Using Auto-Focus at night..

[skywalkerbeth]

Hi everyone

I just got back from a great trip to Europe with my Mom. I lugged
along my new spiffy (and thankfully, ultralight) tripod.. and took
it out exactly once.

I can't say I am entirely happy with the results, and mostly because
of the focus. How can you auto-focus at night time (or even
manually)?

The subject photo taken from a balcony high up above Lake Como,
focusing on the far shore after dark. The lights on the other side
are kind of fuzzy. I'd say it's no less than a mile away, but the
camera just doesn't seem to know where to focus and manually focusing
when it's dark is tough too.

I also seem to find that setting the focus point on "full infinity"
doesn't seem to work, even though clearly it's a mile or more away.

Any thoughts? is it possible my lens needs to be calibrated?

I've seen some stellar night photos where everything is quite crisp
and clear, and I just don't know how to achieve that yet.

Here is the subject photo, it also looks a little grainy, which is weird because it was ISO 100.

 

[Overread]

Autofocus needs good light to be able to work, in the dark like in your shot there is just not enough light for the camera to be able to properly focus - most times it will start going into a hunting mode where it focuses all the way out and all the way in and won't settle.
After that night time photography needs longer shutter speeds to get more light onto the sensor to get a proper exposure. Now to do this the best way is to use a tripod - set the camera up and then take the shot either using a remote shutter release or the timer in the camera. That will remove as much of the shaking in the camera *using mirror lock up and the timer also helps a lot*. This lets you get those long exposures without the camera shaking - to get it hand held is very hard to impossible - a flash can work for closer stuff, but at the range you show it won't help by much.

 

[skywalkerbeth]

oh sorry, should have mentioned - I did have a tripod, and it was about 25 seconds.. but no lock-up. I don't know how to do that...

thank you for the advice!

 

[Overread]

looking again at the shot I am also seeing a lot of "hot spots" (the red dots) as well - an important thing to note, but I don't know much about dealing with them - sorry.
Mirror lock up should be in custom functions in the camera menu - look it up in the manual as its a really useful feature when shooting still subjects.

 

[Alex_B]

what f-stop did you use? it looks like a very small aperture since i can see dustbunnies at your sensor(?)

Also I can see what are either hot pixels or planes in the sky. Hot pixels are normal with longer exposures and can be removed in postprocessing.


it does not appear extremely grainy o me, maybe you did brighten it up a bit since it was a tad underexposed? then you easily get some noise in dark areas.

autofocus does not work in darkness. and infinity is not well defined on most lenses. I would try to set focus to a distance about two thirds into the scene and use something around f/8 or f/16.

All that said, it does not appear really out of focus though, at least not at the resolution posted here. Very bright lights can bleed a bit into the dark parts, and that probably happened a bit. But you can see that the line of faint lights at the water on the left hand side in the distance appears perfectly in focus to me.

 

[skywalkerbeth]

yes, the dustbunnies drive me crazy!

I will try to fix those things and do this again. Not at Lake Como alas!

 

[Alex_B]

Well, you need to get the sensor cleaned. would have done it for you if you were still in Europe

 

[Dubious Drewski]

Any aperture will show the dust spots on the sensor equally. They're too close to the sensor for the aperture size to take effect. And cleaning dust shouldn't be a big deal; a hand held rocket blower or a qTip with a light touch is all you need. It's all I ever use.

And Beth, maybe you should look into hyperfocal distance for your lens. It might help.

 

[Alex_B]

Any aperture will show the dust spots on the sensor equally.

Sorry, that is wrong. From a certain size onwards, the dust shows at any aperture, but even then it is more focused at smaller apertures.

at f/2.8 light from the lens reaches the dust particle from different directions, giving a blurred image of the particle, most small particles will not show up at all. at f/22 the light reaches the particle almost parallel, so it appears with a very sharp border. even the most tiny particles are visible.

just try it.

And cleaning dust shouldn't be a big deal; a hand held rocket blower or a qTip with a light touch is all you need. It's all I ever use.

blowing only works for larger dust particles, and non sticky dust. The q-tip I consider even dangerous.

 

[Dubious Drewski]

You're right about the aperture, I was completely mistaken about that.

But the cleaning methods are valid and effective. Blower gets the non sticky dust and a qtip is perfectly safe with a delicate touch.

Hell, have you seen the kind of abuse sensors can actually take before failing? It's a solid piece of glass that sits over the photosites, so it thankfully not that vulnerable.

 

[Alex_B]

But the cleaning methods are valid and effective. Blower gets the non sticky dust and a qtip is perfectly safe with a delicate touch.

I am not saying a blower has no value, just I hardly ever have that kind of dust in my camera where a blower could help. So all my cleaning is contact cleaning. just not with q-tips Again, I do not claim it is impossible with a q-tip, it only appears more risky to me than with softer materials

And q-tips, similar to paper towels, can scratch glass on the long run if not applied very carefully.

On a side notice, the usual dirt I have on the sensor needs something more adhesive than a q-tip.

Hell, have you seen the kind of abuse sensors can actually take before failing? It's a solid piece of glass that sits over the photosites, so it thankfully not that vulnerable.

Yeds, I agree they are more robust that most people think! But then again, I have seen scratched sensors as well. The average photographer Joe seems not always to have very steady hands or fine mechanical skills

 

[Iron Flatline]

Well, focusing on a tripod is actually pretty simple, esp. if you're focused on something miles away - in this case you would focus at infinity. Zoom lenses have different infinity points, but they're marked on the lens.

Other than that there are zone focusing marks. Set the infinity mark to the f-stop marker (the sideways 8 should line up with the 11 if you're using f/11) and rest assured that almost everything more than 4-6 feet away from the camera will be in focus (this front distance varies on the mm length of the lens.) If you look through you might see it out of focus, but trust physics to work.

The problem with a 25 sec exposure is that a lot of things can have an influence on the image. One thing is slight shake due to gusts of wind, or you walking by. The mirror itself has already been mentioned. But there are also air currents and temperature convections that can affect light sources far away - think of it as a softer version of what you see on the tarmac on a hot day.

Visible Dust makes everything you need. Get the blower, the Arctic Butterfly, and the proper liquid & swab combo to clean your gear. Nothing else comes close.