"Focus on Infinity"

[Lynton]

Sorry for a "numpty" Q - however if you don't know there is no such thing as a numpty Question.

Was out the other night in pitch black and trying to get the lens to focus on a specific object using a torch to light and focus, but failed miserably.

Here's the evidence.


http://i113.photobucket.com/albums/n237/lyntonsmith/stbenetsx.jpg

Then sitting here tonight reading a Photography mag that dropped through the door today, a pro, in a similar situation, though admittedly going for star trails with a boat on the beach in the foreground commented "I try to focus on the subject using my torch (as I did), but sometimes I struggle to do this. (As I did.) If this is the case then I set my focussing to infinity, which usually gives me a sharp picture."

OK, I understand what Infinity is and where it is on focussing on a lens, but how does focussing on infinity ensure (usually?) that an object in relative close foreground (only a few metres away) is sharp?

[david1701]

it doesn't, focussing on infinity ensures stuff far away will be in focus (if using a small aperture)

does the lens have a distance scale on the focussing ring as I tend to get ball park using that and a torch which is usually close enough at small apertures

[Lynton]

david - thanks for the reply - no it doesn't. Is a siggy 17-70 for Pentax.........

Indeed - obviously infinity is a long way away, so do I assume that focussing on infinity and a small ap . I.e. F8 or more gives a DOF long enough to get the foreground interest in focus on a long exp, or am I needing a torch which will need me to wear sunglasses in total darkness????

[towershot]

Focus as close as you can,use aperture priority ,set apeture to f/11, pop on tripod ,and use self timer or remote release.

or get a BIG torch, you can pick em up in lidl for not much.

[Scarecrow]

If the cross in your photo is only a few meters away, did you try propping your torch up against the cross and focussing on that, rather than trying to illuminate the cross to focus on?

[awp]

if your lens has an aperture scale - set infinity to the aperture you are using (on the left of the focus mark, as seen from above the lens when on the camera) - this will give you the greatest possible DOF

[david1701]

if theres no scale you just gotta do it by feel, big DoF really helps here (small aperture+short focal length)

[HoppyUK]

You need light to focus, at least a little, both with auto-focus or manually. There is no getting away from that. At close range a torch may do, or maybe the red AF assist light from a flashgun - somehting like that.

If there is no light, you have to guestimate, and try to cover any error with the extra depth of field you get with a higher f/number. That would be my suggestion for the picture you posted - shouldn't be too difficult, perhaps at f/8-11 or so.

A bit of learning is in order I'm afraid. Check out www.dofmaster.com which will give you depth of field for any combination of lens, camera, distance and f/number. Also get to grips with hyperfocal distance focusing too.