Adventure photography is about telling stories of epic places and journeys. This excerpt from Dan Bailey's new book, Outdoor Action and Adventure Photography, published by Focal Press will give you...
911 Ground Zero. Original 35mm full
frame 16mm fisheye image
The area in red is that recorded on a 22.7 x 15.1mm DSLR sensor
Here's the original image, scanned from 35mm film. The lens was a
16mm fisheye and it's quite evident that the image suffers form "fisheye
distortion". The fisheye results in straight lines that are curved, especially
near the edges of the frame. The horizontal field of view is around 140 degrees,
the vertical field of view is around 90 degrees and the diagonal field of view is
180 degrees. This is much wider than any available rectilinear lens. For example,
a 14mm rectilinear lens (the widest such lens available for a 35mm SLR) only has
a horizontal field of view of 104 degrees (vs. 140 degrees for the fisheye).
This image also suffers from perspective distortion because the
camera was tilted upwards. This is evident by the fact that the buildings "lean
in" towards the center of the image. This is more clearly seen in the image
below, which has been corrected for "fisheye distortion". This perspective
distortion is not a result of the fisheye lens. A normal wide-angle rectilinear
lens would show the same distortion
This the image after fisheye to
rectilinear conversion
In this image the "fisheye"distortion has been corrected to form
a rectilinear image, though the shape of the image is very odd indeed! The
corners have been really stretched out, though that's not surprising since as I
said above, the diagonal field of view is 180 degrees. It turns out that you
can't have a single rectilinear image formed on film with a static lens which has
a 180 degree field of view, or if you did the image would have to be infinitely
wide! So it's no wonder that the corners are stretched so much, and in fact are
slightly cropped.
Luckily the area recorded by the small DSLR sensor does not
require such extreme correction and the stretching is relatively moderate.. You
can now clearly see the perspective distortion of the buildings leaning inwards.
The red frame which shows the image area recorded on the DSLR sensor is no longer
rectangular. We will deal with that later, but first let's crop the image to a
rectangular format (yellow frame), then correct the perspective distortion.
This is the above image, cropped to
the maximum rectangular size
This is the above image, this
time corrected for perspective
As you can see, the top of the image has been stretched, pulling
the buildings back into the vertical. The image has now been corrected for both
perspective and fisheye distortion. All that remains is to crop the area inside
the red DSLR frame lines into a rectangular format.
Here is the final crop, corrected for both fisheye distortion and
perspective, including only that part of the imaged recorded by a 22.7 x 15.1mm
DSLR sensor This view is approximately that which would be seen with a 20mm lens
on a full frame 35mm SLR. Figures at the edges of the frame are stretched, but
that's exactly what happens with a rectilinear lens. The fisheye lens doesn't
stretch objects in this way, but it bends straight lines. Comparing this
shot with the fisheye shot you can see that each has it's own type of distortion
and neither represents the scene in quite the same was as we see it. However
people are more used to seeing rectilinear distortion, so that's what people
usually prefer to see. Note that the aspect ratio of the image has been changed
by the transformation.
Though the transforms here were done on a full frame 35mm image,
they could just as easily have been done on the image recorded by a small sensor
DSLR (i.e. the first image cropped to the red lines). Image quality can be quite
good since we are working with the center part of the image and extreme
corrections are not needed (as is the case for full frame 35mm images).
How to do the corrections
Fisheye to Rectilinear correction can be done
using a number of programs. For the image shown here I used
Panotools. It has two things going for it. First it's probably the
most powerful tool available for image correction and panoramic stitching as well
as a number of other functions. Second it's free! The only downside is that it is
slightly "geekware" in that it does not have a simple user interface, indeed it
doesn't even self install (you have to move files into appropriate directories
manually). There'salso not much in the way of a manual or tutorial. It works
either as a stand alone JAVA program or as a PhotoShop plugin. I used the
PhotoShop configuration. There are graphical front ends available for the
program, but the most user friendly Windows version,
PTGUI, costs $40. To get help with the program, try the
Panotools
FAQ website, this
FAQ or this
Panorama Tools website. If
there is a complete Panotools manual, so far I haven't found it! I'm no expert
with this program, so asking me questions about it isn't likely to get you much
of an answer I'm afraid.
