Hello,
 
I'm sorry, I don't speak french and I don't know if my question has been done yet. Can I use Triplet 345 focal lenght with an F2 of 305 focal lenght? Alternatively what is the best of these:
 
1) 320 fixed
2) 285-330 varifocal
3) 310-360 varifocal
4) 345 fixed
 
I thought to use F1 160mm & F2 305mm (15,4" ) from exclusiv-online: is it a good choice? If not, what do you suggest?
 
 
Thank you for reply

hi,  
a triplet is better because it's more luminous than a varifocale.
A varifocale is more easy to use because you don't have to build something to move the triplet.
 
With a 15.4 panel, a 550 F2 should be better if you don't want to have the corner less luminous. But you need approximately a 500 triplet.
For the F1, i think a 160 is a too short focale...
 
I hope than it can help you, sorry for my poor english  
 
 
edit: you can use this program to decide what kind of lens you need:
http://www.allinbox.com/Programmes [...] Image.html

Agreed with Tijules,
 
550 mm F2 would be much better fro the reason mentionned above.  
500mm triplet will also let more light go through the triplet.
 
All this also depends much on your projection distance and final image size requirements.
345/305 and 500/550 triplet wont give the same picture size at all.
The size of the box wont be the same neither due to focal length requirements.
 
Hope this helps
 
 

Thanks for the answers.
 
I have 15" LCD  not 15,4" and i can't use 500 triplet it's too expensive.  
 
From calcule-image:
 
Setup1
F1 160
F2 305
Triplet 345
 
FOV 52,4
Rendement Lumineux 14.99
Luminosité aux coins par rapport au centre : 36,73
 
Setup2
F1 220
F2 330
Triplet 345
 
FOV 52,29
Rendement Lumineux 9,6
Luminosité aux coins par rapport au centre : 53,07
 
Setup1 seems better than 2  if the gap from F2 & triplet (345-305=40) don't create problems  
 
 

the first solution won't work...
"luminosité aux coins par rapport au centre : 36.73" means that you gonna see darks corners (the brightness of the corners would be only 36% of the center)...

lol i tought "luminosité aux coins par rapport au centre : 36.73" must be small as possible.    
 
 
Many many thanks    

Indeed the 500 mm triplet is quite explansive, but sometimes you can find some good second hand deals and optics don't wear along time  
 
I had started my project with a setup close to your setup 2. I had to switch later on to a F2=550 / 500 mm triplet combination and the luminosity improvement  is really great.  
 
Though setup 2 worked. I had a smaller lcd though (14.1). 50% ratio for brightess seems a bit low to me.  
You probably dont wanna use a condenser with that setup.