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Canon VIXIA Series AVCHD and HDV Camcorders
For VIXIA / LEGRIA Series (HF G, HF S, HF and HV) consumer camcorders.


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Old July 17th, 2007, 02:25 AM   #1
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difference between aperture and exposure.

Im just messing around with my HV20, I'm just a little confused here. I'm no expert when it comes to photography or cameras or anything like that.

I see that you can change the aperture when the camera is in aperture priority mode. I also realized that you can change the exposure, and that is a whole different setting. They both seem to do the same thing so I don't understand the difference between them.

I'm interested in getting a shallower depth of field, I'm not interested in buying any kind of lens adapter but maybe a ND filter.

But my real question is, if I can open up the aperture and turn down the exposure, then what do I need a ND filter for if I can darken the image with a wider aperture. Is this some kind of built in ND filter type of thing or what?
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Old July 17th, 2007, 06:09 AM   #2
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Changing the aperture in Aperture Priority mode forces the cameras auto exposure system to change the shutter speed to compensate and give you a good image. Setting a wide aperture (small f numbers) to give you a shallow depth of field lets in more light so the camera compensates by reducing the shutter speed (bigger numbers after the 1/xxx), which can cause movement to look choppy or jittery, but you can reduce the amount of light by using an ND filter, bring the shutter speed closer to the norm of 1/50th or 1/60th.

Most cameras have an 'exposure' mode that lets you lighten or darken the image, but the camera is still selecting an appropriate aperture (iris), shutter speed and gain setting so you can't guarantee that its going to select a large aperture for shallow depth of field.

Its important to realise that 'manual' exposure modes in consumer cameras only let you select aperture or shutter speed and the camera is still controlling the overall exposure, unless you have an 'AE shift' mode or 'exposure' mode that lightens or darkens the image, but even then, the camera is still controlling the exact combination of aperture, shutter speed and gain.

Only 'Pro' and 'Prosumer' cameras let you switch off the auto exposure and set the aperture, shutter speed, gain and ND settings yourself. Then you can set your aperture for depth of field then tweak the shutter speed, gain and built-in ND filters to get the correct exposure.
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Old July 17th, 2007, 11:49 AM   #3
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Excellent reply, Tony... just a few things to add:

Exposure is the amount of light coming into the camera. There are two primary ways to control that amount: shutter speed and aperture size. Aperture size is simply one way (often the most important way) of controlling exposure.

Jason's confusion probably stems from the way the camera is labeled... "Exposure" in this case is simply a manual override of whatever aperture value has been automatically set by the camera. In other words, it's a manual adjustment of auto exposure; a way to manually change the camera's automatic setting of the aperture value when shooting in the A Auto or Tv shutter priority modes where the aperture is controlled automatically by the camera.

An ND filter helps when you need to cut down on the amount of light without changing the shutter speed or the aperture value. You want to maintain a consistent shutter speed for a consistent appearance of your video between different shots, and you might want to maintain a specific aperture value as that helps to control depth of field. Hope this helps,
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Old July 17th, 2007, 01:47 PM   #4
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wow, thank you for your excellent replies. So a ND filter would still be appropriate for making a shallow depth of field.

I think the thread on the HV20 is 43mm correct? If it is then I should be able to just go on ebay and get any old 43mm ND filter
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Old July 17th, 2007, 03:16 PM   #5
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ND filters come in different 'densities' e.g.

http://www.geocities.com/cokinfiltersystem/gray_nd.htm

so you either need a set of filters or pick one that is going to suit you.

Alternatively, you could use graduated filters using something like the Cokin filter system which comprise a screw-in filter holder that will hold several circular and square filters at the same time, together with an optional lens hood. A graduated filter is clear at one edge and gets increasingly 'dense' across the width of the filter. You can slide it into the filter holder so that it gradually darkens bright areas in an image such as sky or sea, allowing larger apertures for shallow depth of field. With care, you can reduce the contrast in an image and produce some very dramatic effects, but it does limit camera movement somewhat.

