Other than brightness, what sets an f/2.8 lens apart from an f/4 lens? Using the EF70-200mm f/2.8L IS II USM and the EF70-200mm f/4L IS USM, I will show how the two differ by comparing the apparent resolution and bokeh size. (Photos by: Ryosuke Takahashi)

 

The size of bokeh is different

The simplified answer is that there are two main differences.

The most obvious difference between an f/2.8 and an f/4 lens is in their "brightness", i.e. in the maximum amount of light each lens allows to reach the sensor.

Another key difference lies in the depth of field. An f/2.8 lens would usually be capable of giving a more shallow depth of field (and therefore a bigger background bokeh) than an f/4 lens. 

However, we say "usually" because the maximum aperture of a lens alone does not determine the size and quality of bokeh possible. There are other contributing factors, such as lens internal construction (elements used to make up the lens and the way they are arranged together), that could also affect bokeh. As one case in point, in the past, using aspherical lens elements in the lens construction was known to cause a very slight deterioration in bokeh quality. Refinements have since been made to the design of aspherical lens elements to reduce their impact on bokeh.

Therefore, if you compare an f/4 lens whose internal construction is designed to provide optimal bokeh with an f/2.8 lens whose internal construction does not favour bokeh, it's still technically possible for the f/4 lens to give a bokeh that is at least just as big as what you would get on the f/2.8 lens.

However, high grade zoom lenses such as the ones in Canon's L series are already designed to give optimal bokeh, hence, when comparing such lenses, it largely holds true that the brighter the f-number, the larger and more beautiful the bokeh will be.

 

Test 1: Resolution

The EF70-200mm f/2.8L IS II USM and the EF70-200mm f/4L IS USM were attached to the EOS 5D Mark III. Next, the camera was fixed on a tripod with Aperture-Priority AE selected. The resolution at maximum aperture in each lens was then checked.

 

f/2.8 - EF70-200mm f/2.8L IS II USM

 

f/4 - EF70-200mm f/4L IS USM

 

 

There is almost no difference in how fine lines are depicted. The different f-numbers don't seem to affect the resolution significantly.  This is especially so the higher the performance of the lenses used.

 

Test 2: Bokeh size

The EF70-200mm f/2.8L IS II USM and the EF70-200mm f/4L IS USM were attached to the EOS 5D Mark III. Next, the camera was fixed on a tripod with Aperture-Priority AE selected. The bokeh at maximum aperture in each lens was then checked.

 

f/2.8 - EF70-200mm f/2.8L IS II USM

 

f/4 - EF70-200mm f/4L IS USM

 

 

Due to the difference in the depth of field caused by the f-number, the bokeh of the f/2.8 lens is larger. Apart from the magnified area, the bokeh in all other areas also appears a stop larger and smoother for the f/2.8 lens than for the f/4 lens. Therefore, the f-number has a direct impact on the degree of bokeh.

Keen to learn more about telephoto zoom lenses? Check out the following article to learn about the effects of different maximum focal lengths:
Lens FAQ #7: What is the difference between a 200mm and 300mm telephoto lens?
 

 

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