Wading into focal length equivalence

If sifting through Google search results is any indication, I’m wading into fraught territory here. But in looking to acquire more lenses for my Pentax 645, I had to get some basic understanding of focal length equivalency, because my familiarity with what you get, picture-wise, with a lens of x millimeters in focal length is rooted primarily in the 35mm world — and to a lesser degree in Canon’s version of the APS-C world.

This article on Photography Life delves into the subject in more detail than I care to deal with, entering the world of apertures and ISOs and other factors I frankly don’t care about at the moment. All I really wanted to know was this: In terms of the visible image frame of my Pentax 645 with a lens of x millimeters, a lens of what y millimeters on a 35mm would provide approximately (key point) the same visible image frame?

I was thinking that surely someone has produced a usable reference table for this sort of thing, and indeed, some people have — just not with the data points I wanted. And mostly, I’ve instead been hit with overly technical chatter, discussions of crop factors, and other minutia that dances around the bits I actually wanted to know. Like any area of study, experts and expert wannabes do seem to enjoy debating a lot of esoterica amongst themselves. (Get me more into my comfort zone, and I suppose I do as well.)

In any case, there seems to be debate around what the crop factor math actually even is. Thankfully, I don’t need accuracy to the nearest 100th of a millimeter, so using the crop factors from the Photography Life article, I came-up with this table:

APS-C (Canon)35mm645
182735
233545
385975
416380
467090
76117150
81125160
102156200
203313400

The numbers in the third column are the source, and the lens focal lengths I was interested in seeing comparisons of; basically, these are the numbers from the lenses (both fixed and variable) I’ve been shopping for — including the effect of a 2x teleconverter. And since 35mm and my Canon digital are the other cameras I use, that’s what I chose to include in the table.

The math isn’t difficult, of course, and you can easily use Excel to create your own chart with items of interest, using the numbers from the article. Of course, this is assuming that the author’s numbers are correct, and other sources — including this Pentax Forums post — seem to come-up with their own interpretations of reality.

I think I’ll leave the technical analysis to the experts…