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gzp
30-Jul-2014, 18:45
Assuming adequate coverage, what effect, if any, does the film format or digital sensor size have when using tilt/swing or shift?

Thanks

David A. Goldfarb
30-Jul-2014, 19:46
Presuming the same composition, final print size and sharpness requirements, the format size determines the value used to stipulate the acceptible circle of confusion, and therefore the DOF range at a given aperture and subject distance. A smaller format has a larger DOF range for any given composition/print size, aperture and subject distance, so you'll need less focal plane correction with a smaller format.

Emmanuel BIGLER
31-Jul-2014, 04:15
Presuming the same composition, final print size and sharpness requirements, the format size determines the value used to stipulate the acceptible circle of confusion, and therefore the DOF range at a given aperture and subject distance.

Agreed 100% so far, David.


A smaller format has a larger DOF range for any given composition/print size, aperture and subject distance, so you'll need less focal plane correction with a smaller format.

Here, I disagree.
At least if you REALLY want the final image taken with a small sensor to be enlarged as big as a conventional film image from LF.
Do not forget that modern "digital" lenses of shorter focal lengths require not to be stopped down as far as LF lenses with longer focal lengths.

Yes, the value you can choose for your circle of condusion (CoC) roughly scales in proportion with the sensor diagonal or or film format, but do not forget that the best f-number beyond which diffraction becomes a nuisance changes with the focal length hence with the format. It is no longer f/16 of f/22 like in the 4x5" format, it is something like f/5.6 or f/8.
Yes, you can choose to ignore diffraction, and shut down beyond the best f-stop of 5.6 or 8, but doing so you loose the sharpness contest against film LF photography even before actually starting the contest ;)

People using digital MF cameras often complain that their shots are "never crisp". Not because the images are actually fuzzy, but because they want their final images to be as good as a film image from LF. Hence they discover that they have no DOF advantage vs. LF photography!

If you combine both effects: reduced CoC in proportion of the sensor or film format, and reduced f-number to avoid diffraction effects, and again assuming that you REALLY want to get a final print from a small silicon sensor as good as a conventional image from LF, you have absolutely no depth of field gain when using a smaller sensor.

Another effect regarding Scheimpflug settings, if you want to use conventional LF techniques with tilts, is that the actual tilt angles required for the same effect as in LF become minuscule since they scale in proportion with the focal length, at least in the classical 'Scheimpflug-for-landscape' setting.
Imagine that your camera is set at a heigth of 1.5 metre (5 feet) above ground.
You apply a small tilt to one or both of the camera standards in order that the plane of sharpness in object space, according to Scheimpflug's rule, becomes horizontal.
In those conditions, the DoF zone becomes a wedge, the angle of the wedge is governed by the hyperfocal distance, and if you have the same hyperfocal value between film LF and digital MF, it can be shown that the DOF wedge is exactly the same. And of course, with zero tilts, same hyperfocal distance means same acceptable sharpness zone between parallel planes.
Since the hyperfocal distance is H = f2/ (NC), if "c" (CoC) scales llike "f" (focal length) and "N" (f-number) roughly scales like 'f' to avoid diffaction effects, then the hyperfocal distance does not change that much at the best f-stop between the small sensor and the LF setup, you have no gain using smaller formats if you insist on using the best f-number N.

But even without taking this into account, the rule is simple : same hyperfocal H implies same DOF zones in object space with of without tlts. At least within the limits of our geometrical model governed by the hyperfocal distance; this simple model is actually challenged in high performance photography when we always insist on using top-quality lenses at their their best f-stop, i.e. a point where diffraction contributes equally to residual aberrations in the assessment of image quality limits.

For a fixed height of the camera above ground, the required tilt angle is about 60 x (focal length ) / (camera height above ground)

If the lens is a conventional 150 mm standard lens for 4x5", if the height is 1.5 metre, the required tilt angle is already quite small, about 6 degrees.
Now switch to a 70 mm standard lens for a 645 silicon sensor, the angle is divided by about 2, about 3 degrees.
And if you use a 35 mm wide-angle lens for your 645 sensor, the tilt angle is 1.5 degree.
This becomes extremely tricky to set without a live-view monitoring system, even if you have a very precise view camera with extremely sensitive tilt controls.
And in macro-photography, my feeling is that users of 35-mm and MF digital sensors forget about Scheimpflug, too difficult to set, and use focus-stacking techniques.

gzp
31-Jul-2014, 05:11
Agreed 100% so far, David.


Another effect regarding Scheimpflug settings, if you want to use conventional LF techniques with tilts, is that the actual tilt angles required for the same effect as in LF become minuscule since they scale in proportion with the focal length, at least in the classical 'Scheimpflug-for-landscape' setting.
Imagine that your camera is set at a heigth of 1.5 metre (5 feet) above ground.
You apply a small tilt to one or both of the camera standards in order that the plane of sharpness in object space, according to Scheimpflug's rule, becomes horizontal.
In those conditions, the DoF zone becomes a wedge, the angle of the wedge is governed by the hyperfocal distance, and if you have the same hyperfocal value between film LF and digital MF, it can be shown that the DOF wedge is exactly the same. And of course, with zero tilts, same hyperfocal distance means same acceptable sharpness zone between parallel planes.
Since the hyperfocal distance is H = f2/ (NC), if "c" (CoC) scales llike "f" (focal length) and "N" (f-number) roughly scales like 'f' to avoid diffaction effects, then the hyperfocal distance does not change that much at the best f-stop between the small sensor and the LF setup, you have no gain using smaller formats if you insist on using the best f-number N.

