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!