you would think F.29 to be..

so the Gmaster is the same F2.8 rating as the STN 2.8 only the STN your only getting F5.6 when it comes down to actual light hitting your sensor, so it's really a 5.6 not a 2.8.

and you will notice, if your the type who uses an external light meter and shoot with them 2 F-2.8 lenses , your going to get a darker picture using the STM, because your light meter doesn't know your T-stops on your lens, so you would have to make adjustments..

So basically T-stops spec is what you really want to look for, if your really Expecting a lens to deliver the light you want to your sensor.. Now some manufactures are honest Like Sony on the STM they say F2.8  T-5.6, so far they are the only ones who have the balls to actually be honest and print that on their lens.. 

You need to dig further with other manufactures like Sigma , Tamron, Nikon, Canon , none of them will print the Actual T-stops on the lens. You can still find out by checking the specs and looking to what the T-stops are, but they don't make it so obvious,

and you have to dig to find this out..
And you should check this out because you might find out your precious F2.8 lens is not really delivering you F2.8 performance to your sensor and only getting 1/4 the light you though you would be getting..

So remember T-stops is what really counts, Not F-stops..

And some of the reasons for this is that the light that comes into a lens could be getting degraded when going through elements in your lens and gets degraded to less light once it hits your sensor..
This is not just the only reason they are others, but it's mostly because of the design of the lens and what it is TRULY able to deliver when it comes down to actual light at your sensor..

And a long time ago this was discovered when moving making, when they changed their lenses with the same F-stops but notice different results in exposure, so this is why, In photography lenses they label it F2.8 in cinematography, they label the lens T 2.8

and the reason that being is they wanted to know the actual light they are getting, to avoid exposure changes all of a sudden during the movie when they changed lenses..
This is why Photo Camera's lens manufacture's should change it to T-stops to avoid being misled.
Which is what is happening in many cases.
I bet some of you here thought your lens would be a F2.8 but in truth when it comes to real specs, your shooting with a
F5.6 Lens..

So F-stops means how much background Blur,
T-stops means how much light at you sensor..
you can check your lens at SDP.IO/DXOmark
website you can find the true specs of your lens..

This is something you all should check into,  for instance i rented a mitakon f0.95 lens to check it out, and you think it would be a killer in low light shooting right? but i found out it wasn't  better then a 50 MM F1.4 lens, and so the reason was the mitakon f0.95 is really only delivering  T 1.4
So if you want to avoid being misled or let down from something like this start checking the lens T-Stops Not the F-stops , it makes all the difference in the world..


Donny


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## table1349 (Mar 31, 2017)

And for your next assignment please measure the variances in stated and actual focal lengths.   They are not perfectly precise either.   This is photography not neurosurgery.  Close counts.  

FYI.  What lens do you want to know about? Lens scoring versus Optical Metrics | DxOMark


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## Derrel (Mar 31, 2017)

So what is the story behind this Sony f/2.8 lens with a T-stop of f/5.6 when the lens is wide-open at f/2.8? I've never seen a two-stop loss unless it's on a macro lens where effective focal length is reduced at the lens fopcuses down to 1:1, and the lens drops to an f-stop rating (not T-stop) f/5.6; that is SUPER-common among macro lenses, MANY of which are f/2.8 at Infinity, but only f/5.6 or so at 1:1 extension.


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## donny1963 (Mar 31, 2017)

Derrel said:


> So what is the story behind this Sony f/2.8 lens with a T-stop of f/5.6 when the lens is wide-open at f/2.8? I've never seen a two-stop loss unless it's on a macro lens where effective focal length is reduced at the lens fopcuses down to 1:1, and the lens drops to an f-stop rating (not T-stop) f/5.6; that is SUPER-common among macro lenses, MANY of which are f/2.8 at Infinity, but only f/5.6 or so at 1:1 extension.



Well your F-Stop is the depth of field,  and the How ever T-stop is how much light gets to reach the sensor.


