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Old July 16th, 2002, 04:49 AM   #46
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hi Rob,
on professional lenses it's a small ring with stopper, nearest to camera body. I think it moves nearest lens or lens group to camera (not sure about that last one). Never seen such a thing on consumer/prosumer video cameras, only on still cameras.

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Old July 16th, 2002, 11:35 AM   #47
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Sorry Rob, I was not being specific. Since I rent the manual lens (still trying to buy one used, if anyone's selling) I can't remember if it is on there or not--thought it was.

Anyway, I just pulled out my camera with the standard 16x white lens, and discovered that there is essentially built-in macro capability. Up until the halfway mark on the zoom, you can focus down to virtually the front element of the lens. This would be considered macro. And you get very shallow depth of field as a result. It's actually surprising how far you can zoom in and still get this, thanks for inspiring me to try this! On most cameras as I said it only does this at the widest position.

As far as consumer cameras having this function, all of my old ones ((Hi8, SVHS) etc. that had a manual zoom ring used to have a macro button. Usually it was incorporated into the zoom ring button, you would either push or pull it once it got to the wide end and it would allow you to keep moving the element (as you pointed out Margus) which would shift into the macro range. I think with the modern internal lenses on most camcorders, it has been incorporated into the system as described above--they all seem to have the ability to focus extremely close although it's not easy getting there and maintaining focus once you are there!
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Old July 16th, 2002, 03:12 PM   #48
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Thanks, Charles, this stuff is kinda old hat for me. I teach photography and digital photography at a Fine Arts school. I just do it one or two days a week, but it helps me keep sharp about the technical side of things. Which brings be to my next point, our old friend Depth of Field. Since that was the original topic.

Depth of Field (DOF) is dependent upon the following variations:
a. The focal length of the lens.

b. The diaphragm opening (efective aperature, not F-number).

c. The distance from the lens to the object that is focused on.

d. The distance from which the image is viewed.

e. The viewer's personal standard of the permissible degree of sharpness (or unsharpness).

Other variables remaining constant, it follows that:

a. The shorter the focal length of the lens, the greater the DOF.

b. The smaller the diaphragm opening, the greater the DOF.

c. The greater the distance to the object being focused on, the greater DOF.

d. The greater the distance from which the image is viewed, the great the "apparent" DOF.

e. An often used standard of acceptable sharpness is the reproduction in the image of a small point in the object plane by means of a "Circle of Confusion" or disc not greater than 1/100 of an inch. This is often expressed as 1/1000 of the focal length. Sometimes a figure of 1/300 of an inch or 1/3000 of the focal length is used.

The stumbling block in the discussion of DOF is part B. The diaphram opening (effective aperature) not the aperature, effective F number, or relative aperature is the variable in DOF.

I think you, Bill and Justin were on to this earlier in the discussion. The Effective Aperture is the clear opening (aperture) in a lens and is a dimension, inches, mm etc. not an F number. The Effective Aperture in a 35mm format lens is larger, giving you less DOF.

To get the equivlant image size between an EOS lens and a XL1 lens, you would need to back up 7.2 times (the magnification factor). If all other variables remain constant the backing up of the lens will increase DOF by 7.2 times.

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Old August 18th, 2002, 07:39 AM   #49
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Jeff,

My goal is to keep the foreground in focus, while the background would not be as sharp. This is from what I understand (reading your post) done by controlling the aperature size. Is this correct?

So from here while rolling tape could you bring the background into focus? This would be a great effect.
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Old August 18th, 2002, 08:11 AM   #50
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The aperature affects the DoF. Small numerical F numbers are large openings and reduce the DoF. Several techniques could be used to change the DoF. One technique is to use a split field filter. The filter is 1/2 clear (no glass at all) and 1/2 diopter to allow close focusing. This leaves a very distinct line that must be hidden by elements within the scene. This allows for both near and far elements to be in focus at the same time. Rack focus is another technique that rolls the focus from near to far or vice versa. Start with a small numerical F number (F2 or F2.8 and focus on a near object. Then focus to a predetermined point by quickly (or slowly) changing the focus on the lens. This technique is much easier with the true MF lenses and not the white servo AF lenses. Computer aided effects would be a painstaking possability. Apply a gausian blur to portions of the scene, then dissolve the effect off the scene.

Our cameras have inheirently a great amount of DoF because of the focal lenth of lenses we use. That is why a Neutral Density filter (ND) is built in, reduce the amount of light entering the lens and you have to open (numericaly smaller F number) the lens to allow more light to enter the lens. Thus reducing the DoF. The gain can also be reduced to -3db which causes about a stop of light loss also (requiring a still smaller numerical F number). ND filters can be stacked to further reduce the amount of light but a caution here. Someone posted recently about a color shift when stacking ND filters. I believe the cause was the brand of filter used. Not all filters are created equal.

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Old August 18th, 2002, 08:19 AM   #51
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I have the 16x Manual Servo Lens, so this should be easier to accomplish (I hope) from what you have mentioned. So by using rack focus, I could go from my foreground being in focus to the foreground and background being in focus. This would of course be achieved by the correct aperature setting.

Is my above statement on target or am I missing something?
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Old August 18th, 2002, 08:50 AM   #52
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I think with some experimentation this effect could be achieved. You may have to combine the split field technique with the rack focus technique. Use a square split field (Cokin, Singh-Ray etc.) and a square soft focus filter. When mated this would give approximatly 1/2 in focus (near subject) and half soft focus. Remove both filters (someone would need to pull focus also). This would leave perhaps several frames to touch up in post. Going from limited near focus to infinite focus is not easily accomplished. However, respectable results could be obtained with some experimentation and practice. Of course if your George Lucas, just do it in your computer.

