SINAR Information No 8

Information No. 8

Sharpness Control in Depth Simple and Precise.
by C.Koch

Professional photographers are familiar with the problems of sharpness control in depth with the tilts of a conventional view camera. You have to know which way to tilt – You must know how far to tilt – And often, the tilt makes additional adjustments necessary. In fact, sharpness control and tilt or swing procedure have changed little since the era of the wooden studio camera – until the SINAR P came along. The fundamentally new design of this camera brought an equally new procedure for sharpness/tilt control. It is based on a simple and precise two-point focusing system. This new focusing system becomes most obvious on comparison with traditional methods. By «traditional» we do not mean a specific camera of make, but the camera designs and the required focusing procedure of the past. With any method, the intended pictorial effect is paramount. Basically, for depth focusing with the camera tilt (the so-called Scheimpflug principle) we can of course tilt either the camera back or the lens standard. If for instance vertical lines are to appear parallel, the image plane must be parallel to those verticals and any sharpness in depth controlled by tilting the lens standard. However, for deliberate perspective emphasis with sharpness in depth – for instance to make a foreground really prominent – the best way is to tilt the camera back, i. e. the image plane. Between these two extremes we have a wide range of practical compromises. The camera design and operation must of course match these requirements

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The function of the rear standard

(camera back)

The tilting system

Operation

Focusing technique

Arrangement of the swing and tilt axes

Measuring the tilt angles

SINAR P

THE NEW FEATURE:

With the simple precision tilt ad­justment of the camera back you can equally measure tilt angles for sharpness control with the lens stand­dard.

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Sharpness control in depth with the lens standard

(without perspective change)

Measuring the tilt angles

Focus shift and curvature of field:

Lenses are corrected to yield a sharp image in a flat film plane. Oblique rays to the image corner thus come to a focus farther from the lens centre than central rays. Hence, a lens tilt (or swing) shifts the plane of maxi­mum sharpness over the whole ground glass screen, irrespective of the camera design. This causes focu­sing problems.*

Image shift:

On tilting the lens, the point focused usually moves away from the screen centre – which causes further focusing problems.*

The result:

Lens tilts and swings involve secondary image shifts and are thus generally more complex than sharpness control in depth by tilting the camera back.

* Try it yourself by swinging or tilting the lens in different directions. Check and compare the image shift relative to the swing and/or tilt axes!

Focusing technique with the lens tilts

Traditional

You have to stop to work out the right tilt direction.
You have to estimate the correct tilt angle by trial and error.
(And remember: every tilt adjustment needs 3 to 5 ope­rating steps.)
Every time you adjust the tilt angle of the lens standard, you have to correct the resulting focus shift and image shift. This applies equally to the usual central tilt or swing.

The result:

As you adjust the lens tilt, the overall sharpness changes while the focusing point at the same time shifts relative to the tilt axis. So the reference points in the image tend to run away and are difficult to keep under control. Hence you have to rely on cumbersome trial and error procedures.

SINAR P

THE NEW FEATURE:

Instead of trial and error, you mea­sure on the SINAR-p the tilt angle of the image plane without any of these complications. In Fig. F the proce­dure is shown for a tilt about the horizontal axis.

• Tilt the camera back (1 )for optimum overall sharpness. Read off the tilt angle. Return the back to its original position to retain the original image perspective.
• Transfer the tilt angle read off on to the lens standard (2). Tilt this in the same direction as you tilted the camera back to return to its original position.
• After this tilt of the lens standard, refocus the latter to correct the sharpness shift due to the lens tilt. If ne­cessary raise or lower the standard to restore any image shift. (A fine adjustment of the lens tilt angle can be established precisely by a further measure­ment and correction. But in practice, the final sharp­ness adjustment is easier by precision tilting of the camera back as described before; such a fine correction will not significantly affect the perspective rendering but quickly ensures complete overall sharpness.)

The result:

The tilt direction and angle are precisely and simply established even with the lens tilt – without altering a given preselected perspective rendering.
A swing about the vertical axis works in the same way as a tilt about the horizontal axis. Swings and tilts can also be combined, even with indirect parallel displacement, for the vertical swing axis remains stationary. For close­ups focus as indicated in the instruction manual.

To sum up

The SINAR-p design and two-point focusing system** permit easy tilt measurement for sharpness control in depth. The camera has vital practical features: micro­meter drives, «asymmetric» tilt and swing axes in the image plane and large setting scales.

All this make sharpness control in depth simple, pre­cise and easy – with the camera back as well as with the lens standard.

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** The location of a plane in space Is determined by 3 points (or by one ax,s plus one point) The SINAR·p 2·point focusing technique is applied tor the swing about one axis (the first setting point is chosen on this axis. the second one off the axis) If requested. sharpness may be set on a third point, too, by subsequent swing about the other axis

Copyright: SINAR LTD SCHAFFHAUSEN CH-8245 Feuerthalen / Switzerland