Focussing - Very Very basic question

[boccers_2000]

OK, here goes, this is a stupid question (and if I wasnt at work without a camera I could probably work it out).

Auto Focus, you look through the view finder, half press the shutter, hear that beep and red dot flashes (you have locked on something). You push the shutter down, you take the picture.

In the dark where there isnt anything for autofocs to latch onto or say you have some slow glass and want to take the pic quickly, do you go onto manual focus?

If you go onto manual does the camera just take the picture and assumes youve focussed (ie een if you havent)?

I only ask because photographing in low light or quickly can be infuriating sometimes and id like to think I can just switch to manual, get a decent focus and go for it.

Thoughts?

 

[Jacob Sanders]

if you go manual focus the camera will assume youve focussed correctly. you could change the iso or f/stop to increase the light, so its not low lighting and then the auto-focus might kick in again.

 

[lawrie29]

I only ask because photographing in low light or quickly can be infuriating sometimes and id like to think I can just switch to manual, get a decent focus and go for it.

QUOTE]

 

[lawrie29]

I only ask because photographing in low light or quickly can be infuriating sometimes and id like to think I can just switch to manual, get a decent focus and go for it.

 

[Zukabak]

You're correct with your assumption Andrew.
Once in manual focus, the camera won't give a monkey's whether it's in focus or not, it'll just fire the shutter once you press the button.

As long as it looks ok through the viewfinder and you get your little dot, go for it.

 

[boccers_2000]

Well looks like we cleared that one up.

Given it can be diffucult in bad light to know if the focus is spot on but at least it gives you that extra option when you simply cant get autofocus!!

 

[highpriest]

Given it can be diffucult in bad light to know if the focus is spot on but at least it gives you that extra option when you simply cant get autofocus!!

If you have fast glass (f/2.8 or faster), the centre focus point on the 500D will AF in near darkness as it is a cross type focus point which is additionally sensitive at f/2.8 or faster.

 

[boccers_2000]

If you have fast glass (f/2.8 or faster), the centre focus point on the 500D will AF in near darkness as it is a cross type focus point which is additionally sensitive at f/2.8 or faster.

Hmm not sure I follow.

OK I have a 50mm f1.8 so, I keep it in auto focue and manually focus using the centre point and it will focus in really bad light?

 

[Scotty Pro]

Hmm not sure I follow.

OK I have a 50mm f1.8 so, I keep it in auto focue and manually focus using the centre point and it will focus in really bad light?

the 1.8 doesn't have manual focus when in AF, you need a USM lens to do that.

 

[boccers_2000]

the 1.8 doesn't have manual focus when in AF, you need a USM lens to do that.

Ah OK got ya.

 

[HoppyUK]

Changing the f/number won't help, as the lens always focuses at lowest f/number anyway. But it's true that lenses that run to f/2.8 and lower activate extra focusing sensors and are a bit more sensitive.

If you can't use AF, then best of luck with manual. I think you'll find it very difficult. However, I find it's very rare that AF won't work if you use the centre point and look around for something with a bit of brightness and contrast for it to lock on to - a collar or a cuff, an eye or hair band, bit of jewelry etc. Just make sure it's the same distance away as the main subject, lock on to it, and recompose.

Basically I'm saying that AF will probably be better than you will be using manual.

 

[boccers_2000]

Changing the f/number won't help, as the lens always focuses at lowest f/number anyway. But it's true that lenses that run to f/2.8 and lower activate extra focusing sensors and are a bit more sensitive.

If you can't use AF, then best of luck with manual. I think you'll find it very difficult. However, I find it's very rare that AF won't work if you use the centre point and look around for something with a bit of brightness and contrast for it to lock on to - a collar or a cuff, an eye or hair band, bit of jewelry etc. Just make sure it's the same distance away as the main subject, lock on to it, and recompose.

Basically I'm saying that AF will probably be better than you will be using manual.

I guess the issue is when you want to do a long exposure and there is no clink of light to get the autofocus to centre on. In situations like that, if I am able to manual focus then it may be the answer (as in AF the camera wont let you take the shot without having focussed)

 

[HoppyUK]

I guess the issue is when you want to do a long exposure and there is no clink of light to get the autofocus to centre on. In situations like that, if I am able to manual focus then it may be the answer (as in AF the camera wont let you take the shot without having focussed)

The camera will still fire without having locked focus, if you reconfigure the controls for back-button AF - not sure if your camera has that. Or switch to manual of course.

Other ways of focusing in very poor light is to pre-focus, ie take a guess, and use a higher f/number so that depth of field covers any errors (popular technque for night clubs, using flash). Or use the focus-assist beam from a flash gun.

Can you get the flash to put out an AF assist stream from the pop-up flash?