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Author Topic: DXOMark: Sigma 24-105 f/4 DG OS Reviewed  (Read 12185 times)

candc

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #60 on: December 18, 2013, 09:24:34 PM »
I have always been a bit confused about the whole f stop vs. t stop comparison. If I understand it right then a 100 mm f/2 lens has a 50mm max aperture, 100/2 = 50. The lens may pass less light, that is the t stop, an actual transmission or equivelant rating. Does the camera recognize this and make exposure calculations on a lenses light transmission capabilities or does it default to the embedded code of the lens? Is slight underexposure only an issue at max aperture?
« Last Edit: December 18, 2013, 09:28:40 PM by candc »

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #60 on: December 18, 2013, 09:24:34 PM »

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #61 on: December 18, 2013, 10:13:54 PM »
I have always been a bit confused about the whole f stop vs. t stop comparison. If I understand it right then a 100 mm f/2 lens has a 50mm max aperture, 100/2 = 50. The lens may pass less light, that is the t stop, an actual transmission or equivelant rating. Does the camera recognize this and make exposure calculations on a lenses light transmission capabilities or does it default to the embedded code of the lens? Is slight underexposure only an issue at max aperture?

If you have two lenses of similar design - one T 2.8 and other T 3 and put them both on the same camera at the same settings you would notice one is slightly brighter. With the camera in Av mode the metering system would compensate via shutter speed so they look similar. The downside being you lost speed. How important that is depends on what you're shooting - action vs landscape for example.

Since the 24-105L isn't really known for it's action freezing abilities it's not even an issue. For low level light situations it could make a slight difference though. But then again for low light you'd use a faster lens.

However, more light is always welcome so in the grand scheme of things better T stop values are desirable.
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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #62 on: December 18, 2013, 10:46:09 PM »
So you attach an f/2.8 lens to the camera, the max aperture reading f2.8 you see is based on what the lens tells the camera, not what the actual light reading the camera see's based on the focal length?

Yes exactly. F stops only refer to the physical size of the aperture. It does not represent the actual amount of light the lens can transmit.

The maximum amount of light that gets through when the lens is at f/2.8 is what the T stop refers to. By the time that light has gone through the front element, all the elements in the middle (could be several) and out the back and onto the sensor it is no longer at 100% because some of it got reflected along the way. How much got reflected depends on the quality ofthe glass and the coatings.

Easy way to think of it. F stop is just how big the hole is. T stop is how much light is allowed to pass through the hole. If that hole is covered in spider webs not much light gonna get thru! Haha!

Also when you stick a filter in front of the lens it also affects the T stop since the filter is essentially adding another element in the light path.

It makes sense why prime lenses have better image quality than zooms. The zooms tend to have more glass elements thus blocking more light.

No lens is perfect, you'll always lose a bit of light.
« Last Edit: December 18, 2013, 10:54:08 PM by Zv »
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candc

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #63 on: December 18, 2013, 11:15:58 PM »
Okay, I think I understand how the whole lens, camera metering works. The lens tells the camera its max aperture, the camera takes a reading based on that and the light it's seeing, it's actually basing the calculation on the t stop it's seeing relative to the f stop it's given?

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #64 on: December 19, 2013, 12:01:45 AM »
Okay, I think I understand how the whole lens, camera metering works. The lens tells the camera its max aperture, the camera takes a reading based on that and the light it's seeing, it's actually basing the calculation on the t stop it's seeing relative to the f stop it's given?

Close but not really. Forget T stop. The difference between f stop and t stop does not result in slight underexposure even at max aperture.

The camera is basing the exposure on the light it sees. The T value is one factor in that but a very minor one, of magnitudes greater importance is the luminosity of the scene. It doesn't matter to the camera if you have a low transmission efficiency, or the scene is darker, or if you put on a ND filter etc, it adds up to the same thing, longer shutter speeds.

The camera is not basing the calculation on the t stop it sees per se, it never knows what the t stop is and doesn't care, it is basing the calculation on the light it sees, that is all. The difference between the light there is (which an ETTL meter never has a way of knowing) and the light that comes through the lens (which it does), is impacted by the t stop and filters etc, but that difference is not a factor in the exposure calculation.

