Shutter speed control on video digicams

If you ever wondered why small digicams/digirecorders/cellphones don’t have shutter speed controls for video (they usually shoot in high shutter speeds outdoors), here’s an IM conversation I had with JBQ this morning on the subject. Basically, these small sensor cams have to go super-high shutter speed to compensate for their design shortcomings. Adding just shutter speed control on a cam that can’t physically go down to 1/48th or 1/60th under “random point & shoot sunny outdoors conditions”, makes no market sense (if anything, it would be a support nightmare). The chat:

Eugenia: Could you explain quickly why small digicams can’t have shutter speed controls for video that can go all the way down to 1/48th outdoors?
Jean-Baptiste Queru: They have very fast lenses (large relative apertures, i.e. small f-numbers), they need that because of their small sensors. fast lenses -> fast shutter speeds.
Jean-Baptiste Queru: Chances are, they can’t close more than about f/8.
Eugenia: ic
Jean-Baptiste Queru: f/8 is called Av 6 (Av is “aperture value”, f/1 is Av 0, f/1.4 is Av1, f/2 is Av 2, f/2.8 is Av 3, etc…)
Jean-Baptiste Queru: For shutter speeds, there’s the Tv scale (“time value”): 1s is Tv 0, 1/2s is Tv 1, 1/4s is Tv 2… 1/250s (technically 1/256s) is Tv 8. 1/48s is about Tv 5.5.
Jean-Baptiste Queru: In bright sunlight at ISO 100 without ND filters, Av + Tv needs to be about 14.6 for a proper exposure.
Jean-Baptiste Queru: So, if a camera can’t stop down further than f/8, it can’t go beyond Av 6, so it needs at least Tv 8.6 to expose properly, and that’s 1/400s.
Eugenia: So for the Canon S95 lens, how low the shutter speed can go outdoors, on ISO 100?
Jean-Baptiste Queru: well, if depends on how much it can stop down. If it can stop down to f/8, the lowest it’ll be able to get is 1/400s.
Eugenia: the S95 specs are 6.0 (W)-22.5mm (T) f/2.0-4.9 (35mm equivalent: 28-105mm)
Jean-Baptiste Queru: Yes, but that doesn’t tell how much they can stop down.
Jean-Baptiste Queru: (it tells how much it can open up, i.e. the other end of the Av range).
Jean-Baptiste Queru: I assume it’s f/8 because that’s common for such digicams.
Eugenia: ic
Jean-Baptiste Queru: At f/8 you’d already be losing a huge lot of sharpness because of diffraction.
Jean-Baptiste Queru: (and you’d have a huge depth of field)
Jean-Baptiste Queru: So, if you wanted 1/50s f/2.8, that’d be Tv 5.6, Av 3. The sum is called Ev (exposure value), and it’s 8.6. Since the light is Lv 14.6, you’d need to cut 6 units (6 stops, D=1.8, 64x).
Eugenia: And that’s almost impossible on these cams…
Eugenia: why the STILL image mode, in manual control, lets me go to 15″ shutter speed, outdoors, looking at the sun, on ISO 100?
Eugenia: of course it’s completely over exposed, even at f/8
Eugenia: or are we talking about values where the image is not over exposed?
Jean-Baptiste Queru: In manual control, you can set anything you want, but you could end up overexposed or underexposed, and you have to figure out with other means which values will produce a proper exposure.

Jean-Baptiste Queru: BTW, another way to think about it is that small sensor cams can’t deal with as much light, so they need to use higher shutter speeds to reduce the amount of light that comes in.
Jean-Baptiste Queru: The sensor in a Canon 5D MarkII dSLR is 64 times larger than in a 1/3.2″ sensor camera, so it can take up 64 times as much light when setting up a similar shot (that means that the shutter can stay open 64 times longer).

Conclusion: That’s why you should try buying a filter tube for your digicam, and then buy a few ND filters at various strengths. ND filters will act as sunglasses, and will force shutter speeds to go down, by opening up the aperture.

Even with a dSLR, you’d have to stop down a lot to get to 1/50s (at ISO 100 you’d need f/22, and that’s too much). A 3-stop ND would be a good idea in such conditions. The good news is that a 3-stop ND filter in SLR filter sizes is super-common. A 6-stop ND in P&S digicam filter size isn’t (so you’d need step-up rings too).


Glenn Thomas wrote on October 14th, 2010 at 9:41 AM PST:

Interesting. I’ll have to give this another read tomorrow. I actually don’t mind high shutter speeds. Jeff Lew didn’t use any motion blur on his ‘Killer Bean Forever’ animated feature, and watching the movie, you don’t really notice the lack of it.

I saw the other day that Re:Vison effects are one of the companies supporting the Open Effects standard. If they release Open Effects versions of their plugins, hopefully these will work in Vegas 10 and include their Reel Motion Blur plugin, which is quite good for adding natural looking motion blur. I remember trying out the plugin way back in Commotion Pro, and it worked quite nicely.

Still, I’ll get myself one of those filter holders when I get a chance. I’m curious to see if the S95 will work connected to my 35mm adapter also.

Michael C. wrote on October 14th, 2010 at 10:23 AM PST:

Or, one can go to eBay and get himself a used HF100 for about $300, a proper HD camcorder that has a built-in 4-stop ND filter with 1/4-stop gradations.

This does not correspond to you, Eugenia, as you like new and shiny toys. Your kitting out of digicams is akin to kitting out of DSLRs.

BTW, I am sure these cams can go far beyond f/8. My Elura 100 can go to at least f/22.

For my Panasonic camcorder and for my Nikon DSLR I have a variable ND filter, more convenient than dedicated ND filters, but softens image quite a bit.

This is the admin speaking...
Eugenia wrote on October 14th, 2010 at 10:29 AM PST:

Yes, one can go for an AVCHD camcorder (in fact, this is what I suggested to someone recently). However, some people prefer their device to be a still digicam first and foremost, and then a video camera. For those people, they better get down with a filter tube and an ND filter. There’s no way around it.

>My Elura 100 can go to at least f/22.

The Elura was a camcorder. Camcorder lens designs are not the same as in digicams. The HV20 lens for example, beats the hell out of most digicam lenses in abilities.

Glenn Thomas wrote on October 15th, 2010 at 12:24 AM PST:

This is the one I was looking at.
Only problem is, it kind of defeats the purpose of having a camera that will fit in your pocket.

Glenn Thomas wrote on October 15th, 2010 at 12:26 AM PST:

Oops, sorry, I forgot the html again.. I always forget that with this Firefox extension I have running that automatically converts the URL links.

Yanni wrote on October 20th, 2010 at 11:34 PM PST:

Excellent article.

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