Friday, November 27, 2020

TiltSim 2 and the internodal hiatus!

A quick post on a minor tweak I've made to TiltSim 2.

As we know from previous posts, we can successfully model a complex lens by making use of a split/thick lens model, ie:

To register a lens in TiltSim 2 all we need is three pieces of information. First the maximum magnification (M), second the minimum focus distance at that magnification (X) and third the focal length (f).

As an example, let's take the 24mm TS-E f/3.5L II, with an M of 0.34 at an X of 230mm. The hiatus between the two principal nodes is found from solving the equation for t above. Which gives a hiatus of just over 103mm. That is the front principal is positioned 103mm away from the rear principal.

Having 'registered' the above lens in TiltSim 2 we now see something like this:

Here we see the hiatus, which splits the Scheimpflug line in two (shown by the two red dots, one of them on the sensor plane, ie at x=0): in the above at 103mm parallel to the tilted optical axis.

But what if we use a non-retrofocus T/S lens, eg a Canon 135mm f/4L (macro) TS-E, with an X of 486mm and an M of 0.5.

The hiatus now is about -121mm!

In other words, because the 135mm TS-E is of a tele construction, the front principal is behind the rear principal. Plugging X and M into TiltSim 2, we now see something like this:

Here we see that the first principal of the Scheimpflug line remains on the sensor axis, as it should, but second principal, positioned at the hiatus distance, from where the plane of sharp focus and DoFs are calculated, is now behind the first, owing to the tele nature of the lens construction.

As usual I welcome any feedback on this post.

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