Guide to panoramic photography
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2/14
Shim
Updated on February 03, 2017

How many photos to make a virtual tour?

You can shoot a whole sphere in only two photos - to be very quick - but also in thousands to make a very detailed gigapixel. But I like the compromise of eight photos very much because it can be shot and stitched quite quickly and the virtual tour has a great quality on all screens...

Lens Canon 8-15 mm fish-eyeUnlike a simple panorama, when you want to make a virtual tour, you often want to embrace 360° at least but the best thing ever remains the full virtual tour, on 180x360°, from the floor to the ceiling.
The question of the number of photos, thus of the focal to use, is thus asked:

  • Should you use your basic lens and for how many photos?
  • Should you use a very short lens or a fisheye?
  • Circular or full-frame fisheye?
  • Do you need an optically good lens?
  • Do they all stitch well?
  • Do you want to shoot few photos to go as fast as possible or many photos to be able to zoom in the image comfortably?
  • How many photos for a gigapixel project?

 

Key points if you're a beginner...

Here are the key points to remember about the choice of the focal and the lens to make a virtual tour. The rest of this page is aimed at people who want to learn more about it.

THE big question: how many photos do I need to make a virtual tour?

Since it's about shooting a complete sphere of 180x360°, you know intuitively that it depends on the focal used:
With a circular fisheye lens, you need 2/3 photos,
With a full format fisheye lens, you need 8 photos (ideal according to me)
With for instance a 24 mm, you need 22 photos etc. etc...

So the more photos you want, the more time you'll need to shoot the virtual tour but the bigger it'll be and the more you'll be able to zoom in.
Nowadays, I think a good compromise with sensors around 20 Megapixels is 8 photos with a 10mm fisheye if you have an APS-C (Canon 760D or Nikon D5300 or D7200) and 15 or 16 mm Fisheye if you have a full format sensor (Canon 5D Mark III or Nikon D610/D750/D810).
Caution! If you're tempted by a 14 mm, this focal (for 24x36 sensor) is often expensive, doesn't stitch as fast and as well as a fisheye and you'll need more photos without gaining quality. I don't recommend it, except if you only have it at hand!

Next key points: What photo lens to choose? suivre

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How many photos to cover a sphere?

You can probably guess right away that the shorter the focal, the lower the number of photos you'll have to take to shoot a complete sphere. So if instinctively you'd think of the most common short focal, 28 mm (in 24x36), you'll still have to shoot more than 30 photos to cover this sphere completely. Quite an amount of work!

 

The tree best fisheye of the market : Nikkor 10,5mm DX, Canon 8-15mm and Samyang 8mm (for Sony or Fuji).


You need to remember that it has an advantage that we'll detail later about the possibility to zoom in the image but we'll only talk about the number of photos to shoot to cover a complete sphere here, sticking to the facts.
Besides, we know that for two consecutive photos to stitch easily and automatically, they must present an overlap surface of at least 15/20% - 30% being ideal -. With those two facts in mind, you can now calculate the number of photos you'll need in order to cover a complete sphere. It goes from two photos shot with a circular fisheye to hundreds shot with a teleobjective lens...

Choice of the number of images

A very simple formula can give you quite an accurate idea of the number of photos you'll have to shoot depending on the final panorama you're expecting with a manual panoramic head, keeping in mind that it is not important that all overlaps have the same percentage :

 

How many photos?

N = F / ( 70 / 100 × HFOV *)

N = number of images to shoot;
F = field of view of the panorama;
(70/100 = only 70% of the photo is effective since 30% are used as overlap zone).
* HFOV = field of view of the lens. This field of view is the real field of view of the lens. So either on the smaller side if the camera is in a vertical position (also called portrait mode), either in the width if the camera is placed horizontally (also called landscape mode).

Caution: catalogues almost always indicate the field of view of the lens in its diagonal. A rule of three enables to find the angle you're looking for very easily.

HFOV = MFOV × W or H / SD
HFOV = effective field of view of the lens;
MFOV = field of view given by the manufacturer for a lens;
W or H = width or height of the sensor format;
SD = diagonal of the sensor format.

Example of the case of a 15 mm fisheye lens in format 24x36 - manufacturer data SD = 180 ° - and the camera is set vertically:
HFOV = 180 °× 36 / 43.27 | HFOV = 150 ° in height | HFOV = 100 ° in width.

In our example, this formula, if you want to get a 360 ° panorama with a 15 mm fisheye lens in portrait position, tells us that we should take:
N = 360 ° / ( (70 / 100) × 100 ° ) so:
N = 5.14 images so 6 photos with a large overlap.
And you see that you only need one photo towards the ceiling and the floor to close the sphere so 8 photos altogether.

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Table of several focals/number of photos



 
Equivalent 24 x 36 ( Divide the focal by 1.5 or 1.6 to get the equivalent APS-C).

  Field of view/
diagonal
Photos/Ranges/Tilt Nb
of photos
8 mm FE - Circ.
2 or 3 (1)
2/3
15/16 mm FE/FF
6 + Z + N (2)
8
 
14 mm (3)
110°
2 x 8 + Z + N
+-30°/+-90°
from
12 to 18
 
17 mm
100°
2 x 8 + Z + N
+-30°/+-90°
18
 
24 mm
84°
2 x 10 + Z + N
+-30°/+-90°
22
 
35 mm
64°
4 x 15 + Z + N
+-17°/+-55°/+-90°
62
 
50 mm
45°
6 x 18 + Z + N
0°/+-25°/+-50°/+-75°/+-90°
128

 
(1) - With a circular fisheye lens, you can as well shoot two photos only, but they're barely stitchable. It is better to shoot three.
 