« Last Edit: December 19, 2013, 12:10:00 AM by privatebydesign »
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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #65 on: December 19, 2013, 12:16:43 AM »
Okay, I think I understand how the whole lens, camera metering works. The lens tells the camera its max aperture, the camera takes a reading based on that and the light it's seeing, it's actually basing the calculation on the t stop it's seeing relative to the f stop it's given?

T stop is just a theoretical (measured in a lab??) value.

When you half press the shutter the meter is activated and the lens does nothing really, it's already wide open to allow as much light in as possible. The camera then does it's calculations based on the amount of light hitting the sensor.

When you fully press the shutter the aperture then adjusts to the required setting and the shutter moves to achieve a correct exposure.

The only time you might notice T stops is if you were using M mode and switching lenses for the same scene. For example lets say you set up a shot at f/2.8 1/60 ISO 100 and it looks perfect. You then switch lenses and set it to the exact same settings and notice the shot is under exposed slightly. No big deal you just crank up that ISO or adjust shutter speed and you're back on track!

An example where you might encounter this situation is say you're using the 24-105L and shooting at 24mm and say f/8. You decide you're not a fan of the 24-105L at the wide end so you switch to your 24L  or whatever and try again. Surprise surprise it looks better now!

That's basically what this is all about. The Sigma lens would be slightly brighter and of course better. But the amount is fairly small.
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dilbert

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #66 on: December 19, 2013, 08:24:28 AM »
Okay, I think I understand how the whole lens, camera metering works. The lens tells the camera its max aperture, the camera takes a reading based on that and the light it's seeing, it's actually basing the calculation on the t stop it's seeing relative to the f stop it's given?

T stop is just a theoretical (measured in a lab??) value.

T stop is not theoretical. It is the measured transmission of light through the lens.

Quote
The only time you might notice T stops is if you were using M mode and switching lenses for the same scene. For example lets say you set up a shot at f/2.8 1/60 ISO 100 and it looks perfect. You then switch lenses and set it to the exact same settings and notice the shot is under exposed slightly. No big deal you just crank up that ISO or adjust shutter speed and you're back on track!

Correct.

Quote
An example where you might encounter this situation is say you're using the 24-105L and shooting at 24mm and say f/8. You decide you're not a fan of the 24-105L at the wide end so you switch to your 24L  or whatever and try again. Surprise surprise it looks better now!

That's basically what this is all about. The Sigma lens would be slightly brighter and of course better. But the amount is fairly small.

The amount isn't fairly small. It's half a stop or about 25% difference in the amount of light.

Going form f/4.0 to f/5.6 means that the amount of light halves. In this case the Sigma is an f-stop of 4.2 and the Canon is 5.1 which is roughly half a stop. So instead of 1/60 (Sigma), you'd be using 1/45 (Canon.)

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #66 on: December 19, 2013, 08:24:28 AM »

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #67 on: December 19, 2013, 09:00:06 AM »
T stops and f stops all are pretty easy to understand in principle but....

Ever since I first started using the 24-105 in 2005 I found that if you shot from the camera's meter, or suggested exposure in manual, this lens underexposed the frame by about one third of a stop compared with say a fast lens such as the 50/1.4, so the image was slightly more 'dense' and the histogram more dense and to the left.

Now according to DXO the 24-105 has a t stop of 5.1 against an f stop of 4, so it looses two thirds of a stop.

Using the same source the 50/1.4 has a t stop of 1.6 against 1.4 so it is one third of a stop less; so; the difference between the two lenses actual transmission is one third and this is exactly the difference I find in exposure between the two, and this includes using a hand held incident light meter and setting the camera shutter speed and aperture manually.

So far so good; this difference equated exactly to the t stop differences between the lenses.

However there is a problem. The 24-70 f4 IS which by the same source has a t stop of f4 against f4 does exactly the same thing. Compared with a faster prime with less elements it underexposes by one third of a stop. So does complicated element lenses such as the 70-300L. Also I believe Edward Lang (eml58) found that his 200-400L underexposed on the same meter reading compared with the 400/2.8L .

So what's going on ? By the definition of what a 't' stop is, lenses of equal t stop should expose the same, but it seems to me that the more elements that are added the more the lens is likely to underexpose. Yet this is just what t stop is supposed to measure.