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## donny1963 (Mar 31, 2017)

Derrel said:


> So what is the story behind this Sony f/2.8 lens with a T-stop of f/5.6 when the lens is wide-open at f/2.8? I've never seen a two-stop loss unless it's on a macro lens where effective focal length is reduced at the lens fopcuses down to 1:1, and the lens drops to an f-stop rating (not T-stop) f/5.6; that is SUPER-common among macro lenses, MANY of which are f/2.8 at Infinity, but only f/5.6 or so at 1:1 extension.



Also here is a B&H photo spec on that lens.. 

Sony  FE 100mm f/2.8 STF GM OSS Lens SEL100F28GM B&H Photo Video


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## Dave442 (Mar 31, 2017)

In general I have not seen a huge difference between f-stops and t-stops. I also see that the lens in question is a special portrait lens with the following element as noted in the lens specs:

_"Optical design incorporates an apodization element that helps to improve the quality of bokeh. This element, which resembles a radially graduated ND filter that tapers from clear in the center to more dense around the edges, produces more circular out-of-focus highlights for more pleasing selective focus and shallow depth of field effects"._


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## donny1963 (Mar 31, 2017)

Here is a couple videos that explains all this..


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## Derrel (Mar 31, 2017)

I've known what a T-stop is for well over 30 years. Claiming that a 100mm prime lens with an f/2.8 maximum aperurte transmits T-5.6 is a quite a claim...and a claim that I am asking for some explanation on. I did not see the T-stop listed on the web page you linked us to. As dave442 wrote, I too have NOT see a radical divergence between the f/stop and the T-stop; typically 1/3 to 4/10 of a stop is about where the level of light loss is in most modern zooms; a full two-stop loss, from f/2.8 to T-5.6? That seems like a very cherry-picked example. f/2.8 to T-3.3 or so is pretty common among 70-200 and 24-70mm zooms.

But yeah...an f/2.8 zoom lens will most likely have a T-stop that's 1/3 of a stop or 4/10,maybe even 1/2 of an EV lower, as its maximum-aperture's acrtual T-stop. Well within expectations.


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## donny1963 (Mar 31, 2017)

Derrel said:


> I've known what a T-stop is for well over 30 years. Claiming that a 100mm prime lens with an f/2.8 maximum aperurte transmits T-5.6 is a quite a claim...and a claim that I am asking for some explanation on. I did not see the T-stop listed on the web page you linked us to. As dave442 wrote, I too have NOT see a radical divergence between the f/stop and the T-stop; typically 1/3 to 4/10 of a stop is about where the level of light loss is in most modern zooms; a full two-stop loss, from f/2.8 to T-5.6? That seems like a very cherry-picked example. f/2.8 to T-3.3 or so is pretty common among 70-200 and 24-70mm zooms.
> 
> But yeah...an f/2.8 zoom lens will most likely have a T-stop that's 1/3 of a stop or 4/10,maybe even 1/2 of an EV lower, as its maximum-aperture's acrtual T-stop. Well within expectations.


it's right there in the specs on the page  

here is  a picture

So this lens is really extreme, and your actually getting  T 5.6 for best light at the sensor, NOT 2.8


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## donny1963 (Mar 31, 2017)

by the way in that video the second one  there is more lenses mentioned what the spec really are..


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## pixmedic (Mar 31, 2017)

donny1963 said:


> Derrel said:
> 
> 
> > I've known what a T-stop is for well over 30 years. Claiming that a 100mm prime lens with an f/2.8 maximum aperurte transmits T-5.6 is a quite a claim...and a claim that I am asking for some explanation on. I did not see the T-stop listed on the web page you linked us to. As dave442 wrote, I too have NOT see a radical divergence between the f/stop and the T-stop; typically 1/3 to 4/10 of a stop is about where the level of light loss is in most modern zooms; a full two-stop loss, from f/2.8 to T-5.6? That seems like a very cherry-picked example. f/2.8 to T-3.3 or so is pretty common among 70-200 and 24-70mm zooms.
> ...




your example is a lens designed specifically for portraits, and moreso, the bokeh aspect of background rendering. 
they added an element to exaggerate the bokeh affect so of course thats going to affect the light getting to the sensor. 
I suspect that the reason the t-stop is not buried on the page is because its a non issue, not because of any overt bravery on the sellers part.  
any knowledgeable photographer is going to understand what this lens is for, and the implications of the added element. 

