Auto-focus and f-stops

Call me a complete novice (because I basically am, having only been doing this for less than 6 months) but I have another question here. This could be elementary photography, but I wouldn't know!

Whenever I'm taking some night shots - I recently really noticed this while photographing fireworks - at certain f-stops I couldn't get the 18-55mm kit lens to auto-focus!? I had my XT mounted on my tripod and would change my f-stop for different shots, but whenever I'd go back to the fireworks it wouldn't focus. I also noticed that it seems to have something to do with the f-stop versus the zoom factor, or at least it seems that way to me.

Is this a common phenomenon, or do I just not know what I'm doing? I thought I did, until I started running into this problem!

Please help with this...

Everyone starts where you are. Well, your lens defintely does not do its best work in night time/low light shots.

The more stopped down you are, the less light let in. So when you are in low light situations, its harder to lock autofocus. And that's probably what happened with you. It even happens with super expensive lens if there is very little light.

grego wrote in post #2379378
Everyone starts where you are. Well, your lens defintely does not do its best work in night time/low light shots.

The more stopped down you are, the less light let in. So when you are in low light situations, its harder to lock autofocus. And that's probably what happened with you. It even happens with super expensive lens if there is very little light.

I'm tired and out of it, but doesn't the lens AF before stopping down, so there should not be a coorelation.

grego wrote in post #2379378
Everyone starts where you are. Well, your lens defintely does not do its best work in night time/low light shots.

The more stopped down you are, the less light let in. So when you are in low light situations, its harder to lock autofocus. And that's probably what happened with you. It even happens with super expensive lens if there is very little light.

Is it better in these cases to manual focus? My only problem with manual focus is lack of experience. I think it looks like it's perfectly focused, and it looks great on that tiny LCD, but when I upload it onto my computer it looks like I handed my camera off to a 3-year-old.

I've practiced some with manual focus today and I may want to try it again. Auto-focus is usually just so easy!

In some cases with the auto-focus I'd run through every couple of f-stops just trying to find one that would take with auto-focus and I was getting very discouraged because it wasn't wanting to lock in!

coreypolis wrote in post #2379391
I'm tired and out of it, but doesn't the lens AF before stopping down, so there should not be a coorelation.

Well, you'd pick your settings before you try to photograph.
Or maybe Im reading you wrong.

SBCmetroguy wrote in post #2379394
Is it better in these cases to manual focus? My only problem with manual focus is lack of experience. I think it looks like it's perfectly focused, and it looks great on that tiny LCD, but when I upload it onto my computer it looks like I handed my camera off to a 3-year-old.

I've practiced some with manual focus today and I may want to try it again. Auto-focus is usually just so easy!

In some cases with the auto-focus I'd run through every couple of f-stops just trying to find one that would take with auto-focus and I was getting very discouraged because it wasn't wanting to lock in!

It's tough to AF on your camera without a different focusing screen. You can try. But it is challenging. Your lens defintely has limitations. Low light is one of those for this particular lens.

grego wrote in post #2379403
Well, you'd pick your settings before you try to photograph.
Or maybe Im reading you wrong.

but just because you tell it f/22 instead of 2.8 the lens still doesn't close down till the shutter button is depressed (or ADEP)

coreypolis wrote in post #2379409
but just because you tell it f/22 instead of 2.8 the lens still doesn't close down till the shutter button is depressed (or ADEP)

True, but when pressing on the shutter to focus, i would think its activated. But hey, i know im not always right.

Corey, just let me add... the photos on your website are amazing. You're my new idol.

Can anyone tell me what lenses ARE good with nighttime photography?

Fireworks.. if there is nothing going off in the sky at the moment then the sky is completely black, and the AF system needs a high contrast target to focus on. This is exacerbated by the fact that it is night. Also note that if you focus on a white wall during the day, you'll get similar effects.. it will hunt and miss.

My technique for fireworks shooting - with lenses supporting a distance scale, turn the scale to infinity and then switch to MF.

With the kit lens, which has no distance scale, set it to autofocus. Usually there will be some distant city building around with bright lights. Pick something FAR away. Set center AF point (see manual on how to do this). Point at city building and half press to AF, you will hear the beep. Set the lens to MF, now you can start shooting. What it does is make the lens focus at infinity (which the fireworks usually are, even if you are standing exceptionally close as I was one night). Leave it on MF all night and don't bump the focus ring by accident, or you will have to re-focus.

If you don't mind screwing up the kit lens, cut or mark a white notch in the lens. Take it out in daylight and autofocus on an object very far away. Mark the front part of the barrel (which you turn to focus) with another white mark so that the two marks are next to each other.

At night, switch to MF and turn the white marks to line up with each other.. this is your infinity marker.

SBCmetroguy wrote in post #2379422
Corey, just let me add... the photos on your website are amazing. You're my new idol.

thanks!

Lightstream wrote in post #2379430
Fireworks.. if there is nothing going off in the sky at the moment then the sky is completely black, and the AF system needs a high contrast target to focus on. This is exacerbated by the fact that it is night. Also note that if you focus on a white wall during the day, you'll get similar effects.. it will hunt and miss.

My technique for fireworks shooting - with lenses supporting a distance scale, turn the scale to infinity and then switch to MF.

With the kit lens, which has no distance scale, set it to autofocus. Usually there will be some distant city building around with bright lights. Pick something FAR away. Set center AF point (see manual on how to do this). Point at city building and half press to AF, you will hear the beep. Set the lens to MF, now you can start shooting. What it does is make the lens focus at infinity (which the fireworks usually are, even if you are standing exceptionally close as I was one night). Leave it on MF all night and don't bump the focus ring by accident, or you will have to re-focus.

If you don't mind screwing up the kit lens, cut or mark a white notch in the lens. Take it out in daylight and autofocus on an object very far away. Mark the front part of the barrel (which you turn to focus) with another white mark so that the two marks are next to each other.

At night, switch to MF and turn the white marks to line up with each other.. this is your infinity marker.

there we go, now we're talking

Just wanted to clear up something that has come accross as confusing to me. The lens will always autofocus with the aperture wide open, no matter what you set it to. If you set the aperture to f22 on an f2.8 lens, the lens will autofocus at f2.8 and only after the shutter button is depressed will the aperture stop down to f22. Lightstream pretty much nailed the problem you're having. Focusing in low light can be tough and if you're just trying to focus into a dark sky....it's going to be next to impossible.

What I did was just focus on one of the fireworks at the start, then switch it over to manual focus. left it there for the rest of the shots

http://savagebasher.ma​ckycorp.org/fireworks