Tilt trig

[couldabin]

Yes, but the negative will be scanned. I'm attaching a 600 dpi scan from a test shot (HP5). I'm thinking if there had been people, they'd be reasonably clear.

 

[Maris]

There is a much more accurate way of getting the tilt right than calculating its angle and setting it on the camera.

The ground glass of a properly made 8x10 view camera can be adjusted to at least 0.1mm plus or minus. A powerful focussing loupe helps. You can actually see the sharpness come in as the tilt changes. When the sharpness on the ground glass is right the tilt is necessarily right. Lock the tilt control at that point. This works just as well for cameras with accurate tilt scales, inaccurate tilt scales, or no tilt scales at all.

The only cameras where there may be a difficulty are those with strong zero detents for the movements. It can be difficult to set a stable adjustment that is just a tiny fraction off a zero detent.

 

[couldabin]

You have a better eye than I. I am certain I cannot reliably distinguish between a focal point 100 ft away and one 104 feet away using a 12" lens.

 

[thomnola]

OK, I'm where I can measure my camera now, and I did make one miscalculation -- I should be measuring the size of the front lensboard frame, rather than just the lensboard. So the physical distance the top moves is greater, but not a lot. Using the revised size (8" frame, as opposed to 5" lensboard), I estimate the top of the frame will be 0.080" further away from the back than the bottom. I had no idea such a small misalignment (0.58 degrees) shifted the plane of focus 45 degrees. Your run-of-the-mill spirit level cannot resolve much less than 0.3 degrees; if you want the plane of focus to be within 10 degrees of parallel of the back, the front frame has to be within 0.1 degrees of parallel (assuming the same focusing distance and lens). I hope I have this math wrong ...

I have no idea what you are talking about I don't believe that 1/2 a degree shifts the plane of focus 45 degrees. I shoot in a studio situation all day every day and I would venture to guess that 75% of my tilts are less than 5 degrees and that is not to shift the focus 45 degrees, more like 10 - 20 degrees. I am more of a visual person than a math person so I just use what the ground glass tells me to use. Anyway, I have to agree with an earlier poster who said you were overthinking it. Using the DOF Master app on my iphone: 8x10 film with a 300mm lens at f16 the hyperfocal distance is 93.3' which takes care of anything from 46.6' to infinity. I guess I'm lazy and I let others do my math for me. Regardless, have fun and keep us posted on how everything turns out.

 

[couldabin]

I do promise to post an update when the project is done. But for the time being, I want to say I appreciate your candor. I think there are a lot of people who don't believe 1/2 degree shift of the lensboard can move the plane of focus 45 degrees. I was among them, which was the impetus behind my initial post. I was hoping someone would demonstrate it isn't so; that hasn't happened. Without going into the trig, a mental experiment demonstrates the "leverage" at work (and if I am misconstruing the Scheimpflug rule, I hope someone will set me straight):

The point of focus is 100 ft in front of the film plane. If you want a 45 degree plane of focus through that point, you extend that plane toward the camera until it intersects with the vertical film plane. You now have a big right-angle triangle -- 100' on the horizontal, 100' on the vertical, and a longer (about 141') diagonal. The Scheimpflug rule says that the tilt of the front lensboard to create that sloped focal plane will be achieved when the lensboard points to intersection (100' down) of the film plane and the focus plane. Since the lensboard is about 12.12" away from the film plane, it takes a mighty small angle to have it intersect at 100'. Like, about 0.6 degrees.

The closer the point of focus, the steeper the tilt of the front lensboard required to achieve 45 degrees focal plane slope (for two reasons: the vertical leg is that much shorter, and the lensboard is further away from the film plane). If the subject were 15' away, the lensboard tilt would be on the order of 4 degrees; at 8', 8 degrees.

That's the theory, as I understand it. Again, if I'm confused about this -- which is entirely possible -- I'd welcome correction. I'm just learnin' here ...

