Experimenting with Lens AFMA

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?
2. 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)
3. 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

Thanks in advance!

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?
2. 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)
3. 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

Thanks in advance!

1. Impossible with a flash. You need continuos lighting.

2. The more precise the better but most walls can be used and you can use a small level or bubble level on your cam. You use a product like Lens Align if you want to be more precise. I just use a wall. I'll put a level on it to make sure it is not out badly as some older homes can be.

3. A mixed bag. Your guess is as good as mine. One Canon document says 50X, another says the distance you normally shoot at, 3rd party says 20X and others MFA at MFD. In the manual Canon says for best results to MFA at shooting location

I came across an article that explains when a lens settles down so I use the recommend distances which are manual due to the lens lengths and crazy distances you need at 50X.

http://s449182328.websitehome.co.uk/focal/dl//Docs/FoCal%20Test%20Distance_1.1.pdf

On line calculator. I did adopt FoCal and it is pretty accurate. I just try to level both the target and cam as best I can. It does really matter because if you MFA at different distances you will get different results thus Canon's at location recommendation which is impractical unless you are shooting in a studio at the same distance all of the time. The newer cameras that have a short and long end settings helps with that problem.

http://www.reikan.co.uk/focalweb/index.php/online-tools/test-distance-target-size-calculator/

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

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Zee Char wrote:

1. Impossible with a flash. You need continuos lighting.

2. The more precise the better but most walls can be used and you can use a small level or bubble level on your cam. You use a product like Lens Align if you want to be more precise. I just use a wall. I'll put a level on it to make sure it is not out badly as some older homes can be.

Well, that's what I was struggling with; I used the bubble level on the tripod and the target to make sure they are parallel vertically. But aligning the middle of the lens to the target (horizontally) wasn’t so easy - I might use the wrong approach/technique. Tried many times, but I always felt like I had to very slightly tilt the camera upwards or downwards to be spot on middle, but I didn't want to tilt it.

3. A mixed bag. Your guess is as good as mine. One Canon document says 50X, another says the distance you normally shoot at, 3rd party says 20X and others MFA at MFD. In the manual Canon says for best results to MFA at shooting location

I came across an article that explains when a lens settles down so I use the recommend distances which are manual due to the lens lengths and crazy distances you need at 50X.

http://s449182328.websitehome.co.uk/focal/dl//Docs/FoCal%20Test%20Distance_1.1.pdf

On line calculator. I did adopt FoCal and it is pretty accurate. I just try to level both the target and cam as best I can. It does really matter because if you MFA at different distances you will get different results thus Canon's at location recommendation which is impractical unless you are shooting in a studio at the same distance all of the time. The newer cameras that have a short and long end settings helps with that problem.

http://www.reikan.co.uk/focalweb/index.php/online-tools/test-distance-target-size-calculator/

Thanks for the link. Had seen that one before, but wanted to understand the thinking bedind the calculations .

Cheers,