Back Focus Help!

[Damian Grady]

I'm having a bit a trouble setting the back focus on my EPIC-X.

I have the RED Focus to help me but I'm struggling with it. I'm using the RED 5.0 Pro Touchcreen for reference and also activating magnify but there seems to be a fairly big range where the chart looks in focus before it moves out.

Having thought I had set the back focus properly I tried it with a Canon 24-70mm lens and the focus shifts a fair bit from 24mm to 70mm.

Am I doing something wrong? Are there any other methods for getting the back focus set correctly?

Any help would be very much appreciated.

Cheers

 

[Max M.]

i think the design of the 24-70 lens is so that it doesn't hold focus when zooming in or out. This has nothing to do with the Epic, but with the canon 24-70.

 

[Damian Grady]

i think the design of the 24-70 lens is so that it doesn't hold focus when zooming in or out. This has nothing to do with the Epic, but with the canon 24-70.

Thanks for the quick reply Max. It also happens with the 24-105mm. Same deal?

 

[Max M.]

Yep same deal. Looked up the explanation:

http://www.lensrentals.com/blog/2011/02/photo-lenses-for-video/4

 

[Brent J. Craig]

Most stills lenses don't hold focus when zooming. That is one of the reasons cine lenses cost 10 times more.

 

[Jacek Zakowicz]

Most stills lenses don't hold focus when zooming. That is one of the reasons cine lenses cost 10 times more.

Yes but I can guarantee you that every lens can have focus set properly on both ends long and wide and it has nothing to do with how they hold focus. Also the OP asked about setting the camera not the lens....

 

[Rob Ruffo]

The better your backfocus, the less difference you will see between ends of a non-video zoom, but likely you will never get to none. The 70-200 Ls can get to none (only SLR exception I have found).

 

[Robert Hart]

Suggested in another thread.

Suggested in another thread.

Harking back to 35mm groundglass adaptor days and non-parfocal lenses, it seems best to set backfocus for the wide view with the lens witness mark set to inifinity. Wide lenses or zooms on wide present the most narrow "depth-of-field" at the focal plane end of the path from subject through lens to final image.

Witness marks on the focus barrel of consumer level lenses may not always be precisely truthful. In this event and without a proper test rig and charts, here follows a backyard method of setting backfocus which should get you in the ballpark you can trim from.

For best accuracy in setting backfocus set aperture ( iris ) wide ( to its lowest number ) . Set the lens focus by the witness mark on the lens barrel to infinity focus, not end-of-travel of the barrel rotation. Point at a clearly defined object like a communications tower on a hilltop about a half mile or three quarter kilometre away. Use ND filters to allow you to set the lens iris wide open without flaring the image. If you have no NDs, wait until time of day when light levels become sensible for that aperture setting. With the lens controls already preset, adjust your camera backfocus until the sharpest image is found. If you are backfocussing with a zoom lens, set the zoom travel to its widest view.

There is another wrinkle which affects the SI2K and maybe less critically, the RED camera familiy as well. The Optical Low Pass Filter and Infra Red filter are a combined transparent panel which is fairly thick. The refractory effect of this panel slightly affects the flange to focal plane distance. Wide lenses are the most adversely affected. For best precision at a professional level, individual IMS/PL-Mount adaptors were collimated with shims for individual lenses.

The SI2K is a 2/3" or Super16mm approx sensor area with 2048 x 1152 pixels, so focus errors are more of an apparent issue. However the RED camera family, has a much larger sensor area. My imagining of why RED's own lenses work so well is that they may have been engineered to allow for the OLPF/IR layers.

One might assume the larger sensor area is more forgiving but the RED camera's task is to deliver 4K or thereabouts of performance. Therefore its sensor needs to be presented with an image throw of comparable optical resolution because the pixel size is likely to be similar to the SI2K but there are many more of them.

My usual caveat applies. Take notice of the wisdom of others before heeding my comments.

 

[Elizabeth Lowrey]

Thanks for that very informative reply, Robert. I am using Canon still zooms and feel I need to do some back focus adjustment. Was just waiting until I found the time to research around these parts the way to properly do it. I appreciated the tips.

 

[Nils Ruinet]

I'm having a bit a trouble setting the back focus on my EPIC-X.

I have the RED Focus to help me but I'm struggling with it. I'm using the RED 5.0 Pro Touchcreen for reference and also activating magnify but there seems to be a fairly big range where the chart looks in focus before it moves out.

Having thought I had set the back focus properly I tried it with a Canon 24-70mm lens and the focus shifts a fair bit from 24mm to 70mm.

Am I doing something wrong? Are there any other methods for getting the back focus set correctly?

Any help would be very much appreciated.

Cheers

Wait a second, I thought the Red Focus did only work for PL mount ?
Anyway, some focus assist false color modes can also be activated while in magnify mode, to help you find the sharpest focus possible.
And regarding the 24-70mm, as others pointed out, that's unfortunately a lens issue, it's the case with most of those DSLR zooms. If you look on the lens barrel, you'll often see there are different witness marks for the different focal lengths... So, never check your focus at 70mm if you want to shoot at 24mm !

 

[Jacek Zakowicz]

You don'need any witness marks to set the flange depth with a zoom lens. You just get it in focus on both zoom extremes at the same spot preferable short distance

 

[Robert Hart]

True. I agree on the method of setting backfocus using a closer target using the extremes of focal length. Anyone who uses ENG cameras for a living should have this method as second nature.

That applies if all things are equal, a given zoom lens has no uncertain histories, is perfect and is parfocal, otherwise all bets may be off. If things are less than best, then infinity by the witess mark at fully wide with the zoom is the safest to be assured the backfocus is adequate if not best. Some owners who have bet their shirt on the new camera and have yet to raise the ready for a good lens set may be making do with used older lenses and making sometimes rough improvisation to get things done. My backyarder comments are more applicable to them.

On the long end of a zoom, when actually shooting, you can more easily chase the focus by eye against a softer background. On the wide end where there can be much detail and clutter and an operator is under pressure to get the shot, he or she will not always observe that the subject is slightly soft until the larger display tells uncomfortable truths much later.

 

[Jacek Zakowicz]

True. I agree on the method of setting backfocus using a closer target using the extremes of focal length. Anyone who uses ENG cameras for a living should have this method as second nature.

That applies if all things are equal, a given zoom lens has no uncertain histories, is perfect and is parfocal, otherwise all bets may be off. If things are less than best, then infinity by the witess mark at fully wide with the zoom is the safest to be assured the backfocus is adequate if not best. Some owners who have bet their shirt on the new camera and have yet to raise the ready for a good lens set may be making do with used older lenses and making sometimes rough improvisation to get things done. My backyarder comments are more applicable to them. Parf
parfocality has no
On the long end of a zoom, when actually shooting, you can more easily chase the focus by eye against a softer background. On the wide end where there can be much detail and clutter and an operator is under pressure to get the shot, he or she will not always observe that the subject is slightly soft until the larger display tells uncomfortable truths much later.

Parfocality is the ability of the zoom to hold the focus in mid range not the extremes so it's irrelevant to flange adjustment at the extremes. As long as the lens back focus is set correctly it works just fine for flange adjustment - does not need to be parfocal