CurtPalme.com Home Theater Forum

[km987654]

How much better is the focus of a PJ with scheimpflug over a PJ without? Does it make that much difference?

[Gino]

I'd say you'd be very lucky to have a lens that was mounted in such a way that it gave perfect focus from corner to corner, edge to edge on your screen. Scheimpflug allows you to get this so focus is uniform.

[stefuel]

This has all to do with being able to follow manufacturers instructions and use tools. NEC for example, gives very exacting mounting instructions. If you follow those instructions to the letter, you will get perfect mechanical focus. Most of the time, following the manufacturers specifications does not allow for maximum raster useage which is a whole other arguement. Some here feel that manufacturers instructions are for the "ignorant" to follow. Choose your poison wisely. You have to live with it. That's all I have to say on the subject for fear of entering a pissing contest with those who are obviously smarter than me

[winduptoy]

Most lenses are f:1.1 or so. Depth of field is razor thin. The tiny DOF is a big reason reason why physical placement of the screen and PJ are so critical. (Graham Johnson's CRT Setup DVD shows how you will give your measuring tape quite a workout.) Scheimpflug allows you to shift the plane of focus until it falls exactly on the screen plane.

[garyfritz]

[jkruger]

I was amazed at how much better focus I was able to get with continuous sheimpflug adjustment, the 12xx sets I had before the 1209s only had shims with which to adjust and being able to get it just right makes a big difference.

[Tom.W]

You can find more info on Scheimpflug here..

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

Same principle is used in a view camera.

http://www.trenholm.org/hmmerk/FVCADNDM.pdf

[Gino]

[zGman]

The only time there is perfect geometry (eg no lens tilt required) would
be on-axis projection, and then only on the green. the red and blue would still
be projecting at an angle.
Consider the typical front ceiling mount setup where the PJ is located with
lenses at or above the top of the screen - lens tilt will be needed to get the
top and bottom equally in focus.
I haven't seen a PJ that doesn't have some provision for this, but many are
static - which makes fine tuning almost impossible.
G

[km987654]

So it seems that in theory Scheimpflug improves focus. In practice is the difference visible. Can anyone produce two images with and without Scheimpflug say with a static test pattern??

[Tom.W]

[km987654]

Well Tom I am not sure why you post on this forum but for me its because I don't know or don't have the gear to find out. In this case both.

If I had the gear to test this I would test it myself and yes it does matter what forum members recommend. Again perhaps you don't value members input but I do.

[timf]

[JustGreg]

[jkruger]

[JustGreg]

[jkruger]

That is so true. I didnt really understand it either until I started doing it.

[Tom.W]

Maybe this might help.

http://www.moesrealm.com/hometheater/crt-focus-guide.html

Here's an excerpt from the link...


Note: Mechanical Aim is Not Scheimpflug (Lens Flapping)

Novices confuse the two, but mechanical aim is not the same as lens flapping (aka Scheimpflug). Mechanical aim of the CRT/lens assembly is akin to taking a telescope and physically pointing it at something. Lens flapping adjusts the mounting angle of the end lens without changing the direction the telescope is pointed. Lens flapping doesn't change the direction of aim (well just a little), but compensates for the planes of the screen and phosphor being non-parallel. If the lens were kept parallel to the phosphor surface, it would be impossible to focus throughout the screen at the same time. The flapping places the lens into a plane intermediate between those of the screen and phosphor and that makes global focus possible.

Rough Optical Focus
Now that mechanical aim of the guns has been done, it is time to do an initial, rough optical focus.

Display a crosshatch pattern. Adjust optical focus using the two knobs. The front most knob on the lens controls edge optical focus. The rear most knob adjusts center optical focus. Dial in center focus first. Then adjust the outer (front knob) focus to while watching the corner lines of the crosshatch flare inward and outward. Try to minimize flaring using the outer focus knob. Go back and forth between the two optical focus knobs to get both center in focus and outer edge minimally flared. Use of binoculars aids this process tremendously.

Rough Scheimpflug (Lens Flapping)
Once rough optical focus is completed, you can perform lens flapping. The following is primarily for projectors which have continuously variable lens flapping. For projectors with lens flapping rings, simply set the lens rings per the installation manual specified settings for your projection distance.

Display a focus pattern. Look ONLY at the center of the top edge. Adjust center optical focus to make that edge sharpest. Note the position of the focus knob. Pay attention to the center of the bottom edge. Adjust lens center focus to make the bottom edge sharpest. Is the lens center focus control in the same position as when the top was sharpest? If so, vertical flapping is correct. If not, adjust vertical flapping using a wrench. Note the wrench position when the bottom edge is focused vs top edge. Set the vertical flapping to put the wrench in exactly halfway between the two positions. Horizontal flapping is performed in an analogous fashion paying attention to only the left and right edges.

Now go back and refine center and edge lens focus. Once that is done you should have the lenses in fair optical focus and flapping. Advanced owners may consider the 3 x 5 card technique fine focusing technique presented later in this document for even better optical focus and Scheimpflug.

[PJMoore]

KM -

I had Sonys that used wedges and rings that had three discrete settings: S, M, and L. These three settings represented a "reasonable" compromise that provided "acceptable" viewing over a wide range of throw distances and throw angles. My Marquee, however, has continuously adjustable lens plates, so no "compromise" is necessary. As a simple matter of optics, it stands to reason that the S, M, and L settings would each be "perfect" at one throw distance and throw angle and that any other throw would have some degree of error that would range from "a little" to "a lot" depending on the situation and how picky the observer is. What chance is there your setup would be one of the "perfect" ones? About zero.

So back to your questions. 1 - continuously variable flapping makes a difference, since the throw distance and throw angle are variables from theater to theater; and 2 - you cannot make a definitive comparison, per se, between continuous and discrete flapping, since any given projector will have one or the other, but no both. Any difference observed would therefore be attributable to multiple factors.

I can tell you this. Mooneyass described a means of making the Sony continuously variable. While not nearly as easy to use as my Marquee, I did notice the overall image was noticably crisper across the entire screen when I used continuous flapping on my Sonys. I quit using the wedges entirely after that ( I have had 5 Sonys).

I hope this addresses your questions. If this is the deciding factor between your choice between projectors to buy, I can tell you that it is just not a good barometer to be using. Brand reliability, parts availability, and Curts pecking order are better "barometers" for picking a CRT projector.

[garyfritz]