Of course, if all you want is shallower depth of field then you can do it the easy way with your feet ! Just walk away from your subject and zoom in. As you zoom in, the focal length of your lens increases and the depth of field decreases, throwing the background out of focus - its basic optics !

Tony
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Old July 17th, 2007, 06:23 PM   #6
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I've been using PD170s for a few years and recently bought the HV20 as a vaca cam and intro to HD. I tried the Av and Tv modes and noticed that if I tried to lower the exposure in Av, that the shutter compensated giving me a brighter image than I wanted. Conversely, the Tv mode seems to do the same thing adjusting aperature when I set the shutter at different speeds.

Don't think I like the camera giving me an image it thinks is better but I know it's consumer. But, based on Chris and Tony's comments I'm not sure how the camera will react to an ND filter. With the filter on, won't the camera just try to compensate and brighten the picture via aperture or shutter?

Also, since the camera still adjusts exposure in either Av or Tv, is one preferable to the other?
Thanks all.
Bob
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Old July 18th, 2007, 03:04 AM   #7
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Bob ...

Using an ND filter in 'Aperture Priority' mode causes the camera to change to a slower shutter speed - but you have to be careful not to let the speed drop below 1/50th (PAL) or 1/60th (NTSC), although some cameras will not do this, or give you the option of preventing it.

In 'Shutter Priority' mode the ND filter causes the aperture to increase to let in more light, giving you a shallower depth of field but with possibly lower resolution because lenses are not at their best at wide apertures.

The 'AE shift' or 'Exposure' mode allows you to force the auto exposure to make the image darker or lighter regardless of the mode it is in, so it gives you some control over shadow or highlight detail. Some cameras allow you to control AE shift on-the-fly, but most only allow you to pre-set it in a menu.

So a rough rule is - use Shutter Priority mode when you want motion sharply defined or you want to do slo-mo effects or stills in editing, use Aperture Priority mode when you want to control depth of field, and use AE shift to lighten or darken the image.

The best thing to do is put the camera on a tripod, connect it to a big TV and play with the different exposure modes, watching how detail in the shadows and highlights are captured and how the depth of field changes, especially when you zoom in and out.

I record a lot of stage shows, and I use full auto on my Sony FX1 to catch backstage and close-up rehearsal action because there is no time even to think about exposure. But for recording the actual show, I use full manual control and constantly adjust the aperture and gain to prevent the stage lighting from burning out the detail in actors faces.

Tony
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Old July 18th, 2007, 02:27 PM   #8
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Ok. I'm really going to show my stupidity or least lack of knowlege, but I can't let the discussions go on anymore about ND filters without asking. What the heck is an ND filter. I think I may have heard it referred to as a Neutral Density filter. I also wasn't sure whether it is something we can screw on to our lens like ultra violet & other filters or not. I see you mention changing a setting of the ND filter. Is there a setting somewhere in the menus on the camera to do this? Under what circumstances should I be thinking about purchasing an ND filter or changing settings on what sounds like a built one, in our cameras. Sorry for the stupid question. Just send me to an online encyclopedia, if that's where I should go. Who knows, maybe there might be someone else here who is in the dark on this like I am. I feel that sometimes if you don't ask, you might not get an answer.
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Old July 19th, 2007, 03:05 PM   #9
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John ...

Its a good question.

Think of an ND filter as sunglasses for a video camera.

Just as you put on sunglasses to protect your eyes and stop you from squinting, so you screw an ND filter on to a camera to protect the CCD/CMOS chip(s) from getting too much light and burning out the highlights, or to force the auto exposure to open the iris (the aperture) and increase the depth of field, as discussed above.

So, you might ask, why doesn't the auto exposure simply close the iris to decrease the amount of light ?

The reason is that as the iris gets smaller a diffraction effect occurs which starts to reduce the resolution of the chip, so there is a limit to which you can close the iris before the image quality falls apart, especially with HD. (I'm no expert on this, but if you want to know more I can refer you to an ex-BBC man who can write you a thesis on the subject).

