Focusing in low light conditions

[Bluepoole]

Hi all,

When shooting with my Canon 17-85mm lens, and also with my 430EX II Speedlite attached, I found that the lens struggle to focus in very low light conditions, so some of my photos turn out somewhat out of focus (not completely blurry, but obviously not great). I try to use manual focus or even adjust the focus slightly with the lens' focus ring, but I struggle to see if the focus is correct, because the scene through my viewfinder is dark.

I shoot in manual mode, aperture at f4.0, shutter speed 1/100 and ISO 400 or 800.

Is there a way to overcome this? I thought of changing the camera's AF point to the middle of the frame (as oppose to Auto select), because it might try to focus on light coming from the side of the frame, instead of on the subject which is in the centre, but very dark.

Also, which drive mode and which metering mode would your recommend under such circumstances?

Thank you in advance for all the replies!

[Big Mike]

Firstly, make sure that your 430EX is set so that the AF assist beam emits. It is supposed to send out some red light in the same pattern as your AF point, which can really help you to focus.

The ability to focus, lies mainly with the camera. For example, where your 350D or 1000D might have problems, something like a 1D mk IV would probably be much better.
Also, the max aperture of the lens is a factor. The larger the aperture, the more light that can get into the lens, and thus to the AF sensor. That would also make your viewfinder brighter, making manual focus easier.
These entry level DSLR cameras have rather small viewfinders compared to the more expensive models and even film SLR cameras. This is another factor that makes them harder to focus manually.

If you can't (or don't want to) get your flash to emit the AF assist beam, then you could use something else for the same effect. A laser pointer or a flashlight might be enough to give the AF something to see and focus on.

[Gaerek]

I'm not sure if it will help in this case, but one of the first things I did when I got my camera was set it only use the middle AF point. I've had this camera over 2 years and I have only ever switched to something but that a handful of times.

[ANDS!]

Mikes suggestion of the flash light is a good one. Depending on what kind of setup you have, always carrying a miniflashlight with you, or rigging something up to stay on your camera where it is always pointed at your subject will help you grab those shots you are struggling to miss. A camera doesn't need much to grab that focus, just enough to resolve contrast in the area you are looking at.

[Goontz]

Quote: Originally Posted by Gaerek

I'm not sure if it will help in this case, but one of the first things I did when I got my camera was set it only use the middle AF point. I've had this camera over 2 years and I have only ever switched to something but that a handful of times.

Wow, really? Do you center all of your shots (and maybe crop unwanted space later), or just use the focus-lock a lot?

[Big Mike]

I also use the centre focus point about 95% of the time. On my particular cameras, the centre point is more sensitive when used with F2.8 lenses. But I also don't like the idea of the camera choose which point (or points) to use. I like to know exactly what I'm focusing on.
For when I'm focusing on something that I don't want centred...I focus then recompose. I lock the focus by deactivating AF. (I have my cameras set up so that AF it turned on with the * button, rather than the shutter release).

[Gaerek]

Quote: Originally Posted by Goontz

Quote: Originally Posted by Gaerek

I'm not sure if it will help in this case, but one of the first things I did when I got my camera was set it only use the middle AF point. I've had this camera over 2 years and I have only ever switched to something but that a handful of times.

Wow, really? Do you center all of your shots (and maybe crop unwanted space later), or just use the focus-lock a lot?

Err, no offense, but are you serious?

1. Center subject, half depress shutter
2. Re-compose shot, keeping shutter half depressed
3. Fully Depress Shutter to take shot

It's not that difficult...

I should also mention that the center point of most cameras is also the most accurate. Most cameras have two types of af points. Without going into technical details of it, the center point is what's known as a cross type af sensor, and most cameras only have one. The other points are typically less accurate anyway. I believe some of the newer high end cameras actually have more cross type sensors, but I know I only have one.

Here's a good tutorial on how autofocus works, if you really want to get into the technical details.

And by the way, this is a quote from that same tutorial:

"Further, since the central AF sensor is almost always the most accurate, for off-center subjects it is often best to first use this sensor to achieve a focus lock (before recomposing the frame)."

