Focus and flash

[MamaMonkey]

It's another Me problem, I'm sure!  I have a Canon 6d mark ii and shoot regularly with Canon 24-70 2.8 ii.  The 24-70 rarely leaves my camera.  I also love my 70-200 but I find it hard to get sharp photos even with shutter speeds of 1/1250 (but that's probably a different post!).  Anyway, meet my 4 year old son.  My settings are 3.2 1/160 and ISO 160. Focal length 70mm. I was using OCF and my max syn speed is 160. Should I be using high speed sync to get more photos in focus when dealing with little kids?  It was probably every 3-4th photo that has acceptable sharpness.  I do photograph newborns with the same set up, but my f stop is always 7.1 and I never have focus issues.  I have it set at 7.1 for the family photos and then just leave it there.  I was testing what 3.2 would look like with my 4 year old as when it's just the newborn in the photo, I would love to try a wide aperture for more shallow dof.  I back button focus as well.  Here is SOOC and 100% crop.  Thoughts? 

[Brian]

Try this again without the stupid Back Button Focusing Technique!
 

I swear, BBF for cameras is like Frequency Separation in PS; it is not the “end-all-and-be-all” that it’s made out to be. If you move slightly at 1/160, especially with a High Megapixel camera, you will get shots like this.

”But I didn’t move!!” 

Each time your heart pumps blood, your hand moves slightly.  Same thing goes if your breathing technique needs work  

Now, yes…if you have lights that don’t have a high enough flash duration, it won’t freeze the subject when your shoot slower SS. Especially lights like Alien Bees. Those suckers are slow.

But before blaming your gear, try consistent focusing (whatever Canon called Continuous) and try things again. 

Finally, your lens may need to be sent to Canon to have it tweaked. At 1/160th, you should be using image stabilization as well. Especially hand-held. Yes, even if you BBF. Bah!! I hate that stupid technique, almost as much as Famien hates LR. 

[Brian]

If you want a more shallow DoF, you also might have to lower your flash power. Remember, Aperture Relates to Flash Power, Shutter Speed controls ambient light. So what lights are you using? 

[MamaMonkey]

On 1/24/2024 at 4:17 PM, Brian said:

If you want a more shallow DoF, you also might have to lower your flash power. Remember, Aperture Relates to Flash Power, Shutter Speed controls ambient light. So what lights are you using? 

It's a digibee 400.  Not the best light on the market, but I only pull it out 3-6 times a year.  If I lower my flash power then I will need to increase my IsO, right?  I've always adjusted the flash power to light the scene in situations like this one.  

[Brian]

I had to look up Digibee. Never heard of one until now.  It might be another version of the Alien Bee. Alien Bee lights are slow. The are almost a flash from like the 1800's, the "watch the birdie!!!" Fooompah!!! The light hangs around and that causes motion blur, since the flash duration is so long. That's why they came out with the Einstein Light. Basically the E640 is a "Fixed" Alien Bee light.

Golden Rule of Flash:
 

1. Aperture controls to Flash Power that's used.
    
2. Shutter Speed (and ISO) controls to the Ambient Light. (But mostly Shutter Speed.)

The quicker the flash duration, the more is "Frozen," even at slower shutter speeds. I also like using Rear Curtain Sync on my camera, it's WAAAY more forgiving. Basically, the flash fires off just before the 2nd shutter curtain closes, helping to freeze your subject. If you use Front Curtain Sync, the flash fires at the beginning of the shot, and then lets more ambient in and that can cause blur, especially if your lights are slow. I see that the Flash Duration is 1/1450 @ full power with the Digibee. You can't think like a Natural Light Photographer, that means you have a lot more power being thrown at your subject if you don't dial things down, which could be contributing things. Also, your camera's built in meter is COMPLETELY WORTHLESS when using an external light like a DigiBee or Alien Bee, or any other off-camera strobe. It's recommended to have a hand-held meter like a Sekonic to help you dial things in faster.

It's been YEARS since I've messed with this stuff. I recommend trying to fiddle and freeze stuff with a toy that moves back and forth. Maybe something like a clock. Something straightforward, won't complain and get bored...and most importantly will stay in one place. Then fiddle with your lights and modifiers that you use. Keep in mind that a Modifier will eat a little light, and it varies to each one used. Alien Bees were also not known to be consistent. They would go all over the place in terms of power output if you made a change. So I'd fire off 4-5 test fires if you make a change to your light.

My advice? Fiddle with the lower power settings on the DigiBee. See what 1/64 does or even 1/16th power. This should allow you to use wider apertures, provided you take the Inverse Square Law into consideration. You half or double the light power moving towards or away from your subject. So from 1-2 feet, you lose 1/2 the power. Then 2 to 4, then 4-8. Think of your subject sitting on a big dart board and they are in the center of the dart board, on the bullseye. The rings around the dart board are the distances. So that's why 4-8 feet away from your subject is the same light power and aperture. As soon as you move closer or further away, you need to make a change to either your Aperture Setting or Light Power Output. It doesn't matter where you stand, as long as you stay within that 4-8 feet area, the Aperture and Light Power Output stay the same. Make sense?