Red back focus

[Ryan Hamblin]

Does the back focus on the red tend to be accurately set from factory?

 

[jimhare]

On most but not all. Mine was a bit out, plus the back focus bolts were INCREDIBLY tight. Had a to get a heavy duty mother hex wrench to budge it.

Jim

 

[michael zaletel]

Adjusting my back focus made all the difference in the world.

-shooter

 

[Gabriele Turchi]

Guys i apologies for this maybe stupid question...

Does the back focus is needed only for Zoom?...I am using some nikon lense and the focus on infinity is a lot out...

PS:as you can see i have never performed a back focus operation!...

sorry

G

 

[martinnoweck]

gabriele,

backfocus of the camera has to be checked / adjusted regardless using a zoom or primes.

regards,
martin

 

[Shawn Booth]

And check BF often

 

[Chris Bell]

I wish RED would publish a document outlining the back focus procedure. Clear and concise documentation is badly needed.

Chris

 

[Brent Carter]

We are actually working on a "how to adjust your backfocus in the field" video....

In the meantime, hope this helps: http://www.youtube.com/watch?v=pRicc1HU44c

Doug did this as part of his installing the Nikon Mount tutorial but it obviously applies to the standard PL Mount.

Thanks, BC

 

[Steve Sherrick]

Having a reference lens you trust is important. If the lenses are all out of whack, you're going to drive yourself crazy.

 

[Chris Bell]

Brent,

In addition to the video, a document describing the back focus procedure is needed.

 

[Shawn Booth]

BLOODSHOT.TV has a BF video article on the way

 

[jimhare]

I followed Doug's method, but now a lot of users are saying to set the back focus based on infinity, not 3 feet.

Haven't gone back to try it again but I'm definitely getting mixed results so far.

We are actually working on a "how to adjust your backfocus in the field" video....

In the meantime, hope this helps: http://www.youtube.com/watch?v=pRicc1HU44c

Doug did this as part of his installing the Nikon Mount tutorial but it obviously applies to the standard PL Mount.

Thanks, BC

 

[Douglas Underdahl]

Thanks Brent, I appreciate your confidence in my video.

Well, you can set your lens at infinity, then point your camera at the farthest thing you can see, like trees in the distance or the moon, etc. Or you can set the lens at 3 feet and check that distance, as I did in the video. It really depends on how accurate the test lens is.

What the technician does is start with a metal collar. It's a solid lump of metal. I have one from Arri that's chrome plated to reduce error brought on by corrosion. I don't know what it's made of but my guess is a stainless steel that has low expansion and contraction due to temperature.

Then you take a gauge and standard (like a piece of polished granite that is flat to, say, .0002" or something) and zero the gauge by putting it into the collar, which is set on the standard.

Now the gauge will read zero when it measures something that is, in this case, the same height as the collar from Arri, or 52mm.

Then you use this gauge to set your collimator. My favorite was the one I used at a couple of rental houses, made by Chrosziel. It had a lovely video read display. But later I used a Richter which you had to bend over and stare into. The collimator sends light into the lens, bounces it off a mirror, onto a reticle and then compares that image to another reticle. You use the gauge to set the mirror, so that it also reads zero when set to 52mm.

Then, you put the lens on infinity, put it on the collimator, and turn the focal plane micrometer until the reticles match. The matching image depends on the type of collimator you use. Chrosziel had a couple of bars on either side, and one that moved in the middle as you turned the micrometer. The Richter would go in and out of focus and be either red in one direction or green in the other.

So, you discover that the lens, when set to focus at infinity, is focused on the collimator at 52 minus .07mm. So the lens is off. You reset the lens so that instead of focusing .07mm off, it focuses at .00mm or dead on 52. How you do this varies from lens to lens and prime to zoom. I learned how to do this on a few lenses.

So, now you have a correctly focusing lens. You know because you set it with the collimator, and you set the collimator with the gauge, and you set the gauge with the chrome collar from Arri. So it all goes back to that chrome collar.

NOW you take that correct lens, and use it to set the back focus of your RED like I did in the video.

