Experimenting with Lens AFMA

Steve Balcombe wrote:

Kahrom wrote:

Hi,

I am trying to do AFMA, and its my first time doing it, so apologies if the questions seem trivial.

For the target setup I am using the SyderLensCal, a Canon 6D and the 24-70 f/2.8II L. My understanding is that the target should be well lit, and the target should be "exactly@ in front of the lens using the middle focus point. Could someone please clarify some bits:

1. The target should be well lit, but none of the guides ever mention using a flash/speedlight. Is there any reason for avoinding flashes?

You're testing AF, which happens before the exposure, so good ambient light will mean you are not unwittingly testing something else.

Ahh, makes sense, haven't thought about the order of operation before. Thanks.

1. With regards to target alignment, how much precision is really required? I am having a tough time to both a) align the middle focus point precisely on the target's center point and b) keep both camera and the target parallel . I am wondering if there is an easier way of alligning them ( besides using a horizontal rail and putting both on it, don't have such setup)

It does need to be accurate, but no more accurate than you can judge by eye. It shouldn't be difficult if you have a tripod and somewhere convenient to place the target.

1. Target distance: I have seen recommendations from 30x-50x the focal distance. Not sure what are the pros and cons of using higher/lower values

The thinking behind this is to test the lens as close as is practical to infinity focus. At 30x-50x you'll notice that the focusing ring is very close to the infinity position - but at the same time the target is large enough in the frame to be usable. The exact figure isn't critical - anywhere in that range is ok.

Some people suggest that if you normally use the lens at closer distances, e.g. if it's your specialist portrait lens and you never use it for landscapes, then test at your preferred distance instead.

OK, didn't think about the "close to infinity" position. Thanks for the explanation Steve.

HTH

-- hide signature --

https://www.flickr.com/photos/stevebalcombe/