March 13, 2017 at 11:15 am #1763
Have you ever noticed the small button at the back of your camera with the label AF-ON.
If not take your camera and look at it. What does it do? It does exactly the same operation that the half-shutter-press does, viz., acquire focus using auto-focus. That sounds redundant, why should the camera manufacturers build an additional button to duplicate the auto-focus function, especially the half-shutter-focus is so convenient. There are lots of reasons for it, and practically, all of them are non-technical. There is plenty of material on the web on this. I discovered my reasons during my last trip to Bandipur. Reviewing the pictures, I found that that not all my pictures are in great focus despite using a best-in-class camera for auto-focus, the Canon 7D Mark II and a best-in-class lens, Canon 400mm prime. My first reaction was to suspect my lens or my camera. But then I realized that when the lens focused, it delivered impeccable results. Hence it must be my shooting style. I have become over-dependent on the technology to deliver me the best auto-focus every time. It did auto-focus but not where I wanted.
The autofocus issue is more apparent in tele lenses than in wide-angle and normal focus lenses for obvious reasons. First, you need much more steady hand, higher shutter speeds, image stabilization and camera support for delivering a sharper image. Second, the depth of field is very narrow, the closer you are to the subject. For example, the depth of field for a 400mm lens at F8 focused on a subject 10 meters (33 feet) away is just about 0.38 meters or a little over 1 feet. If that is a tiger, there is plenty of things that can go wrong with focus. You could be focusing on the grass in front or on the stretched toe of the tiger than on the face. It has been determined that inability to acquire first focus is one of the main reasons for not getting sharper images.
How does the Back-Button Focus help in fixing this issue?
1. It helps you focus on focusing before clicking. No more unmindful clicking leaving shots to chance. Since you cannot click pictures using the AF-ON button, you make a conscious effort to acquire accurate focus exactly on the eye or face of the subject. I personally use center-point focus to acquire focus and then shift the camera to compose.
2. Because the AF-ON button is conveniently located in the back, you can have a firm hold on your camera while focusing, thereby helping you focus where you want.
3. You can keep the AF-button pressed even after releasing the shutter and your focus will hold. There is no chance of losing focus on that bird in flight. Your Autofocus will continue to keep focusing on a moving subject even when you are not clicking pictures. This will help you click any moment that you want and you will get subjects in focus every time.
Give it a try next time you are out to photograph subjects using tele lens.
Add your suggestions and ideas based on your personal experience on the Topic on YPS Discussion Forum.April 11, 2017 at 6:04 pm #1862
I discovered this function after Satish mentioned it at Bandipur. Want to start using this feature after have read your article. Couple of questions –
Can one zoom in & out, once the focus is locked?
Do you also change the setting to spot metering ?April 11, 2017 at 11:46 pm #1863
Thanks Prema for reading.
To your first question – Can one zoom in and out once focus is locked? You can zoom in and out, but the question is whether the subject will still be in focus. Depends on the lens. As per specifications most of Canon L series lenses are supposed to. But I will never trust that. If I zoom in or out, I will focus again.
Do you also change to Spot Metering? First spot metering should be used with care. Unless I have a reason to use Spot Metering, like back-lit subjects, I will not use spot metering. Since we will be recomposing after acquiring focus, there is no point in metering just on that point. The answer is NO.April 12, 2017 at 9:32 pm #1871
Thanks again! I have to get used to the back button focus. Today I suddenly found my camera not focusing when I used the half shutter press. Thought I had ruined it. Only later did I remember that I had changed to the back button focus : )April 13, 2017 at 4:11 pm #1887
Back button focus is good for all kinds of photography like Landscapes,etc or only for wild life?April 13, 2017 at 10:05 pm #1889
It is most useful when there are many elements in the foreground and you want to achieve focus on a specific subject. More than anything, Back-button focus forces you to separate the focusing and shooting functions so that you will consciously produce pictures with focus on the subject that yo want to be in focus.April 17, 2017 at 3:13 pm #1895
Super sir……have not tried this one anytime….will do it during my next trip out for clicking…..thanks for the informative write-upApril 17, 2017 at 9:17 pm #1898
Thank you for the article and all the clarifications .
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