Camera setting advice

What is the lowest aperture you would use to photograph a 19 month old running outside with 85mm lens?

wide open is the lowest I would use. Its not about aperture as much as it is about technique. Your keeper rate goes down a bit wide open, so if you want every shot to come out go to f8. But with good technique there is no reason you cant get some at f1.8.

What do you want in focus, and how far away from you is the subject? The aperture is controlling your depth of field- see the DOF calculator here. At f1.8, you have about 6" in focus at 10 feet. That's usually enough to get the face in focus if you nail the focus point. You would have to stop down to get the whole body in focus. Depending on the lighting outdoors, you may have to close the aperture because you hit your shutter speed ceiling as well. More distance from your subject will increase the depth of field as well- at 30 feet and f1.8 you have almost 2'6" in the focus plane.

Probably 10-15 feet is what I am shooting. I am just missing focus with 2.2-2.8.

brian4646 wrote in post #16867056
Probably 10-15 feet is what I am shooting. I am just missing focus with 2.2-2.8.

What is your technique? Are you using servo mode and back button focusing?

servo

brian4646 wrote in post #16867150
servo

Try using back button focusing along with it. Less stutter finger when you separate the two functions.

I've never used it. I'll have to try that.

I use BBF but I have yet to master Servo mode. If I really wanted to make sure I got keepers I would stick with f4 to be on the safe side. If it was something repeatable just for personal pics of my own kids, I would shoot wide open just to work on improving my skills.

i have to try the bbf.. but my short right thumb hinders that...

brian4646 wrote in post #16867056
Probably 10-15 feet is what I am shooting. I am just missing focus with 2.2-2.8.

What shutter speed are you getting? With f/2.8 your focus/depth of field should be reasonable, but with a moving kid you'll have a problem with motion blur if the shutter speed isn't quick enough. If so, you'd need to raise your ISO...?

no problem with shutter
I'm thinking f/4 is my target with the 85mm

BBF makes big difference.

f1.2

Over Easter I was shooting just such a scenario but in my case it was TWO toddlers and one 6 year old.

Set f/1.8, ISO 100, SS 1/4000 (bright sunlight). BFF, Servo.

Started off missing, ended up with a bunch of keepers. In my case I knew I wanted to shoot wide open for the separation and I wanted a SS no slower than 1/250 since those kids were MOVING. Mother nature took care of that.