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Is a 5.6 a 5.6 on every lense?


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#1 Phil Gerke

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Posted 25 February 2008 - 09:00 PM

Perhaps this is very basic, but my education on lenses is pretty much DIY and experiance, both mine and others.

I was speaking with a DP friend today and we were on the topic of lens speed etc... It came up that his understanding was this; if you had a 50mm 1.4 lens verses say an 85mm 2.8 and you stopped them both down to 5.6 the 50 would still be faster and allow more light, though he never said how much. This does not make sense to me at all. How could you ever get consistent results? My understanding is that a lenses maximum aperture is just simply that, its maximum. It does not allow any more light, but that does not affect how much light is being let in at other apertures. F stops are known values right? How could a meter work otherwise?

This was brought up on account of a shoot we are doing with a Brevis 35 adapter and EX1 and trying to determine what stop we are actually lighting for, or what the ISO is. T stops were brought up and it was clear that I am not all together clear on the difference between F stop and T stop. Can someone please help?

Thanks so much!

Phil

Edited by Phil Gerke, 25 February 2008 - 09:01 PM.

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#2 Adrian Sierkowski

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Posted 25 February 2008 - 09:13 PM

F stop is just an equation relating to the aperture opening. There's math for it, which i forget. Whereas a T stop is the True amount of light going through the lens, i.e. it's adjusted for the reflection/refraction and general loss of light through the myriad glass elements in a lens.

If, for example, your 50mm and your 85mm were both measured in F stops, and assuming the 85 has more glass elements in it, with the same transmission characteristics of the 50mm, then a F5.6 on each would be different, whereas a T 5.6 would be the same. At least as far as I know.
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#3 Phil Gerke

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Posted 25 February 2008 - 11:26 PM

F stop is just an equation relating to the aperture opening. There's math for it, which i forget. Whereas a T stop is the True amount of light going through the lens, i.e. it's adjusted for the reflection/refraction and general loss of light through the myriad glass elements in a lens.

If, for example, your 50mm and your 85mm were both measured in F stops, and assuming the 85 has more glass elements in it, with the same transmission characteristics of the 50mm, then a F5.6 on each would be different, whereas a T 5.6 would be the same. At least as far as I know.


Ok, that makes sense, but how do you know how much light is lost if the lens is measured in F stops? If I have a lens and the actual transmission is not the stated F stop how do I know how to light and expose for it? Cine lenses are rated in T stops correct? Thats the problem, we doing something with an adapter and will be using still Nikon lenses. However I believe a PL mount adapter and some Zeiss is in talk....
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#4 Scott Fritzshall

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Posted 25 February 2008 - 11:51 PM

Keep in mind that the differences between F-stops and T-stops on most lenses are very small; fractions of a stop. If you've got a zoom lens or you happen to know that your lens has a ton of elements in it, then you might compensate by opening it up a hair over your desired stop. Otherwise you should be able to just go with the F-stops as marked without any problem.

And yes, an f5.6 on any lens is the same as an f5.6 on any other lens, minor light loss due to lens elements notwithstanding.

Edited by Scott Fritzshall, 25 February 2008 - 11:53 PM.

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#5 Adrian Sierkowski

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Posted 26 February 2008 - 12:13 AM

Yeah, I mean effectively it's a non-issue, especially if you're on video. Just use your zebras as you normally would on the camera as a video camera doesn't really have an "asa," such as a film stock does. Or so i'm told.
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#6 Chris Keth

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Posted 26 February 2008 - 12:20 AM

Because f-stop is a proportional measurement (of effective aperture diameter to focal length), it scales to any lens you can rent, buy, build, or imagine.

Think of aperture in these very simplistic terms (which you can actually test): a tube with a soft, even light source at one end. If you have a 1 inch diameter (aperture) tube 1 inch long (focal length), it will allow a certain intensity of light to pass fully through the tube. Now imagine a tube 5 inches long, or 5 times the focal length. To allow the same intensity of light through the tube, it would have to be 5 inches in diameter to keep the same proportion of focal length to aperture.
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#7 David Mullen ASC

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Posted 26 February 2008 - 01:37 AM

While there is a slight difference between lenses in terms of f-stops due to transmission loss, there shouldn't be a difference in terms of T-stops. But a lens' maximum aperture doesn't affect its light gathering capability when stopped down. If you had a T/1.4 50mm lens and a T/2.8 100mm lens, and your meter says "f/5.6" then that's what you set the T-stop of either lens. Otherwise, you'd have to input a lot more data into your light meter...

It's like asking if one car can go to 150 mph max and another car can go to 100 mph max, when both cars are driving at 50 mph... then is the car that can get up to 150 mps max actually going faster? No, it's going 50 mph. So a lens that can open up at T/1.4 and another that can open up to T/2.8... does it make sense that you'd get more exposure on the T/1.4 lens if both lenses are set to T/5.6? No, because T/5.6 is the exposure in both cases.

