PELENG 8mm - Focus Issues...

[Matthew B Clark]

Hello. I got some 16mm Tri-X 7266 footage back from the lab, which I had shot on my K3 with a bunch of primes (Peleng 8mm; Pentax SMC Takumar 50mm/1.4; Carl Zeiss Jennar 135/3.5) and the stock Meteor zoom (17-69mm), and the tests confirmed a hunch from previous footage....it's near impossible to "see" the focus correctly during the shoot using my Peleng 8mm. Almost all the other lenses, especially the stock zoom, seem to be DEAD ON just by zooming in and eye-balling the focus. I make mistakes due to moving the camera sometimes while using fixed primes, which obviously can cause a really unintentional blurring in and out look, but that's operator error.

 

What I'm asking is...when using the widest angle lenses like Peleng 8mm, why is it so incredibly hard to actually "visualize" the focus? I keep thinking I'm nailing it while shooting (sometimes it just ALL LOOKS THE SAME through the lens), and then footage comes back and its "sort of off". Not horrible, but many shots I'd be unhappy with using in any real project. Kind of bothers me until I get a few tips on how to accurately use the Peleng. Because I need that for my wide shots. But it's just completely making me unconfident when my footage comes back all wonky. Here is a link to the shots. You can obviously tell which ones are the Peleng (not just for the focus issues, but because you can see which ones are wider - the recording studio shots etc). Anyway, this test came out pretty darn bad overall. But I'm learning how to treat Tri-X as a result. Which is nice.

 

Any suggestions on maintaining a REAL focus using that Peleng? It just all looks the same through the viewfinder on that one. Do I need to break out the measuring tape for all those shots? I guess so. Is this a collimation issue? It's a new lens. Although....I did have a band spit beer on it....like directly on it for a shot during a shoot once. But I cleaned it all off using wipes and learned not to do that one again.

 

Video uploading to Vimeo....will return when my non-free account has "converted it" in 45 mins!

[Matthew B Clark]

Here is the very bad test footage in question. Embarassed. But then again, I kind of just whipped out the camera and took a fast reading with all existing light. So I'm not too mad at myself. But still, messing up the Peleng focus is driving me pretty crazy since I genuinely cannot see it.

 

[Dan Dorland]

I've experienced this exact thing! So many times I've focused just a tad too close. All you can really do is eyeball AND use the scale whenever possible, and tape measure for closer subjects/wider apertures.

[Satsuki Murashige]

Sounds like a lens (or possibly camera mount) collimation issue. I'm guessing the lens never gets sharp even at infinity focus, right? That's the usual symptom. The reason is that wide angle lenses have shallower depth of focus than normal or telephoto lenses, so you will notice a collimation issue sooner with a wide lens.

 

Depth of focus (not depth of field) is the range of acceptably sharp focus behind the lens, where the image is projected onto the film at the focal plane. If a wide lens is a few tenths of a millimeter out of tolerance (collimation), the images it produces will be soft. The solution is to have a lens technician check the collimation of the system by precisely measuring the flange focal distance (distance from lens mount to the focal plane) and reset either the lens, the camera mount, or both back to spec by using special shims.

 

Since the K3 has an M42 mount which is a screw mount, you can test this yourself by slowly unscrewing the lens a few turns while framing up a focus chart. If the lens gets sharper, it is out of collimation and is too close to the focal plane. If it gets softer, then the lens is too far from the focal plane.

 

Ideally, a properly collimated system will have sharp infinity focus at the infinity mark on the lens barrel, and perfect focus at all the other distance marks on the lens. The benefit of setting both camera mounts and lens mounts to the same strict universal standard is the interchangeability of camera bodies and lenses, all with perfect focus. That's the gold standard.

[Matthew B Clark]

Thank you for the replies! I appreciate the information. Collimation is likely the culprit (looks like). Although my personal understanding of lenses needs collimation as well. I can at least get that onto the focal plane of my mind for now!

[Robert Houllahan]

The removable mount on the 8mm Peleng seems to slip loose over time, I tend to loosen the set screws and then rotate it back and forth to reseat it and then tighten the set screws again and it sharpens up nicely.

 

It's Russian.

 

;-)

[Matthew B Clark]

The removable mount on the 8mm Peleng seems to slip loose over time, I tend to loosen the set screws and then rotate it back and forth to reseat it and then tighten the set screws again and it sharpens up nicely.

 

It's Russian.

 

;-)

 

Ah! Good point. Especially since mine was shipped with the incorrect mount and was sent a replacement to put on myself. Don't know exactly how I could screw that up, but I guess it's possible too. And yeah, good call. I'll try to just shift that around, plus a few other ideas that kind folks have mentioned on and off the board....thanks all. Much appreciated as always. It'd take lifetimes to achieve a few steps without you. Don;t underestimate the value your input gives certain people. Best,

 

Matthew