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Old 09-01-2008, 08:45 AM   #1
David Rasberry
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Default F-stop vs T-stop

This question has come up a couple of times, so I thought it would be useful to post the definitions for those who don't know.








f/number

Definition: Setting of lens diaphragm that determines amount of light transmitted by lens. * Equal to focal length of lens divided by diameter of entrance pupil. * f/numbers are, for convenience and by convention, placed on a scale in which each standard f/number step (f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64 and so on) represents a doubling in the amount of light transmitted e.g. f/4 transmits twice as much light as f/5.6; conversely, f/16 transmits a quarter of f/8. * Since f/number is usually calculated from simple physical dimensions, different lens designs, varying focus and the use of accessories may all affect the actual amount of light projected: one lens set to e.g. f/8 may not give quite the same exposure as another lens set to f/8.



T-number

Definition: f/number of a lens corrected for the light loss during transmission through the lens. * f/number of a perfectly transmitting lens which would give the same illuminance on the axis as that produced by the test lens. * Equals the f/number divided by the square root of transmittance (assuming a circular aperture) e.g. if transmittance is 50% (only half light entering system exits the system), square root of a half is 1/C2, so T-number is one stop more than the f/number, so a relative aperture of f/4 with transmittance 50% is a T/5.6 lens. * Also known as T-stop. Assuming the ideal The f/number of a lens is defined by simple geometry (one length divided by another) so it assumes that the lens passes all of the light entering it. But no lens does: each interface between media of different refractive indexes causes a loss. Modern lenses are amazingly efficient so losses are in practice very small and, at any rate, losses are automatically compensated by through-the-lens metering. T-numbers are important in film industry, where TTL metering is not common.


So the T stop is an accurate corrected transmission reference for a particular lens as opposed to the theoretical maximum. Both use the same exposure scale
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Old 09-01-2008, 09:02 AM   #2
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Nice. So it's almost impossible to compare lenses from different cameras as we don't know how well the lens elements transmit the light, or even how many elements are in each lens? In essence F1.6 on an EX1 may transmit more light than scarlet at T2.8 but it could in theory transmit less light or an equal amount?

Would the only way to tell be to ask a Sony engineer and a Red engineer exactly how much light the lenses transmit at a given aperture? Or to take the lenses of both cameras and do a bench test?

???
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Old 09-01-2008, 09:37 AM   #3
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Quote:
Originally Posted by DefStef View Post
So it's almost impossible to compare lenses from different cameras as we don't know how well the lens elements transmit the light, or even how many elements are in each lens? In essence F1.6 on an EX1 may transmit more light than scarlet at T2.8 but it could in theory transmit less light or an equal amount?
Not to derail the conversation - but at that point even an accurate comparison of the lenses would be a bit meaningless as there are many other factors that go into the camera's sensitivity - such as the sensors themselves, the DSP, etc.

Thanks for the explanation, David.
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Old 09-01-2008, 09:46 AM   #4
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Reds are and Scarlet will be calibrated in T-stops. So the correction is already there. The EX1 at F1.6 would more likely be close to a T2. But the other unknown in the equation is relative sensitivity of the sensors. Bench testing with a logarithmic chip chart and a scope is the best way to determine a practical ASA/ISO speed for a video camera. Not sure how that would apply to a Red raw sensor, maybe scoping the monitor feed would get you in the ballpark, or use the histogram meter as on Red One if that is implemented on Scarlet.
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Old 09-01-2008, 04:17 PM   #5
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Quote:
Originally Posted by David Rasberry View Post
The EX1 at F1.6 would more likely be close to a T2.
That is useful information for me as I've used the EX1 extensively.

Especially when Jim is hinting that Scarlet's lens may be better than T2.8 after all.

Since Mysterium X will, according to Red, have higher DR than Mysterium, we can also hope it will have better sensitivity even in the Scarlet version, which will have higher pixel density than the Epic version.

But we all know about the sensor, so why I'm repeating it? I don't know.

I suppose Scarlet's proof will be in the pudding.
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Old 09-01-2008, 05:12 PM   #6
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Excellent post David! Great job...

