How do I focus in low light for long exposures?

Question

I've been trying to take some long exposure shots at night with my Canon 450D. The problem I'm having is that I can't seem to get a shot in focus. The low light conditions prevent AF from working at all, nor is there enough light for me to get the focus correct by hand. I've tried through the viewfinder and the LCD, but they're both so dark I can't tell if I'm in focus or not.

Perhaps this is a lens issue? I have the 18-55mm ƒ/3.5 kit lens and the 50mm ƒ/1.8 EF lens; neither of them have focus markings, so I'm not sure if I'm hitting ∞ or if my lenses are even capable.

Thank you very much for the suggestions. I’m not able to shine a light, as I’m hoping to get landscapes and night sky shots, and I don’t have anything powerful enough to reflect back. I tried focusing with the 50mm ƒ/1.8 wide open, but I couldn't see anything. I don't think lens speed is going to make a difference.

It sounds like trial and error is my best shot here, which is unfortunate.

Side note: It sucks that the 450D has no way to display the focal length. This info is recorded in the image metadata, so it would seem like it should be available.

I have two more options that I came up with:

1. Set up during the evening, when there is enough light to focus, then take the shot at night. Potentially impractical due to time, but I see no reason it wouldn’t work.

2. Use another camera. I tried again with my PowerShot S90, and got significantly better results. I was able to manually set the focus to infinity, and the wider angle / smaller sensor meant that I didn't have to worry as much about depth of field causing blur. While the S90 only allows for 15s exposures, CHDK removes that limitation.

Answer 1

Lots of good advice and things try here, but whilst faster lenses are generally good for night shots, you will get a limited improvement in AF performance so you will hit the point where it's too dark to AF.

Faster lenses will also give you a brighter viewfinder which enables better manual focus, but again up to a point, with the default focus screen you see no improvement past f/2.6

Lighting the subject to focus is a good idea but not always possible, using the depth scale isn't possible in your case so I'm going to suggest something that's not been mentioned so far.

Focus bracketing

Just as exposure bracketing uses multiple exposures to overcome metering problems, focus bracketing uses multiple shots to overcome AF problems. Start with your best guess for the focus and shoot two shots either side by moving the focus ring slightly and then examine the shots on the LCD. It's time consuming to work this way for sure, but it's a good technique to employ as a last resort. I only wish camera bodies included this as a feature as it would become faster and more accurate.

If this still doesn't work for you consider recomposing to increase your depth of field to give yourself a better chance!

Answer 2

You've mentioned that using a flashlight or similar isn't an option, because things are too far away. However, with the effects of hyperfocal distance, I think you'll find that it may be more usable than you're giving it credit for. According to this calculator, if you focus your 50/1.4 on something 28 feet away, and then expose at f/16, everything from 28 feet to infinity will be acceptably clear.

The laser pointer option mentioned in a comment has promise, as well... though I haven't used that myself, to attest to it. And of course, try to keep yourself from shining it in the eyes of some random hiker that you don't realize is there.

If shooting at f/16 (or f/22 or f/32 or whatever) is unacceptable for some reason, other things to try would be Live View with the ISO cranked up, and test exposures, likely also with the ISO cranked up, and the aperture wide open. Once you've got at least half-way-decent focus, then you should have something good if you're shooting a few stops more closed down (f/5.6 on the 50, or f/8 on either lens).

Finally, one thing that could help you get a focus point at a decent distance with a flashlight/torch is a retroreflector. These are special devices that reflect light back towards the way it came from, specifically. So if your retroreflector is, say, 28 (or 200) feet away, and you put your light source close to your camera, shine it towards the reflector until it lights up, you then would have a nice high-contrast point in your viewfinder or Live View. Now, I say these are "special devices", but they're really quite common -- generally all the reflective things (signs, markers, etc) on roadways are retroreflective, as are many tail lights, license plates, and other night-time safety reflectors. So there may well be some already in your environment (and remember that they needn't be in your frame to make use of them -- just aim at them, focus, and then compose your shot), and failing that, one can by retroreflective tape or other objects pretty easily.

I hope that helps... Happy shooting!

Answer 3

What I do...

I do 2 things in this sort of situation (and I have a 3rd suggestion to try):

1. Shine a light on the subject. If the subject is close, a hand-held torch often works fine.
2. Focus manually. This brings its own hazards, but if the machine can't do it then the man wil have to.
3. I have never actually done this, but in theory you could focus based on the distance to the subject - if your lens has distances marked on it.

Answer 4

For night sky imagery under a clear sky, manually focus on a bright star using "live view". This definitely works with a 450D and any lens below F5.6 — I do this quite often, being an astrophotographer and using a 450D.

