Focus accuracy issue with Intrepid 8x10

[Sam L]

It appears the focus is off on the camera.

I did a more controlled test where I taped newspaper to my paper cutter set on edge at 45 degrees to the camera. I carefully leveled the camera, front and rear standards and focused on the vertical pen line I drew through the newspaper:



As you can see, the focus is about 2 grid lines (1/2" grid) in front of the line I drew. Since the grid is held at 45 degrees to the camera, the focal plane would be about 0.7" in front of where I expected it to be based on the image on the ground glass. This is consistent with our portraiture experience where we got the eyebrows instead of the eyes.

My thinking was the next step is to adjust the camera such that the ground glass shows the center of focus is 2 grid lines in front, since that's the photo it takes. Or maybe I should be talking to Intrepid about a defective camera?

[nolindan]

From your newspaper test it looks like the ground glass is closer to the lens than it should be.

So, you are in luck: it isn't (or shouldn't be) hard to add some shims (i.e. bits of cardboard) to move the ground glass back a bit.

The thickness of the shims would be equal to the distance you have to move the back to go from one of the focus points in your newspaper/line/grid photograph to the other.

[Drew Wiley]

Don't forget that 8X10 film in a conventional holder (versus a vacuum or adhesive holder) can at times distinctly sag or bow in the middle. Most people making only small prints might not notice that risk, but it can certainly be there, and is obviously exacerbated when seeking very shallow depth of field.

Likewise, holders themselves can sometimes be warped or perhaps off due to wear, especially old wooden ones. To check film plane accuracy relative to both the ground glass and film position in the holder, I took an 18 inch bar of machinist's precision ground steel flat stock and drilled a central hole in it accepting a depth micrometer. You need to check not only for the correct depth, but consistent depth on all sides.

[Bernice Loui]

Take the time to read this previous discussion about Standard Distances for film holders.
https://www.largeformatphotography.i...r-film-holders

There is a industry standard for film holders and the camera MUST meet these specifications. If the camera does not meet these specifications (Ground glass to film holder seating area), this could be at the root of the focus accuracy problem which is essentially a defect in the camera.

Best way to check/test this is with a GOOD depth micrometer set up to make this precision measurement with good accuracy.



Bernice

[Ari]

That looks like a useful tool. Can you calibrate the focus on one of these cameras? I assume you would move the ground glass resting position a bit, perhaps by shimming it?

Yes, you'd move the seating of the ground glass forward or back, depending on how much you're off.
The tool I linked to would be able to tell you exactly how much you're off by, and in which direction to adjust your focus, or GG in this case.
I had it happen to a Kodak Master 8x10, which is made of aluminum, and had to remove material in order to get the GG closer to the lens.
And as some have pointed out, your film holders could be part of the problem, or entirely the problem.
As Bernice suggested, a good depth gauge will help determine if the camera or holder(s) is at fault. Then you can adjust accordingly.

[Lachlan 717]

Until you check it with another lens, your conclusion of it being the camera at fault has very limited credibility.

[Bernice Loui]

Explain why another lens would produce a different result?

Bernice

Until you check it with another lens, your conclusion of it being the camera at fault has very limited credibility.

[Tin Can]

Perhaps the easiest way for most, change one variable at a time

Few here have the tools and training to to check 'T', so they don't

I have many Starrett measuring tools and used them every day for decades

I have 3 straight edges, micrometer sets, vernier calipers', feeler gauges and more

Since we did work for USA Govt all our measuring tools were sent out for professional calibration at least once a year, then checked before and after each usage, recorded in writing. If the next usage of the tool failed pre checks all tests prior needed to be repeated. Very expensive, finger pointing, job loss

Gulf War I, I was testing our engine parts at Southwest Research San Antonio for the war effort

All the import parts were grossly out of spec, I failed them all

I soon found out they were all shipped, ignoring the tests!!!

A decade ago members here, 'experts' told me don't worry about my way out of 'T' 11X14 Deardorff back

I sent it to Richard Ritter to install his Bail Back and match a set of his holders to it

Flatness of all parts is also critical

[Tobias Key]

Perhaps this is a stupid idea, but have you tried inserting and removing a film holder then re-checking focus? Just wondering if the camera might be moving when you are loading the holder.

Also are you sure the film holder is seating properly to the camera?

Both easy things to check before you start taking the camera apart.

[Kiwi7475]

The easiest check would to measure with calipers the distance to the bottom of the film holder and see if it doesn’t match de distance to the ground glass. This has been discussed before, for example:

https://www.largeformatphotography.i...and-film-plane