Zeiss ZFs and focus marks

[jimhare]

I was finally able to adjust the back focus for my Zeiss ZFs. inch:

I have it spot on 1 meter on both the lens and sharp focus.

Here's the thing, when I check it at other marks, 3m, 5m etc. it's not even close.

The thing that caught my eye is that it seems to be off by a factor of 1.6... 3m in the real world was just under 5m on the lens. 2m in the real world was around 3m on the lens.

See where I'm going with this?

I wouldn't have thought the focus offset would follow the zoom caused by the sensor but I'm an idiot.

Anyone care to enlighten me or share your own ZF experiences?

Thanks!

Jim

 

[Steve Gal]

Hope you are using the widest lens to set your back focus to. Unless the lenses are not working properly, the markings should be correct if you've adjusted the back focus correctly.

 

[Roberto Lequeux]

Just tape them and write them over as precisely as you can with a permanent thin tip marker and go have a beer.

 

[jimhare]

Hope you are using the widest lens to set your back focus to. Unless the lenses are not working properly, the markings should be correct if you've adjusted the back focus correctly.

Hmmm. I used my 50mm. Should I go back and use the 28mm?

I'll repeat the exercise and see if I get different results.

Thanks!

Jim

 

[jimhare]

Just tape them and write them over as precisely as you can with a permanent thin tip marker and go have a beer.

Heh, that would work too!

 

[Steve Gal]

28 may work. will work definitely better than the 50mm but the wider the lens is the better.

 

[Roberto Lequeux]

Why is the widest the best for back focus adjustment?

 

[Steve Gal]

The wide lenses have a smaller back focus depth of field.

 

[Roberto Lequeux]

aha... thanks for clarifying

 

[jimhare]

I could try my Tokina 11mm but I would exactly call it a reference lens!

I originally tried the 50mm f/2 as it's the highest quality lens I have.

Just tried the 28mm. Lined it up at 1m. It did match again at 2m but appears to go slightly passed infinity.

Then put the 50mm back on. 1m doesn't line up (only about 3% off the mark) and 2m still requires me to go to 3m on the lens to get it sharp.

Now these are ZFs not MPs so I don't have high expectations. Would just like to know if this is normal.

Thanks!

Jim

 

[Steve Gal]

no not normal. Back focus is still off.

 

[Evin Grant]

Ideally you should be calibrating for infinity on your widest lens. The 28mm will do at f2.
Try finding a tree or other object at infinity out your window.

 

[Joel Kaye]

Hmmm. I used my 50mm. Should I go back and use the 28mm?

Go with the widest lens. That should solve it.

 

[Ryan E. Walters]

35mm or wider?

35mm or wider?

So I'm in the process of calibrating my backfocus as well. However, the widest ZF lens I currently have is 35mm. I can go and rent just about any other focal legnth Nikon lens from the local photo store. Should I go rent the widest possible even though it will not be a ZF lens, or will it be a big enough difference between brands that I should just stick with the 35mm?

 

[Steve Gal]

brand doesn't matter. has to be set to it's proper specs though. Obviously a better quality lens will give you an easier time because of an overall sharper image to deal with.

 

[jimhare]

AAAAHH! This is driving me crazy. It seems drastically different for each lens and only the 28mm seems to have accurate focus marks through the range.

Okay, going to start over. This time I should start with the 28mm, open it to f2 and calibrate the back focus to infinity?

Then what? What if my other marks on the same lens don't match? What if my other lenses completely don't line up with their marks?

Knowing what to expect will go far in knowing whether I've arrived! :biggrin:

Thanks guys.

Jim

 

[chuck colburn]

The wide lenses have a smaller back focus depth of field.

Perhaps you mean Depth of Focus as opposed to Depth of Field.

 

[Ryan E. Walters]

Okay, going to start over. This time I should start with the 28mm, open it to f2 and calibrate the back focus to infinity?

Then what? What if my other marks on the same lens don't match? What if my other lenses completely don't line up with their marks?

Yep- do that and then you should be set. I calibrated my Red and ZF lenses today and once infinity focus was set on the 35mm the other marks worked fine. (I double checked at 5 feet, 3 feet and 12 inches.) The 35mm lens hits its marks, as did the 50mm lens. The 85mm lens was a little off, but not by much. (I think if I used an even wider lens it probably would have been better. So I think I'll go rent a 17mm lens and try it out.)

 

[Pawel Achtel]

Now these are ZFs not MPs so I don't have high expectations. Would just like to know if this is normal.

It is normal. Still lenses do not need to have their back flange distance accurate or be consistent from lens to lens as they rely on auto focus or eye focusing. That's one of the drawbacks of still glass. You can have them adjusted, I belive. Arri Australia has a back focus measurment machine (same as the MTF machine) and all my MPs have been confirmed to be within +/- 1 micro meter. So, I can use the 14mm to adjust any PL mount on Red. I am not sure if they have Nikon mount, you can ask. I expect that, once back flange distance is adjusted to be correct and the same on all lenses, then the distance marks will magically show the correct thing.

If you didn't have that secret meeting last Wednesday, I was planning to play with your ZFs while distracting you with MPs :bleh:

Oh well, we have an excuse to have another one shortly then.

 

[jimhare]

It is normal.
If you didn't have that secret meeting last Wednesday, I was planning to play with your ZFs while distracting you with MPs :bleh:

Hey Pawel, any time you want to trade, I'm ready my friend! :tongue:

But seriously, thanks for the great info, you are always a wonderful source for reliable info.

And about the secret meeting, just because you can't read your calender doesn't make it secret! :wink:

But I agree, let's get together real soon and see what's what.

Cheers,

Jim