50 C-Sonnar f1.5 optimized - question to owners

[kichu1979]

After a week of shooting and seeing the results my C-Sonnar is going to Zeiss for f1.5 calibration. I have too much front focused shots, which I tried compensate knowing of the feature but the hit percentage was relatively small.

I've seen the earlier thread here on the issue and I know how this affects f2.8-f5.6 shooting with this lens.

My question is how manageable is f2.8-f5.6 shooting with f1.5 optimized Sonnars. Please do not make it f1.5 vs f2.8 thread as this is not what I need from you. I'd be only interested in f1.5 Sonnar users what you do when you have to shoot this lens stopped down and how you manage focus shift then.

When do you have to be safe and when (shooting distance) will DOF cover the shift ??

Any opinione more than welcome. Thanks.

 

[Keith]

I'll bump this because I'd like to know more about this subject also.

I've considered a Sonnar at various stages but wouldn't have a clue as to whether my shooting style would suit 1.5 or 2.8 optomisation.

 

[kichu1979]

from my observation, the front focus or point of best focus is aprroximately 5 cm (or 2 inches) before subject when shooting wide open at a 1m distance

the more you stop down then front focus is less and less, also depth of field
is larger so at around f2.8 the lens is back into game (effect is minimized)
and completely gone around f3.5

so when shooting f2.8 optimized lens wide open or close to wide open the only option not to see front focus (except for leaning, over-turning focus ring and all that tricks), would be to be to stand at least approximately 2 metres from the subject so the larger DOF would cover the front focus

DOF for 50/1.4 lens focused at 1m is approx 3,5 cm - half in front of subject, half after the subject

DOF for the same lens/aperture combination at 2m is approx 13cm, 6,5 fron, 6,5 back

so generally speaking the more you are going to shoot the lens wide open or near wide open at a 2 mtrs and closer distance the more you are going to experience the focus shift wich results in front focus of the lens

for me this is 90% of my shooting so I will be sending the lens to f1.5 optimization and I will probably buy 50/2 planar as a second general purpose 50mm

as a general rule for shooting the lens and considering the fact that focus shift is approx 5cm in the worst scenario, your depth of field for a shot, which depends on focus distance + aperture combination, would need to create DOF that is large enough to cover the 5cm fron focus
you need depth of field that has 5-6 cm area of focus behind the subject

for dof calculator click below
http://www.dofmaster.com/dofjs.html

the dof calculator has f1.4 and f1.6 lens, but I used f1.4 for the calculations to be more on a convervative side (minimum for f1.4 should be enough for f1.5)

 

[john_s]

I had one of the first ones and I did some careful tests. I found that around a distance of around 2.8m, the focus was:

f1.5 150mm in front
f2 100mm in front
f2.8 spot on
f4 50mm behind
f5.6 DoF took care of it

You can use DoF calculations to determine if there is enough DoF at these distances to satisfy you. Apparently there are users who find it fine at f1.5. This is about the same sharpness as in a postcard sized print on 1930 quality film (barely).

The problem if you have it optimized for f1.5 is that then at f2 it will back focus around 60mm, at f2.8 around 150mm and at f4 around 190mm. If this is within your DoF criterion, go for it. At least the focus inaccuracy would all be at the same direction (back) which might be easier to adjust for. A bit of back focus is preferable if you're taking portraits and focusing on the nearest eye in my opinion.

Most if not all f1.4 type lenses have some focus shift. It's just that this one has a lot.

For digital apparently it's more critical, but I don't have an M mount digital camera (just as well, probably). I have noticed that even Erwin Puts when measuring the absolute resolution using Leica digital equipment does focus bracketing or something like it to get it just right. I think that any sharpness that you can't get using the rangefinder is irrelevant. I think that the MTF graph on the Zeiss web site which shows good sharpness at f1.5 is very misleading, quite naughty in fact.

My tests were done with 25 speed film, tripod, cable release and the usual target on the floor, camera about 1.4m above the floor and pointing down at 30degrees. My M6TTL had just come back from rangefinder adjustment and the fact that it focuses spot on at f2.8 with this lens suggests that it's about right.