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Old May 17th, 2004, 06:34 AM   #1
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Shutter speed - basic question

This is a bit embarrassing, but I've just realised that I'm not sure exactly what slower shutter speeds change on a camcorder. I'm quite familiar with using slow shutter speeds for long exposures on 35mm cameras, and I understand that a fast shutter speed will reduce the light collected by the CCDs - but what is the point of shutter speeds slower than 1/50 sec? (I'm a PAL user) After all, an interlaced image is grabbed every 1/50 sec .... isn't it?

Am I missing something obvious here?

Pat
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Old May 17th, 2004, 06:46 AM   #2
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It is used to let more light in for the cases when you don't have
the ability to add lights and need to get the footage recorded.
In any serious production you will want to add light instead, ofcourse.

Keep in mind that in the digital camera world the shutter is not
tied to the framerate as is the case with a film camera! Yes you
will loose resolution going below 1/50 (unless you have a Canon
running in frame mode, which works in a different manner) on
most cams, but if you are running and gunning and need to get
something dark on tape then it is another tool available to you.

It might even be seen as an "effect" perhaps.
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Old May 17th, 2004, 06:56 AM   #3
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Pat,
Donít worry, this comes up quite frequently on other forums as well (Simplydv.com for example Ė a great UK site)
Even though your camera is capturing 25 frames per second Ė we donít need to get into interlacing here Ė the shutter speed will determine just how long one of those frames is allowed to be exposed to light.
A slower speed allows more light to the film/ccd. However, in video it will certainly cause any motion to be blurred depending on how slow the shutter speed is. You might need this facility if for example you wanted to shoot some landscape or interior in very low light and didn't want to switch in extra gain (which would make the image grainier through introduction of video "noise")
The other use is to use a higher shutter speed to try and use the inter-frame blurring altogether - when a motion such as a golf swing need to be studied later, frame-by-frame. However, played back normally, it will give a juddery feel to the sequence because there isnít enough motion blur to fool the eyes.
Hope this helps somewhat

Robin.
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Old May 17th, 2004, 08:37 AM   #4
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Just to add to previous replies, on consumer cams notably, any shutter speed slower than 1/50 sec.(PAL) will force the cam to completely ignore one of the two fields normally recorded in interlaced mode (odd and even) in order to allow the first field (odd) to be exposed longer than the usual 1/50 sec required to make a 25 frames-per-second stream.

For example, at 1/25 shutter speed, the odd field is exposed for 1/25 sec on the CCD and then duplicated to replace the even field. At this speed, the exposure time is the same as the frame rate, so motion blurr is not too bad, but the vertical resolution is cut in half because you discarded one complete field out of two.
You'll get noticeable stair steps on every fine diagonal line.
The resulting deinterlaced motion effect is very close to that of film (24 fps).

If you select a slower speed, such as 1/12.5, then the exposure is prolonged over two frames which starts to show as strobed movement, and the motion blurr becomes evident. It can create an interesting effect, for instance, on a busy street corner at night: everything standing still will be clean while people walking and cars driving by will be blurred, but colors will be stronger and picture noise can be reduced.
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Old May 17th, 2004, 10:51 AM   #5
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For a nice use of slow shutter speeds, check out Hal Hartley's "Book of Life." It's on DVD.
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Old May 17th, 2004, 12:26 PM   #6
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Thanks for the quick replies. It's becoming clearer now - what I was missing before was an exposure that spans more than one frame. I certainly do want to experiment with the blurring of movement on night shots - in fact with the low light performance of the PDX10 that's probably all I can do with night shots. I think the shutter goes down to 1/6 sec, but don't have it with me to check right now.

And yes, I was aware of simplydv.com, but it doesn't seem as active as this forum. This seems one of the best in terms of knowledgable people, volume of activity, and useful content.

Pat
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Old May 17th, 2004, 12:50 PM   #7
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On my Digital8 TRV320, the Slow Shutter option offers a choice between 1/30, 1/15, 1/8, and 1/4 sec. That's nice.

What's NOT nice is that selecting the Slow Shutter option disables all other manual control except focus. So I'm stuck in auto-iris which opens wide and pushes the gain up until night resembles daylight!
So much for clean night city skylines!

