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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Another back focus question...

Abraham Lim

Abraham Lim

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So I installed the Nikon mount and set the focus chart at 3ft.

20mm Nikon 2.8 slight offset
50mm Nikon 1.4 dead on
50mm Nikon 1.2 dead on
85mm Nikon 1.4 dead on
14mm Samyang 2.8 slight offset

I did not adjust the back focus, because the longer lenses will have more critical focus issues. I assumed if I corrected for the 24mm, the longer lenses would all be offset. Is there any other solution or is it just some lenses have been knocked around a little more and will just be off.

Any advice would be appreciated, thanks.
 
Well, the depth of focus is much shallower with the 20mm and the 14mm, since they are wide angle lenses. So, you could have a slight deviation in your flange focal distance that would only show up on those lenses.

Question: are the 20 and 14 out by the same amount, in the same direction? If so, I suggest that you change the flange focal distance so that they are correct, then re-measure the whole range. Use the 14mm first.
 
I guess I have to join the gang.
I`m standing here trying to perfect my back focus.
I have been able to make the numbers on my lenses spot on, but I seem to have a problem
with infinity.

My lens goes to 2 meters and then infinity, I have it rigged up about 3 meters apart.
If I turn the focus ring all the way it gets noticeably soft, but if I then pull it back just a little bit it seems to reach infinity.
It`s really annoying because I love to be able to just pull the focus ring all the way over and shoot if I`m in a hurry.

It`s pretty weird since I can get all my lenses to be spot on except for the infinity.
Any help or advice would be appreciated!

EDIT:
I can adjust back focus until I reach infinity, but then my numbers are off.
 
Last edited:
Stian, I may not be correct about this, so take it with a grain of salt. I think still lenses are made to focus past infinity because of thermal expansion. Someone with more knowledge about this may be able to jump in.
 
Steve Sherrick said:
Stian, I may not be correct about this, so take it with a grain of salt. I think still lenses are made to focus past infinity because of thermal expansion. Someone with more knowledge about this may be able to jump in.
Click to expand...

Thanks!
That does make some sense, but it also makes me wonder why they dont act like that on my DSLR.
I would love some more info about this!

I have my R1 hooked up to a 24" monitor, maybe I should do the same with my DSLR to see if I can notice anything I havent before.
 
Steve Sherrick said:
Stian, I may not be correct about this, so take it with a grain of salt. I think still lenses are made to focus past infinity because of thermal expansion. Someone with more knowledge about this may be able to jump in.
Click to expand...

I have now done some tests on my DSLR, and you are probably absolutely right.
These are brand new lenses I bought to use on both R1 and DSLR so I guess I havent
used them enough on my DSLR to notice yet. I`m pretty sure this wasnt an issue on my old Nikon DX lenses.
I bought new ones to get aperture rings.

I still dont like it, but to know there is nothing I can do actually helps in a weird way.

Thanks a lot Steve!
 
Abraham,

I just want to make a clear point:

Depth of Field is the area in focus "in front" of the lens, Depth of Focus is the area in focus "behind" the lens.

For Back focusing your R1, you are dealing with Depth of Focus. With Wide Angle lenses the Depth of Focus is swallower than that of a Telephoto lens. So it is more critical to get the Wide Angle lens correct in the Back Focus, as the longer lens will have a larger Depth of Focus and should be ok.
 
It is correct that photographic lenses are constructed to focus a tad beyond infinity at room temperature, I got this confirmed by a retired Rollei engineer (who is repairing collector's items these days).

He also confirmed that flange distance adjustment is more critical for wide angle lenses.
 
Peter Borosh said:
Abraham,

I just want to make a clear point:

Depth of Field is the area in focus "in front" of the lens, Depth of Focus is the area in focus "behind" the lens.

For Back focusing your R1, you are dealing with Depth of Focus. With Wide Angle lenses the Depth of Focus is swallower than that of a Telephoto lens. So it is more critical to get the Wide Angle lens correct in the Back Focus, as the longer lens will have a larger Depth of Focus and should be ok.
Click to expand...

Thanks Peter for that clarification. So I should adjust the back focus or flange focus for the wide angle lenses, as the flange focus distance will be larger on the longer lenses? Thanks to everyone who posted. I was offline and in the air for a while.

I'm going to wait until I get home to do it.
 
Douglas Underdahl said:
Well, the depth of focus is much shallower with the 20mm and the 14mm, since they are wide angle lenses. So, you could have a slight deviation in your flange focal distance that would only show up on those lenses.

Question: are the 20 and 14 out by the same amount, in the same direction? If so, I suggest that you change the flange focal distance so that they are correct, then re-measure the whole range. Use the 14mm first.
Click to expand...

Hey Doug, the 20mm read at 2m and the 14mm Samyang read at 2.5. Also, got the rail mount quick so I was able to take it with me on the plane, thanks.
 
