Stephen Eastwood|Beauty and Fashion Photographer

Focus, Shutter Speed, Focus techniques?

Q. How to focus properly on the eyes when shooting FULL BODY under controlled but not so well lit conditions that can more or less let you have ISO 250, f5 & 1/50?
tripod or IS is available.
Focusing on the eye first and recomposing is ok but not enough.

I know focus around the edges of a lens tend to get softer but does it happen to everybody or is it just me.

and part 2
if using strobes?

OK there may be several issues at play here. First that is low light, and a slow shutter, IS and tripod only help to account for you and your movement not the model, a slight move will create a loss of sharpness, maybe not a full ghosting, but a loss of that tack sharp effect that you can have at 1/2000th or when using a strobe to freeze the action.

Next, A focus recompose may be creating a loss of critical focus because of depth of field. Lets say you have a 135mm lens at 135mm and are at f5 your depth of field is slim, you focus on the eyes with the center focus point than recompse the shot which for you was only a move of centermeters but at the point of focu may have been a move of inches or more. Think of a record spinning, the inside barely moves while the end makes a great distance around, so you may actually be better off moving the focus point up closer to the area while composed correctly. This presents a problem as some cameras do not have enough focus points spread far enough to cover the frame, and often in low light there is not enough contrast for the focus system to lock especially on the outer less sensitive focus points. You may start to get a sense of how the manual focus ring works on your lens, and once you figure that out you lock on and knowing that you recomposed and it through the focus off foward or back you know a slight twist of the lens manual focus would correct it.

So what we have are two main issues. First you may have a slow shutter movement of the model and or you. Tripod and IS only fixes you and while 1/50th is seemingly fast its not as fast as the 1/1000th or more that a strobe flash duration freezes action at so its hard to expect it to be as sharp. And depth of field makes a difference, especially at longer focal lengths.

If using a strobe and a fast enough shutter
All the above rules apply yet a strobe or flash will freeze the action, I always find this to be more crisp than even a 1/5000th shutter for some reason, but thats opinion. The flash will fire at a duration generally between a slow 1/750th and a fast 1/5000th so the only issue here is that you are at a shutter/aperture value that does not allow the ambient light to effect the exposure. How do you know? If the flash does not fire are you seeing an image? or of course you can use an ambient light meter to check the available light and make sure that it is well below your settings, just be mindful of the max flash sync of you camera. If you are mixing the two and have enough ambient light to actually expose an image even at a stop below it may well be causing a ghosting to occur through movement of either you when shooting or the subject or both. To eliminate this follow the steps above carefully.