T-stops, shutter speed, video (originally posted in Panasonic is Developing a New, Smaller 35-100mm

[bikerhiker]

Note from Amin: These posts were originally posted in this thread and moved here to their own thread.


The speed of the lens is dictated by the circumference of the aperture opening based on the f-stop formula. The purpose of the f number is to show the size of the opening of whole of the lens so to speak to allow light to expose the sensor to form an image. It is this opening that dictates how much light passes through. Which is why f/stop is so important in photography. But there seemed to be a drive to go with T-stops even with professional photographers and that's where the confusion comes in. Somehow you can bend physics by just magically raise or lower T-stops, but reality only dictates that the physical opening (the size of hole of the lens and the length of the lens) dictates the amount of light that hits the sensor.

For example. Let's take a close look at 2 lenses. 35mm @ f/2.8 and 200mm @ f/2.8. The 35mm @ f/2.8 has an aperture opening of 12.5mm (35mm divided by 2.8), whereas the 200mm @ f/2.8 has an aperture opening of 71.42mm. But then you wonder, how the heck you say 200mm @ f/2.8 allows the same amount of light as 35mm @ f/2.8 when you have a bigger opening with the 200mm lens?!? 71.42mm hole vs 12.5mm hole. If you look closely, you will notice that the 200mm lens is LONGER than a 35mm and this length acts like a long extension tube which reduces light, so the amount of light that 200mm lens lets through will eventually be the same as the amount of light as a 35mm lens of the same aperture opening. The T-stops rating basically identify the "APERTURE" blades efficiency by how accurately it stops down the lens to that specific aperture. The confusion comes, in which I'm noticing at lot in those so called marketing guys standing around in trade shows, that they use T-stops as though it replaces f/stops. And with this confusion, they don't realize why the lens is designed this way and why is it so big. There's no confusion if a guy actually knows what he is talking about. This stuff is even taught at least in photography classes 2 decades ago. It's common knowledge, it is immediate that the opening of the lens and length of the extension of the tube do not mirror a constant f/2.8.

Having said that and just by looking at the lens itself, this is more likely a 35-100 f/3.5 to 5.6 or f/4 to f/5.6 simply by doing the f/stop formula calculation and by knowing Panasonic has the 12-32 pancake version. The openings of 35 @ f/4 and 100 @ f/5.6 requires only a short extension tube like the photo illustrated. Another possibility is a f/2.8 to f/4 lens, but the length of the lens is too short -- hint hint take a look at the Olympus most hated 12-50 to give you an idea about lens length.

 

[Cruzan80]

Not sure what you are going on about. The T-stop is the amount of light hitting the sensor, and will always be greater than the equivalent f stop (no 100% transmission). They can replace fstops (look at the video industry, where you need to match exposures more closely using various lenses). Length of a lens can vary, look at mirror lenses for a long focal length for short body. Or it isnt internal zoom, like the 12-32. Looking at it, they could easily fit a 36mm opening in that size (100/2.8). Just becaise it is unlikely doesn't mean it isnt possible.

Sent from my LG-P769 using Mu-43 mobile app

 

[speedandstyle]

http://www.43rumors.com/panasonic-says-new-35-100mm-compact-zoom-will-have-constant-f2-8-aperture/

It is not an f2.8.

As for t-stops - what?? t-stops are the measured amount of light through a given lens and f-stops are measured through a mathematical ratio of focal length and aperture diameter. t-stop have nothing to do with the efficiency of an iris but rather the inefficiency of the glass elements. t-stops are also only useful for movie/video use. The f-stop difference between two lenses is usually only a 1/2 or maybe 2/3 EV. For movie production this makes it hard to splice two pieces from the same scene that were shot with two different lenses, noticeable difference in exposure. Thus they started measuring the actual light of the lenses and called them t-stops{t stands for transmission}. For photography t-stops are almost meaningless!

