Why is 1/48 best (and "only") for 24p?

[hifipj]

Hi all,

I have been experimenting with manual exposure on my HV20 and using the lock exposure method in Shutter Priority mode described here and in Barry's tutorial here:

http://www.dvxuser.com/jason/hv20/

My question is this: why does everyone feel so strongly you must keep the shutter speed at 1/48 when shooting 24p, and why is it such a bad thing if your shutter speed varies during shooting?

I know the obvious reason to shoot 24p at 1/48 is for improved low-light performance, and that this is the slowest speed you can shoot at using 24p. But it seems the pros here feel really strongly in locking the shutter at 1/48 when shooting, even in bright light, which in bright daylight would be overexposed without using an ND filter (external or the camera's). So why is it bad when shooting 24p and using manual exposure, in daylight, if your shutter speed goes up from 1/48?

And correct me if this is wrong, but the most the HV20's shutter can stop down to is F8, right? When the camera display says it's gone to F11, F16, etc., anything beyond F8, what it is really doing is keeping it closed down to F8 and applying the in-camera ND filter, yes? And coming from long ago 35mm SLR shooting this is probably another dumb question, but if this is the case, why can the HV20 not stop down to F11, 16, 32? Are all HDV camcorders like this?

Thanks,
Patrick

[Erik Bien]

Gee, I'm kind of intimidated to post now that I'm a "Senior Member" ... I guess it's because I've just reached the magic threshold of 100 posts here (but that just means I need to get out more; no expertise expressed, implied or inferred ...)

Maybe if I can link to explanations by actual experts like David Mullen he'll explain it better than I could!

[um3k]

This page explains why the HV20 (and most other compact digital cameras) can't be stopped down past F8. Basically, if you stop down too much, the diffraction becomes larger than the pixels, making the image blurry.

[Flyingsheep]

I never really realized that... but after reading this I did figure out that my cheapy $250 panasonic's aperture can go as small as f/16 before it starts applying the ND filter. That really sucks for the HV20 though because I would like to use 1/48th all the time on purpose so that the motion looks smoother at 24p. Are there ANY workarounds (besides using a faster shutter speed)? Or are you stuck with the built-in ND filter which pretty much reduces the quality when used?

[Erik Bien]

I don't think it's entirely correct to say the ND filter "reduces the quality" ... most 'professional broadcast' cameras have multi-stage ND filters built in. You can always add additional ND filters using step-rings or a matte box.

Also, as shown quite clearly by the link in um3k's post, just because your Panny is capable of stopping down to f/16, actually shooting at that setting will probably result in a substantial loss of fine detail on playback.

[24Peter]

Hi all,

My question is this: why does everyone feel so strongly you must keep the shutter speed at 1/48 when shooting 24p, and why is it such a bad thing if your shutter speed varies during shooting?

I know the obvious reason to shoot 24p at 1/48 is for improved low-light performance, and that this is the slowest speed you can shoot at using 24p. But it seems the pros here feel really strongly in locking the shutter at 1/48 when shooting, even in bright light, which in bright daylight would be overexposed without using an ND filter (external or the camera's). So why is it bad when shooting 24p and using manual exposure, in daylight, if your shutter speed goes up from 1/48?

And correct me if this is wrong, but the most the HV20's shutter can stop down to is F8, right? When the camera display says it's gone to F11, F16, etc., anything beyond F8, what it is really doing is keeping it closed down to F8 and applying the in-camera ND filter, yes? And coming from long ago 35mm SLR shooting this is probably another dumb question, but if this is the case, why can the HV20 not stop down to F11, 16, 32? Are all HDV camcorders like this?

Thanks,
Patrick

Patrick - a couple of points need clarifying: First, the shutter doesn't stop down; the apeture does. Think of the shutter as the blinds on a window. The duration of time the blinds are open is the shutter speed. Then think of apeture as the size of the window; the larger the F#, the smaller the window (I know in real life the size of the window is constant but just imagine it was variable.) The combination of these two determines your overall exposure (there's also gain on the low end and ND filters on the high end which further regulate overall exposure.)

1/48 SS in 24p is usually selected b/c it gives the most "natural" look for on-screen motion (I say "natural" b/c most of us are used to that look from watching movies for the past x-number of years). (I don't think low light performance has anything to do with it, though that may be a by product when compared to shooting 60i at 1/60th). The HV20 is capable of slower or faster shutter speeds in 24p, but the further you stray from 1/48th, the more "unnatural" on-screen motion will look. At the high end, at 1/120th or above you'll get that stroby "Saving Private Ryan" look - very useful for some scenes but very distracting (to me at least) for most others. At the low end, 1/24th or slower produces excessively blurred motion on faster moving objects. I wouldn't use it for anything other than almost static subjects. For me, the acceptable range of SS for natural looking motion for shooting 24p is 1/32nd on the low end and 1/100th on the high end (though that's pushing it a bit). (BTW - the HV20 -like most camcorders has an electronic shutter - not a mechanical one like your SLR still camera. The sensor effectively turns on and off for the duration of the SS you select.)

Unlike the shutter, I think the apeture on the HV20 mechanical (i.e., blades) up to F8. As you have correctly pointed out, any apeture beyond F8 on the HV20 is electronically controlled - i.e., electronic ND filters. (The issues of diffraction and overall size of the lens determine this.) You can supplement this effect by adding external filters to the front of your lens.

So... you can choose any SS you want when shooting 24p (as opposed to CineMode). Just make sure you like the end result and how it looks.

[SenorKaffee]

Very good post - fixing the speed to 1/48 (1/50 PAL) because "it makes mah video look like mooviez" is limiting the artistic freedom. "Saving Private Ryan" and "Gladiator" look like movies, although they play with this tool.

[Ian-T]

Very good post - fixing the speed to 1/48 (1/50 PAL) because "it makes mah video look like mooviez" is limiting the artistic freedom. "Saving Private Ryan" and "Gladiator" look like movies, although they play with this tool.

Your right they do. But (as you already know) they do it in certain scenes and not throughout the whole film. Particulary all the battle scenes have that stuttery high shutter look. It's a cool effect...sort of gives the scene more adrenaline.