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Old September 5th, 2004, 10:05 PM   #16
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Hi Jeff,

True, focal length is part of the formula but makes very little difference, provided subject size in frame (and f-stop and image size) stay consistent.

Look at the photos on this page:

http://www.luminous-landscape.com/tutorials/dof2.shtml


The pictures of Gromit on this page tell the whole story- because the car is equally blurry in pic A and pic B, because the DoF is the same regardless of focal length. Of course, the background looks blurrier in A because it's bigger!

http://www.vanwalree.com/optics/dof.html

A little further down the page is what Charles was talking about- blowing up the car to the same size and comparing the two pictures- you can see the "blurriness" is consistent.

(Interestingly, this page criticizes the first link I posted!)


From this page:

http://www.normankoren.com/Tutorials...F_focal_length

"DOF expressed in distance is nearly independent of focal length."

You can see by the chart that DoF remains almost constant (within 2mm) from 50mm to 1000mm focal length. With the short focal range of 20mm DoF only increases to 217 mm.

From this one:

http://www.cinematography.net/Pages%...dImageSize.htm

"Image size and f-stop are the main determinants of depth of field. Focal length of the lens does not matter, although on the extreme ends of focal length, image diffraction and interference play some part. If one moves an 18mm shot close enough to make the subject exactly the same size as it was with the 250mm lens, then the DoF is identical assuming the f-stop and circle of confusion choice are exactly the same."

Our whole teaching philosophy at DVcreators.net is "all practical, real-world techniques, no fluff, no theory"! So it's ironic that I am the one that brought up such an impractical, though perhaps interesting diversion.

For anyone besides an obsessed technogeek reading this: "If you want your background fuzzy, zoom in!"
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Old September 6th, 2004, 03:54 AM   #17
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The key to all this is that we are assuming the same subject size in frame. My example of blowing up the center of the image was not making this assumption, rather the same subject photographed from the same distance. In this case, the focal length changes and the DoF changes with it. A wide-angle version of the scene (in which the subject is perhaps 1/10th the height of the frame) and an extreme telephoto (in which the subject's head is seen in close-up) will deliver different depth of field characteristics.

An example: given 35mm 1:33 as the shooting format:

A 17mm lens focused at 6 feet will deliver a depth of field from 4'2" to 10'10".

A 100mm lens at 6 feet will deliver from 5'11" to 6'1".

So, if we had another subject positioned at 10', it will be in focus on the 17mm, but not on the 100mm. And given a perfect recording medium where resolution is not an issue, by blowing up the center of the 17mm shot, that subject at 10 ft should be sharper on the 17mm than on the 100mm.

However, if we were to match the image size of the first subject as shot on the 100mm using the 17mm, we would have to march in to 1'. And our subsequent depth of field would be from 11.4" to 1'1"--in other words, very similar to what we saw with the 100mm (roughly one inch on either side).

Or am I missing something?

(p.s. thanks for the inquiry Josh--I'm about to ship out to Montreal to work on a feature until the beginning of October).
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Old September 6th, 2004, 06:55 AM   #18
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This is a confusing part of DOF for many photographers. The arguments makes assumptions about the position of the camera in relationship to the subject and subject size. In my experience motion picture, video and portrait photographers think of DOF in terms of a constant or fixed subject size. Landscape, sports and wildlife photographers think of DOF not in terms of a constant subject size, but rather how to minimize or maximize DOF for the scene in front of them. The way each of these groups thinks, in terms of their own photography, effects their perception of how DOF works. Josh, you may want to review the article we have posted for our community here at DV Info.net.
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Old September 6th, 2004, 07:12 AM   #19
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another optics question

I am posting in this thread because of the vast skill and experience of those who are posting in it.

My question is this.

I am shooting out the front window of a vehicle moving at freeway speeds. When I zoom in fully, the lane stripes move as though the vehicle is travelling very slow (looks like 5 to 10 mph). When I pull back to full wide, the vehicle then appears to be travelling at the same speed that my eyes see.

What confuses me even further is the fact that the illusion of faster speed is seen by the naked eye with a narrower field of vision. Exactly opposite of how the camera behaves.

Thoughts, comments?