Put the camera on 800 ISO and an exposure time set (in M) of at least 10 seconds. Put the lens on 'Manual". Put the focus ring near the infinity mark, locate a bright star in the sky, aim and put the star on the LCD screen, center it, and then press the screen magnification to 10x. Then focus more accurately until the star is a pinpoint. Try to find a star that is not too bright (and not too faint).

Answer 5

To focus at night, you need a pretty fast lens. The 18-55/3.5 kit lens is definitely too slow to be of much use at night. An f/3.5 aperture is pretty small in that environment. The 50/1.8 is better for night use, but still not great. That lens has some pretty basic gear-based AF, so it is rather slow, and not particularly accurate.

A better night lens would be the Canon EF 50mm f/1.4 lens, which is about 2/3rds of a stop faster than the f/1.8, which is a fairly considerable amount of light. The f/1.4 also uses USM AF (ultrsonic motor), which is a better focusing drive that is faster and more accurate than what you get with the 50mm f/1.8. You are not guaranteed to nail every focus attempt with the f/1.4, but you should get a much higher hit rate. Anything wider than f/1.4 will obviously help, although once you get that wide, lenses get rather expensive. (For perspective, the Canon EF 50/1.4 is about $350, while the EF 50/1.2 L is about $1500.)

There are other brands that make fast wide lenses as well. You might look into Sigma, Tamron, Tokina, Zeiss, etc. to see if you can find a decent fast lens for a better price than the Canon 50/1.2 L. Suffice it to say, you need a "faster" lens to be more effective at night. I would say f/2-f/1.8 is probably the smallest aperture you could probably use at night to get any kind of shot, but if you need autofocus, at least f/1.4, with f/1.2 or even an f/1.0 being ideal (if you can find one...some older lenses exist at this extremely wide aperture, and some even wider at f/0.95 or even f/0.7...although they do tend to come for a price.)

Answer 6

The value of this answer will obviously depend on what you're trying to photograph, but are you able to use an external source of light so you can manually focus first?

I'm assuming that you're taking long exposure shots of an object at a particular distance - therefore the focus needs to be accurate (and not ∞). If you have a torch or other light-source, you could light up the subject to focus manually, then turn it off to take the exposure.

Answer 7

The problem with using hyperfocal techniques for astrophotography is that the assumed "acceptable circle of confusion" is not adequate to render stars as sharp points. To compound the problem, variations in temperatures will affect the different materials of your camera system (lens elements made of varying optical materials and densities, metal helical collars, plastic lens tube, etc.) and the skies are usually best suited for astrophotography when the temperature is much lower than for what your lens was optimally designed.

For night sky shots here is what I do. Once my camera is set up on the tripod I enable live view and set my lens to manual focus. I start with the brightest star in the sky, point my camera at it and center it on the screen. I do a rough focus and then repeat at x5 and x10 magnification (I'm not sure if the 450D has this). That will get the focus near enough that some dimmer stars that may not have been visible in live view before now will be. Re-point your camera to a dimmer star and carefully refocus (x1, x5, x10). Everything in the sky except the Moon should be as sharp as your lens is capable of at this point. For shutter speeds I use a rule of thumb of 600 divided by effective focal length (include your crop factor if applicable). When using a 17mm focal length I can expose for around 30 seconds and the stars will appear motionless when viewing the entire scene. At a 100% crop the stars will appear as very short trails. By the time I'm at 640mm (200mm x 2X extender x 1.6 crop factor) I'm down to less than 1 second for shutter speed and push the ISO up to compensate. With night sky shots in relatively dim (light pollution free) skies, your exposure level choice will determine how many stars are visible. Only the brightest stars will appear at lower exposure levels and each successive exposure level will increase the number of visable stars in the shot.

Answer 8

Your camera supports manual focusing in "Live View" mode. Even with some very dark subjects the light magnification is enough to get excellent focusing. This is one of the uses of Live View recommended by Canon.

Answer 9

The best way to focus at infinity is by using a so-called Bahtinov mask or a Hartmann Mask. A problem you will have to deal with is tripod motion during the long exposure. Also, if you photograph the sky, the rotation of the Earth will cause the stars to become small trails. It is almost impossible to take perfectly sharp pictures without using remote control. The camera only has to rotate by about 1/250 th of a degree during the exposure to produce visible unsharpness when using ordinary 50 mm lenses. The slightest tripod shaking due to wind will ruin any chances of getting sharp pictures during long exposure. The best way to minimize the impact of wind is to take many short exposure pictures and then combine them. The pictures can then be aligned perfectly before being added up.