So that I understand

CamDiver

CamDiver

Active member
Sorry if these seem mundane to some people,
Can someone please explain the meaning behind the terms "Fast" and "Slow" when people are discussing lenses. For my part I believe it relates to the fstop characteristic of the lens, an f1.6 being faster due to the wider apeture therefore allowing for faster shutter speeds as opossed to an f2.8 for example or do I have that in reverse? OR am I completely off base?

Also the terminology T3 or T2.5. I take it the T is just a substitute for the f in fstop? Could someone explain why in some cases we refer to Tstops as opposed to fstops in others.

Thanks,
Mark.
:beer:
 
CamDiver said:
Sorry if these seem mundane to some people,
Can someone please explain the meaning behind the terms "Fast" and "Slow" when people are discussing lenses. For my part I believe it relates to the fstop characteristic of the lens, an f1.6 being faster due to the wider apeture therefore allowing for faster shutter speeds as opossed to an f2.8 for example or do I have that in reverse? OR am I completely off base?

Also the terminology T3 or T2.5. I take it the T is just a substitute for the f in fstop? Could someone explain why in some cases we refer to Tstops as opposed to fstops in others.

Thanks,
Mark.
:beer:
Click to expand...

You're right about the term for "fast lens". I'm not sure about the "T". I'll let the others to explain.
 
The speed of the lens refers to the aperture.A lens that's fast means that the iris inside the lens can open wider and thus let more light in (fast). A fast lens design is more complex and thus more expensive to make. As a general rule, a wider lens is faster than a longer lens(telephoto).
 
I was under the impression that F-stop refered to light levels that go into the lens, while T-stop refers to light levels that exit the lens. I'm probably wrong.
 
These seem to get asked a lot. Maybe we need a cinematographer's dictionary in addition to a RED dictionary. :p

In technical terms, the f-stop (or f-number) expresses the diameter of the entrance pupil in terms of the effective focal length of the lens. For example, if the focal length of the lens (say a 50mm lens) is 8 times bigger than the pupil diameter (in this case, the hole the light passes through has been closed down to 6.25mm in diameter), the f-stop is f/8. As you open up the lens, the hole gets bigger, and the ratio goes down. Conversely, as you close down a lens, the f-number gets bigger, and the less light per unit area reaches the image plane of the system. Basically, it's all math. :)

Shutter speeds are arranged in a similar scale, so that one stop on the shutter speed scale corresponds to one stop on the aperture scale. Opening up a lens by one stop allows twice as much light to fall on the film (or in this case, the sensor) in a given period of time, and therefore to have the same exposure, you must have a shutter speed twice as fast (shutter open half as long). So you are correct that in relation to the shutter, the stops are described as "faster" or "slower" if they let in more or less light, respectively.

T-stops are similar, but they aren't the same. All lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), so for exposure purposes, t-stops are used instead of f-stops on cinema lenses. The t-numbers are adjusted so that the amount of light transmitted through the lens at a given t-stop is equal to that going through an ideal non-absorbing lens set at that f-stop. (The t in t-stop stands for transmission).
 
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If the F stop number is low then the light can get to the sensor quicker.

Imagine shooting on an SLR you would leave the aperture open for longer in order to let more light in, as long as the subject stays still you can allow more information to enter even if you're using a high F Stop number.

But when filming you need to have very fast lenses with low T Stop numbers so that the information hitting the sensor is letting in enough light so that it doesn't record the scene darker than should be.

I believe the difference between an F Stop and a T Stop is that the T Stop measures the ammount of light exiting the rear of the lense whereas the F Stop is a measure of the ammount of light entering the lense. The T stop therefore is the same as the F Stop with lense transmission loss included. T stop is used more in Film because of the nature of the aqcuisition of the image, because it's the exposure onto the film or in Red's case sensor that is important.
 
Wow,
So much information in such a short time! I'm impressed. You guys live here or what?

Cheers,
Mark.
 
Yes, the T-stop is the actual light going threw the lens, where the f-stops is the theoritical light passing threw the lens, if you want to get really anal. The T-stop takes into account that a lens (the glass component) are not perfectly transparent, thus the difference. You could say the closest the t-stop is to the f-stop, the better the lens.

Again, same thing, different scale. :beer:
 
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f stop is based on some mathmatical equation from way back when ??
length of lens ( mm) divided by diameter of lens (aperature) ..
back then if you had a 1 inch long lens ( i think 1 inch is 50mm) and if the diameter is 1 inch = f1 ...
1 inch lens ( 50mm) and 1/2 inch diameter ( 25mm ) =f2 ..
1 inch lens ( 50mm) and diameter is 1/4 ( 12.5mm) = f4
glass absorbs a little light and f stop does not take that into account so the light before going into lens may not measure the same as light coming out real of lens ...
T stop measure the amount of light out the rear that hits the film plane
 
Barry_Green said:
You'd use T-stops to calculate precise exposure.

You'd use F-stops to calculate precise depth of field.
Click to expand...

Hi Barry,

That is correct, remember that DOF is calculated from the front nodal point of a lens and not the film plane. In most cases there is a minimal difference but with physically long lenses such a zoom it does make a difference.

That is why sometimes people believe that a zoom has less depth of field than a prime, what happens is the zoom shows a tighter framing. Backing off the zoom to the same image size will result in the same DOF.

DOF charts made for a precise lens are corrected, from the basic 'thin' lens DOF calculation.

Stephen
 
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