Lens Decentering Question

[wogears]

<p>Hello!</p>

<p>Went out on a very dark day with my D600 and the Rokinon (Samyang) 85mm f1.4. I was using fairly wide apertures (f2-2.8) to get fast shutter speeds at lowish ISO. When I looked at the results on my computer, I saw some odd unsharpness. Fortunately, I had actually taken a shot of an interesting <em>brick wall</em>! The image was sharp in the center, but the upper left and lower right corners were pretty bad for about a quarter of the way into the center, while the upper right and lower left were very good. I am assuming that this is a decentering issue rather than curvature of field? I am also assuming that I should shoot at f5.6-8 if I want better corners?</p>

<p>Thanks,<br>

Les</p>

[mag_miksch]

<p>and you are sure you shot exactly at a right angle?</p>

[rodeo_joe1]

As Mag says, you need to shoot with the camera

plane-parallel to a flat subject to check for

decentring. If the camera was parallel to the wall,

then yes, what you describe above would indicate

decentring.

 

Another good test is to take a shot, then revolve the camera 180 degrees around the lens axis (I.e. turn it upside down), then take another shot with identical framing. If the same corners are unsharp (relative to the camera and swapped on the subject) then that pretty much confirms decentring.

 

My first sample of Samyang 24mm f/1.4 was quite badly decentred. It was replaced without question by the supplier.

[wogears]

<p>I was careful to have the edge lines parallel in the viewfinder, so the alignment may not have been perfect, but it was very good. I'll go out with a tripod and the electronic release when it gets warmer <g>, and do a real test, but I'm confident that the problem is with the lens itself.</p>

[allan_jamieson2]

<p>I've seen something similar a few years ago with a new Samyang 14mm lens, took it out to test it with very simple setup, heavy tripod, D800 with mirror locked up and lens set at f8 and infinity pointing towards a bend in a river with trees on the far side. I wasn't expecting to see parts of each image out of focus all in the same area, a lens like that should have been in focus all across that scene, it went straight back to Amazon, end of story. </p>

<p>Harder to be sure with an 85mm lens than the 14mm lens though but a bit more testing would soon confirm this if you put it on a tripod and take a few different images at different apertures, if each image is consistently weak in the same areas it would seem likely that you do have a decentred lens.</p>

[steve_bingham]

<p>You really should test it! A simple method is a newspaper on your garage door. Be very careful you are at <strong>90 degrees from the door. Brick walls are fine, but being exactly 90 degrees is vital.</strong></p>