Helicoid 13-17mm to short.

[Rogier]

Hi,

I am hacking my Holga 120 Pan and just took my first test shots.
Turns out that the 12-17mm helicoid is to short. Since it only brings objects into focus that are 5 - 25 ft away while fully extended.

My question is what is a better choice.
1. 17 - 31 mm helicoid
2 15mm extension tube with the 12-17 mm helicoid.

My main concern is the "throw" off the helicoid needed. Does the longer helicoid need more turn than my current short one?
Worry is that the 17-31 mm version might be hard to adjust if the length changes dramatically with only a small turn...

Looking forward to your reply :-)

Rogier

[sevo]

If it fails to reach infinity (or past) at minimum extension, it is too long, so neither solution would help.

[Rogier]

Hmm I am getting it wrong?

No focus at all at shorter lengths. And focus nearby when fully extended.
17mm tube extension brings 5 - 15 ft in focus.
12mm tube extension brings nothing in focus...

My assumption is that I would need a tube off at least 17mm to get started.

??

[sevo]

If there is close focus at full extension and focus somewhere past infinity at minimum, infinity must be in between.

[Rogier]

Thanks I will examine my negatives as soon as they are dry.
Here is the first one...


Test-Shot-Holganon-17mm by Rogier Diver, on Flickr

[Rogier]

Here we go 12-17mm Helicoid extended at 1/4th seems to be close of infinite focus :-)


2012-10-18-2 by Rogier Diver, on Flickr

[sevo]

There you are - you need to shift the lens out by something around 1.25mm. If the threads are deep enough, you could get there with spacers. Otherwise you may have to re-seat the camera side mount. Extension rings won't do (the slimmest I've ever had already were 3-4mm), nor will a 17mm helicoid (you'd want a 13.25mm one, but these don't seem to exist).