84 replies to this topic

[Michal1]

Here you can see some photos from the build of my vacuum back for Pentax 6x7:

https://www.astro.cz...l Bilek/vacuum/

Here is the pressure plate from behind:

https://www.astro.cz...CF2902.JPG.html

The brown thing is a "wall" that was cut from a piece of linoleum. And here is the "ceiling" that was glued on the top of the wall:

https://www.astro.cz...CF2906.JPG.html

Paint was used to seal the pores between the glued parts. In order to convince myself that there are no pores remaining and the air goes only through the holes drilled in the pressure plate, I put the pressure plate in a pot with water and let the air pump blow air into the device and I watched where bubbles go out.

[xonefs]

Here you can see some photos from the build of my vacuum back for Pentax 6x7:

https://www.astro.cz...l Bilek/vacuum/

Here is the pressure plate from behind:

https://www.astro.cz...CF2902.JPG.html

The brown thing is a "wall" that was cut from a piece of linoleum. And here is the "ceiling" that was glued on the top of the wall:

https://www.astro.cz...CF2906.JPG.html

Paint was used to seal the pores between the glued parts. In order to convince myself that there are no pores remaining and the air goes only through the holes drilled in the pressure plate, I put the pressure plate in a pot with water and let the air pump blow air into the device and I watched where bubbles go out.

interesting. so how high is the "wall"? then the pressure plate can still move freely- how does it look from the size assembled? what kind of plastic is the ceiling?

 

are there any photos of other designs floating around- how did hutech make theirs?

 

I was originally thinking to seal the whole pressure plate to the back somehow and just put a hose fitting to the back. that would allow more even holes across the whole thing and how it looks like some of the commercial ones are done but not sure how to seal it and still be flexible.

 

I was thinking a neoprene gasket should theoretically be airtight 

Edited by xonefs, 24 January 2023 - 02:14 PM.

[TxStars]

Under normal conditions the pressure plate free floats with enough force to hold the film against the film rails but not compress it.

The vacuum is just keeping the film from buckling / curling or moving during the exposure.

Sealing between the back of the camera body and the pressure plate would likely keep it from free floating.

This could be done, but would likely require shimming or a machined hard stop to prevent the vacuum from pulling the pressure plate away from the film rails and closer to the camera back.

Tho if it is just minimal vacuum it may work and not cause any issues..

Edited by TxStars, 24 January 2023 - 02:36 PM.

[xonefs]

Oh yeah I didn’t think of that. I’m curious how hutech did it and how the commercial modded pentax 67s were done

[Michal1]

interesting. so how high is the "wall"? then the pressure plate can still move freely- how does it look from the size assembled? what kind of plastic is the ceiling?

 

are there any photos of other designs floating around- how did hutech make theirs?

 

I was originally thinking to seal the whole pressure plate to the back somehow and just put a hose fitting to the back. that would allow more even holes across the whole thing and how it looks like some of the commercial ones are done but not sure how to seal it and still be flexible.

 

I was thinking a neoprene gasket should theoretically be airtight 

The wall is between 1-2mm high, if I remember well. There is no need to make the chamber big. It just has too distribute the vacuum from the air outlet to the holes in the pressure plate. The ceiling is under 1mm thick. I don't remember exactly, but it looks like I cut it from an ice cream box lid. And the outlet hub is made of a snap fastener

The pressure plate can move up and down a bit. It can't move left and right. For Pentax 6x7, the pressure plate is supposed to move left and right, in order to set the plate either in the position for a 120mm film or a 220mm film. This is supposed to accommodate for the fact that the 120mm film has a backing paper while the 220mm film hasn't. With a vacuum back, we always use film without the backing paper, so there is no need to move the pressure plate left and right.

 

Here you can find another design:

https://www.horoleze...-roll-films-220

and here another:

https://www.cloudyni...trophotography/

[xonefs]

interesting thanks.

 

I just put a cheap roll of 120 in the press back to burn and test film flatness. it looks pretty good and flat so far. I may be able to avoid the vacuum mod if I can just control the humidity and maybe dry air injection would suffice. it would simplify my life to not have to deal with a vacuum mod or converting rolls to 220. 

 

Edited by xonefs, 26 January 2023 - 12:41 AM.

[Michal1]

This makes perfect sense! There were many users who did not have problems with film flatness. I recommend you to first make some photos with cheap equipment (e.g. the basic lenses), before you invest a lot of money and effort in something better. Getting a good film might be the main problem. Remember that the E200 film, that I used and was the primary choice several years ago, is no longer in production. The E100 film is not that good according to my tests. But maybe you will be happy with it. Try and see.

[xonefs]

The E100 film is not that good according to my tests. But maybe you will be happy with it. Try and see.

I've been having some pretty good results...



and even in narrowband (ha+oiii filter)



If e200 was a lot better than thats too bad and wish I had it, but this seems workable pushed 1 or 2 stops depending

Edited by xonefs, 26 January 2023 - 10:46 AM.

[SteveInNZ]

I'd be interested in some exposure and equipment details of those please.

 

Steve.

[xonefs]

both are an svx102t at f5.2. I dont remember time on first one since I bracketed a few but I think 40min to an hour (it's only 5* above horizon). no filter pushed one stop. 

 

heart nebula was about 2 hours same scope with an idas nbz pushed two stops, the moon was also out during that one I think.

 

light editing once scanned but some of the slides look really good as is, better than how they photograph/scan with my setup. 

 

ive been using tetenal e6 chems

 

I've had more come out well as well

 

 

 

sticking with 35mm might be a lot simpler and is going well so far but we'll see.

Edited by xonefs, 26 January 2023 - 06:06 PM.