So I tried again today shooting with the 100mm portraits of my walking moving baby, and again maybe only 1 out of every 20 images was in focus and sharp. I shot outside at f 2.8, and 160th at iso 200/250/320, i also tried at f4, all either blurry, or not sharp.
You are probably experimenting a combination of subject motion blur and AF "issues".
- Use a speed no lower than 1/500 sec for a randomly moving baby, 1/1000 is probably better,
- Ensure a good understanding of the AF of your body. Typically I would select only one AF point with good sensitivity (meaning the cross ones pretty close to the center of the viewfinder) and strive to keep the closer eye of the baby within that AF sensor.
I have a 15 months baby girl myself, and when taking images indoor, it takes my D800 to the edge in terms of ISO and AF.
This is in fact one of the most technically challenging subject you can find because:
- With a 100mm lens on a small subject like a 80cm tall baby, you are at pretty close range where AF typically performs worst on most lenses,
- the movement is random and hard to predict,
- you are often in pretty dark indoor conditions.
It wouldn't be reasonable to expect a 100% success rate, but 1/20 is definitely on the low side.
Since I am a Nikon shooter, I don't know much about the Canon 100mm macro other that it is optically excellent. What matters most here is probably AF speed when used in combination with your body. Macro lenses are typically not optimized for AF speed.