BTW here is a thread I started on the difference between t-stops and f-stops.
https://www.mu-43.com/showthread.php?t=60244

 

[bikerhiker]

Not sure what you are going on about. The T-stop is the amount of light hitting the sensor, and will always be greater than the equivalent f stop (no 100% transmission). They can replace fstops (look at the video industry, where you need to match exposures more closely using various lenses). Length of a lens can vary, look at mirror lenses for a long focal length for short body. Or it isnt internal zoom, like the 12-32. Looking at it, they could easily fit a 36mm opening in that size (100/2.8). Just becaise it is unlikely doesn't mean it isnt possible.

Sent from my LG-P769 using Mu-43 mobile app

Going with Panasonic is very video focused company and that they are not a camera company until recently. My take is that, because there's a marriage between video and stills that a video centric representative may have implied that this new lens can have the same light transmission capabilities as the 35-100 f/2.8 and that he may confused the answers. Assuming he really knows what he's talking about. I've met people in trade shows that do express lenses in T-stops because they are assuming you are using a DSLR capable video (my D800) for video production purposes. Mirrorless lenses have a shorter lens flange than non-mirrorless. This has been discussed at CP+ by Sigma and Canon.

You can bend physics sure, but at a higher price and letting the camera does all of its automated optical corrections -- that's something Panasonic and recently Olympus has started to do. I'm not sure this is what Panasonic is targetting its market consumer for a GM1 telephone lens. A $2000 lens? Nah. f/stops is intrinsic in the service industry for repairs. Replacement overnight? Nah..

 

[bikerhiker]

http://www.43rumors.com/panasonic-says-new-35-100mm-compact-zoom-will-have-constant-f2-8-aperture/

It is not an f2.8.

As for t-stops - what?? t-stops are the measured amount of light through a given lens and f-stops are measured through a mathematical ratio of focal length and aperture diameter. t-stop have nothing to do with the efficiency of an iris but rather the inefficiency of the glass elements. t-stops are also only useful for movie/video use. The f-stop difference between two lenses is usually only a 1/2 or maybe 2/3 EV. For movie production this makes it hard to splice two pieces from the same scene that were shot with two different lenses, noticeable difference in exposure. Thus they started measuring the actual light of the lenses and called them t-stops{t stands for transmission}. For photography t-stops are almost meaningless!

BTW here is a thread I started on the difference between t-stops and f-stops.
https://www.mu-43.com/showthread.php?t=60244

f/stop basically determines the amount of light transmitted by the lens based on a formula f number divided by aperture. The variation you talked about in regards to noticeable difference in exposure between 2 lenses of the same f/stop is due to
1, Different lens design
2, Varying focus
3, The use of accessories

Can all effect the actual amount of light projected. This variation of difference in exposure is automatically compensated by through the lens metering (TTL) in the camera.

T-stop or T-number is a f/number of a lens corrected for the light loss during transmission through the lens. T-stop is extremely important for film and movie making because the light loss of older lens design can significantly impact the depth of field needed to be used and the number is an accurate transmission reference rather than the stated theoretical max.

You loose about 1% per air to glass surface of a well corrected lens with coating, so say a 6 element prime lens (2 surfaces) loose approximately 8-12% of light. Now contrast that to a zoom lens with say 20 elements (2 surfaces) will loose approximately 40% to 50% light, but with excellent coating and element bonding might get you down to maybe 35%.

So a prime lens set to T/4 will be about f/3.8 to f/3.6 and a zoom lens set to T/4 will be about f/3.2 to about 2.9. The background will be a little bit sharper with a prime lens at the same T stop assuming both lenses are equal in optical sharpness. T-stop number is always slower than the f/stop number. So in this case, EVEN if you have 2 lenses set to the SAME T-STOP, the background sharpness due to depth of field will be different. Both T-stop and f/stop use the same exposure scale.

