Technology during the past 10-15 years has advanced if not revolutionized photographic creativity to previously unimaginable levels. One product of this transformation is that modern-day photographers often take multiple, overlapping images to capture a larger scene and later stitch and crop them into a final composition (Figures 1 & 2). Often the process is considered synonymous with creating panoramas, although the technique can be used to create a composition of any desired crop ratio (Figure 3).
I’ll cover the process of capturing images for stitching in a separate tutorial. But one important aspect to consistently getting good results is to avoid major parallax errors in the final, stitched image. If parallax can’t be avoided, then the software used may not be able to stitch the photos successfully, or very well. By rotating the camera-lens combination around the lens’ no-parallax-point (NPP) we can effectively eliminate parallax errors. The NPP is equivalent to the lens’ entrance pupil, which is the location of the optical image of the physical aperture as seen through the front of the lens. In this tutorial I’ll try my best to explain how to establish the NPP for a lens.
Ask the Manufacturer or Consult the User Manual
First, you might be able to get data from the manufacturer on the distance of the optical center of their lenses from the sensor plane or camera-lens interface in millimeters (mm). For example, Zeiss reports the entrance pupil distance from the focal plane in the lens’ specification sheet. Armed with this information, attach the lens to the camera and measure the distance provided so you know where it occurs along the lens barrel (but remember that the actual NPP is in the center of the lens!).
If it’s a prime lens, I recommend marking the NPP on the barrel, or if possible apply gaffers tape to the barrel and mark the NPP position on the tape for future reference. Since the NPP will vary with magnification, a cheat sheet is needed if a zoom lens is involved (Figures 4 & 5; I like to record NPPs for focal lengths printed on the zoom ring). Then mount the camera on a pano slide (also referred to as a NPP or nodal slide) that has a distance scale marked in millimeters, making note of where along the scale the NPP occurs (Figure 5). This is the mark you’ll use to align the slide on the tripod head to control for parallax.
If you’re lucky, the tripod mounting plate or L-bracket that you purchased for your camera will have been designed to mount on the base of the camera or accessory grip so that it’s center coincides precisely with the camera’s sensor plane. If that’s the case, just mount the camera on a pano slide and position the slide on the tripod at the point where the slide’s scale equals the NPP/entrance pupil distance provided by the manufacturer. Then the camera will be correctly positioned on the NPP of the lens until/unless your using a ballhead and reposition vertically off level (see Axis of Rotation and Tripod Heads near the end of this tutorial for how to compensate when using a ballhead or multi-way panning head – this is where marking the NPP on the lens is handy).
Incidentally, if you own the same camera and lenses I use, you can’t necessarily use the data shown in figure 4 to establish the NPP unless you also use the same camera L-bracket I do (Really Right Stuff BGE11-LB). You’ll also notice that the NPP with the same lens and focal length vary with the camera’s orientation. That’s because for this particular L-bracket, the side and bottom mounting plates are not aligned in the same location relative to the camera’s sensor plane. Finally, if you purchase a new camera and tripod mounting bracket, the NPP distances you previously used may no longer work (even though you might be using the same lenses, and regardless of how you initially established the NPP for your lenses) unless the brackets for both cameras are aligned with the sensor plane. This is another reason to mark the NPP on your lenses so that you’ll have an easier time re-establishing it with new gear.
Rough Method for Determining the NPP
Unfortunately, lens manufacturers don’t often report the optical center point of their lenses, particularly at different focal lengths for a zoom lens. So now what? Although it’s not difficult to precisely determine the NPP yourself, you can roughly approximate it by viewing the front of the lens with the aperture engaged, visually estimate where you see the aperture along the barrel and measure the distance (mm) from that point back toward the sensor plane. Mark this point/plane on the lens. This is the plane you will use when positioning the camera on a pano slide that in-turn is mounted on the tripod head.
Unless your depth perception is quite poor, it’s surprising how close to the actual NPP you can get by using this crude, visual method.
Precisely Estimating the NPP
There are many tutorials available on the Internet if you want to more precisely measure the NPP yourself. Stitching software has gotten very good in recent years, thus it’s not critical that you be absolutely perfect. But the closer you can position the camera relative to the lens’ true NPP the less risk there is that the software won’t be able to perform the stitch, and the less rotating, cropping or warping you’ll have to do on the final stitched result. This is especially true if stitching images taken using wide-angle or ultra wide-angle lenses.
The process to precisely determine the NPP involves the following considerations and steps:
1) Use a tripod and head that’s perfectly level and locked down to prevent any vertical movement. I highly recommend using a fully adjustable gimbal or pano-gimbal head (Figure 6) for doing stitched images and to determine a lens’ NPP, but a multi-way panning head (Figure 7) or ballhead (Figures 8) will work if you take added precautions noted below.
