How can I improve focus using both manual and auto on a candle flame?

Question

I got my new tripod and 50mm F/1.4 sigma on my Nikon D7000 and was testing it out doing some shots of a flame on a candle.

The below image is the sharpest i was able to produce. In review of my 50+ shots I noticed very many out of focus. This is the main factor I was trying to concentrate on to get a clear sharp picture of the flame.

Using Manual Focus

A few things I noticed and could use help with are the act of manual focusing on the flame. I would try to use my eye and view the flame in the eye piece. I would turn the auto focus ring back and forth dialing it in to a point in between both extremes until the flame looked sharp. I noticed there was considerable travel in the focus ring while still looking sharp to my eye. How can I tell when the flame is in focus? When and how do I detect where to leave the focus ring in order to get a crisp and sharp photo?

Using Auto Focus

I took about half my shots using Auto Focus and found a great many of these shots were also not sharp. I guess the auto focus had a hard time with the only light being the candle. I turned on the lights and used a cardboard post to try to get the camera to lock cardboard and then turn off the lights and light the candle. This was also not a help and my attempts seemed very crude. Should I and how can I use auto focus in this situation?

Also, I found it annoying that when I tried to get the auto focus to lock on a particular point on the cardboard the camera kept selecting some other point of focus on. At least this is what i gather was happening because the camera placed a black little square box on a section of the frame other than where I intended it to. How can I control this box so I can tell the camera what I want to focus on?

Answer 1

Theories have been advanced here concerning the lack of brightness of the flame, its flickering, small depth of field, air turbulence, and other things. All are interesting and plausible.

Let's check some of them.

Brightness of flame

This image (unretouched; converted from RAW to JPG for uploading) was taken with a 50 mm lens at a distance of about 100 cm (3 feet). The exposure was 1/200 sec f/3.5 at ISO 1600. The flame is certainly bright enough!

Flickering and air turbulence

You can see even in this short exposure that the edge of the flame is soft. This was a steady flame; there cannot have been much flickering in 1/200 second. The softness is because there is no edge to the flame! The following is the same image, brightened by 2 stops.

The enhancement shows a corona of light surrounding the flame. It is not a partially illuminated backdrop: it emanates from the candle itself. The light is blackbody radiation from the combustion byproducts (soot and gases). The transition from the burning (bright) part of the flame to the burnt corona is sharp, but not extremely so.

The details apparent in the wick demonstrate the focus is spot on.

Depth of field

Even at f/1.4 (where depth of field would be least), with a subject distance at 100 cm, the DoF is about 1 cm in front and behind the flame: more than enough to have the entire flame in focus. (In this image, at f/3.5, the depth of field extends about 2.5 cm--one inch--before and behind the wick.) Thus, establishing a decent DoF is not a consideration.

Softness at wide apertures, mis-focusing (solved by focusing on the wick), and movement of flame are much more likely causes of an unsharp image.

Conclusions

Candle flames are bright enough to allow adequate control of the exposure: you can take rather short exposures at moderate f/stops without using extremely high sensitivities. However, as diffuse radiant balls of gas, they are inherently not sharp and will not appear to be so, no matter how well focused.

Answer 2

How can I tell when the flame is in focus? When and how do I detect where to leave the focus ring in order to get a crisp and sharp photo?

In general you know that the flame will be in focus when the wick is in focus, so I would tackle this as follows:

1. Set the candle in the location you plan on taking pictures of it. If possible set it in a location without a lot of air movement (a steady flame will be easier to take a properly focused picture of)
2. Set your camera/tripod where you plan on taking the picture
3. With room light on dial in your camera settings and AF on the wick. I'd shoot for an f-stop of 4 to 5.6 because the 'mid' f-stops on a lens will generally produce sharper results than f-stops on the extreme ends.
4. Turn off AF on your camera
5. Turn the room light off and light the candle
6. Take the picture(s)

I think this'll give you the best chance for in-focus pictures with the least amount of 'fiddling' with it. As long as you don't move the candle, the camera, or change lenses (e.g. alter the variables), your shots should always be as in focus as they are going to be.

Should I and how can I use auto focus in this situation?

Once you eliminate the need for the camera to focus 'on the fly' it almost doesn't matter any more. I'd say use the AF to get the wick in focus, and then turn it off as you won't need it again (as long as you don't move anything).

How can I control this box so I can tell the camera what I want to focus on?