There are also a number of commercial programs which can make corrections for
image distortions, for example checkout
ImageAlign (though note it costs $300).
Perspective correction can be done by most
popular image editing programs. I used Picture Publisher 8.0's transform tool for
this example, but you could just as easily use PhotoShop using the crop tool's
perspective option.
One of the problems of small sensor DSLRs used with regular 35mm lenses is
that it's very difficult to get a really wide angle image. The small sensor
results in an effective "lens multiplier", so lenses act as though they have a
longer focal length as far as field of view is concerned. Using the common 22.7 x
15.1 mm sensor size (which is used in the Canon EOS D30/D60/10D bodies) results
in a 1.6x lens multiplier. So a 20mm lens gives the same view on such a DSLR as a
32mm lens does on a regular full frame SLR. A 17mm lens gives the view of a 28mm
lens and a 14mm lens (the widest available) gives the view of a 22mm lens.
Nikon have announced (but have not yet made available as of 03/03) a special
lens which only covers the small sensor (not the full 35mm frame), which is a
12-24mm, corresponding to 19-38mm in normal 35mm terms.
Is there an alternative? Well, the answer is yes. Since we are working
directly in the digital domain, not on film, we can do a lot of image
manipulation. Fisheye lenses have VERY wide fields of view, but distort the image
in an unnatural looking way. However in the digital domain such distortions are
fairly easily corrected, or at least transformed into the more normal looking
distortions given by rectilinear lenses!
There are two types of fisheye lens. The first has a 180 degree field of view
in all directions and results in a circular image on 35mm film. These lenses
typically have a focal length around 8mm. These lenses are not so useful in this
application. The second type of fisheye is known as a "full frame fisheye" and
they cover the entire 35mm frame, with a 180 degree diagonal field of view.
Typical horizontal coverage is around 140 degrees, typical vertical coverage is
around 90 degrees and typical focal length is 15 or 16mm.
Nikon and Canon (and other major brand) fisheye lenses are not cheap,
but there is a Russian (actually Ukranian) Zenitar 16/2.8 fisheye available for
many cameras at a cost of around $150. which is a decent all manual lens. It
comes in both Nikon and Pentax screw mounts, with adaptors for a number of other
cameras such as the Canon EOS line. The best way to find a vendor of this lens is
to do a web search or look on Ebay. It's typically not stocked by the usual mail
order houses such as Ritz, Adorama or B&H.
911 Ground Zero. Original 35mm full
frame 16mm fisheye image
The area in red is that recorded on a 22.7 x 15.1mm DSLR sensor
Here's the original image, scanned from 35mm film. The lens was a
16mm fisheye and it's quite evident that the image suffers form "fisheye
distortion". The fisheye results in straight lines that are curved, especially
near the edges of the frame. The horizontal field of view is around 140 degrees,
the vertical field of view is around 90 degrees and the diagonal field of view is
180 degrees. This is much wider than any available rectilinear lens. For example,
a 14mm rectilinear lens (the widest such lens available for a 35mm SLR) only has
a horizontal field of view of 104 degrees (vs. 140 degrees for the fisheye).
This image also suffers from perspective distortion because the
camera was tilted upwards. This is evident by the fact that the buildings "lean
in" towards the center of the image. This is more clearly seen in the image
below, which has been corrected for "fisheye distortion". This perspective
distortion is not a result of the fisheye lens. A normal wide-angle rectilinear
lens would show the same distortion
This the image after fisheye to
rectilinear conversion
In this image the "fisheye"distortion has been corrected to form
a rectilinear image, though the shape of the image is very odd indeed! The
corners have been really stretched out, though that's not surprising since as I
said above, the diagonal field of view is 180 degrees. It turns out that you
can't have a single rectilinear image formed on film with a static lens which has
a 180 degree field of view, or if you did the image would have to be infinitely
wide! So it's no wonder that the corners are stretched so much, and in fact are
slightly cropped.
Luckily the area recorded by the small DSLR sensor does not
require such extreme correction and the stretching is relatively moderate.. You
can now clearly see the perspective distortion of the buildings leaning inwards.
