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I'm aware this is flirting with video rather than photography, but the question really relates to how each world can be understood by the other.

Very briefly - I am an amateur photographer who actually works in the TV/film industry, but not in that capacity. I had a brief chat with the focus-puller on a movie the other day & we quickly realised that our conversation had a translation barrier.

I wanted to quickly understand how they were managing to get such a good picture in relatively low light. They had a long depth of field on a very large sound stage; one I'd equate to maybe f6 - f8 or more, yet they appeared to be able to pull in a lot of light for such a small apparent aperture.

The simple question, "How are you getting so much light at such an apparently small aperture?" was met with an impasse as we reached, "..but we don't use f-stops, we use t-stops" & neither of us knew how to translate that quickly & simply to the other's language, though each of us understood what the other system was in theory.

Our brief conversation ended at that point - no-one gets to hang around just idly chatting for long on that kind of movie, so we never did establish whether they were actually capable of getting a lot of light in through a large aperture whilst preserving DoF or whether they could ramp up their ISO far enough to make an amateur photographer weep.

I'm aware I'm potentially asking for guesswork at this point, but...
Could you have a lens with a very wide aperture that can still have a high DoF, or is that 'breaking' physics. Alternatively, is it possible they actually are capable of what I would consider 'far too high' ISO & still stay noiseless?

From comments - could it be simply that as they're using relatively long exposure times, 1/24s, the problem 'fixes itself'? As the conversation was interrupted, we never did get to discuss that as an option.

For the sake of argument, let's consider this to be an unlimited budget question.

I did later get to see the lens box for one of the cameras, a range of 8 primes from 24mm 2.2 to 135mm 2.4.

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    Moving images use the same technique for shooting in low light at narrow apertures that still photography uses. Slow shutter speeds (e.g. 1/30 or 1/24). The difference is in the way the human visual system process moving images versus still images makes motion blur less an issue in moving images. – user50888 Jul 29 '17 at 13:52
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    @Myridium You can have a lot of depth of field (with some compromises) at any aperture. Look up "hyperfocal distance" and/or read about DOF at dofmaster.com. Note that "in focus" in this context means "relatively in focus". You won't get optimal sharpness for the lens, and generally DOF is based on a handheld 8"x10" print, so requiring more detail will mean you effectively get less DOF. – StephenG Jul 29 '17 at 21:31
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    @StephenG - I don't think that's in the spirit of what the OP was asking. Of course you can focus at/near infinity and get infinite depth of field. My comment meant, all other factors considered, a wider aperture will result in a smaller depth of field. This is a physical principle you can't cheat. – Myridium Jul 29 '17 at 23:56
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    Possible duplicate of What is T-number / T-stop? – Myridium Jul 30 '17 at 1:22
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    @dannemp: the human vision system does not run at any particular frame rate. Humans are able to discern events faster than 1/1000th of a second, e.g. a camera flash. – whatsisname Jul 31 '17 at 1:05
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There's nothing here to grasp. T-stop is the same thing as f-stop, except the manufacturer or a third party has physically tested the lens to find how much light it transmits, rather than making a theoretical calculation based on the size of the exit pupil, which is where the f-stop measurements come from.

When a manufacturer specifies T-stops on their lens, they are making a guarantee that the lens has been physically measured and confirmed to allow the specified amount of light through to the back of the lens. The real measurement of light transmission will always be lower than the theoretical one, because some light is absorbed by the body of the lens, or otherwise lost during its journey. Consequently, at a given f-stop on a lens, the T-stop will be higher. This affects exposure, but not depth of field.

In videography, it's more important to make sure exposure is consistent than it is in photography, because editing film after the fact is time-consuming and expensive. Different shots from the same scene may be taken on different days and under different lighting conditions, and even with different lenses. Measuring the real T-stop value of lenses allows you to interchange them without unexpected under- or overexposure.

If your question was "How are you getting so much light at such an apparently small aperture?" and the response was "..but we don't use f-stops, we use t-stops.", then what you received was a non-sequitur. They use the same exposure triangle as photographers, and so would have to be using a combination of slow shutter speed and high ISO. As pointed out by some other answers/comments, videography can get away with slower shutter speeds and higher ISOs than photography.

Depth of field is determined entirely by f-stop and magnification of the subject at the focal plane (on the sensor). To increase depth of field, you either have to:

  • Decrease magnification - i.e. use a smaller sensor with pixels more densely packed or reduce the size of the subject in the frame, which is not a compromise that can be made while filming or
  • Decrease the size of the exit pupil (increase the f-stop).

If a good depth of field is critical, then you want a well-designed lens where the T-stop is not too much higher than the f-stop. If it is a lot higher, then you have to compensate by decreasing the f-stop and the depth of field with it.

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  • Re: non-sequitur - I didn't include the entire conversation... of course. – Tetsujin Jul 30 '17 at 6:25
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    One easily observable example of f-stop and t-stop difference is catadioptric lenses. The front mirror blocks light from entering. – user50888 Jul 30 '17 at 15:20
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Speaking about noise.

