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Newbie question: chromatic aberration

Mike 'Fireman' Ross

Mike 'Fireman' Ross

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Old Cooke 25-250, @250:

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I'm new to cine lenses... Back focus??

Thanks

Mike
 
One of the truly wonderful things about Reduser is that you can get a question like this answered by a Glassoholic like Evin Grant. :-)
 
its the 250mm part of the 25-250...... :)

push any lens to its extremes and weird thigns happen. I bet that lens, @ 150mm @T8 is a CHAMP!

we will find out this week.....
 
Thanks Evin... this was at t16 so not an iris issue... guess it might just be a characteristic of the lens, but I'd heard that being a whisker off on back focus can cause CA, especially on zooms.

I'll check back focus again and shoot some charts at various stops and focal lengths, just to investigate.

And yes Timur, looking forward to it!

Mike
 
lateral vs. axial

lateral vs. axial

Lateral chromatic is about the same size (distance from the image center) no matter what stop is used, when open you get some extra width because of the extra rays, but the central rays are still in a rainbow.

We have a cooke 40-400 with very bad lateral chromatic, I asked cook rep about that and he said that the three tube TV camera was to be adjusted to compensate by changing the size of the raster on the three tubes. in that one the rainbow seems larger than the coc by some.

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

I added lateral chromatic comp to the de-bayer program I am working on, it works to some degree if the lens is centered on the image.

Axial is a larger problem, but seems less common, you cannot compensate by adjusting the RGB image sizes, it makes a halo around all image points even in the center. You might be able to differentially sharpen the RGB in planes to reduce the halo, what it is is unequal focus, where the red and blue focus are at another distance from the green, so its harder to do anything about, but stopping down does help with axial by making the dof deeper and so the RGB match better.

In a zoom if you zoom the lateral chromatic may very with fl mm, so you would need to adjust the compensation for each shot made, and if you zoom, you would need to animate the compensation values frame to frame.
 
Evin Grant said:
... CA can also be a function of sharpness, most all lenses will show a tiny bit wide open, even Master Primes.
If you stop down a bit it should get better.
Click to expand...

Evin, I respectfully disagree. I've tested the 14mm, 35mm and 75mm at f/1.2 on a projector at ARRI and there was no CA visible at all even down to 200 lp/mm - well beyond the resolving power of any sensor or film. That's why I could not resist buying them :)
 
Pawel Achtel said:
Evin, I respectfully disagree. I've tested the 14mm, 35mm and 75mm at f/1.2 on a projector at ARRI and there was no CA visible at all even down to 200 lp/mm - well beyond the resolving power of any sensor or film. That's why I could not resist buying them :)
Click to expand...

Probably very true. Unfortunately a single Master Prime costs more than my entire rig; I couldn't afford a *case* for a Master Prime, hence the old Cooke :-)

(the 'old Cooke' in question being the Super Cine Varotal 25-250mm f2.8... late 1970s / early 1980s vintage)

Mike
 
To drift the topic slightly, how important IS back focus?

I mean, I can see it's pretty damned important if you're operating in a traditional movie shoot, using tapes, scales, FF and the rest.

But if you're operating single-handed in an EFP style, pulling your own focus entirely by reference to the viewfinder... if it's sharp it's sharp, if it's in focus it's in focus, right? Wrong? Obviously (?) if the back focus is *grossly* off it can cause other optical problems (?), but if it's just a wee bit out, no big deal?

Thanks

Mike
 
Mike 'Fireman' Ross said:
To drift the topic slightly, how important IS back focus?

I mean, I can see it's pretty damned important if you're operating in a traditional movie shoot, using tapes, scales, FF and the rest.

But if you're operating single-handed in an EFP style, pulling your own focus entirely by reference to the viewfinder... if it's sharp it's sharp, if it's in focus it's in focus, right? Wrong? Obviously (?) if the back focus is *grossly* off it can cause other optical problems (?), but if it's just a wee bit out, no big deal?

Thanks

Mike
Click to expand...

Well, if you want to be in focus at infinity it's essential.
 
D Fuller said:
Well, if you want to be in focus at infinity it's essential.
Click to expand...

Doh. Good point. So... put on the widest angle you have (so wide depth of field but shallow depth of focus), set on infinity, and adjust accordingly?

Mike
 
If you are planning to zoom at all- the BF has to be there.
Even if you use zoom as variable prime it's easier to work if you don't need to refocus(much) every time you reframe.
Its all about speed , accuracy and convenience of focus, especially with 35mm format.
 
Mike 'Fireman' Ross said:
Doh. Good point. So... put on the widest angle you have (so wide depth of field but shallow depth of focus), set on infinity, and adjust accordingly?

Mike
Click to expand...

That'll work. (as long as the lens you're using is properly collimated itself)

The problem I always ran into doing it that way is that I really wanted to set back focus at infinity, rather than trust distance scales. That meant doing the back focus adjustment outside. And that made it really hard to work at a wide enough f-stop. I figured out that by raising the shutter speed to 1/1000 or so I could make it work. But in the end, I bought Jaycek's Optimator. It makes it very easy to check the back focus when and where I want to, and it just gives me a lot more confidence that I've got it right.
 
Thanks guys. We dragged the thing out today and gave it a decent test and the images were *beautiful*. Test results will be up in due course on the 'vintage zooms' thread. The only area where it was less that gorgeous, and the CA appeared at the limits, was on that last 50mm... @250mm was indeed the problem!

Mike
 
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