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Depth of Field on LCD projector

5481 Views 12 Replies 5 Participants Last post by tvted, Jan 2, 2006

BLloyd

Discussion Starter · Dec 27, 2005

This is probably a stupid question but I've tried several searches and haven't come up with an equally stupid answer

I'm familiar with the concept and the use of a lens' iris to control Depth of Field (dof) in 35mm photography. My question is do LCD projector lens have a similar dof range in which everything should be considered "in focus"? And if so, other than "eyeballing" a test pattern, is there a way to determine depth of field for a particular lens?

For example, I have a Z2 and the spec for the lens is:

Short Zoom; F2.0-2.54 / f0.85â€-1.09â€

Does that mean the f-stop is adjustable between 0.85 and 1.09? (pretty wide open which means very narrow dof)

Any insight would be appreciated...

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reedl

· #2 · Dec 27, 2005

I think the first F is the f-stop of the lens. So the lens has a 2.0 to 2.54 F-stop.

The second is the magnification factor. So it can be anywhere from .89 magnification (smaller than a normal lens) to 1.09 (magnified a little).

But of course I may be wrong.

Reedl

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BLloyd

Discussion Starter · #3 · Dec 28, 2005

Could be reedl. On camera lenses the f-stop is usually represented by a lower case 'f' which is why I was thinking the other way around.

In any case I'm still wondering how to tell how deep the area that is considered "in focus" is....

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tvted

· #4 · Dec 28, 2005

Depth of Field would imply that there is "depth". The screen is a 2D field so the answer is no. At the F-Stops that the lens provides you are looking at minimum depth of field anyway. The focus would have a small range that your eyes *might* find acceptable but there is still only one point that is best, which is subject to the lens quality.

For example my AE 700 is not uniform in focus across the screen. Overall one side is sharper than the other. I use my XP desktop to focus on the icon text finding the best balance between both sides of the field. Lenses in this price range are not the best.

ted

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Teeh

· #5 · Dec 28, 2005

I read on some manufacturer's website (can't recall which or whether) that the range in f-stop specs for zoom-capable projector lenses means that the efficiency of the lens to transmit light varies as the lens is zoomed. It was further stated that the dof is deeper and the f-stop is the lower number (more light) towards the wide beam setting. Conversely, the f-stop becomes larger (less light) and the dof becomes shallower towards the telephoto or narrow beam setting.

Note: It has been discussed elsewhere that over such a short distance in any relatively clean room, the light drop is not related to distance, but only to lens f-stop and image size. In other words, the famous inverse square law does not apply, assuming a constant image size. Of course the amount of light that hits the screen as well as reflected back is reduced a little if the room's atmosphere has significant fog (vapor), dust or other substances that reflect or absorb light.

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tvted

· #6 · Dec 28, 2005

Quote:

Originally Posted by reedl
I think the first F is the f-stop of the lens. So the lens has a 2.0 to 2.54 F-stop.

The second is the magnification factor. So it can be anywhere from .89 magnification (smaller than a normal lens) to 1.09 (magnified a little).

But of course I may be wrong.

Reedl

Basically you are correct. The second set of numbers represent the focal length of the lens as the zoom range is adjusted - essentially picture size for a given throw.

As the focal length gets larger the lens aperture gets smaller (higher F-stop numbers. F-stop is the relationship between the area of the aperture opening and the focal length.

ted

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tvted

· #7 · Dec 28, 2005

The meaning of the F-Number:
http://en.wikipedia.org/wiki/F-number

An explanation of F-Stop
http://www.uscoles.com/fstop.htm

Quote:

Originally Posted by Teeh
I read on some manufacturer's website (can't recall which or whether) that the range in f-stop specs for zoom-capable projector lenses means that the efficiency of the lens to transmit light varies as the lens is zoomed. It was further stated that the dof is deeper and the f-stop is the lower number (more light) towards the wide beam setting. Conversely, the f-stop becomes larger (less light) and the dof becomes shallower towards the telephoto or narrow beam setting.

This is because the lenses in this price range are not high quality and lack the additional elements that would allow for constant aperture like some photographic zoom lenses.

