"Front Focus," "Back Focus," can this be a function of the lens and not the body.

[trex1]

I have front focus problems with two of my lenses when used with a Nikon D70.

The 35-70mm 2.8 AF and the 50mm 1.4 AF. I took the zoom and the camera to Nikon,

and they told me the problem was with the lens, and not the body, and that the

lens was too old to be repaired. How can the problem be with the lens? Does

anyone know about this problem? Additionally, the manual focus screen that I

installed focuses fine, so why is it that the AF sensors cannot handle this?

 

I plan on visiting the Nikon repair center here in Tokyo, to discuss it more

with them, and try and clear things up, as I was in a rush to get out the other

day when I went there, and could not get a full explanation before leaving.

[alan_chan4]

There has been much discussions on BF/FF yet nobody seems to know why. I am sure the designers are well aware of the issue but they are not telling us. What so strange is that some AF lenses worked fine on AF film SLRs suddenly have problem with some DSLRs. My own speculation is that modern DSLRs have received much lower QC and viewfinder/film-plane/mirror-box/AF-sensor calibrations so we are seeing this problem popping up everywhere. So what the manufacturers do was to offset the problem somewhat by firmware calibration. But still, there is only so much can be done if the AF sensor was not perfectly aligned phyically.

[Ed_Ingold]

The Nikon 35-70/2.8 is in current production. There's no way it's not reparable simply by being "old", even if a non-D lens. Contact an official Nikon service agency.

 

Back Focus problems occur because the focal plane of the AF sensors in the camera are not in the same plane as the film/sensor. The non-standard screen you have installed may adversely affect auto-focus.

[alan_chan4]

The AF sensor sits underneath the mirror box so should not be affected by the focus screen.

[trex1]

Just to clarify, as Alan said the screen has nothing to do with the AF system. In fact the

advantage of the mf screen has been that I can actually focus my lenses now! Just to make

it clear, this problem pre-dated my installation of the mf screen. And also, the place I took

the camera to was Nikon's own official service center in Tokyo, so they must be either

bullshitting me, or it is possible for a lens to not match an af system. However the person I

talked to at the store I bought the camera at said it if I can focus it with the screen but the

af screws up, it is a body problem.

 

So, manyana, I am off to do battle with the Nikon service center. First though I will check

at some stores to see how my camera handles some fast silent wave lenses, just to make

double sure.

[alan_wilder1]

Have you tried the lenses on a properly working film body to see what happens? I own the 35-70/2.8 and have never encountered a back focus issue on my properly calibrated F100 or F4. As I recall the 50/1.4 should work fine. My F100 has a separate mirror stop adjustment posts for the main mirror and AF mirror that can easily be adjusted by the user with a small hex wrench. I've done this myself to get maximum sharpness with my lenses in AF or screeen MF. One needs to understand a little about lens performance at various apertures and run some trial shots with bracketing focus to determine the sharpest lens setting on film (or digital). Once exact focus is determined on the distance scale, adjust both mirrors until the electronic rangefinder (AF indicator dot) and optical focusing aid agree with that setting. To complicte things a little, adjusting the mirrors will also shift the apparent position of the AF sensor focus point in the finder up or down so you shouldn't overdo it. Oddly, there are a few Nikon lenses inwhich the AF and focus screen do not exactly agree but fortunately, most do agree. I think this has to do with how the AF system "sees" the image as opposed to how we focus the image on the screen. I can't imagine the lens being at fault especially if it's with a prime as well as a zoom.

[radfordneal]

I've been puzzled by this talk of "front focus" and "back focus" problems. I've never experienced this myself. I would have thought it would mostly be a body problem, but I can imagine the lens having an effect. Focusing is done at full aperture. If the aperture for taking the image is narrower, I can imagine the point of best focus could be different. Of course, this could only happen if the lens has a substantial amount of abberation - a perfect lens should focus the same from the inner part of the lens as from the outer part. So, do you see a problem when taking an image at full aperture?

[alan w]

Just another thought Darius!