But even without taking this into account, the rule is simple : same hyperfocal H implies same DOF zones in object space with of without tlts. At least within the limits of our geometrical model governed by the hyperfocal distance; this simple model is actually challenged in high performance photography when we always insist on using top-quality lenses at their their best f-stop, i.e. a point where diffraction contributes equally to residual aberrations in the assessment of image quality limits.

For a fixed height of the camera above ground, the required tilt angle is about 60 x (focal length ) / (camera height above ground)

If the lens is a conventional 150 mm standard lens for 4x5", if the height is 1.5 metre, the required tilt angle is already quite small, about 6 degrees.
Now switch to a 70 mm standard lens for a 645 silicon sensor, the angle is divided by about 2, about 3 degrees.
And if you use a 35 mm wide-angle lens for your 645 sensor, the tilt angle is 1.5 degree.
This becomes extremely tricky to set without a live-view monitoring system, even if you have a very precise view camera with extremely sensitive tilt controls.
And in macro-photography, my feeling is that users of 35-mm and MF digital sensors forget about Scheimpflug, too difficult to set, and use focus-stacking techniques.

This is what I was trying to get at in my question, and yes, I was afraid there would be formulas. So, if I understand correctly, if I apply a forward tilt to use the Scheimpflug principle to get a horizontal plane of sharpness, using the same lens, wide open, I'll get the same effect (for that specific amount of tilt) whether I'm using 4x5 film or an APS-c sensor. The amount of tilt to get the same effect would vary depending on the focal lenth of the lens, not the film size.
Thanks for your help!

David A. Goldfarb
31-Jul-2014, 10:27
Another effect regarding Scheimpflug settings, if you want to use conventional LF techniques with tilts, is that the actual tilt angles required for the same effect as in LF become minuscule since they scale in proportion with the focal length, at least in the classical 'Scheimpflug-for-landscape' setting.

This was my main point, but thanks for stating it more clearly and more fully articulating the details.

sanking
31-Jul-2014, 10:48
This is what I was trying to get at in my question, and yes, I was afraid there would be formulas. So, if I understand correctly, if I apply a forward tilt to use the Scheimpflug principle to get a horizontal plane of sharpness, using the same lens, wide open, I'll get the same effect (for that specific amount of tilt) whether I'm using 4x5 film or an APS-c sensor. The amount of tilt to get the same effect would vary depending on the focal lenth of the lens, not the film size.
Thanks for your help!

I never found the formulas of much practical use with LF film cameras, though understanding the principles really helps.

Even though the actual amount of tilt or shift is much less with shorter focal length lenses than long ones the same angle applies, so you have the same principle. Some digital cameras, NEX, a7r, etc. have focus aids (peeking for example) that make tilt/shift adjustment quite easy in live view because you can see the result of movement snap in.

Sandy

neil poulsen
31-Jul-2014, 21:48
One thing that I've picked up is that, as format size decreases, focus becomes critical. Kirk Gittings commented on this, when he was shooting medium format.

And as one considers digital backs of anything over 30 megabytes, very fine controls on swing, tilt, and extension (for accurate focusing) become important. So, cameras like Arca Monolith or Cambo Ultima (both geared movements) become preferable to something like Arca Classic F or Linhof 6x9 cameras originally intended for film.

VictoriaPerelet
31-Jul-2014, 23:04
Google is your friend, main page of this site also.

Here's nice modern overview with interactive calculator(press advanced to include sensor size)

http://www.cambridgeincolour.com/tutorials/tilt-shift-lenses2.htm

Plug in your data and see how focus plane changes.

Emmanuel BIGLER
2-Aug-2014, 06:09
I never found the formulas of much practical use with LF film cameras

I agree in general, but as far as tilt angles are concerned, the formula is so simple (and independant from the sensor size or film size)
60 x (focal length ) / (camera height above ground)
that when I'm setting a tilt for the classical Scheimpflug setup "sharp on ground" the formula prevents me to look for impossible angles ... like 20 degrees if I'm using a 90 mm ...

Another formula not useful in the field but useful to understand why it might be difficult to focus a short focal length with a conventional rack and pinion system, is the amount of travel of the rear standard bringing focus from infinity down to one meter.
rear standard displacement (in mm) = (focal length in mm)2 / 1000

With a focal length of 100 mm, the displacement of the rear standard is 10 mm to change focus from infinity to one meter. This is very easily manageable with a conventional rack and pinion system (e.g. 20 mm per knob turn, a common value to many LF monorail cameras)
But with a 35 mm, this amount of displacement drops down to about 1.2 mm; this is hardly manageable with the same rack and pinion system, 20 mm per knob turn. hence smaller formats use helical focusing systems .. this is not really new ;) but explains why before dreaming of attaching a 6x4.5 cm silicon sensor to a monorail LF camera, you have to think seriously about those figures.

sanking
2-Aug-2014, 07:56
"But with a 35 mm, this amount of displacement drops down to about 1.2 mm; this is hardly manageable with the same rack and pinion system, 20 mm per knob turn. hence smaller formats use helical focusing systems .. this is not really new ;) but explains why before dreaming of attaching a 6x4.5 cm silicon sensor to a monorail LF camera, you have to think seriously about those figures."

One solution is to front mount lenses in helical focusing mounts rather than work with the focusing system of the camera itself. There are many relatively inexpensive medium format lenses on the used market with helical focusing systems that would work nicely.

Sandy