When you shop for a new cam, beware of those hidden facts...
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Old May 18th, 2004, 01:09 PM   #8
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I may have a similar issue with the PDX10, not sure yet. The gain can be limited to 6dB or 12dB but not disabled. I need to find more time to experiment with the camera, but learning FCP has taken all my spare time recently.

Pat
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Old May 19th, 2004, 06:01 AM   #9
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No, you're ok with the PDX10 Pat (and thank goodness you are, cos the camera is pretty poor in low light situations). Slide the chrome slider to the middle position and push 'shutter speed'. Turn the dial to give the desired speed down to 1/3rd sec (PAL). Now you can push 'exposure' and vary the amount of gain, aperture and ND filtration.

What? Gain, ND and aperture all under the wheel turning? Yes - the PDX10 doesn't give you a viewfinfder readout of what the camera's doing, so you'll just have to go by how the viewfinder looks. Not very professional for anything with DVCAM written bold on the side, is it?

tom.
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Old May 20th, 2004, 07:31 AM   #10
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I see, thanks Tom. Yes, it would be handy to know exactly what the camera is doing in terms of aperture, gain etc. But it is at the bottom of the DVCAM range and I guess it is necessary for Sony to differentiate against the more expensive models, other than just the CCDs. I was aware that the ND filters were automatic.

Out of interest do you/others generally leave everything on auto except for when you want to achieve a special effect such as blurring, or a (reasonably well lit) night shot? Or is manual exposure generally regarded as best, providing more control? I'm asking as I understand that the range of 'real' apertures is limited, and there is therefore only very limited control over the depth of field. I would assume that the shutter speed is the main variable, and only for special effects (or scenes too bright for the ND filters).

If you generally rely on manual exposure, do you find that the viewfinder/LCD provides a good enough representation of how the result will appear from tape, or are there any circumstances under which this is not the case?

I do appreciate that experimentation is the real answer, but I'm interested in what others have found.
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Old May 20th, 2004, 08:13 AM   #11
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For my part, I always point and frame in Auto, then switch to manual for fine-tuning because Auto-iris only makes a mathematical average regardless of what composes your frame.
If your background is trees and foliage, your subject in the foreground will be one or two stops overexposed in Auto.
If your subject is in front of a white wall or cloudy skies, it'll be underexposed.
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Old May 20th, 2004, 08:30 AM   #12
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Norm's right - it's often worth asking the camera, "what do you suggest?" by having it set on automatic exposure. Then - and most important this - lock down the suggested exposure for the shot. You can then up it or lower it using the little wheel in combination with your viewfinder experience and valuable zebra helpers.

There's nothing worse than seeing the exposure fluctuate as it constantly tries to make minute corrections. Even a close up of a face in sunlight will make those NDs bob up and down as different amounts of light are relected as the face turns in animated conversation.

As in Norm's case - if the person leans forward and you're in auto exposure the camera will change its exposure to make the background trees correct. Then when your subject straightens up the trees will darken again as the light meter corrects for the face. This looks so amateurish and instantly labels you.

Of all the automations available to you (w/bal, focus, audio level) it's the exposure that should be locked down first as it's give-aways are the most noticeable on screen. The others are important of course, but do start with the exposure.

Pat - even if the PDX /950 used a diaphragm rather than its three ND filters you'd still have precious little DOF control. The reason has to do with the tiny chips and incredibly short focal length lenses employed, so the ND solution to exposure determination is in fact a good one, albeit an unsatisfactory one for any photographer who wants to be informed as to what the camera's dooing at any given time.

The other point is that a Hoodman (H400) is a wonderful accessory for the PDX, although it impedes access to the three-deep touch screen menu of course.

tom.
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Old May 20th, 2004, 09:20 AM   #13
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Hi Pat,

These are some good questions and observations. Experiment with the slow shutter speeds and see what you think. Yes, you can gain a stop by shooting at 1/25 sec. I've shot at 1/30 from time to time on my NTSC PDX-10, but I was not very happy with the results. You essentially lose 50% of the vertical resolution since the same data is being written to both interlaced fields. I suppose it depends on the nature of what you're shooting and the overall effect. But I think it looks a bit too soft compared to 1/60 sec (or 1/50 in your case).