Thanks in advance for your help.

regards,

-gb-
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Old September 6th, 2004, 07:31 AM   #20
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This is one of those tricks that the mind and eye play on each other. The mind needs points of reference to accurately perceive speed. When the lens is zoomed in on the pavement, the eye loses any stationary point of reference.

The mind and eye play the same tricks with the size of objects also. This is why the eye is never an accurate judge of size. Have you ever seen the moon rising on the horizon and it looks huge? Then later in the evening when the moon is higher, it has somehow shrunk or the the earth has adjusted it's orbit and the moon is much, much smaller. What is happening? The mind judges all things on the horizon as being a great distance away and if seen, they must be very large, such as mountains, skyscrapers, the moon etc. But things overhead are perceived differently by the mind because they lack the reference of the horizon. Next time you see a large moon on the horizon, get a quarter from you pocket and hold it next to the moon at arms length. You will actually see the moon shrink to the size of the quarter (it's actual size) right before your eyes.

I've been doing this trick with my photography students for years. It never ceases to amaze them.
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Old September 6th, 2004, 11:44 AM   #21
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Quote:
The diameter of the CoC is determined only by the size of the aperature, not the focal length
That's a confusing statement... the diameter of the Circle of Confusion is actually a subjectively-determined value that is dependent on the size of final blowup. The more blown-up the final image will be, the tighter the circle of confusion needs to be in order to survive the magnification process. 16mm DOF charts use a tighter CoC because 16mm DOF charts are designed to tell you what the depth of field will be WHEN BLOWN UP TO 35mm.

Circles of Confusion are the most variable element in the DOF formula, because the CoC is entirely dependent on the final viewing format. Film cinematographers prefer to see their dailies projected on the big screen, rather than video dailies, because something may look perfectly in focus on the small monitor but be quite out of focus when projected.

The more your image will be magnified, the tighter the CoC needs to be, which is why 35mm has the largest CoC and 1/6" CCD's have the tightest. The CoC isn't dependent on aperture or focal length or anything else; it's a subjective value that is tied to how large you intend to blow up the final image.
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Old September 6th, 2004, 12:41 PM   #22
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Thanks for the info Jeff. I will do the quarter trick. The horizon reference is also possibly the explanation for watching a sunset, or sunrise. Ever notice how 'fast' the sun goes when it gets next to the horizon. It goes from full sphere to completely gone (sunset) in less than 5 minutes. Same statement applies. Does the earth suddenly turn faster when the sun is near the horrizon, of course not.

regards,

-gb-

Ok, enough of my questions. We now return this thread to its 'circle of confusion' regarding 'circle of confusion'. (big grin)
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Old September 14th, 2004, 07:07 PM   #23
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<<<-- Originally posted by Boyd Ostroff : At the end of Spellbound there is a shot viewed from the perspective of Leo G. Caroll as he points his gun at Ingrid Bergman, then turns it towards himself to commit suicide. Hitchcock wanted the hand with the gun and Bergman to both be in sharp focus. His solution was to build a giant hand and gun! -->>>

Just a followup on this. A couple minutes ago I turned on the TV just as Turner Classic Movies was showing the final minutes of this film. Watching that scene again I noticed that while the giant gun was quite convincing, the giant hand was actually very comical and the details of the knuckles and fingernails had an almost cartoon look to them! But even with the gimmick of the oversized props, the background is in focus but the hand and gun aren't quite. It also appears that they used some sort of silk effect to give Ingrid Bergman a soft look.

It's funny what lengths we'll go to in order to realize some crazy idea we've dreamed up... and Hitchcock was no different in this regard.
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Old September 15th, 2004, 01:49 AM   #24
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Ten years ago I shot a short that involved one character looking at his reflection in a spoon, with another character in the background, and both were to be in focus. I can't remember the specifics but I do remember that I had to use both a split diopter and light the set to something like an f11 (this was a 16mm project) to hold the deep focus. It was a real pain; the set was ridiculously bright, we were burning every light we had but the shot worked.

On DV, it would have taken hardly anything to achieve the same (or better) results.
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Old September 22nd, 2004, 02:33 PM   #25
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Michael, your post got moved to Photon Management (lighting).
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