2) Mount the camera-lens combination on a pano slide marked with a distance scale (mm), and position the slide on the tripod head such that the center of the lens is roughly aligned with the horizontal axis of rotation (Figure 8).
3) On a table or similar flat platform, place a couple of relatively thin, straight objects that you can stand on-end, positioning them in-line as viewed through the camera, but at least a couple of feet apart (Figure 9). You also don’t want the front object to completely cover up the back object as viewed through the camera, and want the objects close enough to the lens so you can view them easily. Adjust the tripod distance and/or height, or the platform the objects are on so that the top of each object can be seen through the viewfinder or on the LCD, with the top of the front object preferably near the center of the frame. You might have to re-level the tripod, or re-position the objects so that everything is level and lined up.
Another option is to position the tripod near a window, and draw a thin vertical line with a grease pen on the glass in front a well-defined background reference object outside.
4) Once your reference objects are set and aligned with the center of the frame, slowly pan the horizontal axis of the tripod head back and forth, viewing the objects on the LCD or through the viewfinder. Unless by luck you positioned the lens on the tripod head exactly at the NPP (unlikely), you’ll notice either the rear or front object move relative to the other object as you pan. That’s parallax (Figure 10, also click the video link below showing parallax in real-time).
5) Next, re-center the objects in the frame, move the slide fore or aft 5-10mm, and repeat the panning process. If the apparent parallax worsens (more relative movement among the objects), stop, re-position, move the slide in the opposite direction, and continue panning the camera. But, if the parallax seems to improve (less relative movement between the objects), stop, re-position, and move the slide in the same direction using smaller increments, repeating the process until the two objects remain aligned in the same relative position to each other throughout the field of view as the camera is panned (Figure 11; or click the video link below showing what it looks like when the NPP has been attained). Once this has been achieved, record the distance on the slide’s scale that’s aligned with the center of rotation, and you’ve established the NPP for the lens at that focal length.
Regardless of how you established the NPP of the lens, in the future all you have to do to control for parallax when shooting images for stitching is to position the camera-lens-slide combination on the tripod head at that distance. The axis of rotation is now set at the NPP for that lens and focal length, provided the platform is level. If you might be creating stitched images with multiple lenses or focal lengths (in the case of a zoom), it’s helpful to create a tabular cheat sheet, or jot the data on post-it tape and stick it on your pano slide component (Figures 4 & 5).
Axis of Rotation and Tripod Heads
One final precaution is needed before you start taking photos for stitching. Make sure the tripod is level so that the horizon is appropriately aligned; otherwise you may get a poor stitch, or a skewed orientation that might require you rotate the image as well as crop out much of the composition (Figure 12). Once the camera/lens combination is set at the NPP on the slide and attached to an appropriately prepared tripod, you need to be able to rotate the lens precisely around that point as you pan among frames.
If you use a 3- to 5-way panning head or a ballhead (Figures 7 & 8), after positioning the horizon where you want it (which will result in a vertical adjustment), the camera will no longer be level, nor rotating around the correct axis of rotation to prevent parallax. Instead it will be rotating in front of the NPP (if you pushed the horizon toward the top of the frame) or behind the NPP (if you positioned the horizon toward the bottom of the frame). Depending on the lens, the axis of rotation could well be off the NPP by several inches if you positioned the horizon near the top or bottom edge of the frame (Figure 13).
As a result, you will need to adjust the pano slide fore or aft as needed to re-establish rotation of the horizontal axis at the NPP to compensate for the angle created when re-positioning the horizon (Figure 14). If the lens involved is a prime and you marked the NPP on the barrel, it should be easy to make the necessary adjustment. Assuming the horizon is still level after all this, you are now ready to capture a series of overlapping, parallax-free frames that will form a row in the final stitched image.
If using a fully adjustable gimbal or pano-gimbal tripod head (Figure 6), once leveled, attach the camera-lens-pano slide combination to the head at the NPP marking, and shift the vertical riser of the head to the left or right as needed to center the lens over the horizontal axis of rotation. Now the lens is centered along both axes at the NPP and you won’t have to make any further adjustments to the pano slide after you’ve positioned the horizon where you want it.
What’s more, if you need to capture multiple rows for the final stitched image, by using a pano-gimbal head you won’t have to worry about re-establishing the NPP point when you rotate the lens in the vertical plane (Figure 6; or click on the video below demonstrating how these heads precisely rotate a lens around the NPP). This efficiency is one of the big advantages of using these types of tripod heads over multi-way panning heads or ballheads when capturing images for stitching.
I hope you found some of this information useful for capturing images meant for stitched compositions. If you have any questions regarding the information provided in this tutorial, please leave a comment or contact me at ImagesByBeaulin@charter.net.
© Beau Liddell, ImagesByBeaulin.com, All rights reserved.