I'm planning my first build, an un-split design, and in drawing things up on paper have really started to see how different parts interact with one another. So far I'm working on the trippy side of things and then I'll do the light engine side.

A few details so far...I'll be using a Sony MFM-HT75W which has viewable screen dimensions of 8 13/16" by 14 11/16". I'll use an 18" EF trippy and my throw will be between 13' and 13.5'.

I recognize that adjusting the distance between the field fresnel and the LCD will affect how much of the light cone will fit within the triplet. But what is the recommended distance between the field fresnel and the LCD? And, how much room for adjustment do I need? This will help me in determining what FL fresnel I'll need for my application (550 or 650).

Thanks much...more questions to come I'm sure!

you need to go to the tools section and use the focal calculator

I've done that...and have all the numbers, including the distance the triplet will be from the LCD to be in focus on my screen. But on an unsplit it doesn't make a suggestion on the distance between the field and the LCD. It makes a recommendation of 20mm on a split design and allows you to adjust that variable.

I think I've read that if the fresnel is too close to the LCD, then you can see the "cuts" in the fresnel in your image. Want to avoid that, and maximize the size of the light cone in the triplet. I forgot to mention in the original post, the triplet diameter is 5 1/4" and it is 6 1/4" long.

10-15mm is good. you shouldn't need much adjustment. I would go with the 550mm field fresnel.

Thanks tameone! The 550 is what I expected I'd use based on preliminary drawings. But I had the fresnel a good bit further away and was worried that the light cone was not going to cover enough of the triplet. moving to between 10 to 15 mm will fill a lot more of the area of the lens and still give me room to focus if I change my distance to the screen.

Now on the the hot side of the PJ!!!

O.K. I'm a bit confused and I know...this is not unusual. But I've read a few times now, in a few different plogs, that the pro field fres is not 650mm, but is in fact 610mm. It is advertised as a 650mm (which won't work for me) but if it's a 610mm, then that would work great. Can someone shed some light (pardon the pun ) on this for me?

its in the range of 600-610mm.

Thanks Tameone!

With every answer comes another question! I tried to attach a quick and dirty Rhino rendering to help with my next question but haven't figured out the necessary black magic to put a pic in a post. So you'll have to use your imagination for now...until if crack the picture code.

Picture code cracked!

You'll see two light cones represented on the triplet side, each being driven by a different focal length fresnel, a 550mm in yellow and a 650mm (610mm actual) in orange. On the face of the triplet the rectangles represent the "image" being placed into the triplet by each respective fresnel. As you can see, both fit entirely within the triplet's lens surface, but the 610mm generates a bigger image. The question... which one would deliver the best result and why?

It seems to me that in either case (550mm or 610mm) I'm using exactly the same amount of light from the source, but it is simply being "concentrated" more by the 550mm fresnel. If that is true, wouldn't the end result be the same? Or am I not understanding the physics of this and the image would be less bright with the 550mm?

And on the subject of edge focus...would there be any difference with either solution considering that the larger image (in the lens) is of course closer to the edge of the lens itself?

So many questions...so little knowledge! As always, help is very much appreciated!

Don't know if this is helpful...but here's another pic showing penetration of the light cones into the triplet.

Anyone???

I've done a lot of reading...and learned a great deal, but I could still use some insight. In particular, if anyone has used the 18" e.f.l. lens, what experience do you have in terms of the size of the light cone in the lens.

Thanks!

Im thinking/leaning toward the 550. Personally I would get both lenses and try to play....

Anyone else care to offer a little feedback?

I've always thought a 550 would be best but apparently the '650' mm pro fresnel works well.

Thanks SJ and Tameone. We're all leaning towards the 550, but I'm still hoping that THE optics guru extraordinaire (what I title! ) can tell us WHY the 550 or the 650 is the best choice.

Specifically: 1) would one be brigher than the other, and why? 2) Would one have better edge/corner focus than the other, and why?

I think this would be useful for all of us to know. Nonetheless, I'll probably take SJ's advice and experiment a little. So stay tuned.

well the why is simple. The distance from triplet to LCD is set in stone. The light cone produced by the 550mm fresnel will fill the triplet nicely by the time it passes the triplet for most throws. The 610mm works well for most throws too but might be too long for very long throws throws causing some light to splash around the triplet. someone made a diagram of the light cone from a 500, 550, and 610mm fresnel passing through an 18" triplet at average throw distance.. don't remember where, but it was recent.

I hear ya...and I know that as light passes through a fresnel or an LCD, the light is somewhat diffused. But here's a thought.

Most anyone with experience here will say that an unsplit setup will deliver a better picture than a split setup. I'm certainly not Bill Nye the science guy, nor do I have a lot of experience, but I think that the image is typically better because the light cone is sharper in an unsplit setup because the LCD is, in essence, acting a bit like a light gate. If my "gut feel" is right, then I think my Rhino rendering indicates that there wouldn't be a problem with light spilling outside the triplet. If that’s the case then my two questions may still be valid.

Did you mean something else that I'm missing?

er I feel like a dumbass. if was you who posted the image.. In this thread! I just couldnt see it at work

Oh no...you can't be the dumbass. My wife says that's me!

I know I'm probably splitting hairs with this, but as a new LL addict I just must quench my thirst for lumenknowledge.

lumenowledge!

The main difference of the 550mm and the 610mm is the 550mm is cut with .03 grove spacing and the 610 is .05mm grove spacing. The fine spacing of the 550mm will let you place the Fresnel almost touching the LCD and you won't see the Fresnel rings in the final image. This lets you safe a little space but the box is almost 4' long any how.

The finer spacing of the 550mm also blocks more light buy around 5%.

The size of the light cone will be closer than you think between the two Fresnels. The triplet has to focus on the LCD so it will always be at the same place if the screen and PJ is at the same place. (Let’s call it 580mm)

This means the best place for the arc image from the Fresnel is in the same place inside the triplet. If you adjust each system for the best brightness the focal point will be the same inside the triplet only the lamp will be a little further away for the 610mm fresnel because the total fl is longer.

The longer lamp distance will make things dimmer but the Fresnel is brighter so it is almost a wash. The arc image will be almost exactly the same for both Fresnels and the brightness will be almost the same.

The lamp distance change is only about 15mm for the two systems.

The longer fl system will give slightly more even light.

The longer fl is better for a condenser system only if the condenser fl is on the short side like 7", for a 6x9 condenser I would go with the 550mm.

The 610 is larger and thicker too so it might be easier to frame.


Both are fine.

Glad I cleared that up.

I knew there was a difference in cut pitch, but I didn't understand the effect. Thanks!




I understand how the hot side fresnel affects evenness, but does the field fresnel have additional effect based on its' focal length?



I haven't even given serious consideration to condensers yet. By the time I'm into that, my head will really be spinning!

Thanks for the insight!