Camera Focus @ night

alexisonfire · 7 Apr 2007 at 01:37

Ok this has been bothering me, theres probably an easy explanation, but i really can't think of it

What happens when its too dark for the camera to autofocus, and you can't really see anything out of the viewfinder to manual focus? I mean, the search light is normally strong enough, but for long distance landscapes for example and theres no strong light source, how do i focus? Is it just a case of trial and error?

binaryknight · 7 Apr 2007 at 02:01

if you have a semi decent torch it can help mid distance but not for what your trying

all i can think of at the moment is to try and find 2 constrasting an use that as a focus point then switch to manual focus and compose

edit..... if the landscape is quite far away why not use manual focus and set focus to infinity, seeing as its landscape you will be wanting a small aperture

rpstewart · 7 Apr 2007 at 08:27

For landscapes the easiest way to do it is to set the focus to the hyperfocal distance, that way everything from half that distance to infinity will be in focus.

The hyperfocal distance is different for each lens, body and aperture so you need to use a calculator to determine what it is - I normally use this one but it's offline at the moment. You can print the results out and take them with you so when you set up the shot you can dial in the settings, check the sheet and set the focus distance appropriately.

binaryknight said:
edit..... if the landscape is quite far away why not use manual focus and set focus to infinity, seeing as its landscape you will be wanting a small aperture

Setting focus to infinity isn't the way to go. It does what it says on the tin, focusses at infinity so the front edge of the depth of field can still be a long way away and you end up with an out of focus foreground. The hyperfocal distance is normally somewhere in the region of 10 - 150 feet (with a 50mm lens) so the closest point of acceptable sharpness isn't very far away

SDK^ · 7 Apr 2007 at 08:27

From the Canon side; when an external flash is fitted it fires to assist focus.

Got.soul · 7 Apr 2007 at 14:19

You could try setting your aperature to like f32 and then everything from maybe 10 feet to infinity will be in focus. Youll need a tripod tho! Someone please correct me if im wrong too.

alexisonfire · 7 Apr 2007 at 15:05

Got.soul said:
You could try setting your aperature to like f32 and then everything from maybe 10 feet to infinity will be in focus. Youll need a tripod tho! Someone please correct me if im wrong too.

AFAIK, aperture will have no effect if the camera focus is out, as it only effects areas outside the focus point. If that made any sense?

As for this hyperfocal distance malarki, i've heard of it before, i believe my sigma 10-20mm has some kind of measurement scale for this built in. I still don't really understand how it all works though. Not sure what measurements im suppose to be using to work out anything? Can someone explain it to me as if there speaking to an infant

rpstewart · 7 Apr 2007 at 16:14

alexisonfire said:
AFAIK, aperture will have no effect if the camera focus is out, as it only effects areas outside the focus point. If that made any sense?

The aperture will affect the image regardless of the focus distance. With a small aperture you can compensate for poor focussing (to a degree) because the large DoF can bring the intended subject into focus.

alexisonfire said:
As for this hyperfocal distance malarki, i've heard of it before, i believe my sigma 10-20mm has some kind of measurement scale for this built in. I still don't really understand how it all works though. Not sure what measurements im suppose to be using to work out anything? Can someone explain it to me as if there speaking to an infant

If you look at the scale as you focus on different objects you'll see that it moves as the subject distance changes - it's telling you roughly the distance to where the focus is set. Now you can work that in reverse by putting the lens into manual focus and moving the focus ring to set the scale to approximately the distance you want to focus at. So in theory when combined with a hyperfocal distance chart it should be possible to take a well focussed shot in almost total darkness.

alexisonfire · 7 Apr 2007 at 17:12

rpstewart said:
So in theory when combined with a hyperfocal distance chart it should be possible to take a well focussed shot in almost total darkness.

Thats the bit i don't understand, how to combine these to calculations. Also the scale on my lens only goes up to 3ft, then infinity. How do i manually focus for 100feet away when it only goes up to 3ft? And how do i know what the focus point is.

*confused*

Lets say i have my camera..100ft infront is a fence, 150ft is a table and chairs, 200ft some trees, 300-∞ are houses. Its too dark to autofocus, and i can't see much through the view finder. Lets say im using 10mm @ f/10. How do i use the distance scale and hyperfocal distance chart to manual focus?

rpstewart · 7 Apr 2007 at 20:41

OK, the DoF calculator page that I linked to is back online which makes things easier. According to it the hyperfocal distance for a D40 with a 10mm lens at f/10 is 1.66ft. This gives a near edge of the area of acceptable sharpess of a fraction over a foot with the far edge at infinity.

Therefore if you set the lens so the distance scale is reading as near to 1.66ft as you can everything from 1ft infront of the camera will be in focus.

alexisonfire · 7 Apr 2007 at 20:56

rpstewart said:
OK, the DoF calculator page that I linked to is back online which makes things easier. According to it the hyperfocal distance for a D40 with a 10mm lens at f/10 is 1.66ft. This gives a near edge of the area of acceptable sharpess of a fraction over a foot with the far edge at infinity.

Therefore if you set the lens so the distance scale is reading as near to 1.66ft as you can everything from 1ft infront of the camera will be in focus.

I must be using the program wrong, as i can't seem to get any useful information out of it. Could you take a screenshot of how your hyperfocal distance scale on a 10mm lens looks in DOFmaster please?

rpstewart · 7 Apr 2007 at 21:06

Are you using the download version? The online one's easier I find, it looks like this:

You select the camera, lens focal length, aperture and the subject / focus distance then hit the button. The numbers on the right tell you the distances to the edges of the depth of field and finally the hyperfocal distance.

alexisonfire · 7 Apr 2007 at 21:25

Ohhhh! Thats far easier than the downloaded version. And its helped me spot exactly where im getting stuck.

Subject distance - Obviously if im taking a photograph of a single object, like a person for example, the subject distance is easy. But what would be the subject distance for say a landscape shot? Would i just set it to the nearest thing thats going to be visible in the shot?

Edit: after playing with it i noticed subject distance has no effect on the hyperfocal length?

rpstewart · 7 Apr 2007 at 21:28

Yeah, the nearest thing will probably do. The subject distance becomes pretty much irrelevant when you start dealing with hyperfocal distances though.

alexisonfire · 7 Apr 2007 at 21:29

You missed my ninja edit

Thanks for the help

ICE Master · 7 Apr 2007 at 23:18

I also find it hard to focus on fireworrks int he dark, they go to quick for the camera to auto focus on them. Is there a nack to this?

divine` · 7 Apr 2007 at 23:28

ICE Master said:
I also find it hard to focus on fireworrks int he dark, they go to quick for the camera to auto focus on them. Is there a nack to this?

I've always done it with manual focus at infinity, with an extended shutter time at around 3/4 seconds which I start 'going' when i've heard a rocket and know it's about to go bang