Focal Lengths ...

The image data contains the actual focal length at the time of exposure and I am quite used to seeing it in 1mm increments.

I have recently seen the EXIF data on some images taken on a K3 and that is resolving down to one hundredth of a mm (0.01mm). The K5 is full mm and the K-r 0.1mm

A random example is in last weeks competition: https://www.pentaxuser.com/photo/comp-373/the-fish-fryer-73558 with the focal length of 29.37mm

It that really the case? And does it add anything?

It might really be the case, bit I doubt that it helps us very much.

29mm would have been close enough.

With the front/back focusing issues I am having lately yards would be more than accurate enough

Jim.

Focal length, not focus distance!

It would be useful though to have the focus distance in the EXIF data rather than a meaningless "name".

The term for this is most likely ... spurious accuracy.

The chances of any of the bodies consistently (i.e. non-accidentally) accurately focusing to 1mm, let alone 0.1mm or 0.01 mm is essentially zero I would think. Which doesn't stop the software reporting exact estimated values to many degrees of precision if the software engineers haven't gone to the trouble of rounding those data points to something meaningful.

My hiking watch measures room temperature at 20.75 degrees C. But in the same room measures anything from 19 to 25 degrees according to whether it is on my wrist or not and whether it is covered by my sleeve and whether it is a Monday or not ....

I suppose it depends on how the lens reports the focal length ( and focal distance )

My Signa 17-70 2.8-4.5 went wrong a few years ago by being electrically stuck on a focal length much higher than 17mm so even if I physically set it to 17mm it wouldn't allow the aperture to go below 4.5. took it apart and found breaks in the printed circuit film connecting copper wipers to copper tracks on the zoom rotating barrel. bought a new circuit film off sigma and fixed it.

I can't remember exactly what resolution it was but it was a 4 bit wiper system IIRC so you'd get 16 discrete zoom levels.


Let's see that would be (70-17)/16 = so about 3mm increments

This is a common problem partly caused by systems that allow the number of decimals to be fixed rather than the number of significant figures.

At work our Chromatography data system allows me to fix the decimal places, and to cope with small peaks I need to use 5 decimals, yet some people record all 5 religiously on large peaks. 97.10367% is effectively 97.1% or 97%, but 0.00017%, is significantly different from zero.

With focal lengths I can't see a decimal is useful at all till you get below 10mm

Added to that, even lenses are rarely exactly the marked specification. Apparently, according to Photo Technique, in the 1970s/1980s, say 24mm lenses were likely to be closer to 25mm.

However, 24mm sounds better.

Then we have zooms. Does the M series 24-35mm actually go as wide as the K 24mm? Nominally yes, but in reality maybe or maybe not.

The Ricoh GR is actually marked 18.3mm though, so maybe that's just a bit of honest reporting.

Unless you're using lenses for some kind of scientific testing, the who really cares?

I compose an image to what I can see in the viewfinder. The actual focal length is irrelevant in terms of what number it is and only really relevant as to what rough FOV you can expect.

If bought a 24mm lens and it was really 24.7mm I'm not going to go stomping down to trading standards with steam hissing from my ears.

Who cares? I care, but only in the sense that I'm interested. You must care as well Smeggypants, otherwise you wouldn't bother putting in a reply about it.

It's merely interesting, we don't have to have an emotional stake in it to find it interesting....

Well yes I care on an academic interest level, I do like discussing photography issues on a technical level as well as an artistic level.

In terms of relevance to actually taking a picture then no I don't care.

Sorry if that was unclear before.

I remember many of my old Leica lenses had the real focal length scratched on the back or when the lens head was taken of the 90 and 135mm Elmars with pencil written on the inside of the focusing mount.

For example with the 90mm lenses, I had anything from 87- 92 and with the 135mm lenses from 130 - 134.
it didn't make any difference when photographing, but it was important when fitting the focusing mount.

The better fixed lens cameras had shims fitted when the cameras where manufactured to compensate for the slight variations in focal length of the objectives

Regards, Horst