Very short focal fisheye lenses
and
Stitching/Blending Double-Shot (DS) images

Part (1)

Summary of the study

Sigma and Sunex have recently and almost simultaneously put on the market a new very short focal lens. Both the Sigma 8-mm f2.8 and the Sunex Superfisheye 5.6-mm f5.6 may allow to do spherical panographies by assembling only two hemispherical opposite images (aka Double Shot or DS) thanks to their Angle of View >>180 degrees.
The noteworthy point is that this can be done now on an affordable basis unlike with the previously available but expensive alternatives (4.88-mm f5.2 and 7.45-mm f2.8) sold by Coastal Optical (also known as IPIX lenses) and not quoting the venerable (220 deg FOV) Nikon 6-mm f2.8 monster.

Part 1 (This page)
Appraisal of the lenses performance shall be done in a pure practical approach without Chart measurement: I did not have the chance to put my hands on the lenses and had only some kindly provided sample images for experimentation.
While the Sunex is a fixed aperture model, the Sigma can be set from f/2.8 through f/22 and the capability of this lens shall be evaluated by taking this feature into account. The illustrated results are presented in Part 1 (This page).

Part 2
Evaluation of panorama stitching programs likely to allow DS stitching is presented in Part 2.

Part 3
In the course of evaluation of possible ways to produce panorama with DS, I have discovered an anomaly
I have thus elected to describe the conditions where a strange dysfunction of some blending engines happens seemingly on a random way, when they are faced to blending DS images.
After many hour of experimentation, I have yet not found a valid explanation for this instability and I have resigned myself to write Part 3 in order to give all means to replicate for further investigation by others. I hope to be able to remove this part as soon as the problem is solved.

Note: While the source images from the Superfisheye where provided by Sunex, inc. differnt sources including Sunex he source images shot with the Sigma 4.5-mm were all kindly provided by Roberto Gomez Torres.


Part one

Very short focal fisheye lenses for Panography
Practical and illustrated evaluation of performance

Recent past years are ancient past for quick evolving technology such as digital panography;-)

In the late 90's, real estate panoramic images were most often shot with a fisheye adapter on a PS camera or a seldom and expensive fisheye lens on a 35-mm film camera. With the availability of affordable DSLR camera such as the Canon EOS 300D in 2003 or Nikon D70, the whole panographic community discovered other correspondingly affordable fisheye lenses that could do sharper panoramic outputs due to higher native resolution. This was notably possible while having to shoot and assemble more source images though. The models and types of such fisheye lenses have since spread but none of them could really replace the short focal model built by Coastal Optical (aka IPIX) or the Nikon fisheye adapters as none could simultaneously record (even on a 24 x 36 mm frame) a full circle image with an angle of view of about 183 degrees or more.

These simultaneous requirements relate to "Double Shot" panography that is done by stitching strictly only two opposite hemispherical images.
Early in 2008, Sunex and Sigma have separately but nearly simultaneously put on the market a very short lens that eventually met these requirements on an APS-C DSLR sensor: Sunex explicitly claimed these specifications for the 5.6-mm f5.6 while Sigma modestly announced only 180 degrees of angle of view for the 4.5-mm f2.8-f22.

Our empirical measurements have shown that some margin were hidden by Sigma and the 4.5-mm model has an angle of view of almost 185 degrees. Unfortunately about 182 to 183 degrees (depending from factors such as aperture setting) may be useful for stitching panoramas though. The Sunex 5.6-mm may possibly be used for DS hand-held panography. The Sigma 4.5-mm needs to be put on a more stable and accurate Camera support (e.g. tripod).

1- Sunex 5.6-mm f5.6 Superfisheye

Two Shots Examples (panorama stitched from two opposite hemispherical images):

Click on the image above to view the cubicQTVR.

Read the related article

These panoramas should be viewed on the

Most of this examples were hand-held shot.

These examples were probably not shot by an experienced panographer. The quality could have been certainly considerably better if the shots and stitch had been done by a seasoned and skilled professional IMHO.

