Best settings to focus in low light and low contrast condition

Question

I have been shooting for several years in the north of Canada and I notice that I don't get a really high percentage of keeper when using my Canon 100-400 Mk II or 70-200 F2.8 Mk II with either 5DIV or 1DX2.

Even after careful lens calibration, I often get a 50cm-100cm (20in - 40in) back focus which is problematic when shooting dog's face for example. In this case they are usually coming at 10-15 miles per hour which I try to capture at something like :

• 1/800sec - F5.6 - 400mm - ISO 1600 to 3200
• Subject is usually within 50m - 20m (150ft-60ft) away
• Speed 10-20mph
• temperatures around -20c to -45c (0F to -45F) which makes the camera a tad slow to respond.
• Subject with really low contrast

What would be the optimum way to achieve focus:

• 1 Would you use only one focus point or use an extended area focus?
• 2 One Shot AF to avoid using the tracking that doesn't seem to work anyway or AI Servo?
• 3 Which point would you use? I usually use the lowest points but looking at the picture below that might not be optimum?! a tad confused with that figure.

• A: Cross-type focusing: f/4 horizontal + f/5.6 or f/8 vertical
• B: f/5.6 or f/8 vertical focusing
• C: Cross-type focusing: f/5.6 or f/8 vertical + f/5.6 or f/8 horizontal
• D: Dual cross-type focusing: f/2.8 right diagonal + f/2.8 left diagonal f/5.6 or f/8 vertical + f/5.6 or f/8 horizontal

At that point I would take any advice! Maybe I am just not using the tracking properly... not sure! Also note that the cold condition makes it challenging to keep the tiny points on the subject which is why I usually end up using extended area focus which might be harder on the camera focusing wise.

Thanks for your input!

Answer 1

Your camera is not back focusing at all. It is focusing exactly where you told it to focus.

Your problem is that by the time your camera actually takes the photo, your subject has moved 50-100 cm closer to your camera. But your lens is still focused where your subject was when you locked focus, rather than where it is when the photo is taken.

At 10-15 mph, your subject is moving towards you at 15-22 feet per second, or 180-264 inches/sec (460-670 cm/sec). If the time delay between focus lock in One Shot AF and the time the shutter opens is about 1/20-1/10 second, that accounts for all of your missed focus.

Normally 1/20-1/10 second (50-100 milliseconds) would be quite a bit longer than the expected time between focus lock and shutter movement. But in the temperatures you describe, things start getting very sluggish and it's not inconceivable that it is taking 50-100 milliseconds between focus lock and exposure. It may be taking that long for the lens to move the focus elements to the position instructed by the camera.

What would be the optimum way to achieve focus:

I would first try to find a way to make AI Servo AF work. In AI Servo AF, the camera predicts and compensates for the amount of change in subject distance during the AF blackout between the time the mirror starts moving up and the shutter opens.

• With the EF 70-200mm f/2.8 L IS II, I'd stick to the five dual-diagonal cross-type AF points stacked in a vertical line in the middle of the viewfinder. Regardless of the aperture selected to take the photo, the aperture is wide open during AF and you'll get the advantage of those f/2.8 diagonal lines.
• With the EF 100-400mm f/4.5-5.6 L IS II, the f/2.8 diagonal cross portions of those AF points are not active, so any of the AF points in the center cluster should work equally well.
• I might even try EOS iTR AF.

If I couldn't get AI Servo to work, then I'd "cheat" a bit.

• Intentionally dial in about 50-100 cm of front focus
• Shoot just as you have before. One Shot AF
• When focus locks, it will be focused 50-100 cm in front of your intended subjects
• By the time the shutter opens, your intended subjects will have run 50-100 cm closer to the camera

The AF systems of the Canon EOS 1D X Mark II and EOS 5D Mark IV are highly configurable. They're powerful tools. But like any sophisticated tool with a lot of user controllable options, they require knowledge and skill to be used to their potential.

I'd recommend downloading and reading the following guidebook published by Canon that is devoted exclusively to the AF systems of the 1D X Mark II. The AF system in the 5D Mark IV is so similar to the one in the 1D X Mark II that most of the info in the guide linked below will also be applicable to the 5D Mark IV.

Canon EOS 1D X Mark II AF Setting Guidebook