How can I see if my lens is off center?

Claes · April 25, 2019, 5:30pm

Now, relax. it is NOT complicated at all.
Pick the best Siemens Stern from here: siemens stern at DuckDuckGo
Focus according to the instructions given above.

Quote myself:

If the lens is in proper alignment and pointed directly (lineup isn’t critical, you can eyeball it) at the star, the white and black circles remain circular as they blur. But if the lens is decentered or significantly tilted the center blur will ‘flare’ out in one direction or another as you defocus .

Rico · April 25, 2019, 5:37pm

So I need to setup my camera on a tripod, focus on that stern

and then manually defocus? If it’s going out of focus equally, the lens is fine and if it goes out of focus while flaring out, something is wrong?

Claes · April 25, 2019, 5:44pm

Sounds correct.

Rico · April 25, 2019, 5:50pm

I just recorded a video for you.

https://www.youtube.com/watch?v=W3Ml9JC7mv4&feature=youtu.be

Hope you can see it.

What are your thoughts?

Claes · April 25, 2019, 7:10pm

Avonds, Rico!

Here is an example which I just shot using a “bad” lens.
Actually, it is a tilt-shift lens, pointing downwards (to simulate your dropped lens).

There are two things one immediately notices:

the bottom “beams” are sharp, while the top beams are fuzzy.
the distance from the center to the top “fuzzy circle” is 180 px,
while the distance from the center to the bottom “fuzzy circle” is 120 px.

Verdict: this lens is not aligned!

Do you have a nice, even brick wall available for a test shot?

MvG
Claes in Lund, Zweden

Rico · April 25, 2019, 7:26pm

Goedenavond ,

I can try in the morning. It’s dark now. Which distance should I use?

Claes · April 25, 2019, 7:30pm

Uitstekend!

I believe that a large aperture would be the best
(giving you a shallow DOF).

MvG
Claes in Lund, Zweden

Rico · April 25, 2019, 7:32pm

Kommer att bli bra!

How did the film look though?

Claes · April 25, 2019, 7:34pm

Sorry, I could not draw any conclusions at all from it

Ofnuts · April 25, 2019, 9:01pm

You are assuming that the back and side of the camera are at right angles with the theoretical axis of the lens. AFAIK this is only true for 1) the sensor (but it’s out of reach) and 2) hopefully, the bottom of the camera.

For a proper measure I would:

find an horizontal surface
Put the camera in an L bracket (Arca-swiss, the kind you use to set the camera for portrait mode on a tripod), but mount the camera so that the side of the L is behind the camera, not on the side.
Camera set on the bottom of the L check that the vertical diameter of the lens is indeed vertical
Camera set on the side of the L, check that the diameter of the lens which is parallel to the axis of the L is indeed horizontal.

Measure

(also contributing to the “pixls.us members’ kitchen stand-off”) (L-bracket dangerously on the edge of the counter due to the safety pin at one end)

Rico · April 26, 2019, 6:04am

Thanks a bunch, unfortunately I do not own an L bracket. I do have a smallrig cage. I could try with that I guess

Rico · April 26, 2019, 7:39am

In order:
12mm (FF24)
25mm (FF50)
60mm (FF120)
(M4/3)

Claes · April 26, 2019, 11:48am

Hi again, @Rico.
Which lens did you drop?
Could it be the 12mm AND does it look like that lens points upwards (in your opinion)?

Because the 12mm shot is the only one that I spot
a difference in focusing from top to bottom…

MvG
Claes in Lund, Zweden

Rico · April 26, 2019, 11:56am

Hello Claes,

I used the Leica 12-60mm F2.8/F4.0 lens. Focus was set in the middle (with multiple AF points).

Would you like me to shoot the same picture with a single focus point?

Rico · April 26, 2019, 1:14pm

I just did.

Same focal lengths:

12mm
24mm
60mm

Made sure the camera was level. F-stop is 6.3 (60mm) to 7.1 (12mm) and I used 1 AF point (exactly in the center)

Oh yeah, and to answer your question: Yes it was the 12-60mm lens that I’ve dropped while it was attached to the camera. The wind knocked it over while I had it on a tripod.

Claes · April 26, 2019, 5:25pm

Avonds,

To me, they all look OK.
Does the lens autofocus the way it should?
No funny sounds?

If so, I am out of fresh and/or brilliant ideas…

Mvg
Claes in Lund, Zweden

Rico · April 26, 2019, 5:41pm

Yeah everything works like it should. Just shot another tutorial with it. So I guess the lens is fine then? No need to take it in?

Claes · April 26, 2019, 5:49pm

If you do not find anything wrong with it: no need to send it off to service.

Rico · April 26, 2019, 7:52pm

Thanks a lot for your help Claes. I feel confident enough not to send it in for repairs!