December 22, 2014, 04:41:18 PM

Author Topic: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?  (Read 1669 times)

PhotoCat

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After reading all the rave reviews on this lens, I ordered one myself from B&H.
It is indeed quite a sharp lens even close to the edge at f2.8. My copy is vy sharp on the left
edge and just a little soft on the right edge. At f2.8 on a 5D2, I am happy with it.

However, upon further inspection, I have noticed that there is a funny "hysteresis" on the manual
focus ring. Let me explain: I used live view 10x to establish critical focus on a subject about 15 feet away.
After I turned the focus ring clockwise to achieve critical focus, I noted down the distance scale reading e.g. 10ft.
With the same subject,  I turned the focus ring anti-clockwise to achieve critical focus once again. The distance scale reading is now 4ft! 
I would have expected the distance scale reading to be the same whether I turned the focus ring clockwise or anticlockwise but I was wrong!

Well, that was my first copy of the Rokinon 14mm! B&H was great to give me an exchange.

After more than 2 weeks, I finally got my replacement and I tested the focus ring once again.
It is much better than the 1st lens but there is still some hysteresis on the focus ring.
e.g. anticlockwise to read 10ft and clockwise to read 7ft.

I have also noticed that the reading would become more consistent if I first turn it all the way to infinity and than
turn it clockwise only to achieve critical focus.  However, if I passed the critical focusing point and tried to turn the ring anticlockwise to back track the focus, this hysteresis effect is noticeable again.

So my question is: should I send it back again or live with it?  Is it normal for this Rokinon 14mm lens?
Anyone has similar experience to share?

Any help would be appreciated!
« Last Edit: July 02, 2014, 11:22:01 PM by PhotoCat »

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Jim Saunders

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #1 on: July 02, 2014, 11:28:27 PM »
In a machine tool it would be called backlash, but one perspective is that if you can get usable shots then living with it is an option.  Whether or not you want to is your choice, depends on what you paid for it compared to its competition I guess.

Jim
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distant.star

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #2 on: July 02, 2014, 11:45:39 PM »
.
Did you buy a lens to make pictures or to measure distances?
You're offended? Oh, really! Life IS offense -- get used to it.

PhotoCat

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #3 on: July 03, 2014, 12:16:37 AM »
.
Did you buy a lens to make pictures or to measure distances?

A repeatable distance readout is vy important to me for this manual focused lens. I lot of time I need to preset focus on the hyperfocal distance or on a particular distance and live view maybe too troublesome to use for every shot.
The absolute distance reading on the lens is not vy important but consistency is.

I have a workaround already, which is to "reset" the lens to infinity every time I focus but I am just
wondering if this copy is a lemon or it is a normal behaviour for this lens.

Tks Jim for pointing out backlash. Just wondering if it is normal for this lens. Tks!
« Last Edit: July 04, 2014, 01:35:46 AM by PhotoCat »

Djaaf

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #4 on: July 03, 2014, 08:59:01 AM »
A repeatable distance readout is vy important to me for this manual focused lens. I lot of time I need to preset focus on the hyperfocal distance or on a particular distance and live view maybe too troublesome to use for every shot.
The absolute distance reading on the lens is not vy important but consistency is.

I have a workaround already, which is to "reset" the lens to infinitely every time I focus but I am just
wondering if this copy is a lemon or it is a normal behaviour for this lens.

Tks Jim for pointing out backlash. Just wondering if it is normal for this lens. Tks!

Errh... Isn't that the effect of the huge DOF you got with this lens ?

Approaching focus from 30cm to 2m or from infinity to 2m is different.

Since you may well have a DOF of half a meter or something like that even a f/2.8, you'll achieve focus with at least a meter of difference on the scale if you try to achieve it from min focusing distance or from max focusing distance.

So, to me it seems quite normal, but i may be wrong. :)

Djaaf.
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lintoni

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #5 on: July 03, 2014, 09:07:19 AM »
The depth of field on a 14mm lens is so large that it's not surprising that you're achieving 'critical focus', even at f2.8, within such a large spread. Using a hyperfocal distance of 4.5m (approx 15ft) means everything from just over 1.5m (approx 5ft) to infinity should be in focus, at f2.8.
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Logan

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #6 on: July 03, 2014, 10:03:38 AM »
sounds like backlash to me. can you rotate the focus ring without moving the lens elements, if so, by how much? a helicoid(?) and a ballscrew are basically the same thing, i wonder if you could tighten something up inside the lens to fix it, maybe machine some new pins or whatever rides in the helicoid (i know its not really a helicoid but ive seen it called that i think?)

could also be the large DOF, although if you were testing on a ruler you should have been able to center the focus and avoid the large "critical focus" range.

what does hysteresis mean? where do you know such a word from?

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #6 on: July 03, 2014, 10:03:38 AM »

PhotoCat

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #7 on: July 03, 2014, 11:40:49 AM »
Thanks all for your help! So I went back to test the lens again just to be sure.
Lens set to f2.8 and with a subject 16 ft away in live view x10 in the centre.

It is definitely not a large DOF issue because the critical focus point is reasonably sharp on the
subject. I can easily centre the focus without a doubt.

The problem arises when I try to pull focus by turning the ring in one direction
but I passed the critical focus point a little bit then I
tried to turn the ring in the reverse direction to re-obtain critical focus.

The more I do this the more I find the distance readout (during critical focus) all over the place
from 4ft to 5ft to 6 ft to 8ft to 10ft.


i.e.
hit focus in one direction (readout 6ft) -> passed focus a little (may be half an inch rotation on the ring) -> reverse rotation ->
hit focus again in reverse direction (readout 8ft)  -> passed focus a little  -> reverse rotation -> hit focus again (readout 9ft) etc


So far, I think Logan's theory makes most sense: (also Jim)  (Many thanks!)

