Manual Focus Lens - Focus off? What is happening?
I own a 50mm Nikkor f1.4 AIS. I had begun to test this lens today to see just how sharp it is at f1.4, when I came across something interesting. I was using a chart and shooting at it from an angle so as to get only one line in focus and then zoom in on the digital image and check its sharpness. When I focused on a point (I did this MANY times, tripod and not) the focused area in the image was always closer to me than where I focused ( I have better than perfect, around 20/16 vision). I am confused now, because if the lens is focusing to my vision, which in the viewfinder should be aligned with the focal plane where the sensor or film sits, should not any error be corrected when I focus? I tested my other lens the 28mm f2.8 AIS as well and found that exactly the line I focus on was the only thing in focus all the way open. I narrowed the problem down to this particular lens. What is going on?
Im actually not positive what version this lens is. Its a 50mm Nikkor f1.4. It has a silver ring in front of the aperture with only a single black dot at the selected aperture position and the focus point distance. There are smaller aperture numbers on the end of the lens closest to the camera so I can read them through my F3 viewfinder. The aperture numbers are color coded to the DOF guide scale. I cannot find an image of my lens online anywhere. It has block grips on the barrel, most of the ones I see online have two rows of these, mine has three. The lens also gets a little larger as it goes towards the front element and dips down into the front element. The serial number is 2825811.
This is the closest image to the lens that I can find anywhere. However, mine also has the smaller aperture numbers below the larger ones to be viewed in a viewfinder. The aperture ring also looks a bit different.
It's either an AI or AIS.
If you look at the lens mount with the meter prong at the 12 o'clock position. There's a groove at ~9 o'clock. If it's an AIS it will have a small groove just above and slightly inside the groove. If there's no groove, it's an AI lens.
Does the lens focus correctly on the F3? You can check at the film plane by putting a ground glass or even frosted scotch tape on the rails and check focus there.
If they don't agree, the image at the rails is correct. There's a possibility that the set screws holding the focusing ring weren't tight enough and allowed the ring to move slightly. The fix is this: peel the rubber grip off, find the screws, loosen them set the focusing ring at the infinity stop(without moving the barrel) and snug the screws down. They're right around 1mm screws, use a small screwdriver.=)
Just saw the pic, The lens in the pic is a non-AI lens but yours is a lens that has been AI'd. Likely
from when Nikon was offering the service.
Last edited by John Koehrer; 07-28-2011 at 08:59 PM. Click to view previous post history.
Heavily sedated for your protection.
A 28mm lens will have more depth of field than a 50mm lens, so you may in fact not be focusing the wide angle where you think you are.
"She's always out making pictures, She's always out making scenes.
She's always out the window, When it comes to making Dreams.
It's all mixed up, It's all mixed up, It's all mixed up."
From It's All Mixed Up by The Cars
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Im aware of the depth of field issue. The replicate a shallow depth of field I moved very close at a steep angle on the chart, since my 28mm can focus VERY close, and the depth of field was very similar, the point of sharpest focus was dead on.
Thanks John, now how the hell do I get this rubber off? One thing I did notice about this lens was that it shifts just a tad when I shoot. That must be the conversion issue.
Even though the 28mm seems to focus accurately, I'd still consider a camera body problem (which BTW?): misaligned mirror or badly seated focusing screen.
Try to beg, borrow or steal a longer lens, which will make testing easier.
The AI conversion shouldn't make the lens "shift", whatever you mean exactly by that. If there is any play in the lens mount (loose mount screws or worn flanges), that could also cause the problem.
M6, SL, SL2, R5, P6x7, SL3003, SL35-E, F, F2, FM, FE-2, Varex IIa
What body are you using the 50/1.4 on?
You get the rubber off by sliding a thin piece of flexible plastic under it, a piece of 4x5 sheet film works perfectly. Slide it around all the way to break any glue bond and then slide the rubber sleeve off the front. That lens in a pre AI one according to the serial number, and likely has the factory conversion aperture ring on it. Check for loose screws in the lens mount.
Edit: I suspect he is using a digital body.
If you are using one and focusing manually, the viewfinders on AF type cameras are not meant for manual/visual focusing. You are looking at an aerial image, not one focused on a ground glass. If the diopter setting on the camera is off, or if your vision is not 20/20, or if you have young eyes(under 40+), your eyes can easily focus on the wrong image plane in the viewfinder, and that will make the recorded image out of focus.
Last edited by Bob-D659; 07-28-2011 at 11:56 PM. Click to view previous post history.