Tag Archives: 60p

What shutter speed to use if shooting 50p or 60p for 50i/60i conversion.

An interesting question got raised on Facebook today.

What shutter speed should I use if I am shooting at 50p so that my client can later convert the 50p to 50i? Of course this would also apply to shooting at 60p for 60i conversion.

Lets first of all make sure that we all understand that what’s being asked for here is to shoot at 50(60) progressive frames per second so that the footage can later be converted to 25(30) frames per second interlace – which has 50(60) fields.

If we just consider normal 50p or 60p shooing the the shutter speed that you would chooses on many factors including what you are shooting and how much light you have and personal preference.

1/48 or 1/50th of a second is normally considered the slowest shutter speed at which motion blur in a typical frame no longer significantly softens the image. This is why old point and shoot film cameras almost always had a 1/50th shutter, it was the slowest you could get away with.

Shooting with a shutter speed that is half the duration of the cameras frame rate is also know as using a 180 degree shutter, a very necessary practice with a film movie camera due to the way the mechanical shutter must be closed while the film is physically advanced to the next frame. But it isn’t essential that you have the closed shutter period with an electronic camera as there is no film to move, so you don’t have to use a 180 degree shutter if you don’t want to.

There is no reason why you can’t use a 1/50th or 1/60th shutter when shooting at 50fps or 60fps, especially if you don’t have a lot of light to work with. 1/50(1/60) at 50fps(60fps) will give you the smoothest motion as there are no breaks in the motion between each frame. But many people like to sharpen up the image still further by using 1/100th(1/120th) to reduce motion blur.  Or they prefer the slightly steppy cadence this brings as it introduces a small jump in motion between each frame. Of course 1/100th needs twice as much light. So there is no hard and fast rule and some shots will work better at 1/50th while others may work better at 1/100th.

However if you are shooting at 50fps or 60fps so that it can be converted to 50i or 60i, with each frame becoming a field, then the “normal” shutter speed you should use will be 1/50th or 1/60th to mimic a 25fps-50i camera or 30fps-60i camera which would typically have it’s shutter running at 1/50 or 1/60th. 1/100th(120th) at 50i(60i) can look a little over sharp due to an increase in aliasing due to the way a interlace video field only has half the resolution of the full frame. Particularly with 50p converted to 50i as there is no in-camera anti-aliasing and each frame will simply have it’s resolution divided by 2 to produce the equivalent of a single field. When you shoot with a “real” 50i camera line pairs on the sensor are combined and read out together as a  single field line and this slightly softens and anti-aliases each of fields. 50i has lower vertical resolution than 25p. But with simple software conversions from 50p to 50i this anti-aliasing does not occur. If you combine that with a faster than typical shutter speed the interlaced image can start to look over sharp and may have jaggies or color moire not present in the original 50/60p footage.

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More on frame rate choices for todays video productions.

This is another of those frequent questions at workshops and online.
What frame rate is the best one to use?
First – there is no one “best” frame rate. It really depends on how you want your video to look. Do you want the slightly juddery motion of a feature film or do you want silky smooth motion?
You also need to think about and understand how your video will be viewed. Is it going to be watched on a modern TV set or will it be watched on a computer? Will it only be watched in one country or region or will it be viewed globally?
Here are some things to consider:
TV in Europe is normally 50Hz, either 25p or 50i.
TV in the North America is 60Hz, either 30p or 60i (both actually 29.97fps).
The majority of computer screens run at 60Hz.
Interlaced footage looks bad on most LCD screens.
Low frame rates like 24p and 25p often exhibit judder.
Most newer, mid price and above TV’s use motion estimation techniques to eliminate judder in low frame rate footage.
If you upload 23.98fps footage to YouTube and it is then viewed on a computer it will most likely be shown at 24p as you can’t show 0.98 of a frame on a 60Hz computer screen.
Lets look first at 25p, 50i and 50p.
If you live in Europe or another 50Hz/Pal area these are going to be frame rates you will be familiar with. But are they the only frame rates you should use? If you are doing a broadcast TV production then there is a high chance that you will need to use one of these standards (please consult whoever you are shooting for). But if your audience is going to watch your content online on a computer screen, tablet or mobile phone these are not good frame rates to use.

