aperture and shutter speeds in video cameras

[jli]

After learning about the technical things in photography, I've started to notice things in film/video, like how the bokeh is polygonal, not a perfect circle . It also seems like a lot of video is shot with a relatively shallow depth of field almost always.

So I was wondering about how aperture, shutter, and ISO translate to video. Having an adjustable aperture seems straightforward, but there's no shutter, so does the aperture have to be wide enough to be able to record in real-time? Or, can we think in terms of frames per second? For 30fps video, each frame could only get 1/30 "shutter" at most, minus the processing delay. Is this completely not how video cameras work?

Please disabuse me of my ignorance!

[Boyan]

Hi John,

You're pretty much right on. If you're shooting in 30fps, the slowest shutter you can have is 1/30sec. If the scene is dark, you need to up the ISO or introduce a video light. Scenes in movies are often shot with shallow DOF but it depends on the scene.

All serious videos are shot with manual focus, and in Hollywood there are full time guys who only work the focus as necessary (focus pullers) because the cameraman cant be worrying about framing and adjusting focus at the same time.

Also if you're shooting with a steadicam, you pretty much have to pick a wide lens and shoot with everything is in focus because you can't be adjusting focus on a steadicam cause your fiddling with the focus ring would be defeating the point of a stabilizer.

People who do wedding videos (and I mean the good wedding video guys) dont have the luxury of having a focus puller (or a sound guy usually) so the gig is really tough, you have to do everything yourself and often you have multiple cameras running at the same time with multiple mics and things can get a little crazy :-)

Nevertheless, I can easily say that videography is more challenging than photography in many ways, and I'm very excited about what the future holds in terms of the next generation of video dSLRs. I played around with video a little bit, here's a short wedding highlights video (which I shot as a guest LOL):

Video | B&N Photography Blog

[Raoul Isidro]

You can set and lock aperture and FPS before you record video and set the ISO to auto. The ISO of the sensor will try it's best to render the correct exposure. On Nikon cameras, you can also lock in the ISO even while in the middle of recording by pressing the AF lock key. Releasing the button returns it to auto ISO again.
Cheers!

[jli]

Originally Posted by Boyan

You're pretty much right on. If you're shooting in 30fps, the slowest shutter you can have is 1/30sec. If the scene is dark, you need to up the ISO or introduce a video light. Scenes in movies are often shot with shallow DOF but it depends on the scene.

Thanks Boyan! That was all very informative.

I was watching Lost, and the video had a shallow depth of field. But it's a sunny Hawaiian island! So, are there neutral density filters to cut down on light? Or is it possible to shoot at faster shutter speeds (1/60, 1/120) and throw away every 2nd or 4th frame?

[Boyan]

That's exactly right. To shoot at a shallow DOF outside, you either have to use ND filters, or shoot at really fast shutter speeds (1/1000). If you shoot at fast shutter speeds, you're not really throwing away anything, here's how it works (let's say we're shooting 30fps at 1/120 sec shutter):

1. Frame 1 begins, shutter opens
2. Shutter closes after 1/120 sec (exposure is done, so now camera just waits for the frame to end)
3. When 1/30sec is up, frame 1 is grabbed and saved
4. Frame 2 begins, shutter opens
5. Etc etc

Note that shooting video at high shutter speeds has side effects. The motion of subjects will appear a little choppy and robotic. That's why fast shutter speeds are excelent for dramatic action sequences, and are frequently used in horror/zombie movies. You can find examples of fast shutter video on youtube.

That's why you have to decide what kind of mood you're going for. In majority of cases, you probably don't want fast shutter, and need to use ND filters outdoors if you want shallow DOF and slower shutter.