This series is all about answering questions regarding lens that you thought you knew (but didn’t). In this article, we look at the relationship between the maximum aperture of the lens and the view through the viewfinder. (Reported by: Ryosuke Takahashi)

 

What is a fast (“bright”) lens?

A fast lens refers to a lens with a large maximum aperture, such as f/1.2, f/1.4, or f/2.8.

When a lens has a large maximum aperture, it allows more light to enter it and reach the sensor. At maximum aperture setting, it also lets you to achieve sufficient exposure faster than on a lens with a smaller maximum aperture. Hence, such lenses are also described as “fast” or “bright”.

 

How does this affect the view through an optical viewfinder?

The above images are from the optical viewfinders (OVF) of the EF24-105mm f/4L IS USM and EF24mm f/1.4L II USM respectively. They demonstrate how the aperture of the lens directly affects the brightness of the view through an OVF. A brighter viewfinder view lets you see the details on a subject better. It is also an advantage when shooting in low-light conditions.

Although the difference is relatively harder to see in bright scenes such as outdoor shoots, it will be more obvious when shooting in low light conditions.

What causes this difference?

The illustration below shows how light travels through the lens of a DSLR to form the OVF image.

A: Pentaprism or pentamirror
B: Optical viewfinder
C: Primary mirror

1. Light enters the lens.
2. This light is reflected by the primary mirror (C) onto the pentamirror (A).
3. The image that forms on (A) is what you see when you look through the OVF (B).

The light that reaches the OVF is not affected by your aperture settings, or any other camera settings at all. In other words, the brightness of the OVF view relies on solely on the amount of light that enters the lens at maximum aperture*.

When you use a fast lens, naturally, more light enters the lens, resulting in more light being reflected onto the OVF. The view through the OVF therefore appears brighter compared to if you use a slower lens.

*Know this: Full-aperture metering

Most DSLR cameras measure the scene's brightness using full-aperture metering (also known as open-aperture metering), which means the aperture diaphragm remains at maximum aperture by default. If the camera is set to a narrower f-number, the aperture diaphragm momentarily narrows when you release the shutter, but after that, it returns to maximum aperture. Canon's mirrorless cameras work the same way, but some other cameras might work differently.

Side note: Why some photographers love the view through the OVF

This system also means what you see through the OVF is the same as what you would see with the naked eye. This natural, unmodified and hence “direct” view of the scene is what many photographers love about shooting with an OVF.

 

Does a fast lens affect the view through an EVF?

The image that you see in an electronic viewfinder (EVF) is formed through the following process:

1. Light enters the lens.
2. This light is captured by the image sensor.
3. The image formed on the image sensor is projected onto the EVF.

When you change your camera settings, these are applied to the image that is formed on the image sensor, and hence, the image that you see on the EVF. The EVF also electronically enhances the brightness of the image. The lens’ maximum aperture therefore does not directly influence the brightness of the EVF view.

However, a faster lens can affect the EVF image quality in another way.

The impact of a faster lens on EVF gain

When you are shooting in dark conditions, the EVF automatically “gains” (brightens) to allow the user to see better, but too much gain will cause the EVF image to be grainy.

The EVF will gain less if more light enters the image sensor, which is where a faster lens is an advantage. Less graininess and more light reaching the image sensor also contributes to better AF performance in low light.

 

Read more about the differences between OVFs and EVFs in:
Camera Basics #12: The Viewfinder

 

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