[Goontz]

I'm aware the center's the most accurate and how AF works, I just had honestly never heard of anyone who only uses the center point. I probably also shouldn't have used "focus-lock" rather than getting focus and recomposing, too. Interesting. I don't always let the camera choose which AF point to use, but I don't always use the center. Also interesting, Mike, how you have your cameras set up.

[Gaerek]

Quote: Originally Posted by Goontz

I'm aware the center's the most accurate and how AF works, I just had honestly never heard of anyone who only uses the center point. I probably also shouldn't have used "focus-lock" rather than getting focus and recomposing, too. Interesting. I don't always let the camera choose which AF point to use, but I don't always use the center. Also interesting, Mike, how you have your cameras set up.

It's not that I don't use the others, but you're going to get better results using the center point, and recomposing than using any other single point, or even worse, auto af point selection. I'll try to find it, but I found a blog post where someone tested this out. He had 200% crops of shots he'd taken using various methods of acquiring focus. One was using the closest af point to the subject (meaning not the center point) and not recomposing, another was using the center point and not recomposing, and the last was using the center point, and then recomposing.

The best was using the center point and not recomposing (obviously), second place was using the center point, and then recomposing, third was using one of the outer af points. The results convinced me once and for all.

[Goontz]

That makes sense. Do post if you're able to find it.

[Derrel]

Have you gone into the custom settings menu and enabled the AF Assist Light? That would be the very first thing you could do, so that in low light, the camera or its flash could send out the AF assist beam, to help achieve a focus lock.

There was a post yesterday on this about the Rebel XS, which uses short bursts of white light that emanate from the flash as the source of the AF Assist beam.

[iolair]

I also read that it's typically more accurate to use centre-focus-and-recompose compared to using non central focus points. Though perhaps it depends on your camera (or camera-lens combination).

Can't find the link to the article right now, though :-(

[inTempus]

Quote: Originally Posted by Goontz

I'm aware the center's the most accurate and how AF works, I just had honestly never heard of anyone who only uses the center point. I probably also shouldn't have used "focus-lock" rather than getting focus and recomposing, too. Interesting. I don't always let the camera choose which AF point to use, but I don't always use the center. Also interesting, Mike, how you have your cameras set up.

When shooting pictures of people it's usually better to select a single AF point and put that point on the eye closest to the camera and lock focus. Then recompose if necessary.

This method is how I shoot the vast majority of the time. When shooting people it's imperative to have tack sharp focus on at least one eye. If you don't, the whole image looks out of focus. Most of the time I use the center AF point for this purpose, but I still use my outer points as well. I will select an outer point closest to the eye I want to lock focus on when I'm shooting with a really wide aperture. The reason being that if I lock focus and recompose with a wide aperture, chances are my focus point will shift when I recompose.

I've found that no matter how good the AF system is (like on a 1D), you rarely want the camera selecting the focus point for you on a persons face when shooting a portrait. It will almost always select the wrong focus point as the camera will often times choose the forehead over the eye, or even the tip of the nose.

[Bluepoole]

Thanks guys for all the advice.

My Speedlite's AF assist beam solved my problem! I figured out my mistake - my camera's metering mode is always set on AI Servo, that comes from the days when I only shot fast moving aircraft at the airport, and the habit stuck.

Of course the assist beam doesn't come on in AI Servo mode so I just changed my metering mode. I feel kind of stupid for not thinking so far but that is how you learn!

The flashlight idea is also great, I will take one along on my next shoot.

Of course I also hope to upgrade my equipment within the next 12 months. If all goes well, I want to get a 50D with the 17-55mm 2.8 IS lens or even the 16-35mm L.

[Big Mike]

Quote:

my camera's metering mode is always set on AI Servo

I know what you mean...but just for the record, AI Servo is an auto focus mode, not a metering mode.

Quote:

Mike, how you have your cameras set up.

On my (and most Canon) cameras, it's Custom Function #4 that allows you to move the AF away from the shutter release button to the * button. Some of the newer models actually added another button just for this purpose.

On many cameras, the centre point might be more sensitive (cross type vs linear) but the newer ones have several cross AF sensors. I find that it's just easier to use the centre point and recompose...I don't know about it actually being more accurate. Actually, a very close distances (shallow DOF) it might actually be less accurate to focus and recompose...because unless you rotate the camera precisely around the sensor's position...you are altering the distance to the subject, and thus the accuracy of the focus.