Sharp focus question

[Onestepbeyond70]

I swear i searched on google but could not find it....is there any
formula which, given focal lenght and lens-film distance returns
the distance of the plane of focus?

Thanks!

[Ole Tjugen]

At infinity focus, the distance from the film to the lens nodal point is equal to the focal length of the lens by definition.

[aduncanson]

or 1/focal length = (1/image distance + 1/object distance)

Unfortunately, for real lenses (those not studied in high school physics classes) the Image Distance and Object Distance are measured to the lens nodal points, the exact locations of which are not marked on lenses. This limits the usefulness of the formula.

[Onestepbeyond70]

Thank you, could this be a reasonable method to focus at night?
My lens wide open is f/8, cannot see anything...how would you do?

[Ole Tjugen]

I would focus in daylight, then either mark the camera bed or leave the camera set up until nighfall...

[aduncanson]

Some folks place a flashlight (torch) in the scene and focus on that. If the subject distance is approximately infinite, you could always focus on the moon.

[Julian Boulter]

For night cityscapes I've been thinking about noting the hyperfocal distance for a given lens on my monorail scale in daylight and then setting the standards at that point at night without needing to actually view the image as a rough estimate of focus.

See Leonard's excellent paper here which includes details on setting the hyperfocal distance:

http://www.math.northwestern.edu/~le.../dof_essay.pdf

Julian

[Brian Vuillemenot]

For night cityscapes I've been thinking about noting the hyperfocal distance for a given lens on my monorail scale in daylight and then setting the standards at that point at night without needing to actually view the image as a rough estimate of focus.

See Leonard's excellent paper here which includes details on setting the hyperfocal distance:

http://www.math.northwestern.edu/~le.../dof_essay.pdf

Julian

If you're shooting a citiscape at night, there's bound to be some lights, so why not just focus on those? It's actually easier than focusing during the day, as you don't even need a dark cloth!

[Onestepbeyond70]

For night cityscapes I've been thinking about noting the hyperfocal distance for a given lens on my monorail scale in daylight and then setting the standards at that point at night without needing to actually view the image as a rough estimate of focus.

See Leonard's excellent paper here which includes details on setting the hyperfocal distance:

http://www.math.northwestern.edu/~le.../dof_essay.pdf

Julian

Excellent! I think this is the better idea, im going to try it soon...thank you!

Sometime i shoot at night to allow long exposure for waterscape, an other way
could be to buy ND filters. Do you know if these fit large format lenses?

http://cgi.ebay.com/Green-L-40-5mm-4...QQcmdZViewItem

Or maybe you know some other retailer, possibly in Europe?

[Drew Bedo]

Buy ND filters that match the filter thread size of your lenses. I have a set of 72mm filters that I fit to my smaller diameter lenses with step-up rings.