As for auto vs. manual, well... I don't think my PDX-10 has ever been set for auto! I always use manual personally and the zebra pattern has never been switched off. But by all means experiment with this also and form your own opinion.

I think the LCD screen is really quite good on the PDX-10, it's much brighter, larger and sharper than the screen on my VX-2000 for example. However it isn't a real NTSC monitor and you have to be a bit wary of this. I think the big problem with LCD's in general is that the contrast and brightness vary depending on the angle you view the screen. I've also learned that it's just human nature for one to position themself such that the brightness/contrast are optimal when looking at an LCD. This can really skew your perception of the image.

One thing that can help with this is to properly adjust the LCD brightness. Before going on a shoot I often plug the camera into my mac and put up some NTSC bars. Then I look at the little screen next to my external video monitor and adjust the color and brightness on the PDX-10 screen to match as closely as possible. I also make note of what viewing angle I'm using and try to be consistent with this in the field. You can also run some footage through both the camera and external monitor and use this to tweak the LCD panel settings. None of this is very scientific, it's all subjective, but I find it helps me a lot.

It is very important to turn on the zebra stripes and learn how to use them. Do a search here for "zebra" and you'll find lots of discussion. After you have used your camera for awhile and looked at a lot of its footage you'll gain some insight into what to expect based on what you see on the little LCD screen. As a general rule I try to err on the side of underexposure if there's any doubt in my mind. Once you overexpose the highlights you're out of luck, but there's some hope of fixing underexposure in post.

Perhaps a bigger problem with the LCD and viewfinder is that it can be very hard to judge critical focus in manual mode. Again, some experience will help, but this will not be a substitute for a "real" monitor since neither the LCD nor viewfinder have enough pixels to really judge fine detail.

I agree that an LCD hood helps a lot in bright places. I use the Petrol hood that came with the camera as part of a Sony promotion. It is available very inexpensively however from B&H or other places. I don't find that this hood is a big problem accessing the touch-screen functions although it does make it a bit awkward. Unfortunately the larger problem is that it blocks access to all the buttons on the side of the camera that are situated under the LCD screen (like the VCR controls, display button, volume buttons, custom preset button). Fortunately the custom preset button is just barely accessible and that's the one I need the most. Bad design choice on Sony's part to put these buttons there.

The BW viewfinder is most helpful for contrast/focus judgements I think. These days I find myself using it less and using the LCD (with hood as needed) more. Welcome to DVinfo, glad to have you around! Have fun exploring all the features of your camera.
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Old May 20th, 2004, 01:12 PM   #14
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Wow, lots of useful info here - thanks.

I understand what you mean by locking down the exposure, I'm just not sure how you do it on the PDX10 (I'm at work right now and don't have the camera or manual with me). Maybe it'll be obvious when I get the camera in my hands. I can certainly see how fluctuating exposure would provide a very definite amateur look. And I suspected that manual control would be the preference. In any case I'd prefer that as it gets you more involved with the capture process (never been a point and click person myself).

I do intend using the zebra stripes, and I assume that the 100% setting is most useful for general use. I'm aware from the world of still photography that blown out highlights are impossible to recover, and that erring towards underexposure is best. I'll do a search to find out more.

I will also look into getting a hood for the LCD, although probably not from B&H (being based in the UK). My experience so far has been that the LCD exceeded my expectations in terms of clarity, but that the viewfinder was not as clear as I had hoped. I like the idea of it being B&W, and the focal adjustment is nice, but as you suggest, manual focus is quite difficult to achieve. I probably expected too much from it.

Lots of time needed to experiment. I don't think that Spielberg need see me as a threat just yet ...

Pat
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Old May 20th, 2004, 01:19 PM   #15
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Pat, as to your question about the 100 percent zebras...Others may disagree, but I find it more difficult to set exposure with zebras at 100 percent. I normally use 75%, zoom in on a caucasian face or some other fairly neutral reflectance object, get zebras in the highlights of the face, and then adjust up or down a bit from there as the situation requires. Using 100 percent, you have to zebra out on the whites, and it's easier to overexpose, in my opinion.
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