So why then, the film and movie industry rely on T-stop? The answer I was given was because TTL metering was not as commonly used compared to a film still camera.

 

[speedandstyle]

t-stops were invented long before any TTL metering existed, so your last line is not totally correct. Although you are right that on a movie/video set you use metering from hand meters rather than TTL. This is so that the most accurate metering is done.

Different lens designs can affect the transmission of light and yes air between elements can too. Not sure what you mean by #2 varying focus or #3 accessories. Yes the DOF of two lenses set at the same t-stop could and most likely would be different. However matching DOF is irrelevant and all but impossible once you start using different lenses. Or even the same lens at different distances to the subject. So I am confused why you even mention DOF.

 

[OzRay]

The easiest way to determine whether f stops are the same across lenses is to set up a white background, put a light on it, take an exposure reading and then take manual shots at the same f stop for each lens. Then compare the results and see if each lens transmits the same amount of light ie the same tone for each lens (is the reading the same between 1-255?). My understanding was that when cine lenses, regardless of focal length, were set to the same t stop for a given scene (such as the white backdrop) the results would be the same.

 

[bikerhiker]

t-stops were invented long before any TTL metering existed, so your last line is not totally correct. Although you are right that on a movie/video set you use metering from hand meters rather than TTL. This is so that the most accurate metering is done.

Different lens designs can affect the transmission of light and yes air between elements can too. Not sure what you mean by #2 varying focus or #3 accessories. Yes the DOF of two lenses set at the same t-stop could and most likely would be different. However matching DOF is irrelevant and all but impossible once you start using different lenses. Or even the same lens at different distances to the subject. So I am confused why you even mention DOF.

To explain #2. Some consumer lenses focus by moving the focusing element in and out outside of the lens body itself and the length of the lens grows as a result. This effect is like an extension tube thus resulting in some slight light loss. But this loss is compensated by TTL metering. To explain #3. The accessories are those lens filters -- UV, Polarizer, Graduated, Neutral Density, color filters and the lot that I won't mention here. Anytime you attach a filter on to the lens, you will loose some light transmission because you are introducing a glass element in the front of the lens. If you add a Neutral Density 0.6 filter (2 stops), you will loose 2 stops of light. This is necessary in some photographic applications. But this is of no consequence, because again the TTL metering will compensate for this. How does the TTL metering compensate for this variation of light levels? By telling the camera to vary the shutter speed. So in the camera, the shutter is used to control ambient exposure as well as controlling motion blur. The shutter speed IS NOT FIXED. If you examine this issue, you will notice that it does not matter what light levels are, the metering will control the shutter speed to provide a balanced exposure assuming you lock into a specific aperture. Therefore, you can attach as much filters you want to your zoom lens without creating excessive vignetting and when you stop it down to f/8 or f/2.8, you WILL get the DOF equivalent to f/8 or f/2.8 no matter how many elements are on the lens whether they are in the lens or outside the lens. f/stop is also referred to as Aperture which is related to DOF.

BUT, when we talk about T-stops, the number of elements in different lenses (prime and zoom) with the same T-stop rating can affect the DOF as I had described in my last post.

A movie film camera creating a cinematic film look @ 24fps has a set exposure time of 1/48sec or 1/50sec for each frame. For 60fps, I believe it is 1/120sec. To control the duration of the shutter (changing the shutter speed), we vary the shutter angle in a rotary disc shutter. This variation of the shutter angle creates different shutter speeds. Shutter speeds in film and video cameras control the effect of motion blur.
Steven Spielberg's movie "Saving Private Ryan" is a fine example of a high shutter speed of 1/2000 sec to create that crisp moving motion, or they call the newsreel effect.

But a film still camera or digital still camera like our Olympus does not angle the shutter blades to create different shutter speeds and neither it sets its exposure time fixed at 1/48sec or 1/50sec for 24fps, because you are not making a movie.
With digital SLR, the movie making is via the electronic pulsing of the sensor array to mimic shutter angle and set at a fixed exposure time of 1/48 or 1/50sec, while the shutter blades stay open the whole time. There is also no rule in still photography where you must set your shutter speed to 1/50 sec or 1/120sec for every frame.

