off center focus tips

[iridium7777]

could someone give me a tip on how to do clear off center focus especially when shooting wide open at around f2?

with my dslr i'd just point on which side of the sensor to focus on and that'd be the end of it. with my m6 obviously focusing on center is not a problem, but how to quickly and properly focus off center?

 

[merciful]

There are no tips: you simply focus and recompose. It all depends (of course) on the focal length and distance to the subject whether the recomposing will result in the subject being out of the zone of acceptable sharpness. If you're concerned, stop down.

 

[ferider]

i don't understand?

say i'm focusing on something one meter to my right, put it in focus and then turn left say 30degrees. the arc would now put the subject closer than 1 meter and it would be out of focus. i can see myself putting the subject at 1 meter in front of me, focusing, then taking a step to the left and take a picture. that way it would be in the focusing plane, but this seems like a tedeious way to do it?

What you say does happen but is so small that the focal point stays sharp within
the DOF of the lens.

For example, say you use a 50mm, 35mm format and you focus on the 1/3rd grid intersections.

- A 50mm lens has about 46 deg (diagonal) FOV
- which means you move by about (1/6 * 46 =) 7.8 degrees offcenter
- this corresponds to about 1% focus error, i.e. 1cm for 1m focus distance

At f2 this is well inside the DOF.

Best,

Roland.

 

[merciful]

Ah, there we have it: a technical explanation of why I'm never troubled.

At f2 this is well inside the DOF.

 

[ferider]

Allways wondered if it was noticable on the 35/1.2 Nokton. It should be ...

Roland.

 

[merciful]

Pretty sure I've caused it with the 85/2 wide and close.

Allways wondered if it was noticable on the 35/1.2 Nokton. It should be ...

 

[sepiareverb]

I love your explanations of this kind of stuff Roland. So simple even a moron can get it.

 

[iridium7777]

it's a 35mm lens. here's what i'm talking about.

i focused on her face, turned to my right so she would be to the left and took a picture. when it was developed you could see that focus moved to the wall, which was much more than 1cm, probably 20cm or so. (the photo is cropped, in the original image she is much more to the left)

how would one compose that?

What you say does happen but is so small that the focal point stays sharp within
the DOF of the lens.

For example, say you use a 50mm, 35mm format and you focus on the 1/3rd grid intersections.

- A 50mm lens has about 46 deg (diagonal) FOV
- which means you move by about (1/6 * 46 =) 7.8 degrees offcenter
- this corresponds to about 1% focus error, i.e. 1cm for 1m focus distance

At f2 this is well inside the DOF.

Best,

Roland.

 

[ferider]

Looks like something else is going on:
- either M6 and/or lens are not adjusted
- or this happened during scanning, how does the negative look, maybe it was not flat
- or you got distracted when shooting 🙂 Seriously, though, every so often I have a picture completely out of focus because I look at the patch wrong.

1 or 3 you should be able to check under controlled conditions / other photos.
For 2 check if the negative is sharp.

Roland.

 

[KM-25]

I have always wondered this as well, thanks for bringing it up.

I am going to do a test then using my 5D and 35 1.4 like my range-finder and see what I get wide open and at F2......

...I'm back.

24 1.4 @ 2.0, 30 inches, only in the shot with the subject to the very side is it a little back focused.

35 1.4 @ 2.0, 30 inches, pretty much the same but a tad more.

50 1.4 @ 2.0, 30 inches, a little back focus when moved one third to the right, pretty much back focused about two inches when brought all the way to the right.

So there you have it, you are not imagining it.

I own a 28 cron and 50 1.4.

I will be careful on the 50 from now on....

 

[Gary Sandhu]

More complicated than it seems.
Check here:
http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=004sHD

 

[Chris101]

Don't rotate around your neck, rotate around the nodal point of the lens (about where the diaphragm is.) Then the distance from subject to film won't change.