fresnel problem ,,,?

[boris333]

Fresnel:

Size: 15.5" x 15.5" (~390mm x 390mm)
Largest Lens Square: 15" x 15" (~376mm x 376mm)
Groove Pitch: 0.5 grooves/mm
Rear Lens Focal Length: 13" (~326mm)
Front Lens Focal Length: 8.67" (~218mm)
Material: 92% transmisity optical acrylic


now it says that the rear focal length is 13" ... someone told me that the rear is 8.67" not 13" ??? and when i put the rear fresnel at 8.67" or 13" ,, dosn't work .. but when i put it 4" to the bulb it works ? what is the problem here ? how can i fix this .. the fresnels are from diylabs ...

 

[joe2000chevy]

Where did you get it?

 

[tonydean]

hi boris,

i'm not sure i understand your post? i purchased those same fresnels
from diylabs. i doesn't matter how you have them listed in the post.
alan has that fresnel in two types. one is approx 13" the other a little
over 8". they both look alike.

have you taken each fresnel and checked the focal lengths to see what
he sent you? mine measured pretty much like the specs. if you were
sent two of the 8" inchers and put them in series, is the only thing
i can think that would get you to 4 inches.

 

[JustDavid]

Just a thought, make sure you have the grooved side of each fresnel facing each other.

 

[Crazed_climber]

the fresnels are from diylabs ...

(for Joe🙂)

I emailed alan awhile ago he told me if it has a blackened out sticker the focal length is the 8.67" on, if not its 13"

cheers scotty

 

[boris333]

if i try using only one fresnel at 8" or 13" it works .. but then if i put the other one all i get is a bulb on the screen ? but then if i move rear lens about 4" back and the front one 5" it works ?

 

[Dazzzla]

Hello boris333
This might sound totally irrelevant but how many fresnels did you buy, one or two?

DJ

 

[boris333]

its one fresnel but its splited so i have two .. and then if i put them together then the rear focal length is 3 " ?