How does rear tilt affect front tilt?

[reddesert]

Back in post #18 I attached a dorky little side-view diagram of the effect of tilts on the plane of focus. It's attached again here, click to enlarge:


When you adjust tilts, whether you use axis, base, or asymmetric tilts, you're adjusting the position of the heavy black line that shows the plane of exact focus. Asymmetric tilts are just one way of doing that, whose intent is to cut down on iterations between focus and tilt adjustments.

Once you have that plane, because real scenes have some depth, you need to stop down the lens to extend the forward-back extend of the gray band, which shows the depth of field. When you have used tilts, the gray band is also tilted.

One way of determining the depth of field you need is: after you have the tilts set, adjust the focus as far back as needed, as far forward as needed, measuring the difference between these positions. The distance between these positions can be converted into the f-stop you need to stop down to. This is what the Sinar DOF calculator knob does. You can do this without a Sinar, if you measure and then calculate the necessary DOF, or use the Rodenstock DOF calculator wheel that Bob mentioned.

I'm not saying anything different than what Doremus and others have said, just suggesting that it will be helpful if you look at the diagram.

[Alan Klein]

Back in post #18 I attached a dorky little side-view diagram of the effect of tilts on the plane of focus. It's attached again here, click to enlarge:


When you adjust tilts, whether you use axis, base, or asymmetric tilts, you're adjusting the position of the heavy black line that shows the plane of exact focus. Asymmetric tilts are just one way of doing that, whose intent is to cut down on iterations between focus and tilt adjustments.

Once you have that plane, because real scenes have some depth, you need to stop down the lens to extend the forward-back extend of the gray band, which shows the depth of field. When you have used tilts, the gray band is also tilted.

One way of determining the depth of field you need is: after you have the tilts set, adjust the focus as far back as needed, as far forward as needed, measuring the difference between these positions. The distance between these positions can be converted into the f-stop you need to stop down to. This is what the Sinar DOF calculator knob does. You can do this without a Sinar, if you measure and then calculate the necessary DOF, or use the Rodenstock DOF calculator wheel that Bob mentioned.

I'm not saying anything different than what Doremus and others have said, just suggesting that it will be helpful if you look at the diagram.

What I'm confused with the DOF focusing, is that do you have to move the focus standards? If so, haven't you just destroyed the whole focusing process you did initially? Also, what do you do after you determined the f stop? Do you re-do the initial focusing with tilting.

[Bill Rolph]

What I'm confused with the DOF focusing, is that do you have to move the focus standards? If so, haven't you just destroyed the whole focusing process you did initially? Also, what do you do after you determined the f stop? Do you re-do the initial focusing with tilting.

You don't zero the standards in that procedure, you keep the movements as they were. After finding the necessary aperture, place the focus in the center of the spread between the near and far focus points (the points on the rail, not in the scene).

[Ulophot]

Louis, I was just about to say, "Boy, that sure doesn't look like 75mm to me!" I used one for years, and that is one wi-i-i-de lens. 150 looks right. 75 on the earlier photo makes sense. I'm confident that that's the one Doremus was referring to.

[Alan Klein]

You don't zero the standards in that procedure, you keep the movements as they were. After finding the necessary aperture, place the focus in the center of the spread between the near and far focus points (the points on the rail, not in the scene).

I'm still missing something. I adjust the far focus point on the asymmetrical axis line using the standard(s): front, rear or both. Than I tilt the back standard to focus the near subject. How do I find the center between the near and far focus points when the near focus was done by tilting the standard?

[GoodOldNorm]

Here's the pdf. Hope it helps.

As far as tilt with verticals in the scene goes, it's often better to just stop down. The only amount of tilt you can ever get in a scene is the tilt that gets the nearest foreground object you want sharp and the top of the nearest vertical sharp. More that that is counter-productive.

Best,

Doremus

Hi, Doremus, would you consider writing a primer on using asymmetrical tilts? I found your file, dealing with base tilts a great help, thank you.

[Vaughn]

Does the Sinar DOF calculator assume the amount of enlargement?

Just saw an image of one -- answered my own question (one sets it).

[Jim Noel]

Check this web site. If Merklinger doesn't answer your questions, i doubt anyone can.
http://www.trenholm.org/hmmerk/

[Doremus Scudder]

Hey Doremus Allen posted he used a FL of 150mm for his (latest) example image.

Louis, I was just about to say, "Boy, that sure doesn't look like 75mm to me!" I used one for years, and that is one wi-i-i-de lens. 150 looks right. 75 on the earlier photo makes sense. I'm confident that that's the one Doremus was referring to.

Yep, I was thinking of the image with the fence and barn, which benefits greatly from the DoF that a short lens provides. Sorry for any confusion.

Doremus

[Doremus Scudder]

What I'm confused with the DOF focusing, is that do you have to move the focus standards? If so, haven't you just destroyed the whole focusing process you did initially? Also, what do you do after you determined the f stop? Do you re-do the initial focusing with tilting.

Alan,

You're stuck in a rut about focusing here... Clear your mind of preconceptions and consider the following.

Applying tilt and swing is NOT focusing. Yes you're checking if things come into focus or not to make sure you have the plane of sharp focus where you want it, but this is a separate and different thing than finding where to place your standards for optimum depth-of-field and f-stop.

Let's focus a shot that doesn't need movements using the near/far method:

Step 1: find the nearest object you want in sharp focus. Focus on that with the main focusing knob. Note the position of the standards on the camera bed/rail.
Step 2: find the farthest object you want in sharp focus. Focus on that with the main focusing knob. Note the position of the standards on the camera bed/rail.
Step 3: position the standards halfway between the two focus points.
Step 4: choose your optimum f-stop based on the amount of distance between the two focus points. (This assumes you've figured that out, or have at least copied the table from article on How to select the f-stop the LF home page, https://www.largeformatphotography.info/fstop.html ).

I've used this method for years with excellent results. It's not at all like focusing a smaller camera, where you twist the focusing ring until what you want is in focus and then shoot. No, with this method you concern yourself with the near/far extremes. You never even have to look at focus for anything in between them. This method reliably gives you the optimum compromise between depth-of-field and image degradation due to diffraction while ensuring that what you want sharp is sharp.

Using tilt and swing movements is a separate thing from finding the overall focus for the scene.

What you call the "initial focusing process" is NOT focusing; it's applying movements.

Now, when you want to use tilt/swing, you should do so before focusing. Yes, you'll have to focus on your reference points in order to correctly apply tilt/swing. However, after you have done so, you need to look for the nearest and the farthest object not in the plane of sharp focus and use them to focus using steps 1-4 above. Yes, you'll likely be repositioning the plane of sharp focus a bit in order to get the optimum f-stop for the depth of field you desire. This is not "destroying the whole focusing process you did initially." That's the way it works.

Clear now?

Doremus