Photography 101 & 3D Visualisation – F-Number Aperture.

The F-number on a camera lens relates to the aperture size. The larger the F-number the smaller the aperture is, this means less light is taken in to the camera.

See the two images below, both have the same settings for Shutter speed, ISO and White balance, but very different F-numbers.

F-number 4.0	F-number 16

The F– number 16 photograph is underexposed as the smaller aperture did not let in enough light.

If we wanted to keep the aperture size at 16, but have the photo exposed to the level of F4 photo we would have to trade off another setting.

Camera settings are counted in F-Stops or fractions of F-Stops.

The Standard Full-Stop scale

0.7  1.0  1.4  2  2.8  4  5.6  8  11  16  22  32  45  64  90  128  180  256

This means the two photographs above are 4 F-stops apart.

So we could slow our shutter speed by 4 stops to allow more time for the camera to get enough light without changing the aperture size. The reason we might want to do this is because the aperture size also affects the size of the cameras depth of field (DOF). This is the area that is in focus, if we wanted the whole scene in sharp focus we would need the aperture set to a higher number, notice the cooker in the two photos, the dark  F16 photo is sharper than the F4 photo.

 

Wikipedia has a handy exposure table that will help with trading these settings off:

http://en.wikipedia.org/wiki/Exposure_value

For example, if we want to change our F-number and set the traded off shutter speed, we would find the shutter speed setting we used for the F4 photo (1/125) and move along the row to the F16 to get the traded off shutter speed for the smaller aperture.

The resulting photograph can be seen below.

 

PRACTICAL WITH VRAY

 

The Vray Physical Camera has the same settings as a real camera; you can control both the Aperture and shutter speed.

By default the F-number is set to 8.0 (Photojournalists preferred setting)

As you will be aware rendering packages are more flexible than real life and by default Vray ignores the DOF when it renders. This would be great for a scene like our first photo at F4 because the image would remain sharp and we would not need to do the shutter speed trade off. However, if you want to replicate the shallow DOF you will also need to turn on the depth-of-field option in your camera settings:

Now the VRay camera acts like a real world camera. Therefore increasing the F-number increases the DOF.

 

 

 

 

Well that’s blog one of our photography section covered, next blog will cover ISO settings.