The red frame which shows the image area recorded on the DSLR sensor is no longer
rectangular. We will deal with that later, but first let's crop the image to a
rectangular format (yellow frame), then correct the perspective distortion.
This is the above image, cropped to
the maximum rectangular size
This is the above image, this
time corrected for perspective
As you can see, the top of the image has been stretched, pulling
the buildings back into the vertical. The image has now been corrected for both
perspective and fisheye distortion. All that remains is to crop the area inside
the red DSLR frame lines into a rectangular format.
Here is the final crop, corrected for both fisheye distortion and
perspective, including only that part of the imaged recorded by a 22.7 x 15.1mm
DSLR sensor This view is approximately that which would be seen with a 20mm lens
on a full frame 35mm SLR. Figures at the edges of the frame are stretched, but
that's exactly what happens with a rectilinear lens. The fisheye lens doesn't
stretch objects in this way, but it bends straight lines. Comparing this
shot with the fisheye shot you can see that each has it's own type of distortion
and neither represents the scene in quite the same was as we see it. However
people are more used to seeing rectilinear distortion, so that's what people
usually prefer to see. Note that the aspect ratio of the image has been changed
by the transformation.
Though the transforms here were done on a full frame 35mm image,
they could just as easily have been done on the image recorded by a small sensor
DSLR (i.e. the first image cropped to the red lines). Image quality can be quite
good since we are working with the center part of the image and extreme
corrections are not needed (as is the case for full frame 35mm images).
How to do the corrections
Fisheye to Rectilinear correction can be done
using a number of programs. For the image shown here I used
Panotools. It has two things going for it. First it's probably the
most powerful tool available for image correction and panoramic stitching as well
as a number of other functions. Second it's free! The only downside is that it is
slightly "geekware" in that it does not have a simple user interface, indeed it
doesn't even self install (you have to move files into appropriate directories
manually). There'salso not much in the way of a manual or tutorial. It works
either as a stand alone JAVA program or as a PhotoShop plugin. I used the
PhotoShop configuration. There are graphical front ends available for the
program, but the most user friendly Windows version,
PTGUI, costs $40. To get help with the program, try the
Panotools
FAQ website, this
FAQ or this
Panorama Tools website. If
there is a complete Panotools manual, so far I haven't found it! I'm no expert
with this program, so asking me questions about it isn't likely to get you much
of an answer I'm afraid.
There are also a number of commercial programs which can make corrections for
image distortions, for example checkout
ImageAlign (though note it costs $300).
Perspective correction can be done by most
popular image editing programs. I used Picture Publisher 8.0's transform tool for
this example, but you could just as easily use PhotoShop using the crop tool's
perspective option.
All text and images (C) Copyright 2003, Robert M. Atkins All
Rights Reserved
FWIW, Picture Window Pro includes a lens distortion correction funtion that will fix fisheye distortion. It seems quite easy to use. It also has a perspective correction function, but I usually find that if the buildings at the edges are straight, the building in the center has an enlarged top section...
Question: doesn't image quality get seriously funky around the edges?
Image quality does get seriously funky if you do this on a full frame fisheye image with 180 degrees diagonal coverage. However if you only do the transform on a cropped section, which has a smaller angular coverage, the transformed image is a lot better. I'll try to post an example later of a transformed fisheye image from a D30.
Sounds like a combo I use occasionally, 8mm fisheye on a Nikon D100. Here's a portrait, after conversion in PanoTools to the equivelant of a 6mm ultrawide on 35mm film.
I should have spend more time futzing with the CA corrections on this one.
I think it's clear in the "final crop" picture, that one more step is needed: to strech the image vertically to get the proper ratio. Using the perspective tool to adjust for tilt seems to squish images, making everything look shorter than it is.
It's possible the vertical stretching of this image wasn't optimized, but in general you will tend to get objects at the sides of the frame looking "squat" since they are stretched more horizontally than vertically.
All flat projections distort, just as all maps of the globe distort. Rectilinear projections preserve straight lines, but at the cost of distorting areas. Fisheye projections preserve areas, but at the cost of bending straight lines. We tend to view rectilinear projections as "more correct", but in fact they are just different.
