Wim Koornneef
Posts: 93
Location: Ouddorp, Netherlands
Registered: 15 Nov 2007
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An easy procedure to set the right focus distance of a Tokina 10-17 zoomlens
Posted: 30 May 2008 at 13:07 GMT
updated: 30 May 2008 at 13:28 GMT
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Hello Forum,
Setting a fish lens to the right focus distance is not an easy job.
We know that infinity isn't useful but besides that we don't know what the right distance is.
Just like many others I had in the past a difficult job to find the correct setting for my fish lenses, and especially for my shaved Tokina 10-17mm.
One problem is that the focal plane distance of a fish lens isn't exactly the same for all light entry angles.
The impact of this is that an object in front of the lens, and an object close to nadir with the same distance to the lens, will have a different focus distance.
So it is just if the object in nadir is more closeby, that's weird, but a fish lens is a weird thing 
The Tokina 10-17 mm fish zoom shares this problem with the Nikon 10.5, both lenses are build different than other fish lenses like the Sigma 8mm en the Peling 8mm lens, those lenses suffer less from this focal plane issue.
Besides the focal plane issue there is another and more common problem for zoom lenses, it is the fact that the DOF of the lens depends on the chosen focal length.
It makes quite a difference regarding the DOF if you shoot @10 or @17mm.
This table tells a lot about the DOF of all kind of lenses, try it out and play with it, it will give some insight information about the DOF and the hyperfocal distance of lenses:
www.dofmaster.com/doftable.html
BTW, if you have a Mac, this Widget is very useful.
i360.tv/DOFC_widget_en
Because a fish lens is a different kind of lens the table is not 100% compatible for fish lenses, only for objects exactly in front of the lens the table is useful but for all other angles it is not accurate so the table is nothing more then a fine guide to get a better insight about hyperfocal distance and DOF of lenses and provides some starting points for experiments.
My goal was to get an easy "One focus setting fits all" for my Tokina 10-17 lens and after a lot of trying out I found an adjusting method that is repeatable and is working for me and I guess for others as well.
Here it is:
1) I set my gear (NN5 + EOS 5D) on a tripod in front of a wall at a distance of approx. 115 cm from the wall to the sensor mark on the body the camera. The height of the tripod is the minimum height I use, from floor to lens approx. 150 cm.
2) I set the focal length to 17mm, I switch the Auto focus knob of the lens to on and I push the release button half way down. The camera determines the right focus distance and confirms it with a beep sound,
3) After this initial adjusting I push a couple of times more and while I do it I check if the distance ring is not moving and/or is hunting for the right spot. If it is moving or hunting then the contrast and/or light on the wall isn't sufficient. Some extra light and a brochure or newspaper on the wall will fix this issue.
4) When I am sure that the distance is OK then I switch the Auto focus knob on the lens to off, and I put a piece of black small PVC tape at the left and the right side of the focus distance ring to fix the position of it.
5) That's all, job is done 
This simple procedure is repeatable and it works, I have sharp images from center to edge in all focal length settings !
Of course the sharpness depends on the right aperture, here are some numbers:
@10 mm - minimum aperture setting is f6.3
@11 mm - minimum aperture setting is f7.1
@12,4 - minimum aperture setting is f8
@14,6 - minimum aperture setting is f11
@17 - minimum aperture setting is f13
So when I use aperture f13 I can shoot in all possible focal lenght and I always get sharp images from the center to the edge.
For me this is important because when I change from focal distance I don't need to care about re-focusing and I can fully concentrate on other issues.
BTW, I do the adjusting procedure with the upper arm of the NN5 in horizontal position and I check the sharpness with the camera 25 mm forwards out of nodal point and a downtilt of the upper arm, the downtilt depends on the focal distance from 0 degree to -20 degree.
The reason for the downtilt and the nodal point shift is to point one edge of the image to nadir, that is the point where most sharpness issues occurs.
I hope this procedure is of some use for you, when you have a Nikon 10.5 I guess you can use the same procedure and then f6.3 or around will be fine for you.
Best,
Wim.
PS:
I will anticipate on your question: Why did you choose a distance of 1.15m from the wall ?
The answer: I found this distance by doing some tests from 1 meter to 1.8 meter and by comparing the results.
With 1.15 m the overall range of sharpness across all possible combinations of aperture settings and focal distances is best.
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Shantic
Posts: 312
Location: Puerto Vallarta, Jalisco, Mexico, Mexico
Registered: 12 Apr 2005
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Re: An easy procedure to set the right focus distance of a Tokina 10-17 zoomlens
Posted: 30 May 2008 at 15:38 GMT
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Nice 1st post I bet this will come in handy for many, as I have tried a similar approach using my Sigma 8mm, and it works
Thanks
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Wim Koornneef
Posts: 93
Location: Ouddorp, Netherlands
Registered: 15 Nov 2007
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Re: An easy procedure to set the right focus distance of a Tokina 10-17 zoomlens
Posted: 30 May 2008 at 18:29 GMT
updated: 30 May 2008 at 18:32 GMT
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A small addition:
The Auto focus of the camera is set to the center point so all other measure points are disabled. This prevents that the distance measurement is equalized.
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bigwade
Posts: 569
Location: Netherlands
Registered: 19 Oct 2005
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Re: An easy procedure to set the right focus distance of a Tokina 10-17 zoomlens
Posted: 30 May 2008 at 22:33 GMT
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Hi Wim,
welcom to the forum !
About the autofocus.
It works with the 5D and Tok 10-17. @17
I does not work with the 400/450D series.
For the 450D it is not an issue because with live view you can focus very precise.
With the 400D you have to make tests.
Computer connected is the easiest way.
The 400/450D AF sensor isn't good at lenses lower than 16mm.
IE my 10-22 was completely wrong AF at 10mm at the 450D,
Bad results !!
Have Fun.
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Thomas Krueger
Posts: 224
Location: Genoa, Italy
Registered: 3 Mar 2006
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Re: An easy procedure to set the right focus distance of a Tokina 10-17 zoomlens
Posted: 31 May 2008 at 16:11 GMT
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Thanks Wim, I'm just starting with my fresh shaved Tokina on the 5D!
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Wim Koornneef
Posts: 93
Location: Ouddorp, Netherlands
Registered: 15 Nov 2007
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Re: An easy procedure to set the right focus distance of a Tokina 10-17 zoomlens
Posted: 31 May 2008 at 17:13 GMT
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Hello Thomas,
Success with your lens, I am sure when we meet in Prague that you have lots of good experiences with it.
See you,
Wim.
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EmEss
Posts: 56
Location: Germany
Registered: 19 Nov 2007
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Re: An easy procedure to set the right focus distance of a Tokina 10-17 zoomlens
Posted: 3 Jun 2008 at 17:27 GMT
updated: 3 Jun 2008 at 17:47 GMT
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Hi folks,
if one of you is interested in a german translation of Wims initial post, here it is...
tinyurl.com/6jgrbl
Regards, (translating) Martin
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