Front focus on the planar?

[kipkeston]

I suspected that my recently acquired planar was having either some kind of focus shift or a front focus at wide apertures. I did a test and it seems there is a slight front focus.

tested at: .7m closest focus, M6, on tripod, 250th and f/2, tri-x (sorry nothing high res), scanned on the coolscan V at 4000dpi and cropped

here is the link

Also when I put the 35 biogon on, I had to move the tripod back a tiny tiny bit to get .7 to be in focus on the biogon. That seems to contradict the idea of front focus on the planar? So maybe it was my error in the test.

Does the camera have to be perfectly parallel with the paper for it to be in focus? I'm not sure I had that variable under control.

What do you think?

 

[Ronald M]

forgot the link

Make sure the error if there is one, is not in the camera

 

[mfogiel]

kipkeston
You should place the test chart on the floor and place the camera on a tripod at a distance such, that the diagonal is 70 cm - actually, I'd encourage you to use 1 m distance for easier handling. The camera has to be directed at the focus line at an angle of 45°

Personally, I have tested my Planar with the M7(0.85) and ZI, and the result is within 2mm tolerance - not discernible from human error. In your case, it looks like you are front focusing by 12mm, but I'd check this body with other lenses too, as it may be a body problem and not a lens problem. In any case a 12mm front focus would not be a big deal to manage, and as soon as you stop down a bit, it will disappear in the dof.

 

[kipkeston]

Ok, thank you. I will give it another go. I don't have any other bodies just the 35 to test it with.

 

[Sonnar Brian]

A Planar formula lens has very little focus shift compared with a Sonnar. 1M is probably a better test. I would set up everything on a Tripod, focus both lenses using the RF, and can judge by the finished pictures. However- if you have a loupe, you can also put a film strip at the film gate and look at the focus directly.

 

[awilder]

I would fully expect there to be a little bit front focus at f/2 with the Planar as the lens is normally going to have SOME focus shift due to spherical aberration, usually on the order of about 0.05 mm on a symmetrically designed 6/4 double Gauss layout like the Planar or current Summicron. This would be roughly equivalent to a 1/2 mm turn of the focusing ring and probably consistent with your test if you turned it slightly by 1/2 mm as if to focus behind the subject. I've test several Summicrons by bracketing focus and they all exhibit a slight focus shift stopping down from wide open to f/4 or f/5.6.

 

[kipkeston]

Well I tried again. It seems to be about the same result. 2.8 looks right, it looks like what I would expect, depth behind and in front of the "focus here" but at 2 it doesn't seem to have the "focus here" in focus.

I will try again tomorrow

 

[mfogiel]

If it front focuses by 12 mm wide open, then with f2.8 your dof extends roughly 35% in front and 65% at the back, so you get perfectly symmetrical, however, this does not tell you anything about what is the cause. My Planar has no focus shift and is totally accurate, so as I said before, take another 50mm lens and try it too to get a better idea if it is the lens or the body.

 

[kipkeston]

Well finding another 50 would be difficult. I don't think I know anyone with an M mount anything. I'm not sure why yours doesn't and mine does. Can I send it somewhere to be adjusted?

 

[mfogiel]

My guess is, the fault is with the rf on your body, and then a 1 cm misalignement wide open is nothing to worry about. To test the point, make some portraits wide open from 1.3-1.5 m (this is necerssary to avoid face distortion), focusing on the closer eye, and firing without recomposing the image, then see if you are happy with the results. If it is tha case, just forget about the idea that you have a problem.

 

[Avotius]

...make some portraits wide open from 1.3-1.5 m (this is necerssary to avoid face distortion)

Am I the only one who this face distortion thing never seems to come up on? Most of my portraits are taken 1 meter and in and I never have problems with face distortion except with certain lenses like my canon 17-40 which is already highly distorted.

Anyway, I have a planar 50 and have used it a lot shooting wide open in conditions where you dont get to take your time focusing and never had a problem with this focus shift stuff, which is why I chose the planar over the sonnar. It is possible your body or your lens is somehow out of alignment somewhere.

 

[kipkeston]

Here is the latest test. Scanned at 2000 dpi this time. I hope it's still useful. It looks like the biogon is doing it too! Maybe it really is my camera... I'd rather my camera need adjusting than the lens...

planar

f/2

f/2.8

f/4

biogon

f/2

f/2.8

f/4

for these tests I put the "focus here" in the center of the RF patch

 

[mfogiel]

kipkeston
Check your body, it is highly unlikely that these lenses front focus this way.

 

[Sonnar Brian]

Looking at this batch, it looks like the camera body needs to be adjusted.

 

[kipkeston]

ok, thank you. I will pursue that now.