How do I tell if I got a de-centred lens? (Samyang 14mm f/2.8)

[YellowJersey]

Hi guys,

So I'm about to pull the trigger on the Samyang/Rokinon/Bower 14mm f/2.8 and have heard a number of people complain about getting a decentred lens. This appears to be a common problem. So, how do I tell if I got a de-centred lens?

Lens Rentals.com has a test for this, but they say it's not great for ultra-wide.

Any ideas?

 

[meywd]

take a shot, if its sharp on one side and soft on the other then its de-centered.

 

[docsmith]

Yep...print a good target such as:

http://www.graphics.cornell.edu/~westin/misc/res-chart.html

Do everything you can to make sure you are square to the target: image sensor and target are parallel, with the lens pointed right at the center, same height, etc.

The corners should be equally sharp. If not, you may have a decentered element.

 

[Loswr]

My first copy was decentered, second one was a keeper. I set up on a tripod pointed down at a tile floor, using symmetry of the tiles to ensure the sensor was parallel to the floor. I then took pics with an ISO 12233-type chart in each corner, both wide open and at f/8.

 

[Mt Spokane Photography]

Some of the Samyangs were so bad, that it does not take a expert to see it. Mine was worse than a $10 garage sale lens.

The left side is distorted.

Here is a shot with my EF 15mm f/e of the exact same spot a few minutes later. Fish Eye lenses actually produce a wider angle of view. Both were on my 1D MK III and focus was set to about 3 ft.

 

[martti]

I often get the tips of my Asics in the picture with the Rokinon.
Should I send the lense back or buy another brand of shoes?

 

[Mt Spokane Photography]

I often get the tips of my Asics in the picture with the Rokinon.
Should I send the lense back or buy another brand of shoes?

Personally, I'd send the piece of garbage back!

 

[dcm]

Roger @ LensRentals posted a short article on this topic. You can find the test chart online if you wish to print your own.

http://www.lensrentals.com/blog/2012/05/testing-for-a-decentered-lens-an-old-technique-gets-a-makeover

 

[YellowJersey]

The Lensrentals tip says that their test doesn't work that great for ultra wide. Would putting a teleconverter on it help?

I hear great things about the Samyangs, provided you can get a good copy, which seems to be the sticking point.

 

[Mt Spokane Photography]

The Lensrentals tip says that their test doesn't work that great for ultra wide. Would putting a teleconverter on it help?

I hear great things about the Samyangs, provided you can get a good copy, which seems to be the sticking point.

If the image looks good, that's all that counts. A badly decentered one is pretty obvious. One side looks sharp, the other oof. If you can't see a issue, then there is no worry.

 

[martti]

EVen if you get a good copy of the 14mm f/2.8 Rokinon/Samyang, it still has a generous amount of distortion that you have to correct if you have something with a straight horizontal line in your picture. I found a lens profile for the LR which is doing an OK job though it is for the 5DII.

see here http://joopsnijder.blogspot.com/2012/03/lightroom-lens-profile-for-samyang-14.html