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alanmcd
24-Feb-2016, 02:44

This image was taken with my 65mm lens. The camera was pointing down, and I had some tilt in the front standard, also forward. I was using the stairs as a focus point.
If you enlarge this image and look to the very bottom, you can see that the top of the tread string is in focus.
As your eye travels down, the steps go out of focus.
The timber grid and weeds are out of focus.
The boats, however, are back in focus.
Now I just don't understand how the 'wedge' of DOF created by the forward tilt of the front standard, creates an area which is out of focus, more distant than the closest object, then returns to similar distances further up the image which are back in focus.
What have I missed?

Doremus Scudder
24-Feb-2016, 03:00
When you move the plane of sharp focus around, you can no longer think in terms of "closer/farther" from that plane. In your case, in which the plane of sharp focus is close to horizontal, you need to think of "above/below." What has happened is that the unsharp area is enough below the plane of sharp focus as to be out of the acceptable depth of field.

Placing the plane in such situations takes practice and a little imagination and visualization. I pick three or four focus points with the goal of placing the plane in between the nearest/farthest or highest/lowest elements I want to be rendered sharply. Keep in mind that you can place the plane obliquely in the scene as well (in your case, I would have likely had the plane of sharp focus tilting down to the left foreground and up to a point midway on the bank).

Then, remember to check the areas that are most likely to be outside the DoF to find your focus spread. Keep in mind that "most likely to be outside the DoF" is not necessarily most distant from the camera. In this case, it is the near foreground (where DoF is shallowest) that is underneath the plane of sharp focus!

If you're not choosing an optimum f-stop based on focus spread then read the article here: http://www.largeformatphotography.info/fstop.html

Best,

Doremus

alanmcd
24-Feb-2016, 03:03
When you move the plane of sharp focus around, you can no longer think in terms of "closer/farther" from that plane. In your case, in which the plane of sharp focus is close to horizontal, you need to think of "above/below." What has happened is that the unsharp area is enough below the plane of sharp focus as to be out of the acceptable depth of field.

Placing the plane in such situations takes practice and a little imagination and visualization. I pick three or four focus points with the goal of placing the plane in between the nearest/farthest or highest/lowest elements I want to be rendered sharply. Keep in mind that you can place the plane obliquely in the scene as well (in your case, I would have likely had the plane of sharp focus tilting down to the left foreground and up to a point midway on the bank).

Then, remember to check the areas that are most likely to be outside the DoF to find your focus spread. Keep in mind that "most likely to be outside the DoF" is not necessarily most distant from the camera. In this case, it is the near foreground that is underneath the plane of sharp focus!

If you're not choosing an optimum f-stop based on focus spread then read the article here: http://www.largeformatphotography.info/fstop.html

Best,

Doremus

OK - wow I need to read this about 50 times to get it straight

redshift
24-Feb-2016, 05:41
From the explanation this is the image that comes to my mind. Imagine yourself standing at the deep end of a swimming pool. The pool is empty except for a foot of water way down below your feet. Looking down into the pool toward the shallow end you see that everything is in focus. As you start looking at the bottom of the pool from the shallow end toward the deep end you start to notice the focus on the bottom starts to soften. At the slope to the deeper end things quickly get out of focus. At the deep end, your feet are in focus but the one foot of water at the bottom is out of focus. This sketch may explain my idea a little better. I know it's not 100% accurate.

147037

This image was taken with my 65mm lens. The camera was pointing down, and I had some tilt in the front standard, also forward. I was using the stairs as a focus point.
If you enlarge this image and look to the very bottom, you can see that the top of the tread string is in focus.
As your eye travels down, the steps go out of focus.
The timber grid and weeds are out of focus.
The boats, however, are back in focus.
Now I just don't understand how the 'wedge' of DOF created by the forward tilt of the front standard, creates an area which is out of focus, more distant than the closest object, then returns to similar distances further up the image which are back in focus.
What have I missed?

alanmcd
24-Feb-2016, 20:50
From the explanation this is the image that comes to my mind. Imagine yourself standing at the deep end of a swimming pool. The pool is empty except for a foot of water way down below your feet. Looking down into the pool toward the shallow end you see that everything is in focus. As you start looking at the bottom of the pool from the shallow end toward the deep end you start to notice the focus on the bottom starts to soften. At the slope to the deeper end things quickly get out of focus. At the deep end, your feet are in focus but the one foot of water at the bottom is out of focus. This sketch may explain my idea a little better. I know it's not 100% accurate.

147037

Yes - I suspected that but given that I had such a minute forward tilt on my front standard (i.e. away from the back standard), I can't see it.
The scheimpflug principle would deem it necessary for the front tilt to be quite wild to get something like your drawing as the envelope of plane of focus.

alanmcd
24-Feb-2016, 21:00
When you move the plane of sharp focus around, you can no longer think in terms of "closer/farther" from that plane. In your case, in which the plane of sharp focus is close to horizontal, you need to think of "above/below." What has happened is that the unsharp area is enough below the plane of sharp focus as to be out of the acceptable depth of field.

Placing the plane in such situations takes practice and a little imagination and visualization. I pick three or four focus points with the goal of placing the plane in between the nearest/farthest or highest/lowest elements I want to be rendered sharply. Keep in mind that you can place the plane obliquely in the scene as well (in your case, I would have likely had the plane of sharp focus tilting down to the left foreground and up to a point midway on the bank).

Then, remember to check the areas that are most likely to be outside the DoF to find your focus spread. Keep in mind that "most likely to be outside the DoF" is not necessarily most distant from the camera. In this case, it is the near foreground (where DoF is shallowest) that is underneath the plane of sharp focus!

If you're not choosing an optimum f-stop based on focus spread then read the article here: http://www.largeformatphotography.info/fstop.html

Best,

Doremus

OK - first question Doremus, I've often used the splitting focus distance trick on my other cameras but I know the distance markings on the focus ring are not linear. The markings on my LF camera are linear. Surely you cannot just split the mm distance?

Doremus Scudder
25-Feb-2016, 01:15
OK - first question Doremus, I've often used the splitting focus distance trick on my other cameras but I know the distance markings on the focus ring are not linear. The markings on my LF camera are linear. Surely you cannot just split the mm distance?

Yes, I do. However it's not the distance focused on that I'm referring to, but the actual physical distance of the focusing standard between "near" and "far" focus points (i.e., change in lens-to-film distance). I have millimeter scales on all my cameras and a reference mark or two on the rail or bed so I can easily determine the distance. Distances are usually between 1-4mm.

The article I linked to on the LF homepage explains this all in detail. Yes, I know it's a bit of a slog to get through it all, but really well worth taking the time to digest. In practice (once mastered) it makes things much, much easier.

Best,

Doremus