October 12, 2014

14mm Samyang focusing issues

Although the 14mm Samyang is quite an interesting and creative lens, I have experienced various focusing issues with it. I also found on several web sites that these are not uncommon problems, so I decided to make a few tests on my own, in order to understand how to use this lens at its best.

Please note that the following considerations apply not only to 14mm Samyang, but also to 14mm Walimex, 14mm Rokinon, etc.. This lens is in fact rebranded under different names, but it is exactly the same.

Focus scale accuracy test

The focus scale of the 14mm Samyang has 10 markings, corresponding to the following distances (in meters): 0.28, 0.30, 0.35, 0.40, 0.50, 0.70, 1, 2, 3 and infinite ().

In order to verify the accuracy of the short/medium range markings, I put my Nikon D5100 on a tripod, in front of a focus target (a small postage stamp glued on a wall) and measured the distance from the target to the focal plane (the position of the focal plane is marked on the top of the camera as a circle crossed by a bar).
Positioning the camera at different distances from the target, I have been able to check the accuracy of focus directly on the camera display (using Live View at its maximum enlargement).

I made 4 tests, positioning the camera at the following distances from the target: 0.30m, 0.50m, 1m, 2m.

For each distance I adjusted the focus slowly, starting from the right position (0.28m) of the focus scale, looking at the image of the target on the camera display, until the target appeared sharp.

At this point, I annotated the marking on the focus scale. Comparing the above annotations with the actual target distances, I was able to verify that for all the 4 tests the markings on the focus scale were accurate.

Focus scale hysteresis

I also have read on the Internet about people experiencing issues related to the focus scale hysteresis.
In practice, this defect causes the lens to be focused on a subject at different markings of the focus scale, depending whether you rotate the focus scale from macro to infinite or in the opposite direction.
In order to verify the presence of hysteresis, I repeated the previous tests, adjusting the distances starting from the left position of the focus scale (infinite). I obtained totally different results, as shown in the table below.

distanceof the target (meters)
marking on the focus scale for which the target is in focus
(when rotating the focus ring from macro to infinite) 
marking on the focus scale for which the target is in focus
(when rotating the focus ring from infinite to macro) 
0.30
0.30
0.30
0.50
0.50
0.35-0.40
1
1
0.50
2
2
0.70

This confirms the weird behaviour of this lens: if you focus a subject rotating the focus ring in two different directions, the subject get in focus on two different focus markings!

However, as long as you always rotate the focus ring from right (macro) to left (infinite), the markings on the focus scale are accurate.

Infinity focus test

Testing the accuracy of the focus at infinite is a bit more tricky and is better carried out making a few real shots.
I used the following test procedure:

  • in a clear day, I took several pictures of a generic scene depicting objects at a sufficient distance to be considered at infinite;
  • all pictures were shot in RAW, with my Nikon D5100, at 100 ISO and at the widest aperture of the lens (f2.8);
  • I developed the RAW images in Lightroom and, for each of them, I cropped a 512-pixel square area near the center of the scene.
  • no sharpening was applied at any stage of the process.



I made 5 shots, testing 5 different focus ring positions:

A (extreme left position)

B

C

D

E (focus set to 3 meters)
The first 5 pictures were taken progressively moving the focus ring from the extreme right position (macro) to the extreme left (infinity).

Here are the results:

Focus position A (infinite)

Focus position B
Focus position C
Focus position D
Focus position E (3 meters)

The focus position C seems to produce the most sharp image:

The "real" infinite position of the focus ring

Is the above marking in Samyang's intentions the infinite focus position?  If so, I wonder why they allowed the focus ring to rotate beyond that position.

In order to check the effect of the focus ring hysteresis, I repeated the test, with the following setup:
  • first, I moved the focus ring from the extreme left position (~infinite) to the middle of the scale (~0.5-0.7 meters);
  • at this point, I took 5 more pictures progressively moving the focus ring to positions E (3 meters), D, C, B and A (extreme left).
Here are the results:

Focus position A (with hysteresis)

Focus position B (with hysteresis)
Focus position C (with hysteresis)
Focus position D (with hysteresis)
Focus position E (with hysteresis)
The sharpness is not even comparable to the one of the first set of pictures.

Exploiting the huge DoF

Apart from the above listed issues, focusing a manual 14mm lens is not easy. At a very first sight everything looks rather sharp in the viewfinder, so you are tempted to underestimate the focus, only to discover later that the image is not sharp at all.

However, under some circumstances you can exploit the very extended depth of field of this lens.
The following tables show the hyperfocal distance of a 14mm, for two different circles of confusion:

Aperture
Hyperfocal distance (m)
Closest distance in focus
2.8
2.3
1.2
4
1.7
0.9
5.6
1.2
0.6
8
0.8
0.4
11
0.6
0.3
16
0.4
0.2
22
0.3
0.1
Full-frame (0.030mm circle of confusion)

Aperture
Hyperfocal distance (m)
Closest distance in focus
2.8
3.5
1.8
4
2.5
1.3
5.6
1.7
0.8
8
1.3
0.7
11
0.9
0.5
16
0.7
0.4
22
0.5
0.3
DX format (0.020mm circle of confusion)

From the above data, it is rather evident that the DoF can be better exploited with advantage, especially when working in full-frame (either digital or film).

Conclusions

The 14mm Samyang shows some noticeable hysteresis when adjusting the focus ring. Moreover, the left-most end of the focus ring is beyond the actual infinite focus position.

If the framed scene is bright and the subject has enough contrast, you should be able to manually focus through the viewfinder.

However, in many situations you have to rely only on the focus scale. In such a case, you should follow 4 basic rules:
  1. always adjust the focus rotating the focus ring from its right-most position (macro) to the desired distance;
  2. take into account the real position of the infinite focus marking;
  3. when focusing at short distances, always measure the distance from the subject to the focal plane of the camera (and not from the subject to the lens!);
  4. whenever possible, rely on the very extended depth of field.

References