Again, depth of field is really not applicable in this situation as there is only one plane to focus on - the screen is 2D.

Quote:

Originally Posted by Teeh
Note: It has been discussed elsewhere that over such a short distance in any relatively clean room, the light drop is not related to distance, but only to lens f-stop and image size. In other words, the famous inverse square law does not apply, assuming a constant image size.

The inverse square law applies, because this is not a point source since you have zoom capability which obviates this (don't use the zoom and its applicable). Thus, as you said, screen area is the reference - hence the use of Ft. Lamberts as the unit of measure as it incorporates area as part of the measurement.

ted

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BLloyd

Discussion Starter · #8 · Dec 28, 2005

tvted,

Yes...a flat screen is 2D. The reason I am interested in how "deep" an acceptable focus area I can achieve is because I am interested in building a 2.35 constant height curved screen. Not as radical as some of the torus screens used with CRT projectors, but just enough curvature to give it a three dimensional look. To do so successfully I need to know Depth of Field for my lens. For example if the left and right edges of the screen were curved in 3" from the center, will my lens have a focus area that is 3 " deep? Or at even that moderate curvature would the edges be out of focus?

Thanks for your input....

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tvted

· #9 · Dec 28, 2005

Quote:

Originally Posted by BLloyd
tvted,

Yes...a flat screen is 2D. The reason I am interested in how "deep" an acceptable focus area I can achieve is because I am interested in building a 2.35 constant height curved screen. Not as radical as some of the torus screens used with CRT projectors, but just enough curvature to give it a three dimensional look. To do so successfully I need to know Depth of Field for my lens. For example if the left and right edges of the screen were curved in 3" from the center, will my lens have a focus area that is 3 " deep? Or at even that moderate curvature would the edges be out of focus?

Thanks for your input....

I use my 700 with a Prismasonic H1000 lens for a 120" wide 2.35 screen, though mine is not curved.

The image on the edges are slightly out of focus on a screen anyway due to the angle of the incident light. In other words the light at the edges have further to travel than the centre so focus will always be different. A curved screen will actually help focus in this situation. Another solution to this (if there is an issue) is to increase the throw of the unit thus reducing the angle at which the light hits the screen. Again in this price range I doubt your lens has uniform focus across the field anyway. On top of that should you decide to go with an anamorphic lens you will require some refocus as well.

I've no experience, but I believe that there are formulae for creating curved screens which consider focus.

If 2.35 is your goal (a laudable one

) then I suggest you look through the 2.35 forum here on AVS and ask about curved screen implementation. http://www.avsforum.com/avs-vb/forumdisplay.php?f=117 Some of our members do utilize them.

ted

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noah katz

· #10 · Dec 29, 2005

Depth of field is a standard optical term for the distance, along the optical axis, over which the image is considered in focus.

A smaller aperture increases depth of field by removing the extreme rays which, I think (I'm not an optical guy but I work with them) are the first to go out of focus.

This is the principle of pinhole glasses; putting three fingers together to make a small hole to look through works good in a pinch, too.

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tvted

· #11 · Dec 29, 2005

BLloyd

In case you've not seen this link from the 2.35 forum
To curve or not

ted

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BLloyd

Discussion Starter · #12 · Jan 2, 2006

tvted - Thanks for all the info and advice!! I did see the various threads in the CH forum on curved screens. That's actually what got me interested in the whole idea (this forum is a danger to my wallet

).

To all - Thanks for the info!! Sounds like even with a fixed pixel display like an LCD projector, you should be able to get a decent image, maybe even a little improved in the corners, by using a curved screen.

Thanks!

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tvted

· #13 · Jan 2, 2006

Quote:

Originally Posted by BLloyd
tvted - Thanks for all the info and advice!! I did see the various threads in the CH forum on curved screens. That's actually what got me interested in the whole idea (this forum is a danger to my wallet

Glad to offer what little I could.

You might also do a search for "torus" on the screens (both) as well.

BTW i'm quite happy with my non-curved 120" wide setup - the only geometry errors are the normal pincushing of the Anamorphic. This can be ameliorated by:

1) increasing your throw

2) masking

3) a curved screen

and the most important one:

4) watching the movie

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