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Technische Daten

Dieser Artikel ist leider nicht verfügbar auf Deutsch
January 2010

By Karel Poortman

Are you always trying to achieve that cinematic look when you shoot your videos? Then a Canon professional HDV camcorder is probably the perfect camera for you. Canon’s progressive F-mode gives you beautiful crisp frames at cinematic frame rates. In this article we examine progressive shooting versus interlaced and explain the benefits and the best ways to shoot with 25F.

Both the XH series and the XL H series of Canon camcorders allow you to shoot interlaced as well as progressive video in High Definition on tape. Their PAL versions shoot in 50i for interlaced mode and 25F for progressive modes. With the 25F system you shoot 25 full frames per second (fps).

NTSC and other frame rates

The NTSC system, which is used in the USA and Japan, uses 60i for interlaced video and 24F and 30F for progressive video. Via a service upgrade a Canon PAL camcorder can be expanded to include NTSC frame rates and, vice versa, a Canon NTSC camcorder can be expanded to include PAL frame rates. With this service upgrade your camera will be PAL-NTSC switchable: a very flexible system that supports 50i, 60i, 24F, 25F and 30F frame rates. But what are the differences between progressive and interlaced video?

A beautiful day in Amsterdam – shot in 25F

Click here to watch a film shot with the Canon XL H1S and Canon XH A1S camcorders in 25F.

Progressive versus interlaced: Display

The PAL and NTSC TV systems were originally developed as interlaced display and broadcasting systems. At the time technical and bandwidth limitations caused the image to flicker when displaying progressive full frame video. The interlaced system was a practical, technical solution for creating pictures that moved smoothly with acceptable resolution.

So, how does the interlaced system work? The image is made up out of scan lines – firstly these are broken up into odd and even lines, thereby creating the two half images called ‘fields’. This way every frame is made up out of two fields containing only half of the lines of the original frame - one field contains the odd lines, the other field the even lines.

So, instead of showing 25 full frames per second (which is what the progressive system does), the interlaced system displays 50 fields, or half frames, per second. In the PAL system the odd lines of every frame are displayed first, followed by the even lines.

Diagram showing Progressive: full frame at once versus Interlaced: with the frame split into odd and even laced fields.

Progressive v interlaced: Recording

Originally, video recording was also developed as an interlaced recording system. The same process is used to split up the image. But when it comes to recording there is a fundamental difference between a progressive 25 frames versus an interlaced 25 frames recording: refresh rate and timing.

A 50i video is recorded with a 50Hz frequency. Every 1/50th of a second, one field is recorded. So, one full frame is composed of two separate fields with a 1/50sec interval! A 25F video is being recorded with a 25Hz frequency, with one full frame every 1/25sec, capturing all scan lines at the same time. Got the idea? Take a look at illustration below to explain more...

Diagram showing the difference between Progressive (left) at one full frame per 1/25sec compared to Interlaced (right) at two separate fields at 1/50sec intervals. The two timelines - one I, and one P - illustrate the different break up of a live image.

Progressive v interlaced: the difference in 'look'

Because of this technical difference, you can imagine that 25F progressive video looks very different from 50i-interlaced video. 50i is more fluent due to higher field rates captured every 1/50th of a second. However, it is less detailed with fast moving pictures, as the fields only display half of the resolution. Because of this, interlaced images are said to have a ‘video look’. Most of us are used to this ‘look’, both from watching too many soap operas and home videos!

25F looks more crisp and sharp for movement, with full frames being captured at once, but a little less fluent running at 25Hz. When displaying fast movement, it can become jittery.

Progressive video, at 24 or 25 frames per second, is valued for its cinematic quality and can be very pleasant to the eye. It is the frame rate of motion picture storytelling; where images are much more expressive and characteristic.

When to use 25F?

The choice to shoot in 25F mode on your Canon HD camcorder may be an artistic decision, or it may be that the final medium on which you will distribute your video is progressive.

Some platforms require progressive video, like computer-based playback. If you are producing for the Internet, 25F mode will also give you he best results and the least headaches.

Since computer displays are progressive by design, and are not capable of correctly displaying interlaced video, interlaced video material needs to be de-interlaced for progressive playback. Most conversion software does this automatically when choosing a format for computer playback. When you don’t de-interlace video shot at 50i, the image may look like this (see below).

 

Example of an interlaced image displayed on a progressive screen.

For a direct transfer to film, you obviously need a progressive picture. Film runs at 24 frames, so when you shoot your original video at 25 frames you will either have to slow down your video by 5%, or shoot at 24F to get a perfect match. You might think this slowing down will be dramatic, until you realise that PAL speeds up both broadcast and DVDs by 5% from their original 24 frames to get 25.