Images kindly provided by John Law
Original QTVRs by Sunex

NEW (Feb 24): Some sets of pairs of images have been made available by Sunex in this thread from the Panoguide forum on February 24th. As John Houghton and other that posted messages on this thread I have got the dark shadow line on the seam and could not find a way to really avoid it. All the stitchers and blender combination that I tried failed to remove it while it was slightly more intense in some cases.

Three Shots Examples

These panoramas should be viewed on the Sunex Web site

NEW (Feb 24): Some sets of three images have been made available by Sunex in this thread from the Panoguide forum on February 24th. All the stitchers that I have tested in Part 2 give exactly the same typical result as the one that was got from the sample images shot with the Sigma (native resolution being of course slightly better with the Sunex superfisheye).

Four Shots Examples (all with camera hand-held)

These panoramas should be viewed on the Sunex Web site

Conclusion and personal opinion extracted from my previous Test report (22 January 2008):

I have been experimenting with Sunex superfisheyes for approximately two months now and then. I had suspended my appreciation by waiting for the finalized production model. I was eager to see what this lens would produce as a unique double-shot device to make panoramas from DSLR images. I am now not sure to grasp anymore what the intentions of Sunex are. After initially introducing a panoramic rotator for double shot, it appears from their site that this is not the case anymore as only the three-positions rotator remains available. Furthermore, beside three-shots, they show now mainly hand-held double-shot output panos in the gallery on their website and these are not really convincing, to say the least;-)
The first initial strategy was clear to me and implicitly simple to understand IMHO: thanks to more than 180 degree of angle of view, just supplant the comparable but very expensive Coastal Optical (aka IPIX) fisheye for double-shot "à la IPIX".
Ironically, this fisheye can be used in double-shot mode with non-Sunex panohead anyway. I assume that the quality standard has raised to a much higher level since the IPIX era and that this cannot been reached with a double-shot method on a systematic and repeatable manner. With only the "three positions" rotator presently made available, Sunex shall have to compete with much better, numerous and cheaper performers that can be used the same way. The unique advantage of simultaneous 185 deg Angle of View and full un-cropped circular fisheye image, is lost.
The Sigma 8-mm on an APS-C DSLR, the same Sigma 8-mm or the "shaved" Nikkor 10.5-mm or Tokina10-17 or Sigma 10-mm (if they were "shaved"), when fitted on correspondingly FF cameras (without adapter) are probably preferable in every sense since they offer better sharpness and greater output resolution.
Adding the Sigma 4.5-mm to this list, all of them will give full EXIF data transmission as well as aperture/focus full manual or automatic control. Of course the cost of these combos shall vary and this may count for a fair comparison. While it is definitely a good lens, I believe that the Sunex superfisheye comes probably a little too late to be an absolute winner. I might use it, but not for more than two shots to produce an acceptable 360 x 180 panography, knowing now from experience that technical care and experience is unfortunately required to do so.


2- Sigma 4.5-mm f2.8 EX DC circular fisheye

This fisheye lens differs mainly from the Superfisheye from Sunex described above on at least two points that are valid when it is fitted on a compatible recent camera (Sigma, Canon or Nikon mount) : The focus and the aperture can be both set by auto-control from the camera or manually.

The aperture setting has a big impact on the image quality especially for the region near the edges of the circular image.

A hurdle that could hamper the endeavor for DS panography with this exotic Sigma lens is the light fall off toward the edge of the image circle particularly when the aperture is set at full open (f/2.8).
This so-called "vignetting" has been found in fact really moderate and can be corrected enough by most of the conversion or image editing graphic tools from my extensive experiment. Prior to the correction, cropping the rectangular image to the square circumscribing the image circle may well be mandatory for a more satisfactory output quality though.

An object QTVR movie (1.7 MB) inter-actively illustrates the influence of the aperture setting on the light fall-off a.k.a. "vignetting".

We cannot see clearly here the influence on the angle of view as the "vignetting" hides this aspect somewhat. This was more clearly observed on other fisheye lenses: (1) and (2) the angle of view is reduced with higher (f /) aperture i.e. reducing the diaphragm diameter.

