In the last post I discussed a depth of field hack that can easily be used to help manually capture perfect deep focus, landscape stacks, ie when non-macro focus bracketing.

The hack was based on knowing the location of the (infinity) front principal from the PhotonsToPhotos (PTP) Optical Bench Hub, and ignoring any lens extension, as the hack is aimed at non-macro, wide angle photography, where the magnification and lens extension impact will be small.

But what if your lens isn’t in the PTP database?

In this post I show a quick approach to
**estimating **the front principal location, from a single, simple measurement.

Although it is difficult/’impossible’ to directly measure the front principal (H), it is relatively easy to measure the location of the on-axis entrance pupil (EP) location, P in the diagram below. At least for a ‘normal’, non fisheye lens.

In the above example, of the Sigma 12-24, at 12mm, we can see (by observing the ratio of the exit to entrance pupils), that the lens is,
as expected, a retro focus one, with a pupil magnification (p = Exit Diameter
divided by Entrance Diameter) of about 6. Thus, using the thick lens model, we know that EP is offset, ie in front of the front principal H, by F(1-1/p): about 10mm at a focal length of 12mm.

The value of p can be measured, eg taking an image of the lens from each end, or ‘eyeballed’ at a reasonable level of accuracy.

As was shown in a previous post, estimating the EP, which is the pano rotation location, can be accomplished very simply using a light source. At a push, one could also, whilst on a tripod, locate the pano pivot location and measure the EP from the sensor plane directly.

Whatever approach you use, by estimating/measuring the position of the EP you can get an estimate of the location of the front principal from knowing the pupil magnification (p), ie the front principal is located at EP - F(1-1/p). Thus, even if you don’t have a model of the lens, you can consider the lens as a black box and estimate the location of the front principal, by measuring the location of the entrance pupil and measuring/guesstimating the pupil magnification.

As usual I welcome any feedback on this post or any of my posts.

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