So, nowadays most dramatic content is being shot at 24fps or 25fps, including many video clips and commercials. More and more videographers seem to have got the hang of it.

Shooting 25F: Camera handling

How do you avoid flickering or strobe-like images? If you have played around with progressive video before, you have probably wondered why those Hollywood movies, shot at only 24 frames per second look so steady and ‘peaceful’ in comparison with your own. Well, besides a considerably lower budget, that’s because progressive filming is all about camera handling.

The same handheld shot that worked fine in 50i may end up somewhat disappointing when it was made with your camera set to 25F. Due to the lower refresh rate of the image, a shaky camera style is much more unpleasant to watch in 25F than it is in 50i. So you’ll have to make an effort to make slower better-controlled camera moves and take it a little easy on the panning.

Objects that move horizontally through the frame can result in a very noticeable 'stutter' and ruin your shot. Instead, try to follow your subject while leaving the background slightly out of focus. This will create a motion blur that makes everything run smoothly.

You can also change the composition of the shot. For example, instead of having your subject move across the frame, you could have it move at an angle to the camera, say, along a roughly diagonal line.

Shooting 25F: Shutter speeds

In most situations a good shutter speed for 25F footage is 1/50sec. With that speed you get smoothly moving images, while retaining enough detail in slow motion. Using a higher shutter speed will give you lots of detail but it can also cause serious stutter and will definitely be uneasy on the eye. When capturing high-speed motion, as in sports, a smaller shutter speed will give you very crisp images, although they can look somewhat artificial. This, of course, could be exactly the effect you are going for.

A big advantage of shooting 25F is that you can go down to a shutter speed of 1/25sec, which gives you an extra f-stop of exposure. This makes 25F ideal for shooting in low light conditions. With a 1/25sec shutter speed you need to take into account that you will have a lot more motion blur with moving images. The picture looks very smooth but moving objects can lack detail.

 

The daytime image (top) shows 25F at 1/1000sec shutter speed for crisp frames of moving bicycles whilst the night shooting in 25F at 1/25sec (bottom) has lots of motion blur but the bikers on the bottom left are tricky to pick out.

So, shooting at 25F gives you:

  • Cinematic look and movement.
  • Crisp image due to full frame motion capture.
  • Frame grabs at full resolution.
  • Better low light sensitivity at 1/25sec shutter speed, gaining an extra f-stop.
  • No need for de-interlacing when shooting for a progressive medium like the Internet.

Can you get a progressive image from an interlaced chip? Yes, you can! The 25F system was initially received with some sceptism by videographers since the imaging sensors used in Canon’s professional High Definition camcorder line-up are of the interlaced type. Canon developed its own technique to acquire a very high quality progressive picture from its 3CCD system. Since Canon doesn’t reveal exactly how it does this, there has been some speculation about the system just de-interlacing a 50i stream coming from the imaging block.

When you review the image quality of 25F recordings you can easily conclude that this is not the case. The Canon XL H series and the XH series are the only HDV camcorders on the market that have the full HDV resolution of 1.67 Megapixels on their 3CCD imaging chips. By making use of a very sophisticated patented pixel shift system, and some modifications on their CCD chips, a very high quality progressive readout has been made possible - this results in a beautiful and sharp progressive image.

Playback and capture

The Canon 25F recording system uses its own unique way of recording a progressive image. Due to this alternate system you need a Canon playback device to playback and capture your 25F recordings.

The following Canon camcorder models support 25F:

Editing

Canon 25F footage can be handled by most editing packages. The editing system will treat 25F just the same as 25P. Just set up your projects to ‘HDV capturing’ and your sequence preset to ‘HDV1080P’.

Apple Final Cut Pro and Final Cut Express

The first time you drop your footage on a new sequence in Final Cut Pro or Final Cut Express you will get a system pop-up asking you to 'Change sequence settings to match the clip settings?' When this happens you must make sure that you click on ‘yes’. This way you will avoid the need to render your footage on the timeline.

 

A screenshot of the Match timeline pop-up in Final Cut Pro.

Adobe Premiere Pro

For Adobe Premiere Pro 2 you will need to download the following plug-in: Premiere Pro 2.0 editing preset for Canon Progressive scan HDV . Later versions don't need this plug-in. For all editing packages - make sure that your software is up to date!

* The author of this article, video expert Karel Poortman, has prepared a special HD film showing the capabilities of 25F shooting with the Canon XL H1S – you can click through from here to download it (file size is 227 Mb).

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