Zeiss CP and their t-stops

J

James M

Member
I’m thinking of renting some Zeiss CP lenses, having heard, seen, and read only good things about them. I’m a little confused as to their speed though. Their speed is given in T-stops, not F-stops. Now I understand the principle of a t-stop, but what I don’t understand is physically how that maximum (lowest numerical) t-stop on the lenses corresponds to the maximum aperture. My upcoming project requires really shallow depth of field (previously we’ve been shooting 5D with vintage Zeiss lenses as fast as 1.2). Now we’re looking at trying the af101 with CP.2s. Factoring in that FoV issue, I’m trying to figure out if I will be able to achieve anything like the shallow DoF I got with the 5D and the 50mm 1.2.

So basically, my questions is how do those T-stops compare to f-stops in terms of DoF? Is a 2.1 t-stop equivalent to a 1.8 f-stop, or is the depth pretty much the same as f2.8? I really don’t have a clue.

If anyone can shed any light, I’d be awfully grateful.
 
There is no direct formula for how a T-stop relates to an F-stop, that's individually set for each lens design. You might have a case like the Zeiss where it's extremely close (T3.6 on an F3.5) or you might have a huge variation like on the old Angenieux 12-120, where I think f/2.2 equated to T2.8.

The CP's don't list their maximum f-stop, they only list it by t-stop. But in all of them, the t-stop is almost exactly the same or only barely marginally slower than the maximum f-stop on the ZF's.

On my CP's, the 50mm and 85mm are rated at T1.5, and on my ZF's those same focal lengths are rated at f1.4.
On my 35mm, it's rated at T2.1, and my ZF is F2.0.
On my 28mm, it's rated T2.9, and my ZF is F2.8.
On the 18mm, it's T3.6, on the ZF it's F3.5.

So basically they're the same for any practical purposes.

You can get ridiculously shallow DOF out of an AF100 with a fast lens, but nothing will get you the shallow DOF of a 5D with a 50mm f/1.2. The closest you will get to that would be the Voigtlander 25mm f/0.95 lens. That will still be 1.5 stops deeper DOF, but it's the closest you're going to get.
 
Barry_Green said:
On my CP's, the 50mm and 85mm are rated at T1.5, and on my ZF's those same focal lengths are rated at f1.4.
On my 35mm, it's rated at T2.1, and my ZF is F2.0.
On my 28mm, it's rated T2.9, and my ZF is F2.8.
On the 18mm, it's T3.6, on the ZF it's F3.5.

So basically they're the same for any practical purposes.

You can get ridiculously shallow DOF out of an AF100 with a fast lens, but nothing will get you the shallow DOF of a 5D with a 50mm f/1.2. The closest you will get to that would be the Voigtlander 25mm f/0.95 lens. That will still be 1.5 stops deeper DOF, but it's the closest you're going to get.
Click to expand...


Thanks, Barry. Can I ask, how do you find the the DoF on the above CPs? Can you get nice and shallow?
 
I'll just point out that the glass on the CP2s is exactly the same as in the corresponding ZFs or the ZEs: the only difference is the housing

as stated by the very Shane Hurlbut (greatest lens fetishist I know about) over here:
http://www.hurlbutvisuals.com/blog/2010/02/06/still-lenses/comment-page-3/#comment-2366


plus: how to do DoF calculations with one camera as reference

if you know how shallow the DoF is on a 5D, and are wondering about DoF on a 1.9x crop sensor camera, just divide your f-stops by 1.9

a 48mm f/1.8 on a 5D delivers exactly the same DoF and FoV of a 25mm f/0.95 on a 1.9x crop camera (25=48/1.9) (0.95=1.8/1.9)

a 30mm f/1.1 on a 7D delivers exactly the same DoF and FoV of a 25mm f/0.95 on a 1.9x crop camera (25=30/1.9*1.6) (0.95=1.1/1.9*1.6)

(I know the GH2 is 1.9x crop for video, I'm not sure if the AF100 is also 1.9x, or 2x)

you may want to visit my DoF and FoV calculator
http://www.similaar.com/foto/dof/dof.php
 
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Did they make the CP lenses in anything other than PL mount?
I know the newer CP.2 ones are in F, EF, and PL mount, but I have not seen any of the CP ones in anything but PL.
 
JPM Photography said:
Did they make the CP lenses in anything other than PL mount?
I know the newer CP.2 ones are in F, EF, and PL mount, but I have not seen any of the CP ones in anything but PL.
Click to expand...
The original CP's were only PL mount, like all other Zeiss cinema lenses. The CP.2's were released with an interchangeable amount system, the first cinema lenses to offer that.
 
NormanBates said:
I'll just point out that the glass on the CP2s is exactly the same as in the corresponding ZFs or the ZEs: the only difference is the housing
Click to expand...
Yes, they are the same optical design, according to Zeiss themselves. I have both -- I have a full set of ZFs and a full set of CP's. And the CP's are vastly preferable because of the housings. Vastly.

But, if you're looking for a difference in the actual footage, you won't see one.

One other difference in the CP's is that they have 14-blade irises, as opposed to the (I think 9) in the ZF's. But it doesn't really make any difference in the footage, I've shot the same shot on both with extremely out-of-focus highlights and they look either identical, or close enough that nobody would ever care.

If you want the same look at a lower price, get the ZF's. If you want the size, functionality, and other benefits offered by a genuine cinema set, go for the CP's.
 
Barry_Green said:
You can get astonishingly ridiculously shallow DOF out of the 50mm or 85mm at f/1.4.
Click to expand...

Sorry, i may have confused the issues. The lenses I'm considering are CP2 and the max aperture on the 25mm was t2.9, and t2.1 on the 50mm. I played with them this morning (briefly) on an AF101, and yup, you can get some lovely narrow DoF, but that FoV / crop factor does take it's toll and makes short DoF harder to achieve on wides. I also tried a lovely Nikon 18mm 1.8 stills lens, and I did prefer it.... but it wasn't cine if that is an isssue.

I'll put some video online if anyone is interested...?

Finally, the hire co I was visiting had a cheap adapter (I want to say it was Nipon, but I don;t think that was the brand actually) and there was considerable play at the camera mount end of the adapter that meant that a follow focus would have been unusable.... just a warning that you should choose your adapters carefully. But I think most people know that.
 
James M said:
Sorry, i may have confused the issues. The lenses I'm considering are CP2 and the max aperture on the 25mm was t2.9, and t2.1 on the 50mm. I played with them this morning (briefly) on an AF101, and yup, you can get some lovely narrow DoF, but that FoV / crop factor does take it's toll and makes short DoF harder to achieve on wides. I also tried a lovely Nikon 18mm 1.8 stills lens, and I did prefer it.... but it wasn't cine if that is an isssue.

I'll put some video online if anyone is interested...?

Finally, the hire co I was visiting had a cheap adapter (I want to say it was Nipon, but I don;t think that was the brand actually) and there was considerable play at the camera mount end of the adapter that meant that a follow focus would have been unusable.... just a warning that you should choose your adapters carefully. But I think most people know that.
Click to expand...

Kipon.

The less-than-ideal adapter was a Kipon, fact fans.
 
Yeah, the CP.2's took away the 1.5 t-stop on the 50mm and 85mm, their fastest is 2.1.

If you want ultimate shallow DOF, it's hard to beat the Voigtlander 25mm lens. The f0.95 aperture is still sharp, and the DOF is paper-thin at that rate.
 
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