Noise is random, so when taking a picture with high iso you get some random noise that lowers the "quality" of the shot. While filming the noise in each shot is different, you get some pixels of noise in one shot of the video and then in the next one the noise is on other pixels. The overall effect is that you perceive less noise than in a photo.

This has also to do with the fact that you have a longer time to observe a photo than each shot of a video.

So in the end you can use higher iso while filming than while taking photos.

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  • I imagine this also allows post-processing to remove a great deal of noise when the frame is still. – Myridium Jul 29 '17 at 16:08
  • Yep, definitely as this allows to filter the noise from the still parts by comparing the noise profile to the correct values – dannemp Jul 29 '17 at 16:27
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The f-stop is calculated by dividing the focal length by the working diameter of the lens. As an example: The focal length is 200mm and the working diameter is 25mm; then 200 divided by 25 = 8 thus the f-number of this combination is written as f/8. The f/ is an abbreviation for "focal ratio". The f-number (focal ratio) is universal; meaning any lens operating at the same f-number will deliver the same light energy thus the same level of exposure. In other words a lens with a focal length of 800mm and a working diameter of 100mm is an f/8 lens and passes to film or digital sensor the same amount of light energy.

That being said, lenses consist of multiple glass surfaces. Each surface is polished and thus some light is lost due to reflection from each polished surface. Additionally the glass is not perfectly transparent and this adds to the light loss. Thus two different lenses operating at the same f-number will display slight differences in the amount of light passed.

In cinematography, lenses are being interchanged; thus the need to keep the exposure correct is more stringent than with still photography. The "T" stop (transmission stop) is further refined for accuracy, the detent (click stop) on the lens barrel is calibrated using a light measuring instrument. Thus the "T" has higher accuracy. This is needed for better uniformity scene to scene.

Because cinematography deals with moving images, the criteria for depth-of-field and for sharpness is more relaxed that for still photography. No break with physics but the viewing conditions for motion picture are less stringent. Still photography depth-of-field is based on a viewing distance same as reading distance. Cinematography viewers usually migrate to a distance about equal to the diagonal measure of the projected image (corner to corner measure).

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  • So in short, f-stop is a measurement of dimensions whereas T-stop is a direct measurement of light transmission? – Myridium Jul 29 '17 at 16:06
  • @ Myridium - Both have the same roots, the T-stop is further refined by light measurement. – Alan Marcus Jul 30 '17 at 5:39
  • This answer is self-contradictory, "any lens...same f/#... same exposure" "two different lenses... different f/#... differences in amount of light passed." – Brandon Dube Jul 31 '17 at 1:06
  • @ Brandon Dube -- The f-number accuracy is within the latitude of negative films. Positive films slide films are more stringent. The "T-stop" is mainly a Cine thing to preserve the uniformity. Likely modern electronic Cine cameras receive little benefit from "T-stops". – Alan Marcus Jul 31 '17 at 2:51
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The F/# is a geometrical property. It describes, in a way, the apex angle of the cone of light that forms an image of a point. To a first approximation, this also determines the exposure since you are collapsing light from some area onto a point.

However, because both reflection (mirrors) and transmission (lenses) are imperfect, some light is absorbed, scattered, or reflected. The T/# is defined as:

T/# = (F/#) / (sqrt(transmisssion))

Since transmission is always < 1.0, this constrains the T/# to be larger than the f/#, and very close to it if the transmission is ~100%. For modern lenses, the transmission will be in the 90% or higher range, so there is often only a small gap between the T/# and the f/#. I believe in that case, the accuracy of the aperture ring (if manual) or diaphragm motor (if electronic) is more significant than the transmission of the lens.

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Just to add to the other answers, I want to address:

I wanted to quickly understand how they were managing to get such a good picture in relatively low light. They had a long depth of field on a very large sound stage; one I'd equate to maybe f6 - f8 or more, yet they appeared to be able to pull in a lot of light for such a small apparent aperture.

The actual scene, in real life, might not actually be all that low light. When you see behind-the-scenes TV recordings showing the filming process, the sets are usually pretty brightly lit, even a 'candle light dinner' type scene.

In many still-photography situations, the photographer usually has limited control of the scene lighting, and is able to adjust the shutter speed to achieve desired aperture. In filmmaking, the framerate is fixed, so instead to achieve the target exposure, they must control the lights. If you want to shoot a romantic chat by the fireplace at f32, you bathe the set in light so you get the right exposure, and the actors do their job and pretend there isn't 3000 watts of light bearing down on them.

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  • Yeah... but no - I've spent days on the set... it's dimly lit. It does have hotspots but the general lighting level is quite low. I've come to the conclusion I simply wasn't allowing for their 'long' [relative to a stills camera] exposure times. I won't see the same focus puller until Saturday, as he's on the 2nd unit, but I'll check with him then. – Tetsujin Jul 31 '17 at 19:18
  • 2nd unit postponed til next week - we're currently on setup BK [yes, really] ...with at least a day to go on this scene - before I can catch him again to clarify. – Tetsujin Aug 3 '17 at 19:16
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    Parts of the recent BBC series Wolf Hall were actually shot by candlelight because the technology now allows it. – Peter Taylor Aug 6 '17 at 7:55

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