Red Focus question

[John Bishop]

Can anyone tell me the advantage of back focusing the red one with the new RED FOCUS device as opposed to just putting a lens on the camera, shooting a back focus chart, and then adjusting back focus for that specific lens? Thanks

 

[ericyoung]

Well, it's a known "good" reference, whereas any given lens may potentially be "off" from the technically correct FFD if it hasn't been collimated recently. Also useful if someone doesn't own their own cinema lenses, but occasionally rents their camera out to third parties - then one can at least be reasonably confident that you are renting out a properly working camera.

This isn't so much of a requirement if you only ever shoot with one specific lens that you own. However, it's better if both your camera body and lens are set correctly, so that if you ever want to rent a set of cinema lenses, they will all be the same, with focus markings which are correct/near correct, and interchangeable with your own lens on your Red camera body.

Having said that, if you rent all your lenses from the same reputable source which properly maintains their lenses, then you can set your back focus with the widest lens instead of a Red Focus, and the rest of the lenses in the set should be OK too.

 

[John Bishop]

Thanks Eric, We own all of our lenses and never rent out anything. We have a cook 25 to 250 that we have problems with occasionally, and also a 20 to 100 that gives us problems . Both lenses have been to Duclos recently for service so I believe both of them are about as good as they can be. Am I right to assume that for our purposes we could get along without buying the RED FOCUS and just backfocus our cameras using the specific lenses when we think things may be looking a bit soft? Also, I come from a video background and most of the lenses had a back focus adjustment on the lens. We would routinely check the back focus each time we changed lenses. Why is the back focus now on the camera itself instead of on the lens? And is it true that you don't really need to check the back focus each time you change lenses? Thanks!!

 

[Michael Hastings]

John:

In the video world where it is almost always zooms with tremendous range (15x to 18x very common) it is not uncommon for the same lens to be on the camera semi-permanently so you didn't worry about the proper flange distance of the mount/sensors. Video people also normally do eyefocusing so the proper flange distance and lens collimation aren't quite as critical since you aren't using the lens distance marks for focusing. But in the cine world where you might change lenses dozens of times per day, and the focus is often done using tape measures and the focus marks on the lens, it is more important to have the camera mount/sensor right on and the lenses are supposed to be properly collimated so you can switch lenses without worrying. If the mount FFD is wrong, all of the lenses will be wrong (assuming they are set up correctly) and the amount of shift in focus (meaning your distance marks are off) will vary depending on the focal length.

 

[ericyoung]

What Michael said. If your lenses have both been with Duclos recently then they should be good.

Then just use your widest angle lens at the wide end to set backfocus at about 6 feet. Recheck at both ends of zoom on both lenses, and at different distances. Hopefully all should look sharp throughout the zoom ranges and distance focus marks should be accurate or nearly so.

Once done, you shouldn't have to re do the backfocus every lens change - just check it before a new job, or when going from one temperature extreme to another. Eg going from freezing cold to room temp, or room temp to desert heat in the sun.

For your situation, you shouldn't need to get a Red Focus.