Full open aperture (f/2.8) setting:

Double Shot f/2.8

180 deg apart

Four Shots f/2.8

90 deg apart

Click on the thumbnail image to view the panorama in full screen window
The seam area isn't really clean
The seam area is much better defined

Sweet spot (f/11) aperture setting:

Double Shot f/11

180 deg apart

Three Shots f/11

120 deg apart

Four Shots f/11

90 deg apart

Click on the thumbnail image to view the panorama in full screen window
The seam area isn't really clean but better than f/2.8
The seam area is much better defined
The seam area is much better defined

Overview and synthesis:

Both the Sigma and the Sunex allows to perform Double Shot panography: the angle of view of the Sunex may be a bit larger but by a very slight edge if any.

The light fall-off of these two lenses is really moderate and can be equally well be corrected by a graphic software such as Photoshop for instance.
This often called "vignetting" is however accentuated when the Sigma fisheye is fully opened at f/2.8. Then more care must be taken to remove this luminance gradient. I have found that it is best to crop the rectangular image in order to make the correction much more efficient, I shall come back to this point in Part 2 (Software for DS).
Real vignetting is much more disturbingon both the lenses and can give poor result along the seams: the dark shadow that was experienced by the experimentateurs and myself with the Sunex provided sample pairs of images images are typical. The useable angle of view is really too small to provided sufficient margin from slightly misaligned opposite directions for shooting. As the aperture cannot be set on the Sunex this problem cannot be made up for whereas closing down the diaphragm may help the Sigma to give less circle edge dark gradient. In any case this is a problem for both the Sigma and the Sunex lens: the Angle of View is just too small to insure safe Double Shot making especially when less experienced panographers are involved.

The radial compression mapping is easily handled by the stitching software for both of the lenses.

These lenses are surprisingly sharp in the center part and of a decent contrast up to the edge of the image circle. However the sole seam between the two stitched image is lacking from crispness when need be. Most of the demanding panographers would not accept this obvious softness but it may be accepted by less regarding customers.

The Sigma lens gets an edge over the Sunex when closed down to more than f/8 (as the Sunex is fixed at f/5.6) but this is partly compensated by the slightly higher resolution of the output naturally got with the Sunex due to the longer focal length. The latter may possibly allow (but this has to be confirmed on the field) to shoot hand held DS panoramas by skilled photographers when the former doesn't though.

Nevertheless, Double Shot with these lenses needs some skill and experience to yield decent output panographic quality. The examples that are posted on the Sunex site kind of demonstrate this requirement. I am still wondering why they even posted these examples that show clearly that hand shot DS cannot be done if some quality is required;-)

General conclusion:

In a previous paper I have already explained my opinion about a minor flaw of the Sunex Superfisheye concerning the sometimes strong ring that surround the circular image and fools the auto -detection and auto-stich function of all the leading software.

I understand that Sunex optical engineers have aimed at the panographical (if no from the start of the critical design phase, for sure sometime during the prototyping step). I don't know if Sigma has ever heard the plea from their regular and faithful panographer/customer (ah, the old Sigma 8mm f 4.0). As a consequence I would understand that Sigma forgot to push the Angle of View specification some 5 degrees higher to make a better lens, but I sadly think it shall be hard to forgive Sunex for not having knowingly doing it and to have made a confusion between "Angle of view" with "Practical and useable Angle of View": there is a thin dark blueish line between these terms!

If had had the opportunity to have to use one of these lenses, I probably would paradoxically use it only for Double Shot. I own and I am happy user of other combinations of cameras and lenses that produce much better quality from same three shots: I don't see the benefit that I would get from using so either the Sunex or the Sigma.
BTW: I can also produce DS panoramas of a much higher quality but with 120 deg limited VFOV by simply shooting in landscape mode with my 5D fitted either with the Nikkor 10.5-mm or Tokina 10-17-mm!

The answer from the owner of one of these lenses might differ if he has only an APS-C DSLR camera (and especially amongst them Canon EOS with a smaller CMOS sensors): there are no other affordable fisheye lens that allows three-shots panoramas without some sacrifice (e.g. tilting up or down in portrait mode or slanting the camera 30 degrees).