I would like to add one thing from my recent experience with Red One. Many people here are concerned about the T2.8 of Scarlet... The truth is that for majority of the stuff You will end up shooting T2.8 is overkill. I have just shot with Ultra Primes last weekend with natural light only (no set lights what so ever...) and most of the time I had to stop the lens down to T5.6 even with No.9 ND filter. Without the filter I would overexpose even with T22!!! If You are really after T-stop lesser then 2.8 - make sure You are budgeting for good quality ND filters (which are not cheap), otherwise You can forget about DOF... Luckily for us we did not need shallow DOF for our project - quite the contrary - we needed all in focus...

How many of You (that are asking for faster lens) have actually shot anything with a lens faster then T2.8? That is a blazing fast T-stop if You ask me...

Just my one peso...
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Old 09-01-2008, 05:27 PM   #7
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I'm quite Sanguine. I am so sure that RED is going to make a beautiful piece of kit that is way beyond what I ever thought I would personally own...ever. I never really thought I would own a serious camera. Because generally who does! The fact that I could own something that could stand up to any Indie Film in quality is astounding. Now I just need to carry on learning about cinematography for the next fifty or so years and I'll be fine.

Considering that the fixed lens we will be getting---the equiv. 16mm lens is likely to be $150-200 per day to rent.
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Old 09-02-2008, 03:25 AM   #8
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Only minor thing to add, it is the f/ stop that determines the DoF since it its the physical "hole in the middle". The T/ stop is like an inbuilt ND filter. Though of course lens flare wil have the effect of increasing shadow ilumination.
Just a thought.
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Old 09-03-2008, 02:30 PM   #9
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I think modern lenses transmit about 85-90%, so f-stop = (square root 85%)*T-stop, so if Scarlet is T2.8, it will likely be about f2.6.

When comparing to the EX1, also keep in mind the actual size of the hole is focal length*f-stop, so in terms of light sensitivity and DOF, T2.8 on Scarlet will be equivalent to about f2 on the EX1 (assuming similar technology).
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Old 09-03-2008, 03:02 PM   #10
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Here is some useful input on light loss from a similar thread on Reduser:

"A 6 element prime lens at the same T/ stop will give a little more depth of field than a zoom lens if they have about the same sharpness, since the f/ stop of the zoom lens will be more open.

You lose about 1% per air to glass surface of a well coated lens (5% without the coating) so a four group 6 element prime lens loses about 8% of the light (two surfaces per group), but a zoom lens with 20 air spaced elements would lose maybe 40% to 50% of the light (20 elements = 40 surfaces). With better multi coating and some cemented elements the zoom light lose 25 to 35% of the light.

So a prime lens set to T/4 will be about f/3.8 to f/3.6 and a zoom lens set to T4 will be maybe f/3.2 to f/2.9, so the background may be a little sharper with the prime lens. A retrofocus wide angle or anamorphic lens is closer to a zoom in light loss so the difference in depth of field for T vs. f is less (when compared to a zoom).

The center of the frame will be brighter than the corners, so wide angle lenses will have dark corners with the T/ stop in the center of the frame as much as 1/2 to 2/3 stop higher than the corners of the frame (when wide open).

Many f/1.2 lenses are T/1.4 which is 1/2 shop slower. The glass used in fast lenses can transmit less light, and the elements are thicker. To get a T/1.0 lens you need about f/0.87.

The old Baltar lenses were f/2.3 and T/2.5 so less than 1/2 stop difference between f/ stop and T/ stop.

The T stop is ALWAYS slower than the f/ stop, so if your lens is marked in f/stops you are safe to open up 1/3 to 1/2 stop if you use T/stops for your light reading. Some Zoom lenses have more flair than a prime lens, so you may not want to open up enough to compensate fully for the central T/ exposure since the flair washes out the black areas, in that case you would under expose 1/4 stop to compensate for the zoom lens flair making the f/ stop difference between prime and zoom lenses smaller, meaning you open up 1/2 stop rather than 3/4 stop etc.

Uncoated filters can lose 10% of the light for each filter (5% per side), so 3 filters can lose about 30% of the light, plus the loss from the greenish glass used to bind some filters may take almost a full stop plus the filter factor of the filters in the sandwich between the greenish glass. Better filters use "white" optical glass, if you can find them... Last edited by www.DANCAD3D.com : 08-31-2008 at 02:44 PM.
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