Why and when should you Crop your source image?


One of the most popular lens that is used today by the panorama photographers is the Sigma 8-mm EX Fisheye lens. It's a favorite amongst many Panorama Tools users.

Due to clipping, the Horizontal Field of View (HFOV) in portrait mode is between 112 and 118.5 degrees for all the APS-C sensor sized Camera models as the width of the CCD or CMOS sensors ranges presently from 14.8 (Canon EOS 350D) to 15.7 mm (Nikon D70, D2X, Pentax *ist DS and Konica-Minolta 7D).

Four photos equally spaced by 90 degrees horizontally are then enough to cover 360 degrees of Azimuth The 180 degrees (Vertical) VFOV is unclipped. As a consequence, the full sphere is covered in portrait mode with these only four shots.

Other lenses have similar FOV: Nikon 8 mm fisheye (discontinued) and Peleng 8 mm (slight clipping along the longer sensor side though). The rest of this paper shall then apply as well to them.

Sharpness is good at the center but not so good near the circular edge

Several factors makes the information at the center of the photo of the Sigma 8 mm lens much more crispy and contrasty than near the edges.

Light fall-off increase sharply toward the edges and chromatic aberration tends also to blur the image there. The ideal optical properties of the fisheye theoretical lens are also deviated there. A radial compression of around 12 % reduces the actual pixel resolution close to the circular limit (i.e. at 85-90 degrees from center).

The quality is then very much degraded in this outer area of the source image. The center is crisp (that is to say sharp and contrasty) and is called the "sweet spot" of the lens.

More photos to increase quality of the Panorama

When shooting in portrait mode and rotating about the vertical axis that cross the "Nodal Point", the bottom and the top part of each individual image are of the worst quality.

After subsequent stitching and blending, these two zone of the sphere (Often called Zenith and Nadir in reference to the astronomical vocabulary) shall of course be less sharp than the crisp details of horizontal plane.

This type of lens (circular fisheye) is generally used for Panoramas where most of the interest lies in the horizontal plane. The Nadir can be hidden by a commercial logo and the Zenith may be viewed without the need of sharpness especially when it shows sky possibly with cloud as the only details.

However, it happens that some architectural features lie at the Zenith e.g. the dome of a Cathedral or at the Nadir e.g. the precious gold-inserted tiling of the floor of the same Cathedral.

These features shall best be recorded by shooting directly toward Nadir and Zenith Insertion of these extra images at the relevant position within the set of other "standard" images from the horizontal plane should then give the best result.

Stitching and blending may spoil the Nadir and the Zenith inserts

We have seen in another page (click on "A real Case Study" button from the navigation bar at the top), that in certain frequent circumstances, the work-flow succession of warping, stitching and blending of the sources images shall pick parts of the images of different quality to assemble them quite randomly in the Nadir zone or the Zenith zone or both in Nadir and Zenith

The blender engines may do such a "good job" at blending that one can hardly guess that in fact the obtained quality is very degraded. A casual observer cannot know that a much better image could be obtained. But it happens also that some crisp images detail is contiguous to a "soft" area. The border line between these regions is a clear discontinuity that is perceived as visually defective.

As a net result, inserting an extra image to increase the quality can in fact and unfortunately do the contrary by troubling the panorama viewing experience.

This phenomenon happens also when one inserts a Nadir Patch aiming at hiding the tripod and the panorama head from the Nadir. But then, the more crisp part than the surrounding is grouped as a circle around the Nadir point and can be easily "blurred" to equalize the "softness".

Smart Cropping the source images to keep only the "sweet spot"

A simple way to make sure that no soft part of the image coming from the edges of the fisheye circular image will ever be apparent in the panorama, is to CROP out these portions.

The result shall be like a patchwork of selected neat portions of the images.

How and when would we crop?

A tutorial shows How to Smart Crop within PTGui 5:

Two ways to crop:

  1. In an external application (click on "HOW TO: Part I" button from the navigation bar at the top) -to Panorama Tools- such as in your favorite graphic editing program e.g. Adobe PhotoShop. This requires however some extra steps in the work flow and also deprives PTools of images information that could otherwise yield better "PTOptimization" if not cropped out).
  2. Internally to Panorama Tools (click on "HOW TO: Part II" button from the navigation bar at the top). With PTGui 5 it is very efficient but it requires some advanced skills in using the GUI. It is amazing that this powerful means is so under-used by the Panorama Tools Expert User community. Unfortunately this way can hardly be used under the current PTMac version:-((

As previously stated, cropping the source images of a panorama can be done to insure that the expected quality improvement brought by extra Nadir or Zenith image shall be effective.

Furthermore, cropping could also be applied:

In fact, cropping could also be used to remove unwanted spoiling details (from any source image) that would remain after blending. This is feasible when equivalent unspoiled portion is available on another image.

Remark: Stitcher from Realviz has a very handy and versatile functionality within the Application (called "Stencil") for cropping out unwanted details. Unfortunately Stitcher does not support fisheyes images and therefore cannot help directly at preserving Nadir or Zenith crispness.