Can close-up magnification be calculated from Nikon metadata?

[Chris Newman]

I'm grateful that ExifTool gives me access to metadata I can't get from my image processing software.  But recently, at the end of a walk to count butterflies, I found a moth caterpillar hanging from my hat.  I took numerous photos, and thought I could work out the caterpillar's length from a sharp photo where it is straight across the frame.  If I could work out the magnification, I could easily calculate it's apparent length relative to the nominal 35.9 mm width of my D800's sensor.
I used my AF-S VR Micro-Nikkor 105mm f/2.8G IF-ED.
ExifTool gives:
Min Focal Length = 106.8 mm
Max Focal Length = 106.8 mm
Focal Length = 105.0 mm (35 mm equivalent: 105.0 mm)
Focus Position = 0×11
Focus Distance = 0.33 mm
I assume 'Focus Distance' is the distance from the sensor to the subject, but the closest distance at which a simple 105mm lens could focus is 420 mm.   I think I've read that my 105mm Nikkor, in common with some other macro lenses, has a reduced focal length at close focus, which could explain the 0.33 mm 'Focus Distance' shown in the metadata.
If my assumptions about reduced focal length, and the 0.33 mm 'Focus Distance', are correct, is there anything in the metadata I've overlooked that would show the true focal length at the focus distance used, or the corresponding magnification?
Also, do the Min and Max Focal Lengths of 106.8 mm refer to the true focal length of the lens at infinity, with 105.0 mm only the nominal focal length?

With thanks for any clarifications,
Chris Newman

[StarGeek]

You might want to search the forums using Google with Site:Exiftool.org (in forum search is disabled) for previous threads on focus data.  Also, there's the note next to FocusDistance under Nikon LensData01 Tags.

Focus Distances - problems
Determine focus position with metadata

[Chris Newman]

You might want to search the forums using Google with Site:Exiftool.org (in forum search is disabled) for previous threads on focus data.  Also, there's the note next to FocusDistance under Nikon LensData01 Tags.

Focus Distances - problems
Determine focus position with metadata

Thanks for your help.  I achieved the search, but I note that Google has again made its site-specific Advanced Search yet harder to find; I had to make a Google Search for it!  (I also regret the loss of the facility to sort 'Shopping' finds by price).

It seems that any reported focus distance is likely to be rather approximate, so in this case I probably won't get a better estimate of magnification than knowing I was trying to enlarge as much as possible, with a lens that just surpasses 1:1.

I'm still curious as to the meanings and significance of:
Min Focal Length = 106.8 mm
Max Focal Length = 106.8 mm
Focal Length = 105.0 mm (35 mm equivalent: 105.0 mm)
when the lens has a nominal focal length of 105 mm.

[Alan Clifford]

Is this at all helpful?

https://www.scantips.com/lights/subjectdistance.html

[Chris Newman]

Thanks Alan for your response, and my apologies for my very delayed reply (I've been busy with other things, and am more used to forums which send me an email if I get a response - either this one doesn't, or I've failed to enabled that).

It's very appropriate that Wayne Fulton wrote this based on the D800 I'm still using.  Also the link to measured distances vs what some high end lenses report for focus distance (https://www.scantips.com/lights/ttlbl-d.html#chrt) shows my Nikon 105mm VR macro as reporting a 1.5 m focused distance pretty accurately as 1.6 m.

However, my photo has a reported 'Focus Distance' of 0.33 mm, much closer than Wayne gives any information for, and which, as I commented initially, is technically impossible with a true 105 mm focal length lens.  This forum has warned me to doubt the reported 'Focus Distance', and I have good reasons to believe the lens's focal length at very close focus is substantially less than 105 mm, but no information on what it is reduced to.

I know how to carry out the calculations that Wayne Fulton's site performs, but I lack the necessary accurate input data on focal length and distance.

During lockdown I tested the closest (manual) focus behaviour of my AF-P Nikkor 70-300mm at maximum focal length.  My photograph of a ruler spanned 139.5mm; if the sensor width is 35.9 mm as claimed, this corresponds to a maximum reproduction ratio of 0.257.  Exiftool showed a reported 'Focus Distance' of 1.19 m (I didn't measure the true distance).  I was hoping I might be able to access information to correct the reported exif data, and calculate outcomes without having to take measurements over a range of marked focal distance settings, but it doesn't seem to be available.

[StarGeek]

Thanks Alan for your response, and my apologies for my very delayed reply (I've been busy with other things, and am more used to forums which send me an email if I get a response - either this one doesn't, or I've failed to enabled that).

You can either click the "Notify" button in the upper right of each thread or sub-forum, or click "Profile", then mouse over "Modify Profile" and select "Notifications".  There you can click the checkbox for "Turn notification on when you post or reply to a topic".

I didn't even realize that setting existed until I went looking for it.  It's definitely not something I want to set for myself, I have too many posts.  I just have some of the subforums set with Notify.

[Chris Newman]

Thanks for the help with the forum.  I've now <Clicked> [NOTIFY] for this thread, although I doubt whether there'll be much to add to it.