They are a common questions.

Why does HD judder so much more than film?

And why does HD appear to judder more when viewed on on a crt monitor than when recorded to film and viewed at the cinema? How does progressive scan help a video camera picture look like film?

First lets look at the history of cinema to establish why we have arrived at the frame rates and perception of flicker in the cinema that we have today.

Initially the frame rate of motion picture cameras and projectors was around 15 frames per second, this was increased to 24fps when talkies came along. The number of frames per second needed to capture fluid movement is one question, the number of frames needed to display it another.

A film projector must have a blanking phase to allow each frame of film to be transported into the gate. Then the stationary frame is projected. Our eyes are very sensitive to flicker, especially in a dark environment so 24 black flashes per second is very disturbing.

The solution to reducing the duration of the black flashes is to show the same stationary frame two times within 1/24th of a second. So instead of one long black flash between frames we get two short black flashes between frames. This makes the black flashes less objectionable. (In the days of 15 frames per second projection a three bladed rotary shutter was used to create 45 images per second)

So today at the cinema we are seeing the same frame flashed twice, creating a 48 image per second display. (some projectors have three bladed shutters but most are two) For a century both the industry and audience standard for exposure duration has been 1/48th sec per frame (for features shot at 24fps). This shutter speed combined with the frame rate has a characteristic sharpness and judder that is in my view the strongest characteristic of what we call the film look.

So is 24 frames per second OK for actually capturing the motion on set? Actually it isn't ideal, it really is low horsepower. It is not taxed by talking heads and wide shots with not much movement. but when the subject starts to move the cinematographer has to be very aware of the limits the subject can move between frames. If the subject moves too much then they will appear to judder.

Page one of "how to be a cinematographer" circa 1890s is that a "subject must remain stationary in frame or move slowly through frame"!Feature films spend a good deal of time and money making sure that the subject does not move through the frame too quickly. Cranes and dollies help the camera track a subject through a set, but do it so that the subject itself does not move through frame too quickly. Extraordinary devices have been created to accommodate the relatively low frame rate of film.

However look at the background on such a crane or dolly move and if it is in focus enough it will judder. If the audience around you is looking at a juddery background and not the subject then it is a poorly composed or framed shot. But the idea is to look at the subject not the background.

The sharper the subject is the more the perception of judder. One of the tools the cinematographer has on 35mm is a narrow depth of field that pulls the background out of focus. A subject can be running down a corridor being tracked by steadycam, so it is not moving within the frame too much yet the fast moving background will not judder as it out of focus.

This brings us to the main reason HD appears to judder more than 35mm film. Depth of field. Current HD cameras have more than enough, a law of optics relating to the size of their imagers. Larger imager digital cameras will solve this issue.

So the background subject should be carefully considered in planning a shot on HD. High contrast sharp verticals, such as white door frames against dark walls should be avoided on HD in dramas if they can't be thrown sufficiently out of focus. Reducing the contrast of verticals in frame is a smart move.

Older dramas shot on 16mm these days have a similar HD characteristic due to similarity of depth of field and background judder. At the time these dramas were shot it wasn't objectionable, but now it is, to industry professionals at least as we are more attuned to it.

A reason new directors feel that the judder of HD is unacceptable is related to the working environment.

When working with HD we see a live progressive picture from the camera whilst a shot it is being set up, re framed or focus checked. This involves a lot

of swishing and panning around. So the director sees lots of judder during this phase of the shoot and can become overly sensitive to it. Often to the point in a rehearsal of looking at the background juddering rather than the subject!

Also the background judder can be more prominent when watching on a small location monitor from a distance than on a larger screen. The dark black border of the HD monitor forms a strong contrast if the background subject is bright. Also due to the size of the monitor the viewer may not be fully engaged with the subject by not being close enough to the screen.

Whereas on film only the good moves make it to the cinema screen, we don't see the shot being setup, even on film dailies we only see good takes and usually a well composed well blocked move. So it is not surprising a director new to HD can become unnerved as can on their first HD shoot. If the DP rolled a film camera while it was being setup the dailies would also judder!

A single frame of progressive HD at 1/48th second when transmitted on TV is no different in its motion capture of a moving subject than 35mm film at 1/48th second. The only slight technical difference is that in a film camera the image is wiped across the frame by a rotary shutter whereas with HD it is "exposed" from top to bottom.

However despite numerous tests there is no evidence to suggest that this difference in direction of image falling across focal plane creates any difference in judder or motion portrayal.

In respect of comparing HD judder on a crt monitor vs the same images transferred to film and viewed in a cinema, from my experience the crt creates a only very slightly more juddery picture than the same images transferred and viewed at the cinema. If it strobes (so you notice) in the edit it will also strobe in the cinema.

Film can also be made to create juddery images but operators shooting film have experience with shot sizes and camera moves that take account of the effect of movement across frame. There are even tables that chart panning speeds and focal lengths.

Video operators with no experience of shooting either progressive HD or film are liable to pan too fast until they have learnt the ropes. The greater depth of field of HD which creates sharper backgrounds is enough to unnerve even the most experienced (film) operator who may complain that HD strobes more than film. A film camera would also judder if it had the same depth of field as HD.

