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Canon XH Series HDV Camcorders
Canon XH G1S / G1 (with SDI), Canon XH A1S / A1 (without SDI).


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Old November 13th, 2007, 05:33 PM   #1
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Help Needed: Exposure Question

I have a situation where I am trying to create the shallowest depth of field I can. The location is indoors and the lighting is very controlled. The subject is lit by 3 Tota lights and foam core.

I have read all I can about achieving shallow depth of field: opening the iris all the way, using the ND filter and/or turning the gain to -3db to accomplish this, zooming in as much as possible, etc. Zooming in very far is out since the camera is only 2 feet from the subject. I have found that opening the iris helps. To open the iris all the way I needed to turn on the ND filter due to the fact that my subject is very well lit. The only problem is that with the ND filter on, the image is underexposed, even if the iris is fully open. I have increased the gain from -3db to +6db to properly expose the image, but I generally don't like to use the gain if I can help it. I am always worried that increasing the gain will add grain to the footage, but perhaps this is not an issue since the subject is well lit.

I also wanted to know if there is any harm in raising the Shutter speed from 1/60th (I am shooting 30F) to 1/75th or greater to let more light in, instead of using the gain? I know that if I decrease the shutter speed below 1/60th I begin to experience motion blur, but what is the draw back if any of increasing the shutter speed a bit?

Any help is greatly appreciated,
Hugh
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Old November 13th, 2007, 07:23 PM   #2
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Hey, Hugh. As far as I know there is no correlation between gain and DoF. Leave the gain where it needs to be. The only thing you can do to get DoF is zoom in. That's the only way other than buying a mini 35 adapter. Check out the Letus adapters, they are a favorite among this crowd. Other than that you just have to zoom in.
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Old November 13th, 2007, 07:35 PM   #3
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Hugh,

In order to get a a shallow DOF you need to keep your iris wide open (as you've stated) and you need as large a focal length as possible (zoom in as much as possible).

With most video cameras a shallow DOF is virually impossible unless you're completely zoomed in. So unless you can do that in your case and still maintain the composition you're after the only other choice you have is to use a 35mm adapter (as Steve mentioned).

Now keeping your gain at -3db effectively allows you to keep your iris open more so than without. -3db is about 1 stop. So it does tend to help but unless you've zoomed in quite a bit it doesn't really help make things look the way you're after.

Another thing you should keep in mind is that a larger focal length gives you kind of compressed look, in that the distance between subject and bakground tends to be a lot smaller. That look compared to what a 35mm adapter can do at much smaller focal lengths (85mm or 100mm) is very different.
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Old November 13th, 2007, 08:01 PM   #4
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Originally Posted by Shiv Kumar View Post
Hugh,

In order to get a a shallow DOF you need to keep your iris wide open (as you've stated) and you need as large a focal length as possible (zoom in as much as possible).
My problem is that I have the camera zoomed in as much as possible, the ND filter on, the iris all the way open, and the depth of field is what I am looking for. The trouble is that the image is underexposed, so I need to either increase the gain or increase the exposure from 1/60th to 1/75th or more in order to properly expose the image. What are the disadvantages of increasing the gain to +6db from -3db and what happens when the shutter speed is increased from 1/60th to 1/75th or faster?

Thanks,
Hugh
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Old November 13th, 2007, 09:18 PM   #5
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Tota lights are anything but controlled- and three of them? I have to ask you- can you do anything about the lighting? If you can bring down the overall light level a bit that will allow you to shoot wide open without any nd.
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Old November 13th, 2007, 09:21 PM   #6
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take the ND filters off first - if you're in a situation with an underexposed image (not sure if I'm reading your post correctly or not)

slowing the shutter speed to 1/30 will give more light as well but you mentioned motion blur as being a problem -but definitely going to a higher shutterspeed will mean you'll need even more light so that's no good

you could take the gain to 0 or even 3 without too much image noise

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Old November 13th, 2007, 11:20 PM   #7
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Trish,

I guess what me means is that increasing the shutter speed you will lower the exposure and so will not need to use the ND filter.