Even if your lens is loosing some light and it's not what it's rated to be.

Modern cameras has 2 ways to compensate this on top of TTL metering.

1, Shutter monitor sensor (on expensive pro bodies)
2, Sensor grid light fall off mapping (on expensive pro bodies)

The shutter monitor sensor is a special nuisance if you are doing infra-red photography, because it will show up on the sensor on long exposures as some sort of colored haze.

 

[Cruzan80]

To explain #2. Some consumer lenses focus by moving the focusing element in and out outside of the lens body itself and the length of the lens grows as a result. This effect is like an extension tube thus resulting in some slight light loss. But this loss is compensated by TTL metering.

Um, No. Really, really no. Totally wrong. An internal zoom vs a creeping zoom both focus the light at the same point, therefore are completely the same when it comes to the effect on t-stops. Extension tubes change the point where the light is gathered (the convergence point moves farther from the sensor). The glass used may change the amount of light transmitted, but moving the elements doesn't. Not sure where you keep getting these things that you preach as the way things are. The reason the movie industry uses t-stops (as has been mentioned before) is to make sure that when using multiple lenses the exposure stays the exact same. Not "really close", like you get when using f-stops, which are only a a ratio of focal length to aperture, but almost exact. The only reason DoF may change is due to the fact that the relationship between f-stops (aperture size) and t-stops (light transmission) varies form lens to lens. The number of elements may contribute to a different t-stop, or it may be the type of glass with the same number of elements in each, but not because the zoom is moving the focusing element (otherwise any zoom would move the focusing element, or it couldn't zoom!). Any two lenses that have the same focal length, and f-stop to t-stop differential will show the exact same DoF. Nothing to do with the number of elements inside the lens. Again, you are taking one aspect of a situation, and trying to make it into the "RULE" that all other things follow from. Lets say we have one cart, and can hook up different numbers of horses, (one lens, different elements inside). Not every horse (glass element) will be the same, so even if we hook up horse A&B, or horse C&D, we may get a different speed (t-stop), yet we still have the same ratio of horses to carts (f-stops). T-stops don't imply using a fixed exposure time, they simply mean how much light is actually hitting the sensor. I know some studio photographers who use T-stops to make sure they are getting the exact exposure they want.

 

[speedandstyle]

OK I now understand what you meant by #2 "focus adjustment" and I don't think it would. The amount of air would increase between the glass elements BUT it is the change between glass to air and back again that causes the slight loss and not the distance between them. The science term is refraction and air has a different refraction than glass and even the types of glass have different refractions. In fact lens designers use that in designing the lenses.

#3 "accessories" I understand this one now too BUT in rating a lens for it's t-stop you can't consider possible filters that one might use. Those filters themselves are rated and you would have to do the math when setting up your camera.

I still do not understand why you are talking about DOF - the iris size or aperture determines that along with lens design, distance to subject and sensor size. But we are talking about t-stops and how that affects exposure, or at least I am. To me DOF is not even relevant in the discussion of t-stops.

Why are you going on about shutter speeds now? I agree with you about how movie and video cameras work. I shoot video as part of my job every week! I have also worked on TV shows and low budget movies, although not on camera crew. Shutter speed/angle is not what we are talking about. The helicopter blade analogy is wrong! Rotary shutters are cut like pies{see picture and link with video below}.

Subscribe to see EXIF info for this image (if available)

http://www.surfacedstudio.com/tutorials/shutter-angle-tutorial

The angle is how much is cut out of the disc not an angle that the disc sits at. You make it sound like shutter speed/angle dictate the need of t-stops which is wrong. The very earliest cinema cameras used a rotary disc shutter but they used f-stops on the lenses for many years until they came up with the t-stop to compensate for the exposure difference between lenses.

