Focus Finder problem

[javierpuga]

I have a "Focus Scope" focus finder:

http://www.bhphotovideo.com/c/product/123618-REG/Tundra_Security_97FS_Focus_Scope.html


i got it used in a bundle with a bunch of other used darkroom equipment. I understand how to use it, but my problem is that it doesn't seem to be accurate. I will focus an image until i see the grain through the lens, but then, after taking it away, just using my eye, i can see the image is way out of focus.

You can pull the top part of the lens out, so the eyepiece is farther from the mirror, i thought this would adjust it, but it makes it even more out of focus. i would need the lens to be closer to the mirror, if that makes sense.

either way, how could it not be accurate, it's just a lens and a mirror, right? is there something i'm missing? thanks

 

[d_ross]

perhaps post a picture of yours here, as it's hard to say without seeing it. but most likely the top piece isn't actually all the way in, mine has a little groove either side of the insert that matches with two little rails on the inside of the bottom part, and it needs be inserted fully to be accurate.

 

[javierpuga]

it looks identical to the one in the b&h picture. just checked, and mine definitely does not have any grooves or rails.

it's just confusing because it's such a simple contraption, i can't imagine it not working properly.

like i said, i acquired it used, so i'm not sure if it was always inaccurate, or if it's something that happened over time.

 

[d_ross]

if its not adjustable then it should be right, still they are cheap to buy, and invaluable tool I think.