Hugh,

From what you say (the need to gain up to +6db when using the ND filter) there seems to be some confusion. Let me explain my reasoning...

The 1/6 ND filter cuts the light by 2 stops. On the other hand +6db means 2 stops more light. So If by using the 1/6 ND filter you need to gain up by +6db it seems to me like you're simply countering the effect of the ND filter no?

You could simply use a half stop or one stop ND filter in front of the camera lens (filter or matt box) if the 2 stop ND filter is cutting too much light.

Increasing the shutter speed to 1/75 shouldn't have much of a difference (depending on the type of shoot). Higher shutter speeds could tend to make the scence frantic looking (normally a creative choice rather than a last option).

I'd go with the external NDs or as Greg suggests, cut the lights if you can.
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Old November 14th, 2007, 02:17 AM   #8
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Hugh, just take off the ND filter and up the shutter speed a notch. No one will be able to tell. Or bring the lights back a little. Or stick an ND on the lights.
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Old November 14th, 2007, 07:12 AM   #9
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Actually, 1 stop exposure corresponds to about 6 dB of gain. 3 dB is about 1/2 stop.

Depending on the source material, using a faster shutter to manage exposure is usually OK, but motion may have a slightly different look due to less motion blur with the faster shutter. So run a test to determine if it looks OK to you before your final take.

However, depending on your lighting type using a faster shutter might introduce some color variations. This can be an issue if using discharge-type lighting (e.g., fluorescent) operating at the power line frequency.

When using zoom (longer focal lengths) to obtain more depth of field, keep in mind that the perspective changes as well - be sure you are content with the changes.
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Old November 14th, 2007, 07:17 AM   #10
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Thanks everyone for your feedback. Because of the particulars of my setup I don't have a lot of flexibility as far as generating a shallow depth of field, i.e. zoom. All of your suggestions are great. Rather than putting ND gels on all the lights I think that simply raising the shutter speed, in order to let in a little more light, while using the 1/6 ND filter will work better than increasing the camera's gain. Also, thanks for letting me know that the 1/6 ND filter lowers the amount of light entering the camera by 2 stops and that increasing the gain to +6db will raise the amount of light entering the camera by 1 stop. And thanks for clarifying what increasing the shutter speed will do to the image.

Again, thanks for all of your help,
Hugh
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Old November 14th, 2007, 02:54 PM   #11
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Quote:
...that increasing the gain to +6db will raise the amount of light entering the camera by 1 stop.
More precisely, increasing the gain by 6 dB will have an effect on image brightness similar to increasing the exposure by 1 stop. The amount of light entering the camera is not changed by gain. (Gain is a lot like push processing in film and 6 dB of gain would correspond to about a 1 stop push.)

The three main in-camera factors (forgetting filters and DSP presets for the moment) that effect image brightness are shutter and aperture (the exposure) and gain. Generally speaking, a 6 dB change in gain corresponds to a 1 stop change in aperture or a 2x change in shutter speed and within the limits of the camcorder you can trade-off among them to produce ~the same brightness in the final image.

However, because the CCD is an imperfect device, you may encounter some differences caused by the portion of the CCD sensitivity curve you are operating on, image noise due to integration of dark current during long exposures, and other subtle and not so subtle artifacts in the resulting image. The trick is to learn the capabilities of the gear as they relate to your artistic intent.
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Old November 14th, 2007, 05:57 PM   #12
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Don,

I stand corrected about the 3db = 1/2 f stop. I mean I'll take your word for it :).

Hugh,

Not to beat a dead horse but.... you say
Quote:
I think that simply raising the shutter speed, in order to let in a little more light,
Raising the shutter speed reduces the amount of light coming in. In other words, raising the shutter speed from 1/60 to 1/120 will reduced the exposure by 1 stop. Conversely, reducing the shutter speed from 1/60 to 1/30 will increase the exposure by 1 stop.
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