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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 :)

Back Focus & Sensor Cleaning

Dominik Muench

Dominik Muench

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Hi guys,

I've been looking for an answer to these two questions for a while, im sure its been discussed before but i cant find any answer to this atm :/

can anyone explain to me how to check and if necessary adjust the back focus on the RED ?

also, what is the best way to clean the RED sensor, if it needs cleaning at all ? which im sure is the case ? I know my digital slr needs cleaning on a regular basis. Would i approach the red sensor in the same way i would a DSLR one ?

thanks for any help.

dom
 
Using a lens that you know is collimated, put the chart 6ft away from the sensor plane. Now check if the chart is in focus at the 6ft mark on the lens.

http://www.zgc.com/webstore.nsf/products/sharpindicator_7a9

So far I haven't needed to use any liquid sensor cleaner. If I see any dust while checking the sensor under the loupe, I just blow it off. There are some liquid sensor cleaners that work on DSLR's but are not recommended to be used on RED.

http://www.visibledust.com/products3.php?pid=602
http://www.nrdfirefly.com/
 
thanks for that :)
what if the chart wouldn't be in focus ? is there away to adjust the backfocus or would i have to send the camera in ? i know the Viper has a little screw on the lens mount that lets you adjust backfocus.

yeah im a bit scared when it comes to all those "home cleaning kits" like those vacuum spray cans n stuff, i usually just use a blow duster.
 
You should be able to adjust the backfocus yourself.


Mitch Gross of Abel Cine posted some tips the other day:

On the RED, the backfocus ring with the two hexbolts is a clamp, nothing more. The ring that adjusts the wormgear for moving the mount in & out from the sensor is located underneath it. The idea is to loosen the hexbolts just barely enough to be able to move the outer ring, which will then move the inner ring via contact friction. What happens often is that people will loosen the hexbolts far too much, separating the outer ring from the inner ring. At that point turning the outer ring will have little to no effect. Or you might be able to move it one way but not the other, getting the mount further & further out of depth.

Another trick I found is that it can really matter which screw you tighten first once you found correct depth. The definition of frustration is to find perfect depth and then discover that the act of tightening the mount in place shifts the mount back out of focus! Think of the backfocus adjustment as a big screw. By definition there has to be a bit of play front and back between the threads, otherwise they would not turn. When we're talking about a system measured in microns this is a pretty tight tolerance. So turning the screw one direction would put metal to metal contact against one side with air space on the other. Rotate in the opposite direction and the metal to metal contact occurs on the opposite side. YMMV, but I find that -- facing into the lens port -- if I finish my adjustment by rotating CLOCKWISE then I should tighten the REAR hexblot first (just enough so it catches, then tighten the other one). If I end turning COUNTERCLOCKWISE then tighten the FRONT hexbolt first.

It's vital to do this with a decent-sized monitor and the pixel-to-pixel zoom switched on. Otherwise you could be totally soft and have no idea. When you make the adjustment and then lock down you'll see in an instant if anything is going wrong.
 
Excellent chart for testing back focus. Only $49.95 from ASC store.

http://www.ascmag.com/store/product.php?productid=9200&cat=283&bestseller=Y

Sharpness Indicator Chart, Putora 7A9
The SMPTE endorsed 7A9 "Magic Chart" is the most advanced and accurate test chart available, perfect for evaluating the sharpness, resolution and back focus of all types of motion picture, video and still cameras. 10" x 10".

-michael zaletel
(shooter)
 
This is another great chart:

Putora Sharpness Indicator Chart but much more expensive at $425.

-michael zaletel
(shooter)
 
Here is another tool for lens focus calibration.

LensAlign Pro $139.95
LensAlign Lite $79.95

Should be a valuable tool investment.
Details, demo video and reviews; http://www.rawworkflow.com/lensalign/

LensAlign_PRO_002.jpg
 
Mitch,
Excellent advice !!!
If I may, Use a white grease pencil and thoroughly mark the ring per adjustment and put a hash mark as a guide on the body. I find this helps and quickens the procedure so you can see what adjustments you have made.
Remember what Mitch said about Back Focus changing when you tighten down the screws...His trick works and the first time I did it was a process of discovery.
Speak to someone at RED about cleaners before you put them on a sensor. The Artic Butterfly is great...I have one but wouldn't use it unless I HAD TO. Never a good idea to touch or clean a sensor unless it warrants cleaning. I am old school and believe that keeping the Sensor clean is as easy as an air bulb.
 
Hi Dominik,

Australian ex-pat here....

Regarding sensor cleaning... With the Red One, you're not actually cleaning the sensor itself but instead you're cleaning the filter assembly mounted in front of the sensor. Which is a good thing for 2 reasons. One reason being that even if there is dust or dirt on the filter assembly, it's unlikely it will actually show up as a "dark shadow spot" on your images, even at f16 or beyond. This is because of the distance between the sensor & filter, and because of this distance light will "wrap" around and dirt on the filter & won't cause this shadow spot (but possibly at a very small aperture it may cause a larger but more subtle shadow but it would need to be a fairy large dust or dirt particle)
The 2nd reason is the filter protects the sensor itself from scratches & damage which may occur due to poor cleaning. Much cheaper & easier to replace a damaged filter than a damaged sensor. I've cleaned thousands of sensors on DSLR's & Digital Cinema Cameras & I've seen my fair share of scratched filters due to poor cleaning, and unfortunately a few scratched sensors also.....

If there is just dust particles on the filter, you can often easily remove them with a blower bulb. Another good product for dust removal is the "Arctic Butterfly" by VisibleDust.

If there is any oil residue on the filter, do not use any "dry" method of cleaning as it will just smear. A wet cleaning method is necessary then & often requires repeated cleaning passes to completely remove all oil or grease residue. There is somewhat of an "art" in getting a perfectly clean, dust & smear free sensor or filter surface. I recommend doing any cleaning in a dust free chamber or dust free environment & being grounded helps a lot so you don't attract more dust from static with your cleaning movements.

Hope this info helps!
 
I really like the Sensor Swabs and Eclipe solution from the second link in Brent's post. That is if an air puffer can't get the dust off first. That usually does the trick.
 
Jamie,
You are right it is an optical low pass filter that protects the sensor that you are actually cleaning. I just like to heighten peoples awareness using the term "sensor" to prevent people from actually spending to much time in that area unless absolutely needed. I have found that rental companies do a great job at keeping the OLPF clean and have gotten some privately owned cameras that needed some "help". I also use the Eclipse solution in conjunction with the swabs that Brent mentioned. I use the Loop provided in the butterfly kit during a checkout to see if the filter warrants any attention and keep checking the filter during lens changes.
More often than not a quick blast from my air bulb does the trick. ( I have seen some really strong air bulbs in the market these days. Make sure before you blow the filter that the tip is void of any particles or residue that might have been picked up in your kit)
 
Brent - thanks for mentioning my Youtube lens mount videos.

Hey, the 1:1 focus button in build 17 makes this all that much easier.
 
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