Fast Lenses Mean Fast Autofocus
Photos and Text by: Jared Lloyd
Fact: autofocus is dependent on light.
The more light you have, the more effective and accurate your autofocus system will be.
Seems simple enough, right?
But there is more to this that I have come to realize many wildlife photographers don’t seem to be privy to.
An f/2.8 lens, like a 400mm f/2.8, is always going to focus faster and more accurately than a f/4 or f/5.6 or f/6.3 lens EVEN WHEN THE APERTURE SETTING IS THE SAME.
What this means is if I am shooting with a 400 f/2.8 lens that is set to f/5.6, it is going to focus faster and produce sharper images than if I was using another lens such as a 500mm f/5.6 lens that is set at f/5.6.
Why?
Our cameras do not step down the aperture to the f/stop we have set until the moment the shutter button is actuated.
This means that anytime you are walking around, looking through your viewfinder, or acquiring focus, your lens is actually set to its maximum aperture. No matter what.
Your camera does this for multiple reasons. One of which is to allow enough light to enter the viewfinder so you can see clearly. SLR and DSLR cameras even had a “depth of field preview” button for this reason which would force the camera to “stop down” to the aperture setting so you can get an idea of your DOF.
But the other reason is so that your autofocus system functions as fast and accurately as possible.
If the lens is an f/2.8 lens, this is what the aperture will be until the shutter is triggered – one of the many reasons I like to separate my autofocus from my shutter button. Likewise, if your lens is an f/5.6, this is what your lens is set to until you press the shutter button.
Therefore, if I am using an f/2.8 lens, there is going to be considerably more light hitting my autofocus sensors at all times than if I am using an f/5.6 lens.
More light reaching the AF sensors means faster focusing, more accurate focusing, and thus sharper photos.
But just how much light are we talking about?
The difference between a 2.8 and 5.6 is literally FOUR TIMES THE AMOUNT OF LIGHT.
Stops of light are logarithmic. When we go from f/2.8 to f/4 (one full stop), we literally halve the amount of light coming into the lens. And when we go from f/4 to f/5.6 (one full stop), we literally half the amount of light coming into the lens again. So, f/2.8 is FOUR TIMES more light than f/5.6.
It might not be exactly accurate to say that this means an f/2.8 lens is going to be four times faster focusing, four times more accurate, and four times sharper than an f/5.6 lens. However, when you have the ability to switch back and forth between the two, it certainly looks and feels that way.
And this is because the f/2.8 lens is always going to let in more light than any other lens with a smaller maximum aperture like f/4 or f/5.6 or f/6.3.
Autofocus systems are notorious for slowing down and losing precision in low light and / or low contrast situations. If you know you are going to be in one of those situations, then you can use faster glass to improve your autofocus success rate.
Likewise, if you are someone who is constantly struggling with sharpness, color rendition, clarity, and contrast in your images, try using a 400mm f/2.8 or a 600mm f/4 lens. You will be blown away at the difference in image quality you see in comparison with a 200-500, 150-600, or 200-600mm lens. There is a reason working professionals tote these things around in the field.
But not everyone can rush out and purchase a $13,000 lens (although you can always rent).
The good news is that critical sharpness isn’t what makes an amazing photograph.
Light, composition, and story is what makes an extraordinary photograph.
Yes, f/2.8 lets my camera’s AF system work faster and more accurately than a f/5.6 or f/6.3.
Yes, I can tell the difference instantly.
However, this isn’t what sells a 6-foot fine art print that will hang over someone’s stone fireplace. And it’s not what is going to land your photos on the covers of magazines.
A few weeks ago, I had a 20 something guy come up to me with a Sony camera and 200-600mm lens. He was staring at the 400 f/2.8 lens on my camera and wanted to know if I was a professional wildlife photographer. I told him that I was, and he immediately launched into questions about the lens I was using.
“Why would a wildlife photographer want a prime lens?”
Answer: they are faster, sharper, and produce significantly better image quality.
“Aren’t zoom lenses better for animals?”
Answer: zoom lenses are all about versatility. Sometimes that is exactly what I need. But other times, I prioritize low light functionality and my ability to control my depth of field over versatility and adjust how I photograph accordingly.
“Isn’t lighter better for this type of work?”
Answer: lighter weight is always better in terms of ease of use. But I only prioritize lightweight gear when I am in situations where it is an absolute must such as when I am on a boat in rough seas or when there will be MANY miles of hiking because of the compromises I have to make in terms of low light, autofocus, and IQ (image quality).
“Wait! What do you mean an f/2.8 lens is going to always allow my camera’s autofocus to work better!?”
I spent the next 20 minutes talking with him about all of this. I get excited to see people like him in the field. I remember being in college, just starting to sell my work to magazines, with visions of being a professional wildlife photographer dancing in my head. And I always make sure I give folks like him my complete and undivided attention even if I am trying to work, because it would have meant the world to me if someone had done the same when I was that age.
I guess you can thank that kid for this email. It dawned on me that what I was saying really made no sense unless you understand the mechanics of our cameras and lenses. And this is not information that is normally talked about despite the fact that it makes a significant impact on how our equipment performs in the field.