But, you only checked it at infinity. So maybe you should set the back focus at infinity too, like focus the camera on the moon or a distant mountain or trees. Because sure, that focus scale on the lens could be off at 3'.

So set your camera with the lens at infinity, then check it at 3'. If it's on at infinity but off at 3', then you'll probably want to tape over the footage scale on the lens and make your own marks. ACs do this all the time and of course with a large HD monitor and the RED it's much easier than with a film camera with a hard to focus image on a ground glass or fiber screen. So that's a real plus with shooting HD over film, no doubt.

What if you don't have a collar, gauge, standard, and collimator? Well, you can take one of your lenses to a tech, have them collimate it, adjust it, then use it to set your camera. I suggest using a wide lens, like 25mm or wider, because wide lenses have large depth of field outside the camera but small depth of focus, inside the camera, and this small depth of focus at the sensor plane means that you have a finer range, plus or minus, to set the back focus at. So it's more accurate.

 

[Jeff Kilgroe]

but now a lot of users are saying to set the back focus based on infinity, not 3 feet.

Bad medicine there... Best to set with a lens that is known to be accurate with a good chart / focal object at a properly measured distance. Then check other focal marks, including infinity.

Setting BF on a RED One takes a bit of practice it seems. It's hard to tell for sure if something is 100% in-focus on a 720p monitor. The zoom does help, as does edge highlight and focus histogram. Once you check and set your BF a couple times, it's not a big deal.

As has said, use a lens you can trust. If you don't have one, take the camera to a rental house or lens shop you trust that will have the proper equipment or can help you with mounting a good lens for this purpose.

Check your BF every now and then. Especially if your camera undergoes lots of drastic temperature changes or you have traveled with it. The adjustable mount collar can move.

My BF was off a bit when I received the camera. The bolts on my mount collar were a bit loose. I've had the camera for about 10 months now (wow, really?). I've probably checked the BF 7 or 8 times. Only had to adjust it twice -- right when I got it and once again a few weeks ago after the camera was exposed to lots of indoor/outdoor hot/cold temp changes.

 

[Paul Leeming]

I haven't touched any of our cameras' back focus collars at all since I figure Red is collimating them properly before they leave HQ. We have marked that setting with tape and it hasn't budged at all, so we know it's tight.

But I'd still like a cheap(ish) tool from Red themselves to ensure the collimation is/stays accurate.....

Paul

 

[Tyler Black]

I come from primarily a video background and I've only ever owned 1 lens for any particular camera. So if I fiddled with the back focus for that lens, I didn't have to worry about what would happen if I swapped lenses out. But now that I have a RED with an 18-50 and a 50-150, I have a problem. My 18-50 lens has always been sharp through the entire range of focus, however, I just received my 50-150 yesterday and noticed that it doesn't hold focus through the full range of zooming. Isn't the back focus a function of adjusting the camera? The adjustment isn't on the lens, is it? And if it's an adjustment of the camera, won't my 18-50 be off if I adjust the back focus for the 50-150? If this is the case, is this a collimating problem that needs to be corrected by a qualified tech?

Please correct me if I'm completely wrong here. Thanks.

 

[Mitch Gross]

The lenses need to be set to a standard on an optical bench. This involves a precision collimating machine and shims. Once properly set, the camera's backfocus should be correct for all lenses, although it is important to check this on the RED periodically.

 

[Matthew Duclos]

In addition to other topics, I've been considering a creating a short vid detailing what back focus is and why its important. Maybe include some of the many options for tools available for helping obtain proper back focus. Not only for the RED camera but for lenses as well.
Interest?

 

[James Brundige]

in addition to other topics, i've been considering a creating a short vid detailing what back focus is and why its important. Maybe include some of the many options for tools available for helping obtain proper back focus. Not only for the red camera but for lenses as well.
Interest?

great interest.

 

[Jacek Zakowicz]

need the Optimator?

need the Optimator?

In addition to other topics, I've been considering a creating a short vid detailing what back focus is and why its important. Maybe include some of the many options for tools available for helping obtain proper back focus. Not only for the RED camera but for lenses as well.
Interest?

Matt,
Let me know if you need the Optimator for the Video.:thumbsup:
Jacek Zakowicz, optitek-dot-org