KYC #2 – Why is an f/2.8 lens called faster than f/4

The 70-200 f/2.8 is said to be a faster lens that the 70-200 f/4. What does this mean?

Well, the most common answer that I have heard is  – ‘it gives faster shutter speeds’ 

Lets take a moment to try and understand the above statement and see if that really is the reason

Consider a situation where you are shooting with the following settings

  • Aperture : f/4
  • Shutter   : 1/100
  • ISO         : 100

Now, if you have a 70-200 f/4 lens, the only way to achieve a faster shutter speed in the above condition is by bumping up the ISO. So an ISO of 200 in this case would give you a shutter speed of 1/200

However, if you have a 70-200 f/2.8, you can achieve a faster shutter speed by changing the aperture to f/2.8. Thats a bigger aperture size than f/4, thus it helps in getting a faster shutter speed.

Is that why it is then called a faster lens?

Well thats not really why the f/2.8 is a faster lens. 

To understand why the f/2.8 is a faster lens, you need to understand some basics of how your camera AF works

There are different kinds of AF mechanisms used in cameras but most of them are dependent on contrast. They try to work with contrast and in-order to function well, they need as much light as they can to detect contrast. More the light coming into the AF sensors, faster the process of measuring contrast and acquiring AF.  Please note that the AF Sensors in your DSLR receive light from the lens just like the viewfinder does as well.

Needless to say, the f/2.8 lens will always have a bigger maximum aperture than an f/4 lens and the AF acquisition always happens at the widest possible aperture. 

Here is a fun fact: Try this – 

  • Put your camera in Aperture Priority Mode
  • Turn the Aperture value to f/18
  • Now, f/18 is a very small hole, so do you see the image any darker through your viewfinder?
  • NO – Why????

The reason it doesnt seem darker is because your aperture value kicks in only after you click. Remember, focus happens on full open aperture always! The aperture value sets in only after the mirror moves out to let the light through

Thus, an f/2.8 lens will always be able to acquire focus faster than an f/4 lens because it will always focus with more light (owing to f/2.8) and that is why it is called faster – it focuses faster always!


Have you ever encountered a scenario where your lens starts hunting for focus (Those whizzing sounds as the lens tries to hunt for a subject to focus on)? Well that would happen lesser with a faster lens.

A faster lens helps immensely in the field especially under low light conditions where contrast detection is difficult. That bigger aperture comes in extremely handy in such conditions. Now, that isn’t the only advantage of an f/2.8 lens but for the sake of this blog i’ll stop it at that.

Take a look at the following images. If you notice, there is very low contrast because of the light conditions and it is imperative that you try and use the fastest lens you have in such conditions. If I were not on a 500 f/4, this one might not have focussed at all. I like the cheaper telephoto zooms that we have started to get, for e.g. the 200-500 f/5.6 Nikon for e.g. but they will find it difficult to compete with an f/4 in terms of focussing..always!



Even though the Hume’s Pheasant Image looks nice and colorful, believe me it was extremely low light and focussing was only possible because of the wide aperture of the 500 f/4

The below image of the White-Gorgeted Flycatcher was clicked on a very rainy foggy day in the hills of Thailand and I got it only because of the f/4 (well that and a good camera as well, but without the f/4 the camera wouldn’t have been able to do much either)

Regarding the AF of your camera. There are lot of blogs getting ready to discuss the AF modes and ways of using them well.

As usual, please do share with your friends and also please let me know what you want me to cover under KYC!



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  1. siva kumar May 29, 2017 at 7:07 am #

    Sir..Please write a Blog on Metering aspects in Photography

  2. Nitin Dighe June 2, 2017 at 8:42 am #

    Excellent article. It helped me clear my understanding about focusing & f values.
    Looking forward to read more such informative blogs.

  3. Kintoo June 4, 2017 at 10:03 am #

    Excellent I would say !!!
    Thanks Rahul ji.

  4. Ashish June 10, 2017 at 9:48 pm #

    Thanks a lot….👍

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