As with many craft skills where technicians have learnt from experience, they do not always have the background to immediately understand something new.

It may be a surprise to some that very fast "swish" pans are no problem with HD in progressive mode. It is the intermediate speed pans that are a problem, a subject moving across a stationary frame in less than 5 to 6 seconds will judder, if shot on film or HD.

It is interesting that on examination of an interlace video image pan speed of say 4 seconds they are also unsatisfactory. You can't actually see much detail in the frame as it is blurred.

The fast movement however is less disturbing in interlace mode than the same ill timed pan in progressive mode where the image will judder.Blur is the lesser of two evils. Interlace does not judder it blurs.

My point is that fast pans in interlace mode offer nothing for the viewer during the pan, but do allow the camera operator to get away with an awkwardly timed move, say between two subjects across a table in a documentary.

But a fast swish pan will work on HD, as it does on film. It has to be fast that the background is blurred, say 2 seconds.

Moral of the story is that progressive can be used for documentaries where fast camera pans are required, just make sure that they are fast!

Is this the end of the story? Not quite, HD has another surprise when it comes to motion portrayal that is new and where we cannot draw on the film experience for guidance. HD cameras and now many standard definition cameras have the ability to make an exposure of 1/24th or 1/25th of a second duration yet still record 24 or 25 frames per second.

Film cameras cannot do this as they need time for the film transport mechanism to move the film. As mentioned previously 1/48th second exposure often referred to as 180 degree shutter is the industry standard for film and progressive HD. But on most HD cameras the shutter can be turned off enabling an exposure of 1/24th of a second. There is also an intermediate speed of 1/32 at 24fps and 1/33 at 25fps.

The effect of turning the shutter off is that any motion across frame becomes more blurred than if it were captured at 1/48th sec. The common description of this is "smear". It is a different look from the live interlace look of TV but not as juddery as the film look, perhaps somewhere in between.

You could say that smear is the opposite of judder. Only digital cameras can do it at 24fps.

Michael Mann's use of 1/24th second exposure, first in the night scenes of the movie "Ali" and now in "Collateral" has challenged the notion in Hollywood that an image must have a certain sharpness and judder when it comes to motion portrayal.

Unfortunately the use of 1/24th sec exposure is often seen by some as being "video like" and an integral part of shooting digital.

This is not true!

It should be regarded as a new and separate look. Having said that the slower shutter speed was apparently not chosen for Collateral for artistic reasons per se, they needed all the help they could get shooting under the low light conditions. Turning the shutter off to 1/24th is a very useful one stop gain.

The interesting point is that shooting at 1/33rd or 1/24th sec shutter does not seem to have had a huge effect on the audience, although there has been little study of this, but to my eye it looks very "videoish".

Many operators and DPs new to HD make the mistake of choosing 1/33rd or 1/25th of a second to create smear and help reduce judder of overly fast pans. It does do this but at the expense of creating a smeary look for every scene.

One DP new to HD became so fixated he spent many days trying to prove that HD juddered more than 35mm film. He shot a subject holding white cards moving against black backgrounds. Unfortunately he did not shoot 35mm along side as a comparison. Despite all the advice in the world they went ahead and shot the movie with the shutter at 1/33. When it was transferred to film the motion looked smeary.

In my view the judder effect of progressive capture at 1/48thsec shutter is the single most obvious technical factor for an audience that distinguishes film from interlace video. To put a figure on it I'd say 85% of the difference between film and video is bridged by using progressive capture.

In recent years the concept of persistence of vision (an imprecise term in itself) and "after image" to explain how a series of still images can create an effect of motion has been debunked. Notions that the black flash at the cinema enables the mind to "fill in the gaps" and thus helps suspend belief and make for a more theatrical experience are yet to be proven after 50 years.

We probably know less about how we actually perceive motion pictures than we think we do.

This is demonstrated by the ease at which audiences have taken to viewing smeary images. A big surprise to everyone.

The cinema experience is just one form of motion picture display and who is to say that its characteristics are the ones we should ape in the new world of digital?

But there are pitfalls.

MTV and Brit awards were shot in progressive mode and transmitted as live but some viewers complained that it "didn't look real". The progressive judder is associated with movies commercials and dramas.

The interlace capture or "video look" is associated with live broadcasts and soaps.

The companies who run the soaps are aware of the sensitivity of their long established audience and shy away from progressive, where the film look perhaps creates the expectation of certain quality of performance and production value that is not a trade mark nor achievable of soaps!

Perhaps just as well for the film industry whose film look could take on the look of daytime soaps.

HD circa 2005 has three looks, "judder" "smear" and "live"

Digital 60 frames per second capture and display for the cinema and 60fps transmission is likely in the next decade and the audiences of the future will settle down to a new industry standard that will last until the next century.

This audience of the future may find the judder of 24fps movies made in the 20th century hard to bear.

Mike Brennan

For a chart detailing relationships and times of panning speeds, angle of view and degrees of pan, try

http://www.gecko-cam.com/html/Deutsch/Know-How-D/Panning-Speed-D.htm

Or look at the monitor!