I think this will become a moot point if you're 'pre-focusing' as I've described above, but in general it is possible to control which AF point the camera uses. You'll need to check with your manual to discover the specifics of how to do this, though...

Answer 3

Are you sure the real problem is with focusing? A flame is in nearly constant motion, and a typical candle does not produce a whole lot of light. Unless you're shooting at quite high ISO, you're likely to have a long enough exposure to show some movement in the flame. In that case, changing your focus point can't really do much good.

Flames have a few other problems as well. The flame is heating the air around it, so you get a "shimmer" from the turbulence in the air where it's being heated from room temperature to 400+ (or so) degrees. You also typically have some smoke coming off of the flame -- even when it's not directly visible as a stream of smoke, you still have quite a few particles in the air around the flame. These cause both reflections and refraction that reduce sharpness still further.

That's not to say you can't possibly do any better, or that it's not worth working on doing better. It does mean that you've picked a sufficiently difficult subject that having a small percentage of "keepers" should come as no surprise.

Answer 4

The bigger and brighter the viewfinder image is, the easier it will be to focus manually. The D7000 at least has a prism rather than a mirror, but the crop sensor means that you're looking at an image on the focusing screen that's only about 45% of the area you'd be working with using a full-frame camera. Making the image large enough in the viewfinder means that the apparent brightness goes way down. (If you get a chance some day, try manually focusing a medium format camera at a camera shop -- even an old clunker with a plain ground glass screen and an f/4 or slower lens -- and you'll see what a difference in brightness and ease of focusing there is just from having a much larger screen.)

You're also working with a plain screen -- we used to have focusing aids like split-image prisms and microprisms that helped a lot, especially in low-light or low-contrast situations in the manual-focus era. So there's another handicap into the mix.

That doesn't make it impossible -- just difficult. I was eventually able to train myself to focus on the plain ground glass areas of a Pentax Auto 110, but it sure wasn't easy (and there was nothing I or anyone else could do to keep the pictures from looking like they were shot on 110 film -- but I was 17 and not completely unretarded at the time). It can be done, but it takes a little while to train your eyes to see extremely small differences in contrast.

That all being said, and taking into consideration that buying a $40K Hasselblad to take a picture of a candle is perhaps not the most cost-effective approach, the simplest expedient is to focus with the lights on, then douse the lights before taking an exposure. That won't help you take pictures where you can't control the lighting, but there's no sense in doing things the hard way if you don't have to.

You might find it helpful to use a dark cloth or an accessory eye cup to keep stray light out of your eyes while focusing -- ambient brightness kills whatever contrast you have to work with on the ground glass, both by closing your iris down and by front-lighting the screen. Treat pictures like this one (and still lives and landscapes) as if you were using a view camera. Slow everything way down, be deliberate, and give yourself every advantage you can -- it'll make a huge difference in the final result.

And yes, you can focus effectively with the lens wide open unless the lens exhibits significant focus shift (a phenomenon in which the focus point of the lens changes as the aperture changes -- something that shouldn't be a problem with the Sigma 50). The camera does it; why shouldn't you? DoF preview is another matter -- it can be very hard to determine what is an isn't in the range of acceptable focus once you've stopped down and made the image, say, 32 times as dim as it was wide open (that's f/8 compared to f/1.4). Maybe with a couple of long-range military searchlights...

Even if you are faced with focus shift, you can always "focus bracket" when shooting a subject that isn't going anywhere. (It's not like you're burning a ten-dollar sheet of 8x10 film every time you release the shutter.) Relax, take your time -- and cheat whenever you can.

Answer 5

I would recommend getting the rim of the candle in focus, rather than the flame itself. The flame is going to waver and flicker, and it may move closer or farther from you as you are trying to focus. It is rather ephemeral itself, as well, and may not necessarily always have "sharp" edges.

If you try to focus the left and right edges of the candle that are around the flame in the plane you want focused, you will probably have more luck. It should be easier to see when the picture is in focus as well, since the shape and sharpness of the candle itself are going to be static...unlike the flame. Once you have the candle in focus at the right depth, so long as your DOF is deep enough to include the whole flame, it should be "in focus".

Answer 6

Have you tried shooting the candle flame tethered to a computer? It's easy to do and there are free apps that you can use to trigger the camera from your phone. I use an app from onOne software on my IPhone and it works beautifully. Just a thought, you'll see a much bigger image on the laptop screen than on the back of your D7000.