 

[Luckypenguin]

Um, No. Really, really no. Totally wrong. An internal zoom vs a creeping zoom both focus the light at the same point, therefore are completely the same when it comes to the effect on t-stops. Extension tubes change the point where the light is gathered (the convergence point moves farther from the sensor). The glass used may change the amount of light transmitted, but moving the elements doesn't.

Possibly relevant to this, macro lenses (as they approach minimum focusing distance) magnify the image by increasing the size of their projected image circle, such that an f/2.8 lens might have the transmission closer to f/5.6 on the sensor when shot at their highest magnification. I also thought that a much less obvious but still noticeable version of this phenomena occurs when lenses change their field-of-view slightly depending on their focusing distance. This is particularly noticeable on unit-focusing lenses where focus is achieved by shifting the whole lens group forwards or backwards.

 

[bikerhiker]

Um, No. Really, really no. Totally wrong. An internal zoom vs a creeping zoom both focus the light at the same point, therefore are completely the same when it comes to the effect on t-stops. Extension tubes change the point where the light is gathered (the convergence point moves farther from the sensor). The glass used may change the amount of light transmitted, but moving the elements doesn't. Not sure where you keep getting these things that you preach as the way things are. The reason the movie industry uses t-stops (as has been mentioned before) is to make sure that when using multiple lenses the exposure stays the exact same. Not "really close", like you get when using f-stops, which are only a a ratio of focal length to aperture, but almost exact. The only reason DoF may change is due to the fact that the relationship between f-stops (aperture size) and t-stops (light transmission) varies form lens to lens. The number of elements may contribute to a different t-stop, or it may be the type of glass with the same number of elements in each, but not because the zoom is moving the focusing element (otherwise any zoom would move the focusing element, or it couldn't zoom!). Any two lenses that have the same focal length, and f-stop to t-stop differential will show the exact same DoF. Nothing to do with the number of elements inside the lens. Again, you are taking one aspect of a situation, and trying to make it into the "RULE" that all other things follow from. Lets say we have one cart, and can hook up different numbers of horses, (one lens, different elements inside). Not every horse (glass element) will be the same, so even if we hook up horse A&B, or horse C&D, we may get a different speed (t-stop), yet we still have the same ratio of horses to carts (f-stops). T-stops don't imply using a fixed exposure time, they simply mean how much light is actually hitting the sensor. I know some studio photographers who use T-stops to make sure they are getting the exact exposure they want.

You have the freedom to believe in what you want. If you want to work in T-stops with your photography, I'm not stopping you.

Cheers!

 

[bikerhiker]

OK I now understand what you meant by #2 "focus adjustment" and I don't think it would. The amount of air would increase between the glass elements BUT it is the change between glass to air and back again that causes the slight loss and not the distance between them. The science term is refraction and air has a different refraction than glass and even the types of glass have different refractions. In fact lens designers use that in designing the lenses.

#3 "accessories" I understand this one now too BUT in rating a lens for it's t-stop you can't consider possible filters that one might use. Those filters themselves are rated and you would have to do the math when setting up your camera.

I still do not understand why you are talking about DOF - the iris size or aperture determines that along with lens design, distance to subject and sensor size. But we are talking about t-stops and how that affects exposure, or at least I am. To me DOF is not even relevant in the discussion of t-stops.

Why are you going on about shutter speeds now? I agree with you about how movie and video cameras work. I shoot video as part of my job every week! I have also worked on TV shows and low budget movies, although not on camera crew. Shutter speed/angle is not what we are talking about. The helicopter blade analogy is wrong! Rotary shutters are cut like pies{see picture and link with video below}.

