CCC Forum

Nikon has announced the D810a which is optimised for astrophotography. One sentence in the DPReview article intrigues me:

"A virtual exposure preview in bulb and time settings, similar to Olympus’ livetime mode, lets users essentially see in the dark, so that they can frame and focus." You need to focus on something which is billions of miles away? Perhaps one of those on this forum who has knowledge of this type of photography can explain. Maybe it is to do with lenses focusing past infinity?

http://www.dpreview.com/articles/578620 ... phy-camera

It's exactly that. Most lenses allow you to focus slightly past infinity for a few reasons. The first is that AF mechanisms hunting around infinity need to be able to go either side of infinity to more accurately assess focus - allowing them to do so also stops the focusing mechanism whacking against a "hard stop" infinity. It also means that if there's a slight construction imperfection and infinity focus is slightly BEYOND infinity, you can still micro-focus adjust to compensate.

Also, lenses will expand and contract due to temperature changes and giving things the ability to "flex" slightly prevents damage.

I'd be very interested to see how Nikon are achieving the view but think it's not all that necessary - it's easy to frame a photo already using a known star and live view, and likewise focusing accurately to infinity by using a star is dead easy.

Thanks for the explanation.