Most computer screens run at 60Hz and very often this rate can’t be changed. 25p shown on most computer screens requires 15 frames to be shown twice and 10 frames to be shown 3 times to create a total of 60 frames every second. This creates an uneven cadence and it’s not something you can control as the actual structure of the cadence depends on the video subsystem of the computer the end user is using.

The odd 25p cadence is most noticeable on smooth pans and tilts where the pan speed will appear to jump slightly as the cadence flips between the 10 frame x3 and 15 frame x 2 segments. This often makes what would otherwise be smooth motion appear to stutter unevenly. 24p material doesn’t exhibit this same uneven stutter (see the 24p section). 50p material will exhibit a similar stutter as again the number of padding frames needed is uneven, although the motion should be a bit more fluid.
So really 25p and 50p are best reserved for material that will only ever be seen on televisions that are running at 50Hz. They are not the best choices for online distribution or viewing on computers etc.
24p, 30p or 60p (23.98p, 29.97p)
If you are doing a broadcast TV show in an NTSC/60Hz area then you will most likely need to use the slightly odd frame rates of 23.98fps or 29.97fps. These are legacy frame rates specifically for broadcast TV. The odd frame rates came about to avoid problems with the color signal interfering with the luma (brightness) signal in the early days of analog color TV.
If you show 23.98fps or 29.97fps footage on a computer it will normally be shown at the equivalent of 24p or 30p  to fit with the 60Hz refresh rate of the computer screen. In most cases no one will ever notice any difference.
24p Cadence.
23.98p and 24p when shown on a 60Hz screen are shown by using 2:3 cadence where the first frame is shown twice, the next 3 times, then 2, then 3 and so on. This is very similar to the way any other movie or feature film is shown on TV and it doesn’t look too bad.
30p or 29.97p footage will look smoother than 24p as all you need to do is show each frame twice to get to 60Hz there is no odd cadence and the slightly higher frame rate will exhibit a little less judder. 60p will be very smooth and is a really good choice for sports or other fast action. But, higher frame rates do require higher data rates to maintain the same image quality. This means larger files and possibly slower downloads and must be considered. 30p is a reasonable middle ground choice for a lot of productions, not as juddery as 24p but not as smooth as 60p.
24p or 23.98p for “The Film Look”.
Generally if you want to mimic the look of a feature film then you might choose to use 23.98p or 24p as films are normally shot at 24fps. If your video is only going to be viewed online then 24p is a good choice. If your footage might get shown on TV the 23.98p may be the better choice as 23.98fps works well on 29.97fps TV’s in 60Hz/NTSC areas.
BUT THERE IS A NEW CATCH!!!
A lot of modern, new TV’s feature motion compensation processes designed to eliminate judder. You might see things in the TV’s literature such as “100 Hz smooth motion” or similar.  If this function is enabled in the TV it will take any low frame rate footage such as 24p or 25p and use software to create new frames to increase the frame rate and smooth out any motion judder.
So if you want the motion judder typical of a 24fps movie and you create at 24fps video, you may find that the viewer never sees this juddery, film like motion as the TV will do it’s best to smooth it out! Meanwhile someone watching the same clip on a computer will see the judder. So the motion in the same clip will look quite different depending on how it’s viewed.
Most TV’s that have this feature will disable it it when the footage is 60p as 60p footage should look smooth anyway. So a trick you might want to consider is to shoot at 24p or 30p and then for the export file create a 60p file as this will typically cause the TV to turn off the motion estimation.
In summary, if you are doing a broadcast TV project you should use the frame rate specified by the broadcaster. But for projects that will be distributed via the internet I recommend the use of 23.98p or 24p for film style projects and 30p for most other projects. However if you want very smooth motion you should consider using 60p.