"sounds like backlash to me. can you rotate the focus ring without moving the lens elements, if so, by how much? a helicoid(?) and a ballscrew are basically the same thing, i wonder if you could tighten something up inside the lens to fix it, maybe machine some new pins or whatever rides in the helicoid (i know its not really a helicoid but ive seen it called that i think?)"

Yes, I can rotate the focus ring a little (about 1/4 inch on the ring) but the lens element is not changed. (verified
by live view)  This happens at the onset when I reverse the focus ring rotation after achieving critical focus.


Hysteresis is a term used in Electrical Engineering...  but the wiki explanation seems appropriate:

http://en.wikipedia.org/wiki/Hysteresis

"Hysteresis is the dependence of the output of a system not only on its current input, but also on its history of past inputs. The dependence arises because the history affects the value of an internal state. To predict its future outputs, either its internal state or its history must be known.[1] If a given input alternately increases and decreases, a typical mark of hysteresis is that the output forms a loop as in the figure."


My 1st  copy was much worst in "backlash" (much larger "play").   This makes me wonder if this is a
common problem to this lens.  Has anyone else seen this problem b4 on this lens?
« Last Edit: July 03, 2014, 11:57:09 AM by PhotoCat »

sama

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« Last Edit: July 03, 2014, 09:10:03 PM by sama »

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #9 on: July 03, 2014, 11:08:28 PM »
The inaccuracy and variability of the focus ring is well known.  Its the least of your woories.

Logan

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #10 on: July 03, 2014, 11:34:03 PM »
if it is plain backlash, you should be able to get repeatablility by always coming at it from the same way. the focus scale may line up better one way than the other, if so, use that direction. does that make sense? if you focus from 9' then 10', and it is actually 10', then always focus short first before using the focus scale.

Frodo

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #11 on: July 03, 2014, 11:39:47 PM »
I have one of these lenses.
The focusing ring is very smooth and there is no backlash.  I think you would feel backlash when twisting the focus ring back and forth.
It is true that there is variability with the position of the distance scale in relation to the actual focus.  My first lens was about 1cm off along the distance scale.  The focus ring seized and I got a new one under warranty - this one is spot on.
I suspect the issue is the huge depth of field.  I would not rely on differences on the screen even at 10x.  Take a photo where you see "critical focus" when focusing at 10x from the minimum point and another focusing from infinity and a third in between.  Then examine the images on screen (or print).  That is the critical test.  I suspect that there will be little difference between the three images.
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PhotoCat

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #12 on: July 03, 2014, 11:48:55 PM »
Will this help ??


http://www.videofy.me/emadeloc/755452

Wow! Thanks a lot Sama! This is vy informative. This fix is less intrusive than other fixes I have
seen on youtube. (open up the lens and add a shim etc)

If I had an old or 2nd hand lens, I would do this right away for sure.
But this is a brand new lens... and I have a chance to exchange another one too...

So my question is:  Do all brand new Rokinon/Samyang 14mm lenses have this problem?
If so, I will live with it for now and fix it later when I feel more comfortable messing with it.

If not, (i.e. there are many "good copies" out there directly out of the box), I would try to
exchange it once again and hope for good luck.

What do u think I should do?  Live with it and fix it later?  Or roll the dice and try another exchange,
hoping for a good copy?  Anyone has a good copy right out of the box?

I am so surprised that this lens is so poorly calibrated, yet the optical quality is not bad at all...
My 2nd copy is vy sharp on the left edge & centre but a little soft on the right edge about 10% of the frame.
Not bad for a $350ish lens!





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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #12 on: July 03, 2014, 11:48:55 PM »

PhotoCat

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #13 on: July 04, 2014, 12:15:25 AM »
if it is plain backlash, you should be able to get repeatablility by always coming at it from the same way. the focus scale may line up better one way than the other, if so, use that direction. does that make sense?

Yes, it makes perfect sense. Good point. Thanks again Logan for the tips! I will try that. As I had mentioned b4, the distance readout is much more
consistent if I first "reset" the lens to infinity and then turn it clockwise only. So I have to try again by resetting the lens for the other end (short end) of the scale and then turn it counter-clockwise only. 
Compare the 2 and see which one gives me a better reading and consistency.

if you focus from 9' then 10', and it is actually 10', then always focus short first before using the focus scale.

I found that I can't just go from 9' to 10' to obtain more consistency. I had to go all the way to 0.28m first (the min focus distance)  and then turn it anticlockwise only, in order to obtain better focusing consistency.  It is more work starting from 0.28m then from infinity because of the large rotation distance of the ring at short distances.

Well, this workaround works for now but I am not so sure if it will work in a year's time because some screws
might get more lose.  Just don't know...    Same question again: should I exchange it or should I live with it??

Thanks again for everyone who has contributed!

PhotoCat

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #14 on: July 04, 2014, 12:22:00 AM »
I have one of these lenses.
The focusing ring is very smooth and there is no backlash.  I think you would feel backlash when twisting the focus ring back and forth.
It is true that there is variability with the position of the distance scale in relation to the actual focus.  My first lens was about 1cm off along the distance scale.  The focus ring seized and I got a new one under warranty - this one is spot on.


Tks Frodo for the info. Good to know there r good copies out of the box...

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Re: Rokinon/Samyang 14mm f2.8 Focus Ring "Hysteresis" - is this normal?
« Reply #14 on: July 04, 2014, 12:22:00 AM »