 
(2) Z + N = Zenith and Nadir
 
 
(3) even with a 14mm, you need two ranges of photos!
 

Number of photos / sensor size / panorama size

As I also explain in the third part of the photo equipment to make a virtual tour page, the number of photos combined with the sensor size determines the final size in pixels of the panorama that will be used to create the virtual and thus its quality and the zoom-in possibilities.

What file size for what display?

1 - When the panorama that will become a virtual tour is 360° x 180°. It is then an image file (JPEG, TIFF or other) of, say 4,000 x 2,000 pixels. The height/width ratio is 2/1. The width of 4,000 pixels of our example enable to display four cube sides, hence our 360°.
2 – If you take into account that your screen enables to display one side of your panorama, hence about 90 of the 360, and you remember that the standard for a screen nowadays is an average width of 1,920 pixels, then a panorama in the form of a virtual tour must be at least 2,000 pixels per side x 4 hence about 8,000 pixels long when the virtual tour is displayed with the zoom-out at its maximum. With this size, you'll agree that it will be impossible to zoom in the image but its display will look good. If once stitched, the side of our panorama is still over 8,000 pixels then you can admit that we'll be able to start zooming in the virtual tours without seeing horrendous pixels everywhere like there often are in so-called world records!

 

A few examples...

Nowadays, by stitching together 8 photos with a 20 Megapixels sensor, you get an image of a size about 15,000 x 7,500 pixels. It is perfect to zoom-in 2x on an HD screen of today. With a 36 Megapixels sensor, you'll get a 20,000 x 10,000 pixels file. Still only shooting 8 photos, you can zoom in more in the virtual tour...
By shooting two ranges of photos at 24 mm, you'd have had a 25,000 x 12,500 pixels file.


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Full format fisheye, circular or 14 mm?

For who wants to start creating virtual tours, the choice of the focal used is thus strategic. If the focal's too short, the virtual tour will be shot quickly - 2 or 3 photos are enough - but the size of the panorama will be a bit small and if the focal's too long, you'll have to shoot many photos and you'll be able to zoom in a little bit more. Full format fisheye is thus an excellent compromise.
There are two "types" of fisheye lenses, full format and circular fisheyes:

 

Photos fish-eye fullframe et circulaire

 

In both cases, the photo looks very distorted but in the second base, the effective part of the photo is very much reduced. Only half the pixels can be used. It doesn't make big virtual tours...

Caution! If you choose to shoot two circular photos, you'll indeed have your 180x360° in two photos only but even shot with a 20 Mo sensor, your virtual tour won't be so big because of the important reframing - the major part of the photo is black and useless!

 

Différence entree un objectif fish-eye plein format et un 14 mm

 

Especially in height, the - yet - very large 14 mm doesn't embrace a field of view wide enough to only need to shoot six photos vertically. You'll need at least two ranges of eight photos with a lot of overlap, which is useless. 14 mm is far from being the ideal compromise...



My recommendation!

If you want to go fast, take a circular fisheye in three photos in order to have a bit of overlap and if you want the best compromise between number of photos, shooting time and final quality, use a full frame fisheye, hence a 10.5 mm FE on an APS-Csensor or a 15 or 16 mm FE (depending on the brand) on a 24x36.

Last recommendation! To go fast (in two or three photos) it is better to have at least 20 Mo of pixels and if you want to make a nice virtual tour very quickly, the ideal compromise is still, according to me, 6+2 photos with a full format fisheye.


Why 25% overlap at least?

ReouvrementOf course, the software needs enough raw material to be able to recognize the common parts of two consecutive shots to be stitched in order to stitch together checkpoints or recognition points. But there is also a second reason that you wouldn't necessarily think of: harmonization of luminosities or color balance between two photos. Indeed, the success of the stitching will also depend on its level of discretion hence, among other parameters, on the absence of zones between two consecutive photos in the final stitching. There can be many causes since to simple differences of exposure or white balance can be added vignetting, often a problem even when the diaphragm is shut tight. And to do so, it needs at least 25% overlap. Nowadays, I would even tend to recommend 30 to 35%. The bigger the overlap zone and the more invisible the stitching.

Caution! Don't forget to set the positioner on the right number of notches you need!

In the next page, we'll talk about criteria to choose your lens to realize a virtual tour. Suivre


Next page 2/14 : criteria to choose your lens to realize a VT suivre


To be remembered...

You'll have to choose between:

1 - Take as few photos as possible to go fast during the shooting, but it will be detrimental to the quality of the virtual tour. In certain situations, the photographer doesn't have a choice.
2 - Shoot 8 photos with a full sensor fisheye, which remains the best compromise according to me,
3 - Multiply the ranges and thus shoot more photos to offer users a great zoom-in quality.

Fisheye lenses, curiously, stitch much more easily than very short focals like 14 mm. They're also less expensive and require less photos for an equivalent final quality. I really prefer my 10.5 mm fisheye or my 8-15 mm fisheye to my old 14 mm for this kind of work.

The ideal is to have an overlap rate of at least 25% between two consecutive photos in order to make harmonization of lights and colors easier. To stitch details together only, 10% could indeed be enough.


 




     
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