Film-Tech Forum ARCHIVE: What is depth of focus, depth of field and lens stop?

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Posted by Iben Jimenez (Member # 3713) on 04-19-2006, 07:56 PM:

Greetings:
Can anybody explain me what is depth of focus, depth of field and a lens stop?

Thanks

Posted by Sean R. Custer (Member # 3531) on 04-19-2006, 09:08 PM:

Iben,

F-stop is the aperature diameter of a lens, usually selectable but as it pertains to motion picture presentation usually fixed "wide" between f/1.4 - f/2. On a camera lens, you'll see an adjustable aperture ring to the effect of f-stops 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Each numeral down closes the aperture, admitting half as much light. Conversely, opening the aperture from say, f/11 to f/8 doubles the light through the lens. Depth of Field & Depth of Focus are one & the same. They are affected by both the diameter of the aperture and the focal length of the lens. A wide aperture (f/1.4 - f/4) has a shallow depth of field (the amount of foreground and background in focus around the selected plane of focus) meaning, not much aside from what you've focused on will appear sharp. A short focal lengh lens (wide angle) will give more depth of field than a normal focal lenth lens at the same aperture. A long focal length lens (telephoto) will have substantially less depth of field at the same aperture as a normal lens. This is just lens design, and how light is compressed and delivered through any given optical system. At small apertures, severe wide angles have nearly infinite depth of field, whereas a telephoto lens might get at f/32 what a normal focal length lens might render sharply at f/8.

As it pertains to motion picture presentation, your lenses are "fast" meaning they have wide apertures to admit as much light as possible. The lens designs for both anamorphic and spherical (flat) lenses is predominatly in the normal and telephoto range. The focal lenth is entirely dependent on your projection throw. Projectors that are right up against a huge screen might be approaching the benifits of wide-angle lens design in terms of focus. But if you've noticed a movement of a millimeter or less affects your on screen focus - this is why.

To get an idea the focal lenth on your projectors they should be engraved on the barrel, along with the aperture diameter given as an f/stop. Multiply the numeral given in millimeters by 2 to get a rough estimate as to what that lens would be in 35mm photography.

And the price: that big, fast glass ain't cheap!
Cheers,
-SRC

Posted by John Pytlak (Member # 331) on 04-20-2006, 01:15 PM:

Some links:

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

quote:
Many representations of the depth-of-field equations exist. Some are approximate, valid for either the far field or the near field, and some are exact. The great majority of manifestations encountered in text books have in common that the issue of lens (a)symmetry is completely ignored. This is fine as long as an asymmetrical lens is not used at close focus and as long as the limited validity is mentioned, but the latter is rarely the case. The below derivation of the DOF equations makes due allowance for lens design asymmetry

http://en.wikipedia.org/wiki/Depth_of_field

quote:
In optics, particularly film and photography, the depth of field (DOF) is the distance in front of and behind the subject which appears to be in focus. For any given lens setting, there is only one distance at which a subject is precisely in focus, but focus falls off gradually on either side of that distance, so there is a region in which the blurring is tolerable. This region is greater behind the point of focus than it is in front, as the angle of the light rays change more rapidly; they approach being parallel with increasing distance.

Posted by Iben Jimenez (Member # 3713) on 04-20-2006, 02:51 PM:

The reason I post this topic is because supposedly, the reason why the curvature in a screen made for the purpose of focus is not so necessary today as it was in CinamaScope time is that modern lens has a better depth of focus. But I don't understand yet the relationship.

Thanks

Posted by John Pytlak (Member # 331) on 04-20-2006, 03:01 PM:

The main reason to curve a gain screen is for uniformity of illumination, and not for focus uniformity. Of course widescreen processes like Cinerama, Dimension 150, and IMAX Dome used deeply curved screens to "surround" the audience with the image.

Posted by Iben Jimenez (Member # 3713) on 04-20-2006, 03:07 PM:

Yes John, you are right. But I have read the manuals provided by the 20th Century Fox at the time, and it is clear that they used the curve for two purposes: even illumination(silver lenticular screen) and even focus across the screen(curvature radius equal to the throw). Please correct me if I'm not right.

Kind Regards
I Jimenez

Posted by John Pytlak (Member # 331) on 04-20-2006, 03:38 PM:

The 20th Century Fox CinemaScope recommendations may have been specific for the anamorphic lens designs of that era. Modern lens designs usually have some curvature assumed, either of the film plane (due to radiant heating of the film and curved gates) or of the screen, but likely do not assume a deeply curved screen.

Again, gain screens should be properly curved and tilted by ray tracing to reflect the projector's light back to the prime seating area, to achieve optimum uniformity of illumination. Then choose the lens design that gives the optimum focus for that screen and the projector gate / lamphouse used.

Posted by Hugh McCullough (Member # 1542) on 04-20-2006, 04:45 PM:

Sean says

quote: Sean R. Custer
Depth of Field & Depth of Focus are one & the same

A common misconception.
Depth of field has already been explained above, but depth of focus is the space behind the lens where the film plane position can vary slightly and still not cause any loss of sharpness.

Depth of focus is not affected by focal length of the lens.

Posted by Robert Throop (Member # 61) on 04-20-2006, 04:58 PM:

[QUOTE]Depth of focus is not affected by focal length of the lens.
I always thought that depth of focus increased with focal length.
Bob

Posted by Frank Angel (Member # 248) on 04-21-2006, 01:18 AM:

Because there has been a increase in the use of curved screens recently in new builds, Schneider initiated a project to revisit the relationship between screen curature and focus with modern lenses. They concluded that screen curvature does improve focus across the entire screen, especially on larger screens. Some of their findings surprised even them. Obviously if lenses are designed to correct for an assumed curve, then it is important to know the dimension of that curve since it changes with throw distance -- you can't just pull the curve dimension out of the air when designing the frame. If the screen is flat, how does that affect focus with lenses that assume a curve what does that mean for a theatre that has flat screens and vice a versa. The Schneider study addressed these issues.

I looked on their website but can't find any reference to this study, but I am sure I have a white paper that they sent me a few years ago somewhere in the office. I will see if I can find it and post it later.

Posted by Iben Jimenez (Member # 3713) on 04-21-2006, 10:51 AM:

Frank
Where did you find the information about the Schneider study?

Posted by Larry Myers (Member # 738) on 04-24-2006, 06:51 PM:

Yes, depth of focus is not controlled by focal length but rather the f-stop and wavelength of light being used.

A real quick way of figuring this is simply taking the f-stop and square it and multiply it by the main wavelength and times it by 2 for both fore and aft of the film surface.

So a f-1 lens will have a depth of 1 micron. That's 1 squared times .500 micron times 2.

So it goes like this.

f-stop vs microns of depth of focus

f-1= 1 micron
f-2= 4 microns
f-4= 16 microns
f-8= 64 microns
f-16= 256 microns
f-32= 1024 or almost 1mm at 1000 microns to a millmeter
f-64= 4096 or almost 4mm

This is regardless of focal length.

My home theater uses a 8 foot scope screen with a 2.5 inch curve in the middle with a 12 foot throw from the center of the screen using a 60mm Ultra Star and a Super Cinelux Compact Anamorphic. That means the center of the screen is 2.5 inches further in film to screen distance then with a flat screen. The focus is perfect from ear to ear. I really need a 65mm to move the projector back another 10% or about 1 foot for easer film loading of the projector in my small booth.