 

[E. von Hoegh]

I have no idea what you are talking about I don't believe that 1/2 a degree shifts the plane of focus 45 degrees. I shoot in a studio situation all day every day and I would venture to guess that 75% of my tilts are less than 5 degrees and that is not to shift the focus 45 degrees, more like 10 - 20 degrees. I am more of a visual person than a math person so I just use what the ground glass tells me to use. Anyway, I have to agree with an earlier poster who said you were overthinking it. Using the DOF Master app on my iphone: 8x10 film with a 300mm lens at f16 the hyperfocal distance is 93.3' which takes care of anything from 46.6' to infinity. I guess I'm lazy and I let others do my math for me. Regardless, have fun and keep us posted on how everything turns out.

And your working distance in the studio is......?

 

[E. von Hoegh]

I do promise to post an update when the project is done. But for the time being, I want to say I appreciate your candor. I think there are a lot of people who don't believe 1/2 degree shift of the lensboard can move the plane of focus 45 degrees. I was among them, which was the impetus behind my initial post. I was hoping someone would demonstrate it isn't so; that hasn't happened. Without going into the trig, a mental experiment demonstrates the "leverage" at work (and if I am misconstruing the Scheimpflug rule, I hope someone will set me straight):

The point of focus is 100 ft in front of the film plane. If you want a 45 degree plane of focus through that point, you extend that plane toward the camera until it intersects with the vertical film plane. You now have a big right-angle triangle -- 100' on the horizontal, 100' on the vertical, and a longer (about 141') diagonal. The Scheimpflug rule says that the tilt of the front lensboard to create that sloped focal plane will be achieved when the lensboard points to intersection (100' down) of the film plane and the focus plane. Since the lensboard is about 12.12" away from the film plane, it takes a mighty small angle to have it intersect at 100'. Like, about 0.6 degrees.

The closer the point of focus, the steeper the tilt of the front lensboard required to achieve 45 degrees focal plane slope (for two reasons: the vertical leg is that much shorter, and the lensboard is further away from the film plane). If the subject were 15' away, the lensboard tilt would be on the order of 4 degrees; at 8', 8 degrees.

That's the theory, as I understand it. Again, if I'm confused about this -- which is entirely possible -- I'd welcome correction. I'm just learnin' here ...

I haven't double checked your trig, but visualising the angles leads me to think you are correct. Just get what needs to be sharp in focus on the GG and you're good. One of the benefits of 8x10 is that big GG image.

 

[thomnola]

And your working distance in the studio is......?

I'm doing table top shots and I'm typically anywhere from 3' to 3" but it can range from 6' depending on the size of the object. Usually shooting a 150mm lens to 645 digital. Point of clarification: I said ground glass but we use Live View to compose and focus on the computer monitor but the principles are the same.

 

[thomnola]

I will say this: I think it is an interesting exercise to try to figure out the math when using Scheimpflug and I spent many an hour poring over Merklinger's book as well as other texts in trying to get a handle on how it all worked. In the end, Steve Simmons taught me (20 years ago via his magazine) the method that I use to this day which is focus on the farthest point you want in focus, tilt (or swing, whichever the case may be) in the direction of the closest object you want in focus until both points are equally out of focus. Refocus and you achieve Scheimpflug. No math, all visual. Works like a charm!

 

[Maris]

You have a better eye than I. I am certain I cannot reliably distinguish between a focal point 100 ft away and one 104 feet away using a 12" lens.

This is an important observation! Even with a powerful 10x loupe on the ground glass (illuminated by a 300mm f5.6 lens) I can't distinguish between a focal point 100 ft away and one 104 ft away. But the crunch is that if I can't see the difference at 10x there is no way that the film can register a difference that would show up at a 5x enlargement! In effect that loupe gives me a safety margin to focus finer than the unaided eye or the film can see. And beyond that there is no additional sharpness to be had.

 

[couldabin]

Ah. You got me there -- when you said that the ground glass of a properly made 8x10 view camera can be adjusted to at least 0.1mm I mistakenly thought that you were saying you could find focus to within those tolerances. What you're saying is that 10x with a loupe on the groundglass gives you more information as 5x in print; if not twice as much, then an abundance that creates a focal safety margin. My intuition is, no way. I'm thinking the resolution on film is way more than twice that of the focusing screen. But I'll play with that a little and report back later ...