Pro cameras have manually switchable ND filters, while consumer camcorders have built-in ND filters that they can bring in progressively.

So the auto exposure in a consumer camcorder not only has to juggle the iris and the shutter speed, it also has to turn up the gain in low light without producing too much noise and it has to bring in the ND filter in bright light to keep the image sharp - its a wonder that they work as well as they do !

Bottom line is that with a consumer camcorder, you only need an ND filter if you want to fool the auto exposure into giving you a shallow depth of field when you are in shutter priority mode. But you can get the same shallow depth of field simply by zooming in.

In short, don't bother with ND filters.

Just in case you were wondering, the 'shallow depth of field effect' is where your subject, such as a face, is in sharp focus, but everything else, like the foliage in front and the wall behind is blurred.

Keep asking the questions - its the quickest way to learn.

Tony
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Old July 19th, 2007, 03:09 PM   #10
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ND= Neutral Density. Nuetral for color, but allowing less light to pass.
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Old July 19th, 2007, 03:28 PM   #11
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Thanks guys. That was very helpful to my understanding. I do wonder now since it is built into the HV20 and others too, is it actually a physical filter that moves into place between the lens and the sensor. If that is the case, how would it change it's characteristics as it needs to? Maybe like the glasses you see advertised on television that darken automatically as the person steps out into the sun? If it did that, I wouldn't think it could be very precise.
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Old July 19th, 2007, 04:32 PM   #12
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Quote:
Originally Posted by John Hotze View Post
Thanks guys. That was very helpful to my understanding. I do wonder now since it is built into the HV20 and others too, is it actually a physical filter that moves into place between the lens and the sensor. If that is the case, how would it change it's characteristics as it needs to? Maybe like the glasses you see advertised on television that darken automatically as the person steps out into the sun? If it did that, I wouldn't think it could be very precise.
Don't know for sure, I'm not gonna tear my HC7 apart to find out !

I would guess that the ND filter in a palmcorder is a graduated disk or plate that starts clear and gets progressively darker and is gradually moved across the CCD/CMOS sensor.

Anybody know for certain ??
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Old July 20th, 2007, 10:30 AM   #13
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Quote:
Originally Posted by Tony Neal View Post
Don't know for sure, I'm not gonna tear my HC7 apart to find out !

I would guess that the ND filter in a palmcorder is a graduated disk or plate that starts clear and gets progressively darker and is gradually moved across the CCD/CMOS sensor.

Anybody know for certain ??
Hey Tony, when you tear your camcorder apart, would you mind taking some photos of it as you do. We would probably all love to see what a real internal ND filter looks like.
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Old July 20th, 2007, 11:14 AM   #14
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John,
Not sure if I understood Tony's response but an ND filter is not built into the HV20. You have to buy an ND filter lens and screw it on.

I slightly disagree on the value of an ND filter also. I use PD170s professionally and they have 2 built in ND filters. They are invaluable for capturing properly exposed outdoor images. When I bought the HV20, I also got a polarizer and ND filter but haven't had a chance to try out the ND. BTW, the polarizer should always be on this or any camera to protect the lens. I would only remove it for the ND or other filter.
Bob
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Old July 20th, 2007, 01:52 PM   #15
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Bob. I was only going on what some others have said. I'm pretty sure that some have talked about the built in ND filter on the HV20. I personally haven't read the book to find out if it stated their wasn't any built in. As for as lens protection, I purchased the WD-H43, installed it on my 20 and haven't taken it off since. It came with a lens cap, which also goes on whenever I'm not shooting. I have shot about 50 tapes now and don't really see a reason that I willl ever take the wide angle off unless I get a Canon telephoto lens.

As for as the need for an external ND filter, if it would help prevent blowouts when I tape something bright, like a person wearing a white teeshirt out in the sun, I would sure like to have one.
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