Aperture Demystified

The thought for this blog post came while I was having a cup of coffee with a photographer friend and it turned out that he wasn’t aware about the actual physics behind what f/4 really meant for a lens.

We all know that aperture controls Depth of Field and a small aperture value means shallow depth of field whereas a large aperture value (f/16 types) means a large DoF, but what do these values actually mean and does f/4 mean the same for all lenses?

In order to understand aperture and the above questions well, it is very important that we understand what f/4 actually means to start with.

The Aperture value, is the size of the diameter of aperture,

  1. 500mm f/4 = 500/4 = 125  : diameter of the aperture at f/4 for a 500mm lens = 12.5cm
  2. 200mm f/4 = 200/4 = 50   : diameter of the aperture at f/4 for a 200mm lens = 5cm

While we are at it, lets also look at how f/4 to f/5.6 reduces the light coming in by half. Here is a simple math. 

Whats the area of a circle?

Ask any kid and the reply will be  3.14 x R x R.

Now, if the aperture size is the diameter of the aperture, assuming 12.5cm is the diameter of 500 f/4 at f/4, lets see what the area comes to :

Area = 3.14 x 6.25 x 6.25 = 123 approx

Now, lets say you change the aperture to f/5.6, meaning the diameter now = 500/5.6 = 8.9 cm

Lets see the area now :

Area = 3.14 x 4.45 x 4.45 = 62 approx

So, the area reduces nearly to half the amount, and thats why the light coming in reduces by half. 

The amount of light let through by f/4 is the same, irrespective of the size of the diameter.  

This ones slightly more interesting. Think about it for a moment. Would f/4 be the same size for all lenses?

what is f4

At f/4 if the size of the aperture is different for different focal lengths, how does it let through the same amount of light?

The 500 f/4 vs the 200 f/4 for e.g., have 12.5 cm hole vs 5cm hole at f/4. How can that have the same amount of light going through?

Its very simple, the light rays travel a greater distance in a 500mm lens than in a 70-200 mm lens and the total amount of light reaching the sensor is the same for both.

This has to be the case otherwise all the metering would  just not work. For e.g. The external light meters that you get, they don’t know which lens is being used, they just tell you the aperture/shutter required based on the light, right? 

So why was it important for you to understand some of the details of aperture? Well, I believe that unless you understand why the camera behaves how it behaves, it is impossible to be one with it.

A lot more about the aperture will be coming up in a couple of blogs shortly. Till then, thanks for reading this and please share the blogs with your friends as well.

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This entry was posted in Advanced Photography Tips, Tutorials, Uncategorized.

4 Comments

  1. Akshay July 14, 2016 at 12:03 pm #

    One basic question, it is often observed that the capability of making BG blurred for prime lens is much better than zoom lens even if it is on same f-point.
    On f/8, two lenses gives different level of blur BG effect. If the same amount of light is reaching to the sensor then both should give same result, isn’t it?

    • rahul.sachdev@gmail.com July 14, 2016 at 1:03 pm #

      The basic thing to understand here is that :
      The Same amount of light does not mean the same image quality of image and the quality/perception of Blur is dependent on the glass and focal length a lot.

      F/8 transfers the same amount of light to the sensor and that is essential because without that your exposure meter would be different for different lenses. This does not extend to the quality of background. There will be another blog shortly about why some lenses blur better than others.

  2. Ashutosh Shinde July 19, 2016 at 5:58 am #

    Thanks Rahul Sir for sharing this. very useful calculation which everyone of us should understand before hitting shutter button. But need to know more in detail about the correlation between correct f stop, exposure compensation, white balance and metering mode to make a perfectly exposed image. (i am not considering cases where one wants to be creative with light and deliberately over/under exposes the images)

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