Is this true?

[theraven]

Just read this somewhere whilst looking for advise on shooting in extremely low light, is it true? Because if it is it is handy!

Using a Sony A200 with 18-70 stock lens.

[FONT=Tahoma, Geneva, sans-serif]"Let the auto focus "saw" and when it can't find anything to focus on, it [/FONT]should[FONT=Tahoma, Geneva, sans-serif] settle at infinity, now switch to manual and take the shot. Verify this behavior in good conditions."[/FONT]

 

[JSER]

What dummy posted that

Use manual, take a torch, I use a LED Lenser P7, aim that at something and focus on that

 

[theraven]

The issue though it with photographing the sky, i.e. stars, or if the subject is too far away for a torch?

 

[fjrabon]

uh, if the whole point is to get it to 'settle at infinity' why wouldn't you just put it on infinity in manual in the first place?

 

[theraven]

Sorry let me explain, I am struggling to find it, there is no marker and it is an old lens, and it is not at the end as you would expect?

 

[amolitor]

I don't have any actual information, but it seems to me unlikely to work particularly well. The camera body may attempt to set the lens to infinity, and for all I know the lens has a rough notion of where infinity is, but the notion is likely to be pretty rough. If you want to do this, you could focus it at infinity in good light and then make a mark with a sharpie. Then set manual. I think this would be more efficient in actual use, and might be more accurate?

Test it, for all I know "manual focus" on some lenses is just an input to the motor, so there might be no reproducibility at all with a mark, in which case, yuck.

 

[fjrabon]

Sorry let me explain, I am struggling to find it, there is no marker and it is an old lens, and it is not at the end as you would expect?

weird. Ive never used it, but the dpreview of the lens says it goes from infinity to closest focus. Or maybe you're referring to an even older version? At the very least, once you used this 'trick' once, wouldn't you just be able to get some whiteout or something and 'mark' where infinity is?

 

[KmH]

Infinity is a notional concept.

You can manually focus the lens to 'infinity' by looking in the viewfinder. If the a200 has Live View you can zoom in on the rear LCD to refine focus on the stars.

Many lenses that do have an infinity indicator (∞) don't mark the exact point because there are variables, like lens temperature, that make it slightly different from lens to lens of the same make and model.

 

[fjrabon]

Infinity is a notional concept.

You can manually focus the lens to 'infinity' by looking in the viewfinder. If the a200 has Live View you can zoom in on the rear LCD to refine focus on the stars.

Many lenses that do have an infinity indicator (∞) don't mark the exact point because there are variables, like lens temperature, that make it slightly different from lens to lens of the same make and model.

Yeah, I've definitely noticed that on my sigmas with it marked, where it actually ends up is slightly off from the mark.

 

[theraven]

No the a200 doesn't have live view, well I'm going to go out tonight and test this, I'll try the above theory and report back!