David Mullen ASC said:
Sounds like you may need to remark (using tape) the distances on some of your lenses.
Click to expand...

Thanks for the reply David, so you are saying I should just compensate for the offset on the lenses? I just assumed I would have to do that given that collimating seems rather daunting.
 
Douglas Underdahl said:
This video shows me swapping mounts to Nikon on Red One and checking flange focal distance:

http://www.youtube.com/watch?v=pRicc1HU44c

Also check out part one.

How far away is your chart from the sensor plane?
Click to expand...

Sorry Doug, just got back home from Shanghai. Used your video to install the Nikon mount. Will try to do flange focal distance check again by myself with the Samyang 14mm. the chart was 3 ft. away from the sensor plane for the first check.
 
So, the chart was 3 feet and the 20mm said 2 meters, and the 14mm said 2.5 meters? Those are way off and in the same direction, which does make me suspect the FFD is off.

If you find the correct FFD for the 20 and 14, then the question is, will the rest of the set line up? If so, then I'd say yes, you have 46.5mm aka correct FFD for Nikon.

If not, then you have to make some decisions based on what's what. Which is to say, if you can set the FFD so that, say, 5 out of 6 lenses are correct (and the wides are correct) then you can put new footage marks on the incorrect lens and be on your way. Lots of ACs put new footage marks on lenses, usually to add marks between existing marks or to make existing marks more accurate and this of course is very easy with a Red One and a large HD monitor - but you definitely want to get the FFD as correct as possible.

It's assumed by many that the distance marks on Nikon mounted lenses aren't very accurate, but in my experience, with the primes, they are really quite good.
 
Douglas Underdahl said:
So, the chart was 3 feet and the 20mm said 2 meters, and the 14mm said 2.5 meters? Those are way off and in the same direction, which does make me suspect the FFD is off.

If you find the correct FFD for the 20 and 14, then the question is, will the rest of the set line up? If so, then I'd say yes, you have 46.5mm aka correct FFD for Nikon.

If not, then you have to make some decisions based on what's what. Which is to say, if you can set the FFD so that, say, 5 out of 6 lenses are correct (and the wides are correct) then you can put new footage marks on the incorrect lens and be on your way. Lots of ACs put new footage marks on lenses, usually to add marks between existing marks or to make existing marks more accurate and this of course is very easy with a Red One and a large HD monitor - but you definitely want to get the FFD as correct as possible.

It's assumed by many that the distance marks on Nikon mounted lenses aren't very accurate, but in my experience, with the primes, they are really quite good.
Click to expand...

Yeah, that's what I was worried about when I first posted Doug, that the rest of the set would be off. But, I 'll try to get the 14mm Samyang dead on since it was off the most. I'm gun-shy of adjusting the back focus after a horror story from a new BT R1 owner where something fell off after he tried. He ended up sending the camera back to Red, which is not really an option for me in the middle of China. I have a focus chart here with me though and my Ruige monitor has 1:1 zoom in function, so I'll give 'er a go. What's the best distance for the chart from the focus plane?

Thanks
 
You shouldn't have to turn the FFD collar more than a little bit, clockwise/counterclockise. At most, from 9 to 3 o'clock, never more than that and certainly of course never one revolution.

What I've seen happend where the mount falls off, or becomes unregistered from the pins that keep the outer part of the mount from rotating is:

A) inexperienced user tries to adjust FFD with 85mm or longer lens (large depth of focus behind this lens, not good for accuracy)

B) they turn the mount around and around and don't see any change because the lens is too long and the chart is too far away or they don't have a good chart or aren't using the focus 1:1 thing

C) mount falls off or becomes unregistered.

You won't do any of these things, I know!

Best focus distance for chart (I like charts with Seimens star(s)) is the one where you have the most marks for it on your lenses. Usually three feet or one meter is marked on all your lenses and that's what I use You can usually get all the lenses to be within an inch and most dead on.

Some Acs get the FFD dead on (either by doing it themselves using this method, or with a pocket collimator, or by asking the rental dept to do it) then check every lens at multiple distances and adding/adjusting marks with Letraset and fine line marker.

The focus is off in dailies and the AC gets fired no matter what the excuse, is my experience.

Whew.
 
Oh, I should mention that the 1:1 focus punch in thingy is in the menu. I think on the Red One it's a button on the side of the monitor. I need to set up my Epic this way - love that feature.
 
You know back in our NYU days I started as an AC, but never collimated a lens. It was always done at the rental house, and I was never under the gun of a union shoot. I'm glad you're so sure I won't do anything from A-C but you have never seen me with Ikea furniture.

I should have spent more time in the equipment room bugging you. My 1:1 feature is built into the monitor, is that good enough? We used it on the R1 on the first check but on a 14mm, it was still tough to call spot on focus.
 
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