What is trap focus (and why it's not possible on the D800)?

Started Dec 22, 2012 | Discussions thread
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What is trap focus (and why it's not possible on the D800)?
Dec 22, 2012

I asked in a thread but I decided to start a new one. I've never used "trap focus" and wasn't sure what it was. So I Googled and found this http://www.digital-photo-secrets.com/tip/1303/how-to-use-trap-focus-to-get-better-sports-and-wildlife-shots/

I also asked in another thread and had this reply, after setting the camera to AF-ON and for shutter to not take the shot without focus: "Now, one sets the focus to a specific distance via AF-ON or manual focus and fully depresses the shutter button. Once the AF system detects something at the distance it's set for, the shutter trips and creates and exposure."

I understand the idea of what it is supposed to do, but I still don\t undersatnd a few things abotu how it works.

First, reĀ AF-ON. How do you set the focus distance with AF-ON? Do you focus on something else that is the same distance (e.g. a branch where a bird is going to land on)? In that case, you either have to release the AF-ON button before pressing the shutter, otherwise it will focus on whatever it was and take the shot, or move the camera and now focusing on a background, but keeping the AF-ON button, the camera will continue to try to focus (no?), ruining the focus.

So the only way I can see this working is when you use AF-ON to focus on something close to where your target will be. Once you have focus, move the camera so the focus point is aiming at something that is out of focus, like the background, and then press the shutter. If I'm right, it shouldn't take the photo. then once the subject get under the focus point, it will be in focus (assuming you predicted its position correctly) and take the photo. I assume this works in AFS mode only, since otherwise it will just take the shot regardless and only works for the specific situations described (e.g. a tennis player running from the side into the focus point).

Second, re manual focus. I've never tried using AF-ON setup and manual focusing. What happens then? When I use manual focus, the shutter just takes the shot regardless, from what I remember (maybe I remember wrong). If you setup AF-ON for focusing and use manual focus, would the shutter still not take the shot unless it's in focus, or unless you press the AF-ON, or...?

Third, I'd also like to understand why this is not possible on the D800.

Thank you

Nikon D800
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