Friendly Reminder: Back Focus Check

[Phil Holland]

Aloha all,

Just assisted a fellow Reduser earlier this week after getting a somewhat frantic phone call from set. Drove over, checked that back focus, way off.

Remember, especially if you're working with zooms and/or working with a digital cinema camera is new to you, check that back focus.

If you aren't hitting infinity, especially on ultra wide zooms, it's likely due to this.

Have a great weekend.

 

[Mike P.]

I thought just basing back focus off of ultra-wide infinity focus was bad practice?

Also, when using stills glass (which a lot of people who don't check backfocus tend to use on their Scarlets) the markings can be off on the lens(es) or, if using mount adapters, the adapters could be slightly off... OH and if using wides and focusing to infinity, you're really stressing the REDcode, which can have an impact on image quality (which may be seen as a softer/noiser image and not necessarily because the BF is off.)

But, yeah, thanks for the reminder. I've been meaning to do this every few months since getting the camera, but with an EOS mount it's a little more difficult (are there any BF tools for EOS mount cameras?) as you have to free ball it using an ultra-wide and measured off focus distances and make sure your lens has accurate markings... Too many variables and could do more harm than good.

 

[Phil Holland]

This particular situation involved the Tokina 11-16mm f/2.8 and Canon 24-70mm f/2.8 L and an Epic. The Tokina was soft/out of focus at 8 feet at 11mm, which was really causes the panic. Back focus, for whatever reason, was way off on this dudes camera.

I free balled it, we tested every single lens on set, and I vanished into a poof of smoke.

Pretty much had to do that in this situation. They are shooting almost everything at 11-16mm from what I saw that day.

 

[Mike P.]

I free balled it, we tested every single lens on set, and I vanished into a poof of smoke.

That's because you're a wizard. : )

I feel nervous judging off my Tokina. Anything within a given range (close focus to, say, 10ft) is wicked sharp even at 2.8, but after that (10ft to infinity) it does get softer but it's only noticeable when viewing 4k at 1:1. Also, I've read that Tokina 11-16 v1 sometimes misses infinity focus marks, so I'd rather just do it with a proper tool and not worry about whether my glass is sub-par.

 

[Jacek Zakowicz]

That's why lens "misses" infinity mark- so you can go through infinity and make sure you get the best focus. Sometimes I miss the old times when these things went without saying....

 

[Phil Holland]

Heh. In this situation I imagine it's most important to lock critical focus. They weren't anywhere close, but I don't think he's ever touched his back focus before. Likely whoever used his camera last.

It's fairly trivial to accomplish. I used a semans chart and a phone pole. If you are trying to hold focus through the range, it's a bit more time consuming and using a focus tool is better.

 

[Mike P.]

Yeah, I get that Jacek... but is it the same if it reaches infinity before the infinity mark? That seems more indicative of a bad mark than bad back focus (when all other marks are hitting). Conversely, if it stays the same sharpness (or softness; that is to say, not as sharp as everything between close and 10ft) at the infinity mark and passed it, I'd think that was bad back focus.

Anyway I just tested my Tok at 11... 1, 3, and inf, and it seemed to hit each close enough (within an inch). I think there is slight detail loss at inf, but I think that's 6:1 breakdown more than focal flange distance (again, since it hits everything else essentially on the mark).

The other thing is the info passed as metadata always indicates 1cm +/- off of what the mark on the barrel says. I know that the metadata is not the most accurate, but that would account for the 1" of play and again, indicate that the barrel markings are merely "good enough" and not tack (which makes sense with mass produced/machine made lenses).

 

[Jacek Zakowicz]

Marks on still lenses are only for a rough reference.
They are too close together , have no witness lines, etc.
These are the reasons we made the Optitron- you stretch the focus range to almost a full turn so you can make your marks on the marking ring. Then they hold as good as you are marking them. And they are spaced out properly.
BTW the lens should not lose detail at infinity- 4K is 4K, close focus, infinity- why would that make a difference? Unless you are mistaking detail for magnification- but that's not a lens problem...

 

[Mike P.]

The more detail in a scene, the more it stresses REDcode compression. On an 11mm lens focused at infinity is a lot more fine detail that needs to be compressed in the same file size (at 6:1)... Not softer, per se, but more compression-based artifacts which hinders the look/resolving of fine detail (which is almost everything in the scene at those settings -- 11mm at inf).

Or at least that's the impression I was under (and what I thought I had been witnessing... either that or maybe my back focus *is* off ever so slightly).

 

[Ketch Rossi]

Good advise Phil,

however is to keep in mind as Jacek said too, Still glass is not meant to use via referring to the markings, as they are pretty much worthless other then to give a very approximated idea of were you are, and to make things worst none match each other, so in the case of standard still glass, the Back Focus will not matter, as each will hit precise Focus even in their respective Infinity Focus, at different distances.


I'm currently using Canon L glass on the Epic, and have seen this on all L glass used this far, so there is only two ways to Focus, one by hand in Manual Mode, and the other in AF via the touch screen, just not as always spot on and not as fast, but pick your favorite after trials, and never ever trust any markings for focusing on your still glass, unless it has been Cine re-Housed by a professional service.

 

[Jacek Zakowicz]

That could be detail on the 500mm lens or a 11mm lens at close focus or infinity- that only depends on the subject. The camera doesn't care what it's capturing(a fine fabric fibers at 6" away or a forest landscape at infinity) at what distance or with what lens. It looks like you are confusing resolution with magnification...
Anyway just make sure you go through infinity past it so you have the reference for the best focus...
It's just the basic standard procedure....

 

[Phil Holland]

Yep. The issue here was when this fella pulled to infinity (the len's hard stop) even subjects at 8 feet were wicked soft. Wasn't a matter of marks. It was a matter of no more real estate for more rotation. Which is indeed a back focus problem.

I don't trust the marks on still lenses one bit. Not at all. Some cinema lenses too. Which is why I've practiced and use wild pulls for over a decade as well as traditional methods. Marks and measurements are fantastic, but if your actor is more mobile in the Z depth and you like shooting at T1.3-T2 I find there's a always a bit of reason to track the subject's, or at least the point of focus's, movement in that dangerous and wild fashion. That's where good eye/hand coordination come into play as well as a good feel for the lens.

Which again, is a pain using still glass. Much easier via proper cinema lenses.

Love still glass for affordability and compact size. Some of the optics I'm pretty smitten with even. Hate it for nearly every other reason.

It's just the basic standard procedure....

Agreed!