Subscribe to see EXIF info for this image (if available)

http://www.surfacedstudio.com/tutorials/shutter-angle-tutorial

The angle is how much is cut out of the disc not an angle that the disc sits at. You make it sound like shutter speed/angle dictate the need of t-stops which is wrong. The very earliest cinema cameras used a rotary disc shutter but they used f-stops on the lenses for many years until they came up with the t-stop to compensate for the exposure difference between lenses.

I said exposure time, not shutter speed that relates to T-stops. The exposure time of the movie film camera is always set to 1/48sec @ 24fps is it not? Or are you trying to confuse me that shutter speed sets the exposure time on a movie film camera?

 

[kwalsh]

Possibly relevant to this, macro lenses (as they approach minimum focusing distance) magnify the image by increasing the size of their projected image circle, such that an f/2.8 lens might have the transmission closer to f/5.6 on the sensor when shot at their highest magnification. I also thought that a much less obvious but still noticeable version of this phenomena occurs when lenses change their field-of-view slightly depending on their focusing distance. This is particularly noticeable on unit-focusing lenses where focus is achieved by shifting the whole lens group forwards or backwards.

Also worth noting that the bellows correction factor for exposure correction at macro distances also depends on the pupil magnification ratio of a lens. So in theory two lenses with identical focal lengths, identical apertures, identical transmission, identical field of view and identical focus distance can have slightly different exposures in the macro range because of different pupil magnification. Pupil magnification is most easily related to whether a lens design is symmetric, telephoto or retro-focus in design. Is this difference of any significance and any non-macro distance? No, not really at all. But it is important to remember that the simple model of a complicated lens as a pinhole breaks down in some situations.

 

[speedandstyle]

I said exposure time, not shutter speed that relates to T-stops. The exposure time of the movie film camera is always set to 1/48sec @ 24fps is it not? Or are you trying to confuse me that shutter speed sets the exposure time on a movie film camera?

Exposure time and shutter speed are the same thing. The length of time the sensor or film is exposed to light is determined by how fast the shutter is opening and closing. Until recently the shutter speed could not exceed the frame rate - in other words at 24 FPS the slowest shutter speed possible would have been 1/24th of a second. However modern technology allows for shutter speeds slower than the frame rate{mind blown!}. Frame rate on the other hand is how many frames per second are recorded and it has nothing to do with exposure.

You do NOT have to set the shutter speed to 2x frame rate. The reason for setting the shutter this way is that it gives a look that is about what our eyes see, normal looking motion blur. This rule isn't really a rule but rather a guide, a first step. Also it had more relevance back in the day with old film and old tube video cameras but even then it was sometimes broken. With most film cameras, 2x frame rate was the slowest shutter speed or I should say largest angle{180degrees} you could use. Here is a great video showing how you can break this rule.

[video=youtube;dpSFyAt4LWc]https://www.youtube.com/watch?v=dpSFyAt4LWc#t=258[/video]


Finally, here is a little tutorial from the RED company site talking about shutter angle.
http://www.red.com/learn/red-101/shutter-angle-tutorial

 

[bikerhiker]

Speedandstyle said the following -- Exposure time and shutter speed are the same thing. The length of time the sensor or film is exposed to light is determined by how fast the shutter is opening and closing.

This is very odd and that's not what I was taught at least when we are dealing with making movies and videos.

The shutter speed in a movie camera is known as the Frame Shutter Speed. Frame Shutter Speed controls the motion blur of the movie and is usually double that of frame rate and that's it to get natural looking motion blur. It does not control the exposure of light if you're shooting at very bright sunlight @ EV 15 (right at noon). Usually the Frame Shutter Speed rate can not change during capture dramatically due to frame rate limitation in HD-DSLR. I shoot movies with a D800 btw.
So to change exposure, I was told by the instructors @ Nikon USA that only ISO, aperture and filters can be altered to obtain a desired exposure. Are you implying that people at Nikon USA or even at Panavision are completely wrong?!?