Now obviously if your meter said you needed a f/1.4 or f/2.0, you can get more exposure from a T/1.4 lens than a T/2.8 lens, because you can't get any bigger an aperture than T/2.8 on one of the two lenses.

If you're using still camera lenses with f-stops on a video camera, it would be easy to see the difference in transmission between the lenses set to the same f-stop. For example, set each lens to f/4 and put them on the camera attached to a waveform, pointed at an 18% grey card, and see what the differences are in actual exposure as you swap lenses. But of course this brings up the question of why do you need to meter a scene when you're shooting video and have access to things like zebras or even waveforms, etc.
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#8 Phil Gerke

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Posted 26 February 2008 - 02:00 AM

You guys rock, very helpful. T and F stops are clear now, what you all have said brought all my other "knowledge" into perspective.

So from lens to lens a 5.6 will always equal a 5.6 not because of physical size but because the ratio of focal length to diameter is the same, therefore the amount of light transmitted is the same from lens to lens. (ignoring light loss due to elements of course) Thats what the paper towel role is illustrating? So this is why T stops are more specific because its a fixed value, an actual amount of light, assuming equal ISO etc... its not a ratio.

Thanks!
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#9 Phil Gerke

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Posted 26 February 2008 - 02:13 AM

Yes David! Thank you. That is exactly what I thought. I think we may do exactly that with the still lenses, testing and seeing what kind of differences we are getting with some of the longer lenses. Granted most of the final lighting will be with the aid of a monitor but I'll be gaffing and will be working without a monitor much of the time and to be honest I would rather know my values as exact as I can.

Thanks again. This has been great. Its nice feeling reinforced in what you already knew, and yet still expanding it all at the same time.
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#10 Jase Ryan

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Posted 26 February 2008 - 03:04 AM

But of course this brings up the question of why do you need to meter a scene when you're shooting video and have access to things like zebras or even waveforms, etc.


I have a question based on this sentence. David, are you saying if you were shooting a HD project you wouldn't bother metering? I started shooting digital, then shot a few things on film and now go back and forth. I don't like looking at the monitor while I set up too much. Mostly as reference to the frame. But I find using my meter more accurate and consistent from shot to shot within a scene. Otherwise, my values and ratios from the master to the close ups could be all off. Are you saying you wouldn't use a meter to check if you're over or under exposing? But you'd still use it for ratios and consistency?
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#11 Scott Fritzshall

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Posted 26 February 2008 - 03:45 AM

You guys rock, very helpful. T and F stops are clear now, what you all have said brought all my other "knowledge" into perspective.

So from lens to lens a 5.6 will always equal a 5.6 not because of physical size but because the ratio of focal length to diameter is the same, therefore the amount of light transmitted is the same from lens to lens. (ignoring light loss due to elements of course) Thats what the paper towel role is illustrating? So this is why T stops are more specific because its a fixed value, an actual amount of light, assuming equal ISO etc... its not a ratio.

Thanks!

Both F and T stops are actual amounts of light. ISO doesn't come into play at all.

Maybe this will help clear it up for you:
F-stop= (Focal length/aperture)
T-stop= (Focal length/aperture) - (light lost due to elements)

A t-stop is just slightly more specific.
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#12 Sam Wells

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Posted 26 February 2008 - 12:13 PM

So this is why T stops are more specific because its a fixed value, an actual amount of light, assuming equal ISO etc... its not a ratio.

Thanks!


It's not a 'fixed ratio' in the sense that the difference between F stop and T stop will diminish as a lens is stopped down (less air to glass surface area being used to form the image)

-Sam
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#13 David Mullen ASC

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Posted 26 February 2008 - 07:21 PM

I have a question based on this sentence. David, are you saying if you were shooting a HD project you wouldn't bother metering? I started shooting digital, then shot a few things on film and now go back and forth. I don't like looking at the monitor while I set up too much. Mostly as reference to the frame. But I find using my meter more accurate and consistent from shot to shot within a scene. Otherwise, my values and ratios from the master to the close ups could be all off. Are you saying you wouldn't use a meter to check if you're over or under exposing? But you'd still use it for ratios and consistency?


Video's great advantage is that you can see the image being recorded live, so I don't understand why you wouldn't take advantage of that.

No, I don't use a light meter when shooting HD. Why should I be trying to imagine if this face should be one-stop underexposed in this kind of light or one and a half stops, when I can see on a properly set-up and tented professional monitor how dark to make the face? And I can use zebras as a reference guide too.

A video camera is essentially a spot meter.

I remember Bob Prime's saying "A meter makes you a coward; your eyes make you brave." I think the whole goal of cinematography is to minimize the use of meters, unless you are shooting something very technical and need careful notes for matching, pick-up shots, etc.

What I tend to do in HD is set the lens to the f-stop I want to shoot the scene at, then try and light to that level, then at the last minute, adjust the f-stop for the final lighting. I'll use a meter only if the camera is not set-up or if I have a bunch of spots in a row that need to match each other, so sometimes I use a meter for some types of balancing and matching, but not for final exposure.
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