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When shooting a panorama, rotating the camera & lens about the nodal point of the lens is expected to produce higher-quality images (in the sense that they match better at the edges) -- hence the existence of panoramic heads and nodal sliders.

If I've got a nodal slider, though, I still need to find the nodal point of the lens. For primes, I understand that the nodal point will be fixed, so I could find the point once and mark it, but for zooms, since the nodal point moves when I zoom, a method that can be easily replicated in the field would be best.

How do I find this point?

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    FYI the correct term is either the centre of projection, optical centre or "no parallax point", the most widely used term: "nodal point" refers to something else! – Matt Grum Jan 24 '14 at 16:25
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There is a nice video explaining how to do it here: https://www.youtube.com/watch?v=k0HaRZi-FWs. The procedure is the same with primes and zoom lenses and it works in the field as long as you have two vertical reference lines to work with.

I'm not really sure what method you're looking for for the zooms though. You can always set the lens to a certain focal length in the field and then calibrate it. If you want something quicker you simply have to calibrate it for some different focal lengths that you think you will be using and mark them. The movement of the nodal point (as a function of the present focal length) of the zoom lens does not have to be linear. It's not even certain that it will move at all, even though this is unlikely.

In short, perform some measurements at home and mark up the different focal lengths you think you will be using. If you in the field realise that you need another focal length you could either calibrate it in the field as shown in the video or perform some crude interpolation from the data you've already got.

God luck!

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