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Old August 18th, 2002, 09:13 AM   #53
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Well my results have been horrible. I guess I just need more practice. I appreciate your comments and will continue to work on this technique.

Maybe I will hit the Texas Lottery and be able to buy a "George Lucas" style mini studio!
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Old August 18th, 2002, 02:41 PM   #54
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Hi
I'm a masochist and would love to hear somebody to explain me the "circle of confusion". It popped up from time to time in this threat, but everybody avoided it somehow.

I learn from the DoF-charts of the American Cinematographer Manual 8-th edition (The bible) that this is one of the main factors in determinating the DoF, BUT I don't understand what it is and why it does effect the DoF.

I found it also interesting that the charts are given for ALL formats the same value, so there are no differences for 16,35 or 65 mm lenses.

Thanks
Istvan
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Old August 18th, 2002, 05:45 PM   #55
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Hi,

I pretty much defined it in my post above, but I'll give it the complete treatment this time.

Circle of Confusion - The diameter of a circle formed by a lens imaging a true point. The largest circle which will appear as a point to the eye (without producing perceptable unsharpness). A primary factor in determining sharpness to the viewer.

As the diaphragm is closed (stopped down), the circle of confusion is reduced in diameter. However, the presence of spherical aberration, causes the plane of sharpest focus to shift along the optical axis, toward the film plane.

The smallest area of crossing for incoming rays produces not a point, but a circle, hence the term circle of confusion. If the circle has a diameter of 1/3000 of the viewing distance or less, it is considerd by the eye to be a point. Therefore, as the viewing distance increases, a physically larger diameter circle is still considered to be a point by the eye.

This post will explain sperical aberration http://www.dvinfo.net/conf/showthread.php?s=&threadid=2847&highlight=defects

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Old August 18th, 2002, 10:35 PM   #56
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Hi Jeff,
and thank you very much!!!
I think I start to understand at least a part.

But I still have a little question: in the Cinem. Manual they calculated the DoF charts with a Cc of 0.001' and suggest to consider for the 16mm a Cc 0.0005', which would mean to consider as reference much more open lens, actually like 2 F stops more open.
If I understand this well then this would mean less DoF for 16mm then for 35mm at the same distance and same focal length????

Or did I miss something?
Thanks
Istvan
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Old August 19th, 2002, 06:53 AM   #57
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Viewing distance also has to be considered. Does the manual state any viewing distances? If not, then their data is incomplete. Viewing distance is usually stated in a distance so many times the diagonal of the screen. My data is in inches, not mm or cm. As an example, a billboard (larger than your typical 35mm screen) looks sharp when viewed from the road. Yet when viewed at a close distance the dots (circles of confusion) are very large and the image is not recognizable.

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Old August 19th, 2002, 08:38 AM   #58
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Paul:

I've been thinking about your wanting to transition from a frame where the foreground alone is in focus, to one in which both the foreground and background are in focus.

With a film camera, you would achieve this by doing a combination aperture/shutter ramp. As the aperture (T-stop) is dialed from wide (short DOF) to narrow (greater DOF), the shutter speed is correspondingly decreased. The effect is controlled by an external device that calculates the rate at which both parameters are changed so the effect is invisible.

On the XL1, I have not found a way to do this as elegantly--I would think that in aperture priority mode, the shutter should follow the aperture and as you dial it up and down, the shutter compensates. I think that it doesn't do this smoothly enough to manage the effect without seeing changes in exposure, unfortunately. Perhaps someone will be able to figure out how to pull this off?
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Old August 19th, 2002, 11:27 AM   #59
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I've been doing some tests in my short vacation and have achieved
some beautiful DOF shots (as I like to call em). Where my
foreground was in sharp focus and the background was all
blurry. Very very nice. I followed the tips and opened up
the lens too its max. I added two ND filters (I was shooting
outside in the sun). Gain was at -3 dB (where it always is).
Shutter was 1/25 th I think but I can check that if anyone wants
to know for sure.

The trick is to zoom in as much as possible (which is a lot
easier outdoors than indoors). This creates a very nice shallow
DOF.... Now I need to run some indoor tests!

Cheers.
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Old August 19th, 2002, 06:15 PM   #60
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<<<-- Originally posted by Charles Papert : Paul:

I've been thinking about your wanting to transition from a frame where the foreground alone is in focus, to one in which both the foreground and background are in focus.

With a film camera, you would achieve this by doing a combination aperture/shutter ramp. As the aperture (T-stop) is dialed from wide (short DOF) to narrow (greater DOF), the shutter speed is correspondingly decreased. The effect is controlled by an external device that calculates the rate at which both parameters are changed so the effect is invisible.

On the XL1, I have not found a way to do this as elegantly--I would think that in aperture priority mode, the shutter should follow the aperture and as you dial it up and down, the shutter compensates. I think that it doesn't do this smoothly enough to manage the effect without seeing changes in exposure, unfortunately. Perhaps someone will be able to figure out how to pull this off? -->>>

Charles,

Have you had a chance to read the latest issue of American Cinematographer? There is an article in there that talks about the filming of Road to Perdition. Conard L. Hall (DP) talks about how "constently shot at the bottom of the aperture." His goal was to cut down on the depth of field. He shot a lot at "T1.9 to T2.5, which cut down on depth of field, made the focal plane more specific and softened the backlight."

Hall goes on to say, "I like to shoot wide open, with only one point in the depth of field sharply focused." He "feels" this technique gives the imagery an emotional dimension.

I thought that this article described some of the things that I am trying to achieve. Now of course I am not shooting with a Panavision Platinum and Primo lenses, but I still think this look can be pulled off. My thought that being wide open with the correct lighting, you should get a pretty nice look/feel.
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