Front focus problem

[Jon Thornton]

I recently bought myself a Nikon 500mm f/4G VR lens. In combination with my D300 and my Nikon TC-14E teleconverter, it front focuses really badly.

I have tried using my in-camera autofocus fine tune control to fix the problem. I even splashed out on a Lens Align Pro focus calibration target to help with the calibration process. My tests showed that an AF fine tune value of +20 was not enough to correct the problem, so I sent my camera, lens and teleconverter to the Nikon authorised repairer in my city. When they failed to fix the problem, I sent my lens to Nikon directly. They calibrated the lens so that it autofocuses properly without my teleconverter, but with my teleconverter, my camera still front focuses really badly.

I repeated my tests with my brother's D300 and got similar results. The teleconverter and lens front focus on both bodies. This discounts the camera as the source of the problem. I also tested my 200-400mm lens and teleconverter. Although this lens front focuses a little bit with my teleconverter, it can easily be calibrated using my in-camera autofocus fine tune control.

Could the teleconverter be the problem? Is it unreasonable for me to expect the same level of AF accuracy from my 500mm lens as I enjoy from my 200-400mm lens? What would you do if you were in my shoes? Any tips would be greatly appreciated.

[Robert Amoruso]

Jon,

In trouble shooting, eliminating variables or replacing one part for another to narrow down the suspect is a valid method and that is what you are doing. It seems that you have not yet tried another 1.4x tc on your newly calibrated 500mm and D300. If not, borrow someones and see what happens.

Did Nikon get the camera, lens and TC for calibration? If so did they not determine what was the problem and with which component?

I feel that you shuld be able to expect the same level of accuracy with the 500mm as the 200-400mm. It seems that the TC is the problem based on what you are describing.

[Alfred Forns]

Jon there is something wrong with that lens.

Who did you buy it from? First would go to them. I know returning would be tough since they are so difficult to get but what good is something not working properly.

Would try the suggestions Robert made to rule out any doubt but feel there is something wrong. Never ran into a similar problem with the 600 VR or any other. The adjustments should be for fine tuning !!

[Cliff Beittel]

Jon,

I also recently bought a LensAlign Pro, and on my first check of my 600 IS found it was outside Canon's adjustment range. A later re-check, however, showed it was best with no microadjustment at all. My conclusion was that the LensAlign ruler scale is hard to read with long lenses even on a monitor, let alone with Live View. Perhaps that is why RawWorkFlow is planning to offer (perhaps already is) a much longer scaled ruler for use with the LensAlign when checking supertelephotos. Also, Canon recommends its lenses be tested at no less than 50x the focal length of the lens, or roughly 33 yards with a 600mm. I found that impractical and tested at maybe 33 feet.

[Jon Thornton]

I've had some further thoughts on my autofocus problem. I now suspect that the distance between the front of my camera and my imaging sensor is not the same as the distance between the front of my camera and my autofocus sensor.

As I understand it, phase comparison autofocus sensors compare two copies of the image to determine the extent to which the image is out of focus and the direction in which the lens needs to be moved in order to sharply focus the image.

Given that my equipment always front focuses, I reckon that my AF sensor is closer to my subject than my imaging sensor. Even in continuous AF mode, my equipment does not make the necessary refinements to bring my selected point into sharp focus.

Robert, I agree that trying a different TC would be a good idea. I will do this if I can.

Alfred, thanks for the support. As you suggested, I will take it back to the person who sold me the lens.

Cliff, I also tested at about 30ft because I figured that I should test at the distance from which I plan to use my lens. I have found that running a high pass filter across my test images allows me to see the location of the band of sharp focus more easily.

[MikeCornwell]

I hadn't seen a Lens Align Pro and looked online. Very nice, but $140. I have a Canon 600mm and used Artie's pdf calibration pic and instructions from his Mark III users' guide - $20. It improved the focus accuracy of the lens a lot. I recently did a calibration of the 600 and a Canon 2x extender. I'd never liked the 2x extender but there was a huge improvement after calibration and I now never hesitate to use it if I need the reach and have the light.

[Cliff Beittel]

. . . I . . . used Artie's pdf calibration pic and instructions from his Mark III users' guide - $20. It improved the focus accuracy of the lens a lot. . . .

I'm assuming the calibration pic is two dimensional? Seems to me it would work fine, but Canon recommends a three dimensional target.