I want to build a 4X5 camera; how accurate do I have to be with placing the ground glass?

Discussion in 'Large Format' started by derek_reid, Nov 9, 2018.

  1. I intend on building my own 4x5 camera very soon. The body will mostly be made of hardwood. The research I've done states that the ground glass has to be directly on the film plane to achieve proper focusing. The issue is, I'm not sure of exactly how precise I have to be while building the camera. I'm wondering if anyone can perhaps provide building tips and/or suggestions in order for me to tackle this issue while designing and building my camera.

    Many Thanks,

    Derek.
     
  2. AJG

    AJG

    I don't know anything about building cameras but you might think about harvesting a back assembly with ground glass from a used 4x5. There are lots of used 4x5 cameras out there for not a lot of money, including some of wooden construction that would possibly blend with the wood that you plan to use for the rest of the camera.
     
    jakeenn likes this.
  3. "not very", regarding everything else besides getting ground glass and film (in holder) distance to the lens exactly the same. I disassembled a pre WW 2 6x9cm folder and noticed they shimmed the lens that had a focusing helicoid distance engravings and a hard infinity stop with rings cut from newspaper most likely 0.04mm each. So you are aiming for 1/25th mm precision, regarding the ground glass placement. - No reason to wet your pants, since you don't need to achieve this with your jigsaw; make the frame holding the GG a bit too thin and leave space to shim it.

    IDK how you are going to work on your hardwood. - laser cutting Plywood seems to provide a whole lot of precision, but plate photography would have been impossible if you'd need to achieve that. - Just make sure that everything moves and can be locked down too.
     
  4. Tony Parsons

    Tony Parsons Norfolk and Good

    I seem to recall there used to be kits to build your own large format cameras, in the dim and distant past - mayhap you could locate the instructions for one of these, see what advice is offered.
     
  5. The area required for precision is the back where the ground glass must be in the same plane as the film when the film holder is inserted into the camera. I found a Graflok back, mounted it on a board to fit the camera, and skipped the tedious process of shimming the ground glass for my project.
     
  6. Derek

    As others have said, the plane of the ground glass must be the same distance from the lens as the film plane. That is because you focus on the ground glass, and when you then insert the film holder, the film must be in the position formerly occupied by the ground glass for the image to be focused on the film.

    Film holders are described in industry standards, and one of the specifications is 't' - which is the physical distance between the film plane in the holder and the front edge of the holder. The idea is that if manufacturers of both cameras and holders build their products to achieve that 't' distance, then any holder can be used in any camera. Here's a link to a table that shows the standard dimensions of film holders; "t' is the first entry in this table.

    When you get into the exotic and extra large formats, 't' is less important, That is because in those larger sizes, film holders are rarely manufactured, and instead are typically custom made, often by the maker of the camera. Also, a photographer is less likely to own a bunch of, say, 20x24" holders. But 4x5" components are essentially commodity items, and standardization is critical.
     
  7. If you slightly overcut the seating for the GG it can be simply shimmed with paper, thin card or fibre-board material to bring it to the right height. OTOH, scraping material down isn't too hard either. So getting within 0.1mm either way would be a good machining aim. There's about that tolerance in how flat film will sit in a DDS in any case.

    You need a fair amount of 'slop' around the perimeter of the glass to comfortably fit it. You certainly don't want to be forcing the glass into place, or having to drop it in because there's no room for your fingernails to support it. 1mm clearance all the way round is about right, with a land-width of 2.5 to 3 mm for the glass to sit on.

    It's also best if the GG sits on raised lands and not an all-round flat seat. Remember you need some air breathing to avoid causing a partial vacuum and collapsing the bellows. Raised GG seating lands automatically give this.

    I wouldn't get too paranoid about accuracy. Some people happily use Grafmatic holders, which have a non-standard register together with sloppy tolerances between film sheaths that can put the film a country mile away from the focal plane. And still they don't complain of fuzzy pictures!
     
  8. You also have to consider parallelism between the front and rear standards and maintaing it as you move and focus the camera.
     
  9. Your question relates to depth of focus, that is, the distance between the farthest plane in front (toward the lens) of the perfect plane and the farthest plane rearward from perfection within which the image will be in acceptably sharp focus. The answer depends on the focal length of the lens, the f/stop used, the subject distance, and one's definition of "acceptably sharp."

