Correcting a cast introduced into the image by using either an extreme wide angle lens or heavy tilt/shift/swing movements on a technical camera, has been a part of the Hasselblad software platform for many years.In Phocus we call this “Scene calibration”

The scene calibration tool has two correction options. “Remove cast” removes a given lens cast in the image and “Equalize intensity” eliminates exposure differences throughout the image. The following is a description of both.

Remove cast:

If a cast is introduced to the CCD by the lens we can use this utility to remove the (typically) green or magenta cast in the image. The cast appears when light from the lens enters the pixels at an odd angle and thereby spills over to the adjacent pixels creating a cast which is visible in the image.By capturing an image which is representative of the cast, the software can “invert” this image and use this as a temporary calibration file to “subtract” the cast from the image.

As you can see in this example, there is a strong green cast in the left side of the image caused by a swing and shift of the 90mm Schneider lens mounted on a Sinar P2.

Phocus: Scene calibration tool

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In order to record the behavior of the lens we now need to shoot an image which represents the given cast from the lens.The best option is to shoot through an opaque filter placed in front of the lens. It can be a piece of opaque plexi glass or a lens-mounted filter such as an ExpoDisc.(http://www.expoimaging.net)The most important thing here is that nothing is in focus and that the image does not contain any strong colors from the subject (such as a strong colored background or object in the image). Try to eliminate any such objects from the scene before making the cast exposure.

It is not important if this exposure is a couple of f-stops under. Phocus will tell you if there is a problem with the capture when you enable it. If the cast exposure is too dark, do not change the aperture as this will change the behavior of the cast. Alternatively, change the exposure time or increase the lighting.

Capture the 'Cast Shot' as described above and then click the “Create” button in the “Scene Calibration tool”

“Scene calibration” can be selected by clicking on the small triangle next to the tool tabs in case it is not already present in your tools.

Phocus: Scene calibration tool

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Next type in a name for the cast shot you have just created. This will allow you to select the specific cast shot from your list in case you need to re-use it later on.

Your new cast shot file is now visible in the “Scene Calibration” tool and you can tick the “Remove cast” option. Also, make certain you tick the box for the tool itself at the top right-hand corner.

The visible lens cast will now be removed from subsequent captures. You may need to neutralize the image again.

Phocus: Scene calibration tool

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Equalize intensity:

Basically, you enable the “Equalize intensity” in the same way as “Remove cast” with a few exceptions.The purpose of the “Equalize intensity” option is to even out subtle variations in the light in a given scene, e.g in copy stand work or a reproduction scene where you are trying to obtain a completely even light distribution from corner to corner.Even with the most meticulous preparations and use of multiple light sources, obtaining even light distribution can be somewhat of a task. The “Equalize intensity” can help you achieve a uniform exposure with ease.

In this example we will be doing both a “Remove cast” and “Equalize intensity”

Using the opaque capture method mentioned earlier, will record only the cast information from the image and will therefore return an incorrect impression of the light distribution. Hence, this method cannot be used to correctly record the information required by the "Equalize intensity" tool. Instead we have to shoot the actual scene as it appears.

NOTE: it is important in this case to have a clean and even white surface with no significant structure, pattern or spots as any such imperfetions will be recorded in the file.

(small specs of dust and spots in the scene will be eliminated because of the way the file is eventually down sampled)

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As previously; click the “Create” button in the “Scene calibration” tool and type in an appropriate name for the calibration.

As we are doing both calibrations in this example both “Remove cast” and “Equalize intensity” are ticked. You may of course, select either one by itself to suit your purpose.

When applied to the capture you can clearly see the effect of both filters.

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Because the “Equalize intensity” basically works as an anti-vignetting filter it is important to disable this from the Lens Corrections tool in case you are shooting on a H-camera system which has DAC corrections applied.Failing to do so will adjust the corners of the image more than intended as you will have both the “Vignetting” tool and the “Equalize intensity” tool applied to the image resulting in an incorrect correction of the image

Disabling “Vignetting” is important but only applies to H-system cameras.

Finally, neutralize the image and continue shooting with the new corrections.

If needed you can select a previous “Scene Calibration” file from the popup list in the tool.

Phocus: Scene calibration tool

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