Back focusing with DSLRs?

[RedSpectrumPictures]

Hey guys. I have recently learnt a new term, 'back focusing', and was wondering how important this actually is for filmmaking.

I made a quick test video to upload to youtube, so I can get some answers on whether my focus is correct

 

[dvbrother]

Back focusing is mostly in the realm of broadcast cameras with interchangeable lenses. The cameras actually have an adjustable back focus lever. DSLR mounts have no such adjustment. The only time I've heard of adjusting back focus with DSLRs is when you have a PL mount and need to use shims to get the back focus correct. But with regular still lens mounts, I don't think this is an issue.

 

[wgzn]

from your video, are you sure you're not talking about "rack focus"?

 

[RedSpectrumPictures]

No I was actually zooming out and back in

 

[gonzo_entertainment]

Some brutal aliasing in that.

 

[RedSpectrumPictures]

what do you mean by this?

 

[optitek]

your back focus is incorrect- the star is soft on the wide end

 

[gonzo_entertainment]

"Brutal Aliasing"...

The jagged lines when you are zoomed in.
DSLRs are not good with sharp straight lines. You'll sometimes have to soften focus a little to get rid of it in a shot. Telephone wires are a notorious culprit.

 

[kgimedia]

I think you are looking for the term Parfocal. As in a "true zoom".

http://en.wikipedia.org/wiki/Parfocal_lens

 

[nothing]

You cannot check backfocus (flange depth) by zooming in and out with that lens because, like many modern stills lenses, it it not parfocal - i.e. it does not hold focus throughout the zoom range. You need a true zoom parfocal lens, like many cinema lenses, in order to detect flange depth discrepancies by that method.

Even if you could detect backfocus issues with that lens, there is nothing you could do about it since flange depth is not adjustable on these cameras.