This is good, however, how can one go about doing a real fisheye image on a DSLR with the drop factor. That wouold have been more interesting, to me. You can get a Sigma 15-30mm and your wide angle worries are solved (at a price). I have the Zenitar but never use it.
Maybe I'm a little late but actually Zenitar is a line of really Russian lenses (the factory is called KMZ, http://www.zenit-camera.com/ and it's located in Krasnogorsk, a Moscow suburby), Ukrainian lenses are called Arsat (the factory is called "Arsenal") and the factory is located in Kiev.
Another option for remapping fisheye images is provided by RectFish which on top of rectilinear remapping with perspective control also provides a unique "rectangular" projection mode, that minimizes stretching but will get rid of dark corners.
Salisbury Cathedral, with a Zenitar 16mm fisheye lens on a full-frame body, defished with Panotools.
"Image quality does get seriously funky if you do this on a full frame fisheye image with 180 degrees diagonal coverage."
I find that it's not too bad if the image is fairly dark around the edges naturally, or if the subject is right slap-bang in the middle of the image. Here's a shot I took today with a 16mm Zenitar fisheye lens, on a full-frame body. I defished it with Panotools, using HFOV and VFOV values of 130 (probably not strictly correct, but it seems to work). The biggest problem is holding the camera so that it is perfectly level; if it's at an angle, the end result looks weird.
One tip I have is that you should set Panotools to output the image into a directory, rather than onto the screen. It will then produce a full-sized image right out to the edges, whereas if you output the image to the screen, Panotools isn't smart enough to enlarge the canvas to match the new image size.
Panotools seems to have trouble overwriting its own output - it fails to create the image and then locks the output file, so you have to quit - which means that you can only have one Panotools output file per directory.
Also, certain image sizes simply don't work; my setup at least refuses to work with files that are 4368x3912, but is perfectly happy if I resize them to e.g. 4300x2867. Perhaps it is dividing by zero somewhere.
Unless I missed something, Fisheye gives you a wider angle of view and a distorted, circular image, then you straighten and crop it to get it rectilinear - So - you get a view similar to a 20mm wide angle. IF you do not want to use the special effects created by a Fisheye lens, why use it in the first place? Why take a circular image and then turn it rectilinear when the process involves you losing most of the detail by cropping it out?
Hi
Well, actually, depending on the lens and camera used, you may get a MUCH wider angle of view than any rectilinear lens can give you. So, it is a viable option on some cases, when that 12 mm Sigma just will not provide enough field of view even on your full frame camera :-)
http://www.acapixus.dk/photography/angle_of_view.htm
http://www.acapixus.dk/photography/AOV111/index.htm
In my opinion you don't lose all that much by stretching the image out and correcting it for perspective. Here's an example that I quickly created, with the original at the top and the stretched, corrected result at the bottom:
It looks much better sized down, and so I surmise if you start with a 20mp+ original you will end up with something that looks good in large print sizes. The software has since been updated to work with 16-bit files, and it is now much easier to use - you can just open up the RAW original, apply your favourite contrast etc settings, run the file through PTRemap, perform final edits, resize, all without having to manually open and close and delete files. There's a helpful wiki about it here at the Panotools wiki.
I have no formal way of comparing the field of view of the above image with a rectilinear wide angle, such as Sigma's recent 12mm. The Zenitar lens I use is very sharp in the centre when stopped down, and it is compact and point-and-shoot, but I suspect the Sigma lens will perform much better at the edges.
Also, this is more interesting to talk about than a conventional wide angle lens. It's like shooting Cinerama!
Thanks guys for this extremely informative article/thread! I found an Aperture plug-in that will work for me for $29.95: Image Trends Fisheye-Hemi. I haven't tried it out, because I've had the 5D Mark II for a week now and don't have a fisheye lens yet. But since it will integrate so well with my workflow, I think I'll try it out (start with the trial download in case it sucks). I'm waiting for my Sigma 15mm Fisheye, and I'll try some shots.
I can't resist a two-fer: a fisheye and an ultra wide rectilinear lens in one!
, March 07, 2003; 10:39 A.M. 
