This is for an FOV of 110. You can change that by changing the command. Read the parts in BOLD for the specific command instructions. You can include another binding to switch it back to 85 if you'd like. I'm using keys F10 and F12 because I know those are free. If you use those keys for something else like screenshotting, you may wish to change it.

Documents\My Games\Dishonored\DishonoredGame\Config\DishonoredI nput.ini

Find the following sections where you see several bindings like this:

m_PCBindings=(Name="None",Command="GBA_OpenInvento ry")
m_PCBindings=(Name="F5",Command="GBA_QuickSave")
m_PCBindings=(Name="F9",Command="GBA_QuickLoad")

ADD THE FOLLOWING UNDER THOSE:

m_PCBindings=(Name="F10",Command="FOV 85",Control=False,Shift=False,Alt=False)
m_PCBindings=(Name="F12",Command="FOV 110",Control=False,Shift=False,Alt=False)

Find the following sections where you see several bindings like this:

BaseBindings=(Name="F1",Command="viewmode wireframe",Control=False,Shift=False,Alt=True)
BaseBindings=(Name="F2",Command="viewmode unlit",Control=False,Shift=False,Alt=True)
BaseBindings=(Name="F3",Command="viewmode lit",Control=False,Shift=False,Alt=True)

C:\Program Files (x86)\Steam\steamapps\common\Dishonored\Dishonored Game\Config\DefaultInput.ini

(that is just my path)

Find the following sections where you see several bindings like this:

.m_PCBindings=(Name="F5",Command="GBA_QuickSave")
.m_PCBindings=(Name="F9",Command="GBA_QuickLoad")


ADD THE FOLLOWING UNDER THOSE:

.m_PCBindings=(Name="F10",Command="FOV 85",Control=False,Shift=False,Alt=False)
.m_PCBindings=(Name="F12",Command="FOV 110",Control=False,Shift=False,Alt=False)

Would this result in any significant fisheye effect?