Monday, April 23, 2012

Which fisheye lens?

One of the most common questions we get from customers relates to the differences between all these different types of fisheye lenses.  We actually stock four different types of fisheye.  For Canon users they are:
  • Canon 15mm f/2.8 fisheye 
  • Sigma 10mm f/2.8 fisheye
  • Sigma 8mm f/3.5 circular fisheye
  • Sigma 4.5mm f/2.8 circular fisheye
and there's a similar range for Nikon users.


So what do they all do?


Well, the first thing to understand is that ALL fisheyes produce an image which is completely circular, and 180 degrees across - a complete hemisphere.  Like this:


Underscar, Lake District, March 2012 © Stewart Robertson 


The ONLY difference between the different fisheyes is the size of that circular image.  The longer the focal length of the fisheye lens, the larger the image.  Here's an illustration of the images produced by the four fisheyes  listed above, to scale - 15mm on the left, 4.5mm on the right:









But when we look at how big the sensor in the camera is, compared to these circles, then that's where things start to get really interesting.  In the images below, which are still to scale:
  • the red rectangle is the size of a full-frame sensor (1Ds series, 5D series - FX for Nikon users);
  • the green rectangle is the size of an APS-C crop sensor (7D, xxD / xxxD / xxxxD series - DX for Nikon users).




We can see that:
  • The 15mm fisheye is designed for full-frame sensors.  It fills the frame with an image which measures 180° across the diagonal, which is why this type of lens is sometimes called a rectangular fisheye or a diagonal fisheye.
  • The 10mm fisheye is a diagonal fisheye designed for crop sensors.
  • The 8mm fisheye is designed for full-frame sensors.  It creates a completely circular images which just fits into the frame, which is why this type of lens is sometimes called a circular fisheye.
  • The 4.5mm fisheye is a circular fisheye designed for crop sensors.
(Incidentally this shows why Canon introduced their 8-15mm zoom fisheye in 2011.  Clearly it's designed for full-frame sensors.  But it's not immediately obvious why you would want to use any focal lengths between 8mm and 15mm!)

In most cases you can mount a lens on a camera for which it's not designed.  For example you can mount a 15mm fisheye on a crop-sensor camera, or you can mount a 10mm fisheye on a full-frame camera.  But there's really not much point, because the effects you can achieve don't work particularly well if the size of the image circle isn't matched to the sensor.

Here's what you get using each of the different fisheyes on a full-frame camera.



And here's what you get using each of the different fisheyes on a crop-sensor camera.

Hope that all makes sense.  If anybody out there has any questions, do please drop us a line!

10 comments:

  1. Very useful, thanks for this.

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  2. Great article.

    But could use a little more towards a conclusion.

    There are two combinations which are most useful.

    A "Regtangular" fisheye where the diameter of the image circle matches the diagonal of the sensor rectangle, leaving no black corners. This is achieved with a 15mm focal lens on a full frame sensor or with a 10mm lens on an APS-C sensor.

    A "Circular" fisheye where the diameter of the image circle is the same as the height of the sensor, making the whole circle viewable with nothing cropped off. This is best achieved with an 8mm focal length on full frame or 4.5mm on an APS-C sensor.

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    Replies
    1. Andrew - Good suggestion, thanks. I thought the conclusions would be obvious from the pictures, but perhaps not.

      However, there is one small point you've overlooked.

      If you want a rectangular image with no black corners and you want a 3:2 aspect ratio (the same as the sensor), then you want the 15mm lens on full frame or 10mm on APS-C. As you say.

      But if you want a rectangular image with a different aspect ratio (e.g. square, or 10:8, or 16:9 widescreen), then things are slightly different. The best approach is to use a lens with a slightly shorter focal length - 10mm on full frame or 8mm on APS-C. That gives you more options for cropping to a rectangular format of various shapes, whilst still achieving a 180° view across the diagonal. Sure, you'll be throwing away some pixels, but these days most cameras have plenty of pixels to spare.

      I think we can agree, however, that the 4.5mm lens on full frame and the 15mm lens on APS-C aren't particularly useful!

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  3. A well written and well illustrated article - thanks for making this issue clear.

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  4. So glad to have stumbled across this- it's saved me making an expensive mistake!

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  5. Very very useful!!!! Thanx a lot!!

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  6. Hello , Nikon D800 +14-24mm f2.8 is my camera ...which fisheye 180 degree lens better for this body

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  7. Wow this is incredibly helpful thanks! Very simple to understand, Thanks.

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  8. Hi Great article. I am looking at 360 Virtual tour work for my canon 5D Mk11 or Mk111, so you tips on lens etc have helped. Is there an update for Nov 2016?

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