Focusing using thin DOF

Chr1stof

Chr1stof

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Chris
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I am just having a play with my new Canon 50mm f1.8 on a 550d, shooting at short distances in good light (lets say about 2 feet), getting a nice high shutter speed (1/1000 plus) at ISO 100 to minimise shake, but at f1.8 obviously the DOF is wafer thin.

I am using AF but finding it tricky to a get consistent sharp point when using centre point AF. Does AF normally present any issues with accuracy in these circumstances? Is it better to use MF, maybe in live view? I am finding sometimes the AF is fractionally out sometimes, maybe only a few mm but at this DOF its noticeable.

I am just trying to get to grips with the lens as I want to use it to document the stages of pregnancy.
 
AF should give good results but you have to be accurate and hit what you want to be the point of focus. I also try to hit a target that the AF stands a good chance of locking on to and if in doubt I take more than one picture or switch to manual focus.

It's also a good idea to watch your shutter speed and try to avoid either subject or camera movement, try to avoid nodding forwards or backwards as you take the shot as you may not have the DoF to cover these slight movements.
 
I agree with the above and will also say that if I am shooting a static image I will put the frame the shot, put the camera in live view and then zoom in to 10x magnification and manually focus.
 
Good points, the dof is so slim that the effort of pressing the button is enough to move the aim from where the AF has locked onto, I hadn't considered that.
Tripod and extremely still pregnant women is the order of the day :)

I started a thread here asking for some tips with lighting etc but its been a bit quiet so far. I am at a loss on how to consistently light it etc over the coming months!
 
Chr1stof said:
Good points, the dof is so slim that the effort of pressing the button is enough to move the aim from where the AF has locked onto, I hadn't considered that.
Tripod and extremely still pregnant women is the order of the day :)

I started a thread here asking for some tips with lighting etc but its been a bit quiet so far. I am at a loss on how to consistently light it etc over the coming months!
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The usual problem, which you've touched upon, is movement of either the subject or camera after focusing - you'll move half an inch just breathing.

The other thing is focus-recompose using the centre point at a) very close distance, b) low f/number, c) subject off-centre. When you focus-recompose, you tilt the camera very slightly, with causes some movement obviously, but more significantly it shifts the plane of focus.

Imagine the plane of sharp focus as a flat plane of glass attached square to the lens. When you AF with the subject central that plane of glass is exactly on your focus point. But then when you recompose, that plane tilts and puts the plane of sharpness behind the focused point. It's usually okay if you switch to the AF point nearest the subject.

PS Re-post your other thread in the lighting section. You'll get plenty of replies there :thumbs:
 
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