 

As you are able to accurately focus manually, i am wondering if you are hand-holding when shooting in AF, on 'static subjects'.

 

Ive encounterd a similar problem when i got my first AF lens, and finally realised my body movement had triggered the 'predictive focus' of my D70 {which is always active in Af modes, and seems to be sensitive to the slightest movement}, and shifted my intended focus a few inches in front of the subject {sometimes the back}. I never had any problems with MF, and after realising what was happening, i found that i didn't get the focus shift when shooting from a tripod with static subjects with AF.

 

If you are getting this problem using a tripod, then disregard the above.

 

Alan

[Michael R Freeman]

"The Nikon 35-70/2.8 is in current production."

 

Sorry Edward, it's not. Discontinued (for some time now I think). This is not the first post that I have read (here and elsewhere) where official Nikon service departments told users that parts were no longer available for this particular model.

 

Current zoom lenses: http://nikonimaging.com/global/products/lens/af/zoom/index.htm

 

Recently discontinued lenses: http://nikonimaging.com/global/products/lens/af/archives.htm

 

Also, the focusing screen is not in the optical path of the autofocus system, which is entirely in the bottom of the mirror box, so a non-standard screen would not contribute to back/front focus problems (with autofocus).

 

Sorry Darius - unfortunately none of the above helps solve your problem. :-(

 

I can see where a very worn drive gear/pinion within the 35~70/2.8 lens might cause small autofocus errors if there was significant mechanical backlash. It is a very tiny gear. Such wear would not be a problem with manual focusing.

 

Good luck.

[gustavo_maurizio]

The FF/BF issue depends on the camera, lens and effective aperture and subject distance.

Both the D200 and the D2x suffered this problem with large aperture (F2.8 or lower) prime

lenses like the 200mm f2.0 and the 85mm f1.4.

I experienced them myself and Nikon was able to correct the bodies in both cases (both

had Katzeye manual focus screens, so no problem there). The correct procedure is to

mount the camera with a cable release on a sturd tripod. Put a highly contraste focus aid

at 2 meters (6/7 ft) from the lens and add a ruler at 45 degrees to make sure if it is a FF/

BF problem. Since the AF sensors of D2xxx cameras are rather large, use SF and focus.

Mirror up, shoot. Progressively close aperture without refocusing. Keep and write a CD

(NEF and JPG) and send it with a nice letter to Nikon service (copy the service manager).

Repair is free, regardless of warranty status. The problem might be the lens, here it is

MUCH more expensive to fix.

GL

[nimit_jethwa]

Hi Darius,

 

Please note i am Canon users, but my experience tells me that Focusing is achived by the body not the lens. The lens purely has a motor in it, which is driven by the camera's AF processing function.

 

Front and Back focusing are caused by both the lens and body. The body has a focal plane which is supposed to be exactly the same distance as the sensor (but at 90 degrees from the subject). The idea is if the plane gains focus, when the shutter flips up the sensor will capture an image in focus.

 

But in the real world, this is rubbish for the most part of it. Purely because the plane in most cases is never 100% in the same location. This is where front of back focus occurs. The lens also contributes slightly, because the during the building process, if the lens is not tightened to the right specification the focusing ring is slightly off.

 

Hopefully like Canon, Nikon will if they haven't already introduce AF Microadjustment, which allows the user (i.e me) to adjust my focul plane to counteract the effects of front or back focusing, through my bodies menu.

 

Hopefully this way helpful and not aload of babble

[terry thomas photos]

<p>Darius Jedburg:<br>

When I lived in Tokyo in 1966 to 1968 I had better luck with Nikon service at the factory, not the service center across from the central Tokyo Train Station.<br>

I'm sorry but I don't remember the Tokyo district where the Nikon factory was located. My memory cells from 40 years ago say it was on a train line that ran south of the Tokyo Central Station.<br>

I hope this helps.<br>

Terry Thomas...<br />the photographer<br />Atlanta, Georgia USA<br /><a href="http://www.TerryThomasPhotos.com">www.TerryThomasPhotos.com</a></p>