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How do you focus to 10 kilometers
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PostPosted: Tue Jan 31, 2023 1:44 pm    Post subject: How do you focus to 10 kilometers Reply with quote

I have started to take photographs of high flying planes again.

How do I focus. The scale of my lens is a bit off so it gets over infinity.
Is it possible to focus like to 1 km and to 100 meters and some how extrapolate the right position?


PostPosted: Tue Jan 31, 2023 2:17 pm    Post subject: Reply with quote

a. get a better adapter
b. shim the adapter
c. adjust your lens to fit the adapter
d. print your own custom distance scale and stick it to the focus ring of your lens


PostPosted: Tue Jan 31, 2023 2:27 pm    Post subject: Reply with quote

I can only assume that you will be using a very long focal length lens, otherwise the question would be a bit of a non-issue.

The problem is that most super telephoto lenses do not have an accurately calibrated focus scale anyway at the long end; changes in temperature affect both the effective focal length as well as the physical length of the lens, hence most of them are designed to be able to focus beyond infinity to allow for some compensation in the field.

Assuming that you know the high flying planes are near 10km (a reasonable assumption for commercial airliners at cruising altitude, but it does vary a bit), I can't see any alternative than to find a land-based object of similar distance and focus on that first.

I do wonder though why you don't actually focus using the airplane(s) themselves?


PostPosted: Tue Jan 31, 2023 6:33 pm    Post subject: Reply with quote

A landmark exactly 10km from my position as measured on Google Maps. Smile

Although probably more accurate to focus on the plane being photographed.


PostPosted: Fri Feb 03, 2023 8:34 am    Post subject: Reply with quote

visualopsins wrote:
A landmark exactly 10km from my position as measured on Google Maps. Smile


I think I can get about 2,298 kilometers from my upstairs window. Wink
(the blue line)

#1


PostPosted: Fri Feb 03, 2023 11:04 am    Post subject: Reply with quote

kansalliskala wrote:
visualopsins wrote:
A landmark exactly 10km from my position as measured on Google Maps. Smile


I think I can get about 2,298 kilometers from my upstairs window. Wink
(the blue line)


Asking the local authorities for permission to install a couple of large opposing mirror arrays in your valley might be the answer then.

Our you could build your own DIY collimator calibrated at 10km, whichever works out cheaper... Wink

Joking aside, you could build your own simple DIY 10km collimator from an old SLR camera body and lens; you would have to to modify that lens to allow for focusing just a bit beyond infinity so the focusing screen of that camera as seen through the front of that lens has a virtual image at 10km behind that camera, and use that for collimation.
But that would still leave you with the problem of drift under changes in temperature...


PostPosted: Sun Feb 05, 2023 2:38 pm    Post subject: Reply with quote

kansalliskala wrote:

#1


Incredible - somehow this small map seemed to be familiar to some extent, and i checked on google maps. Somehow "Hirsijärvi" rung a bell, and probably I have been there when I was about 20 years old, in wintertime. If I'm right (not completey sure yet) we went there with some friends from Turku; they had their mökki somwhere on the shore of that lake ... Just one or two nights out there, pretty cold weather, a frozen lake and a terrific full moon. And ... well, let's talk about that later when I have confirmed the story.

S


PostPosted: Mon Feb 06, 2023 6:19 am    Post subject: Reply with quote

[quote="stevemark"]
kansalliskala wrote:

Incredible - somehow this small map seemed to be familiar to some extent, and i checked on google maps. Somehow "Hirsijärvi" rung a bell, and probably I have been there when I was about 20 years old, in wintertime. If I'm right (not completey sure yet) we went there with some friends from Turku; they had their mökki somwhere on the shore of that lake ... Just one or two nights out there, pretty cold weather, a frozen lake and a terrific full moon. And ... well, let's talk about that later when I have confirmed the story.

S


the world is surprisingly small Smile


PostPosted: Sun Feb 12, 2023 5:37 am    Post subject: Reply with quote

https://www.flightradar24.com/60.59,10.52/6