I think if you take the majority of lenses into account, the f-stop/t-stop difference will be significantly smaller, and not really a noticeable issue in actual practice.


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## donny1963 (Mar 31, 2017)

pixmedic said:


> donny1963 said:
> 
> 
> > Derrel said:
> ...




Yes and no, if your one of them people who use an external light meter then you apply it's readings to your camera you just might get something different for exposure then what you expected,  Because external light meters don't read T-stops, it's F-stops, so lets say you got a lens that is F2.8 and rated for T3.2 well that's not going to be a huge difference but enough for you to see the difference with your eye when it comes down to it's exposure..
I think all lens should be labled Both with F Stops  Rating and T Stops rating..

if your a cinematographer, you better pay attention to the Tstops more so then the fstops because you will find yourself having to correct thousands of frames of video..
This is the reason why all cinematography lenses are labeled with Tstops for that reason.. 

Donny


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## Derrel (Mar 31, 2017)

Compensating for the T-stop rating is simple. When using an external meter, simply re-set the ISO value to the T-stop loss compared to the f/stop. Just like one does when compensating for light loss due to bellows factor.

The use of external, hand-held light meters today is down to, I would guess, possibly something in the area of 1 still-picture exposure out of every 100,000 still-picture exposures made. I know I've shot 40,000 or so still-picture frames since I last used my flash meter, or my iPhone's app for continuous light metering.

Yes, cinema does require carefully matching exposure variables from shot to shot, to ensure visual continuity, and T-stops have been used on cine lenses for decades and decades now.


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## dunfly (Mar 31, 2017)

If you add focus breathing to the equation, not only will your 70-200mm/f2.8 zoom lens not really be f2.8, it may not even be 70-200mm.  That is why I let the camera take care of the BS and concentrate on the composition.


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## donny1963 (Mar 31, 2017)

dunfly said:


> If you add focus breathing to the equation, not only will your 70-200mm/f2.8 zoom lens be not really be f2.8, it may not even be 70-200mm.  That is why let the camera take care of the BS and concentrate on the composition.



Focus breathing is something else different then F-stops  VS T-stops..

Focus breathing you might notice  slight changes in focal length as you bring your subject into focus.
You've likely experienced it many times before.

Basically when you certin focal length lets say a 24-70 and you zoom all the way to 70 and you frame you shot then put your subject in focus the field of view can move slightly it can get bigger or smaller depending on weather your focusing in further or closer..

this happens on cheap lenses and on the highest end lenses alike, while some higher end lenses are designed on trying to minimize it, it's really hard to eliminate it all together..

Should you care about it?  Maybe not, if you never heard of the term focus breathing, it's probably never effected you in a a away it would bother you.

but some times if your trying to perfectly match up a frame of a shot, and then you re-focus and it significantly changes
the field of view of the lens, it can be an issue for you because then it's suddently thrown off and then you need to zoom back out and re-focus again..
And it's especially of importance if your doing macro work or focus stacking,

the lens i've experienced the most focus breathing is the 70-200 when your zoomed all the way to 200, and then focus the shot it moves a great deal you can see it probaby as much as 20 mm or more.

That lense sucks for focus breathing, how ever the brand new 70-200 they seem to have fixed it but i never bought the new one, i have the older version..

Even the 85 1.4 prime lens i have where there is no zoom, it does it i noticed it when i focus on my subject it moves quite a bit,, and that lens is a very expensive lens too.

i was shocked to see this happen on a prime lens, but it happens to that one for sure..
you can watch it move as you focus in on your subject..

Donny


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## Bebulamar (Apr 1, 2017)

The Sony is a special case and I think that's why they published the T stop. I measured a lot of lenses and found the T Stop is generally less than 1/3 stop different from the f/stop.


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