    I'm no expert on this stuff, but I found your question interesting so took a look at the Wikipedia article on depth of focus. The article has various formulas for calculating depth of focus, including this simplified, rough-justice one that eliminates the focal length and subject distance variables:

    t ≈ 2Nc,

    where t is the total depth of focus, N is the f-number, and c is the circle of confusion (the "acceptably sharp" variable). A commonly used value for c for 4x5 film is 0.1 mm. Using that value, the simplified formula would yield the following depth of focus ranges:

    f/4.5: 0.9 mm
    f/8: 1.6 mm
    f/22: 4.4 mm

    Since those are total ranges, front to back, the margin for error in camera construction would be half the calculated amount. Say about plus or minus 0.5 mm for wide-open shooting. Using the good advice from earlier posters, it seems you could achieve much better precision than that.

    Of course, hardwood will expand and contract a bit with changes in humidity, which could eat up some of the margin for error.
     
  10. Kent:

    Depth of field is the nearest and furthest points of APPARENTLY sharp focus in front of the lens. This is determined by the focal length of your lens, the point focused on the aperture used and the circle of confusion.

    Depth of focus is the area behind the lens where the image plane must be positioned.
     
  11. SCL

    SCL

    Don't forget issues like the expansion coefficient of the wood you plan to use and factor that into your calculations.
     
  12. Bob, yes, exactly right. Depth of focus, rather than depth of field, would seem to the relevant consideration in determining the precision required in camera construction.
     
  13. There are still people putting together kits for 4x5 camera assembly ( e.g., kits for 4x5 cameras - Google Search)

    For something like the film plane, it might be worthwhile to see if parts are available .
     
  14. What everyone else already wrote...

    Preferably, you want the inside surface of the groundglass (the focus plane) at the same point as the emulsion side of the sheet film (the film plane) when in the film holder and inserted into the camera back. It's easy to measure the relative locations of both planes using a depth micrometer with the back removed from the camera.

    Assuming all your film holders are the same, the film plane should be at the same depth when loaded into every holder. Use the depth micrometer to first measure the film plane location with a loaded holder inserted into the back. After establishing the position of the film plane, use the depth micrometer to determine and adjust the position of the groundglass in the back, either by adding shims or removing a bit of material (wood?) from the back's frame, to position the focus plane at the same point as the film plane.
     
  15. The depth of a modern plastic double-darkslide is standardised at exactly 5mm. Unfortunately the thickness of 5x4 film isn't standardised, and neither is its flatness assured.

    Commercial camera registers can vary anywhere in the region of 4.5 to 5mm or more, but most are somewhere around 4.75mm, which is almost exactly the same as 3/16 of an inch.

    I believe this came about because the original glass plateholder register would have been 1/4" with 1/16" allowance for the thickness of the plate.

    In any event, modern DDS filmholders aren't precision-made by any means. The channel taking the film is quite sloppy, allowing for easy insertion and removal of film. Simultaneously allowing the film quite a lot of play in where it sits and with no restraint for it bowing towards the lens.

    In short, you can be as accurate as you like in placing the ground-glass, but still with no guarantee that the film will be in the same plane at the point of tripping the shutter.

    So if you aim for a register of 4.75/4.8 mm or 3/16" you'll be as close as any factory-made camera.

    P.S. anything involving sheet film that required precision usually had a vacuum back to suck the film flat and true. Or had a pressure-plate and facing glass arrangement.
     
    Last edited: Nov 15, 2018
  16. P.P.S. The sheet of processed 5x4 I just measured mics up at a touch under 0.2mm.
     
  17. AS a general rule, the larger the film, the harder it is to keep it flat. Some cameras had a pressure plate that pushed forward when cocked.

    Imagine carrying around 16x20 inch glass plates!*:)

    __________
    * "when cameras were wood and photographers were chromed steel."
     
  18. Vacuum backs or adhesive, not pressure plates.
     
  19. Rapid Omega has a mechanical pressure plate
     
  20. So do Rollei 6x6 ans 35mm cameras, but large format do not.
     

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