And before anyone asks, this isn't just on one copy of the lens; it is universal.

To me it suggests that 't' stop value is not accurate in practice, or at least in this application. Any ideas ?

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #68 on: December 19, 2013, 09:16:46 AM »
Interesting discussion!

Now I think I understand practical purpose of this parameter in DxO stats.

Did anyone see lenses with calculated T-stop equal to F-number on them?
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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #69 on: December 19, 2013, 09:30:48 AM »
Interesting discussion!

Now I think I understand practical purpose of this parameter in DxO stats.

Did anyone see lenses with calculated T-stop equal to F-number on them?

Many of the better primes (particularly the new ones) are much closer.  I've already mentioned the new EF 35mm f/2IS, which has both an f-number and t-stop of 2.  In the past, I would say that many primes are more likely to be close than zooms (where more compromises are made), and that does speak well of the light transmission of the new Sigma 24-105. because it manages to transmit just about (almost) the amount of light that the f-stop suggests.
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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #70 on: December 19, 2013, 09:36:04 AM »
Interesting discussion!

Now I think I understand practical purpose of this parameter in DxO stats.

Did anyone see lenses with calculated T-stop equal to F-number on them?

Many of the better primes (particularly the new ones) are much closer.  I've already mentioned the new EF 35mm f/2IS, which has both an f-number and t-stop of 2.  In the past, I would say that many primes are more likely to be close than zooms (where more compromises are made), and that does speak well of the light transmission of the new Sigma 24-105. because it manages to transmit just about (almost) the amount of light that the f-stop suggests.

How can the T value be the same as the f number if the lens clearly vignettes wide open? Is T value only measured in the center?

Just asking, I have no clue how the T value is actually determined.
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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #71 on: December 19, 2013, 09:46:44 AM »
Interesting discussion!

Now I think I understand practical purpose of this parameter in DxO stats.

Did anyone see lenses with calculated T-stop equal to F-number on them?

Many of the better primes (particularly the new ones) are much closer.  I've already mentioned the new EF 35mm f/2IS, which has both an f-number and t-stop of 2.  In the past, I would say that many primes are more likely to be close than zooms (where more compromises are made), and that does speak well of the light transmission of the new Sigma 24-105. because it manages to transmit just about (almost) the amount of light that the f-stop suggests.

How can the T value be the same as the f number if the lens clearly vignettes wide open? Is T value only measured in the center?

Just asking, I have no clue how the T value is actually determined.

Good question.  I can't see it being anything else, as the average from almost every lens towards the periphery would bring the score down
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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #72 on: December 19, 2013, 10:05:20 AM »
It used to be measured as the density of the negative plus base, fb+f . In today's speak that would equate to the same charge on the sensor. It is not a spot measurement.
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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #72 on: December 19, 2013, 10:05:20 AM »

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #73 on: December 19, 2013, 10:50:34 AM »
DXO gives this lens the same sharpness score (18) as the Canon 24-70 2.8 II and the Tamron 24-70 2.8 VC.  Perhaps I don't understand what their sharpness scores mean, but I'm pretty sure I've not seen any other review claim or show that the Tamron, however much they may like it, is as sharp as the Canon, or that the new Sigma, while a bit sharper than the 24-105, is also as sharp as the Canon 24-70 2.8 II.  What am I missing?

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #74 on: December 19, 2013, 11:03:54 AM »
DXO gives this lens the same sharpness score (18) as the Canon 24-70 2.8 II and the Tamron 24-70 2.8 VC.  Perhaps I don't understand what their sharpness scores mean, but I'm pretty sure I've not seen any other review claim or show that the Tamron, however much they may like it, is as sharp as the Canon, or that the new Sigma, while a bit sharper than the 24-105, is also as sharp as the Canon 24-70 2.8 II.  What am I missing?

Their score summaries are, in my opinion, a joke. I mean have you seen the chromatic aberration summary score they show for the 85mm 1.8 ? It's 3 mu,  that's  about the lowest there is. The Zeiss Otus for example is 6.

Yet use the 85 1.8 wide open in high contrast and we all know what happens.

( Well all with the exception of dxo that is ).

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Re: DXOMark: Sigma 24-105 f/4 DG OS Reviewed
« Reply #74 on: December 19, 2013, 11:03:54 AM »