Determine focus position with metadata?

[11august]

Hello Phil and everyone!

I happen to work on airplane photos taken during an airshow with a Nikon D310 and the huge AF-S VR Nikkor 400mm f/2.8G ED. I'm not the photographer, but he gently agreed to give me some of its original NEF shots for metadata study purpose.

My concern is to determine the focus position. I know that, by simply looking at the metadata, you usually can't have precise and accurate infos; anyway, I found some other clues in the metadata as given by EXIFTool that I thought could be helpful. These are:

1- The tag "Focus Distance", which is a Nikon tag apparently. I read here and here that this tag is not very precise and could be in some cases inaccurate. I especially looked at what was said in the Nikon group in Flickr by the user "WintrHawk" (that promotes BTW EXIFTool) but he does not indicate if there's a way to have precise informations about this matter.
2- The tag "Depth Of Field". I searched in the EXIFTool Tag Names doc where it comes from and how it is generated, without success.

Can you (or anyone else) shed some light on this? And in a more general way, do you know if it's really possible to determine the focus position simply by looking at the metadata?

In my image sample, I have in the metadata as read by EXIFTool (for a 0.03mm circle of confusion, 9.0 aperture and a 400.00m focal length eq35mm) for the tag "Depth Of Field" 12.03 m (54.15 - 66.18 m), and in "Focus Distance" 59.57m.

We know for sure that the airplane is located ~75-85m away to the photographer and perfectly in focus. But while the photographer keep saying that he focused to infinity, I have some doubts!

So my other question is do you know why the (small) difference between the known distance and what EXIFTool said? And what are your thoughts about the focus distance? Do you think that a focus to infinity is really possible in my example? (The hyperfocal distance as given by EXIFTool is 591.68m). I personally think that it is not possible, at the light of the EXIFTool output, but I could be wrong!

Thank you very much for any insight that could help me!

[Hayo Baan]

The focus distance tag is set by the camera and comes from the lens. It is not very precise but does give an indication as to what distance one focussed at. The depth of field is a straightforward calculation based on aperture, sensor size, and focus distance as reported by the camera. The hyper focal distance is also a simple calculation.

So to answer your question: no, you can not determine the exact focus position, only an estimation, when looking at the metadata. Furthermore, not all camera models provide e.g. the focus distance.

[11august]

The focus distance tag is set by the camera and comes from the lens. It is not very precise but does give an indication as to what distance one focussed at. The depth of field is a straightforward calculation based on aperture, sensor size, and focus distance as reported by the camera. The hyper focal distance is also a simple calculation.

So to answer your question: no, you can not determine the exact focus position, only an estimation, when looking at the metadata. Furthermore, not all camera models provide e.g. the focus distance.

Hi Hayo Baan and thank you for your reply.

So when it says in the metadata "Focus Distance 59.57m", should I consider it as an indication that the focus was approximately at this distance and therefore that it's impossible that the focus was at infinity? Or is not a reliable info at all?

[Hayo Baan]

So when it says in the metadata "Focus Distance 59.57m", should I consider it as an indication that the focus was approximately at this distance and therefore that it's impossible that the focus was at infinity? Or is not a reliable info at all?

That depends on the lends/camera; for that lens 59.97m could very well indicate the infinity setting. The only way to know for sure is to test various focus distance settings and compare

[11august]

But if you focus is set to infinity shouldn't the nearest in-focus position be located at the hyperfocal distance i-e in my case 591.6m? So how the hell a subject that is 80m away can be in-focus?

[Phil Harvey]

I don't know about Nikon, but with Canon cameras the lens only has about 6 possible positions as communicated to the camera body, so the position of the focus ring is not very well defined.  Nikons are a bit better I think, but in general the focus measurement is not accurate.

- Phil

[Hayo Baan]

I don't know about Nikon, but with Canon cameras the lens only has about 6 possible positions as communicated to the camera body, so the position of the focus ring is not very well defined.  Nikons are a bit better I think, but in general the focus measurement is not accurate.

Nikons lenses are indeed providing more data, from a small sample I already got 16 different values from 3.16m to 29.85m for one lens and 14 different values from 7.94m to 63.10m for another. But there were also lenses with only a couple different values, but that's probably also due to the limited range I used those lenses in.