Hi Charles, excellent info thanks. You're right the reason I wanted the wide angle lense is if for example I wanted to get a shallow DOF in a room and couldn't back away far enough to zoom in. I've also been doing some playing around and the results I've been getting are good so I'm quite pleased, but I'm going to go to my local camera store to try out the wide angle lense to see if it can give a shallow DOF when zoomed allowing me to be a bit closer to the subject.

Thanks again guys, this is a really interesting thread.

John.

Ok, now I've read these posts I really need a whiskey to recover. Can someone point me to a site that will explain how all this focal length, lens size and stuff really works. I don't understand what a focal length means for one thing. 5.5mm on the 16x lens? Is something in the lens 5.5mm away from something else? Why do you need to be 7x as far away when using a 35mm lens with an EOS adapter as opposed to something like the mini35 adapter?

I really would like to get this all understood, so I know what people mean and what the difference is when they say a 28mm lens, or 72mm lens or whatever. Once I have my camera I plan on doing many short movies and so I'd like to be reasonably up on the play as far as camera geek speak goes.

EEEK!

Ah, this could be a long one. First lets get our millemetres straight. MM can represent the focal length of the lens or it can represent the diamater of a filter or other attachment. Lens caps can be 49mm, 55mm, and filters can be 72mm etc.
Lets talk about the focal length of a lens. The strict definition is the distance between the rear nodal point of the lens and the best axial focus of an object, when the lens is focused at infinity. Huh? Ok. A nodal point is an imaginary point in the lens design. In simple terms it is the rear point at which the light at the back of the lens gets bent slightly to come to focus at a point on the CCD (or film). Measure the distance in mm from where the light bends to the chip. If its 50mm, it is a 50mm lens. That was easy.

What type of lenses are there? Normal, Telephoto, Wide-Angle,Ultra Wide-Angle and Fisheye. A normal lens makes the image appear to have the same perspective as the original scene when viewed from the normal viewing distance. It is roughly equal to the diagonal of the CCD. A telephoto, from the same position, will produce a larger image proportionate to the focal length of the normal lens. A 100mm lens will produce an image twice as large as a 50mm lens. A wide-angle has a greater angular field than a normal lens. shooting from the same position it will include more subject area, but with smaller images. It tends toexaggerate differences between near and far. An ultra wide-angle is a wide-angle have extreme angular coverage of up to 120¡. It maintains rectilinearity with no bowing of straight line. A fisheye is an extreme wide-angle that covers up to 180¡. it has noticeable barrel distortion. Lastly a zoom lens. A zoom lens is a complex lens system with variable focal lengths that are produced by moving (zooming) the spacing of some of the elements within the lens while maintaining the same back focus. The zoom lens allows the image size to change without moving the camera.

Zoom ratios are the last part for now. The 16x is the amount the zoom changes. The 5.5mm multiplied by 16 equals 88mm. The standard XL1 16x AF lens is a 16x zoom. There, not so hard. You'll be up to geek speed in no time.

Jeff

Nice work Jeff. I am embarassed to say that after all these years as a cameraman I wouldn't have been able to define how the numerical formula for focal length is divined, but then again it doesn't come up in practice all that often--well, never, actually!

Aaron, the reason there is a 7.2x magnification between a 35mm lens and a DV lens is the target size (35mm negative vs a much smaller DV chip). Imagine projecting on your wall a 35mm slide of a head-to-toe image of a person. Now walk up to the wall and draw a frame around the person's head. This represents the DV chip. Using an EOS lens adaptor with a 35mm lens means that you are "projecting" a large image onto a small target, the DV chip, and it will only "see" a small area of the original frame. Thus a wide-angle lens like a 28mm will become a telephoto lens when placed on the XL1. To achieve that same telephoto field of view on your still camera you would have to multiple the focal length by 7.2, which would be around a 200mm lens.

Now, still thinking about your SLR camera, the image that you see when you look through the viewfinder is has a different quality than the viewfinder of a point and shoot; it is the actual image from the lens displayed on the surface of a ground glass. It has the depth of field characteristics that the final image will have, and will demonstrate focus and flares and other optical phenomenon. The Mini 35 incorporates such a ground glass, capturing the full frame of the 35mm motion picture lens and presenting it to the XL1 to be rephotographed. It's like a high end version of one of those devices you can get to transfer slides or 8mm movies to video.

Charles... I did not find any macro button/switch what-so-ever.
Perhaps I am not crazy at all? Where you perhaps referring to
another lens?

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.

Margus

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!

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.

Jeff

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.

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.

Jeff

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?

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.

Jeff

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!

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

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

Jeff

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

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.

Jeff

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?

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.

<<<-- 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.

I haven't read that issue at length yet, it's kicking around on my desk somewhere. Most DP's like to shoot fairly open, but it does depend on the project. I was fortunate enough to do some Steadicam work for Roger Deakins last month, and was a bit surprised that we were working at an 8/11 stop for a day exterior walk-and-talk...but then figured that it was a definite choice, and came to understand why he would want a deep focus for that particular shot. I like to work that way also, considering what is appropriate for a given shot or mood.

That's for film though. There is no question that video looks better with shallow depth of field, I think it probably has something to do with the rather harsh edges in a video image. The less of them in the shot, while there is still something in sharp focus, the more attractive the image. But unless you are using something like the Mini-35, it's really hard to get DV to fall off in focus at a wide to medium focal length, regardless of how wide open the lens is (and the supplied lenses, while good, are not sharpest at their widest opening).

Hi Jeff,

<...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....>

The manual calolats first the Hyperfocal Distance for every F.stops and then the "NEAR" AND "FAR" limits for different "Distances from camera to object"

You want me to put the formulas here?
Istvan

Depth of field is the distance between the nearest and farthest objects that appear to be in acceptably sharp focus in the image plane. Depth of field involves one image plane and the area between two target planes (in front of the lens).

Depth of Focus is the distance range through which the image of an abject, in a single plane, can be shifted axially and still have the image appear to be sharply defined. This is the area behind the lens, at or near the CCD (film plane).

So, depth of field is a distance, in front of the lens. Approxomately 2/3 of the distance is behind the image plane (subject) and 1/3 the distance is in front of the plane.

I've moved recently and many of my books are still in storeage. I'm sure many would like to see the formulas. Thanks!

Jeff

OK, so here is the formula, taken from the "American Cinematographer Manual" 8-th edition, pag:698,699

First the Hyperfocal Distance (definition from the manual: Hyperf.Dist of a lens is that focus setting where objects at infinity and half the focus distance are of an acceptable sharpness)

H= F^2 / (f)(Cc)
F=focal length of lens
f=f-stop number
Cc=circle of confusion

Depth of Field
Near limit
(H)(S) / H+(S-F)

Far limit
(H)(S) / H-(S-F)

Where:
H=HYperfocal distance
S=Distance from camera to object
F=Focal length of lens

Istvan

This forum is addictive!

I have learnt so much from this thread it is hard to beleive. I meant to go out and shoot some gritty night time shots an hour ago and I'm still in my way-too-comfortable desk chair!

thats it! I'm leaving!

Hehe, i can imagine and know what you mean.... Thanks for
the compliment though.