 

[kwalsh]

So to change exposure, I was told by the instructors @ Nikon USA that only ISO, aperture and filters can be altered to obtain a desired exposure. Are you implying that people at Nikon USA or even at Panavision are completely wrong?!?

Well that is completely wrong to be sure. Whether your instructors were wrong or you misunderstood them is up for debate.

Indeed, except in the cases already described above, you usually can't push the exposure *longer* than the frame rate but you can make it much, much shorter than the frame rate. In fact your D800 has an S mode for video I believe and you should be able to set an exposure in the thousands of a second if you want. This is pretty basic stuff and has been true on video cameras for decades and film motion cameras for many more decades.

 

[bikerhiker]

Well that is completely wrong to be sure. Whether your instructors were wrong or you misunderstood them is up for debate.

Indeed, except in the cases already described above, you usually can't push the exposure *longer* than the frame rate but you can make it much, much shorter than the frame rate. In fact your D800 has an S mode for video I believe and you should be able to set an exposure in the thousands of a second if you want. This is pretty basic stuff and has been true on video cameras for decades and film motion cameras for many more decades.

Only in M (manual) mode that you have control on shutter speed and aperture with the D800; other modes seem to lock you in the frame shutter speed based on frame rate you use. The idea they convey is that, while you can make the shutter speed faster than the frame shutter speed, you will change the look of the blur. If you want consistent motion blur on your movie, then you would set the frame shutter speed constant. This practise is NO different than shooting HDR, where you would LOCK IN the aperture setting and vary shutter speed to bracket exposures. You can surely vary aperture settings and shutter speed to bracket your exposures, but the merged HDR photo won't look all that sharp; if that's the kind of look you go for. But in practise, if you want a sharp HDR photo, the bracketed photos are done via Tv (shutter speed) and not aperture. Same with movies. If you want to maintain the same consistent motion blur look on the entire movie length, wouldn't you then keep the frame shutter speed constant and change exposure elsewhere? Isn't T-stop f/stop with light transmission loss taken into account?
That's what Nikon USA was teaching. It's not a mistake. Some of this information is in the digi tutor site as well as shooting HD-SLR tutorial.

Here's the link..

http://cinema.nikonusa.com/produce-...to-consider-plus-need-to-know-pointers/#Frame Shutter Speed + Frame Rate

 

[kwalsh]

I see what you are saying, if for artistic reasons you want to constrain your shutter speed to be constant throughout your production then yes of course you have to use other exposure or lighting controls. But that is not a limitation of the cameras at all nor is it in any way related to your frame rate beyond the aforementioned restriction that typically the shutter speed must be at least faster than the frame rate. That's probably why everyone is confused with what you are saying.

And thanks for the link, that explains the 2x rule as being typically the most aesthetically pleasing.

 

[speedandstyle]

Most cameras{especially in an auto mode} will automatically set the shutter speed to 2x the frame rate but that is just to make things simple for the user. Some have limitations and that may be what Nikon was telling you about their camera - it specifically couldn't do it. At work I have 2 Panasonic AF100 pro camcorders and I can set them either by shutter speed or shutter angle{speed 1/24th - 1/250th and angle 10-360 degrees}{not all speeds/angles are available for all frame rates however}{Oh I can also set the frame rate as low as 12FPS and as high as 60FPS}.

Now I do use the 2x guideline for most things so that the motion blur looks natural. As a general rule you can't change the shutter speed/angle WHILE filming although some cameras will even allow that. So when you are filming and you need to adjust the exposure you do it with aperture and/or ISO/gain{proper term for video}. ND filters are also used but you can't switch them in the middle of filming. Many high end camcorders have ND filters built in BTW.

Here are the reasons why you would want to change the shutter speed/angle.
If you go faster than the 2x the image will be sharper with more strobe effect{think stop motion unevenness}. This look gives a retro film look as many old film cameras used shutter angles less than 180 degrees. If you go slower there will be more blur to the images and less strobe. Both are often used for dramatic effect.