Nikon 50mm F1.8 Front focus on D80

[CT 3833]

I bought a Nikon 50mm F1.8 recently. Noticed the lens has front focus. Please refer to a very simple test shot below. I sent the lens to NSC but I am told this lens is not designed for DSLR and therefore is expected to have front focus. They performed a test and had confirmed my observation. And NSC told me that this FF cannot be calibrated.

Anyone has similar experience? Any advice?

Simple test below, camera mounted 45degree to the paper plane, about 30 cm away.
Focus point at x but the sharpest line turned out to be about 3 lines(about 10mm) nearer to the camera.

 

[zac08]

Dun have such a problem on my 50mm... :dunno:

 

[zoossh]

I sent the lens to NSC but I am told this lens is not designed for DSLR and therefore is expected to have front focus. They performed a test and had confirmed my observation. And NSC told me that this FF cannot be calibrated.

i'll be curious to hear from more experts. is it the 50mm f/1.8 AF D version?

as far as i know, "designed for DSLR" can means two things.

1. it has a smaller image circle thus compatible only for smaller sensor formats and will cause physical vignette on formats larger than APS-C size.

2. it has special coatings on rear lens elements to minimize problems with flare caused by light reflected off the sensor that may cause a veiling type of flare which lowers contrast.

so i dun understand why "not designed for DSLR" can be a reason for front focusing? i mean how does the difference between a film sensor versus a digital sensor changes the lens mechanism?

 

[roygoh]

so i dun understand why "not designed for DSLR" can be a reason for front focusing? i mean how does the difference between a film sensor versus a digital sensor changes the lens mechanism?

Same here....

I have the same lens (not sure if D nor not D) and it was accurate on my D200.

Usually the bigger contributor to front or back focusing is the body. The optical path from lens to image sensor has to match the path from lens to AF sensor. Once these are matched then the focus should be accurate independent of the lens.

If the path does not match then the amount of front/back focusing can vary from lens to lens.

Speaking from my own experience only, from tweaking the mirror stop position on my D1x to fix a back focusing problem, as well as tweaking the focusing screen position on my D200 to fix manual focus error problem..

To the TS, do you only have focusing error on this lens? Have sonducted the same test on all your lenses?

 

[vince123123]

Oh my, this is one of the weirdest reasons I heard from NSC staff....and I thought the one given to me on another issue (front focussing on my D200) was weird. Looks like they now blame it on the lens.

I bought a Nikon 50mm F1.8 recently. Noticed the lens has front focus. Please refer to a very simple test shot below. I sent the lens to NSC but I am told this lens is not designed for DSLR and therefore is expected to have front focus. They performed a test and had confirmed my observation. And NSC told me that this FF cannot be calibrated.

Anyone has similar experience? Any advice?

Simple test below, camera mounted 45degree to the paper plane, about 30 cm away.
Focus point at x but the sharpest line turned out to be about 3 lines(about 10mm) nearer to the camera.

 

[geraldkhoo]

Simple test below, camera mounted 45degree to the paper plane, about 30 cm away.
Focus point at x but the sharpest line turned out to be about 3 lines(about 10mm) nearer to the camera.
QUOTE]

Looks like a front focusing problem... mmm... could you test it with a distance of about 50cm away?

The reason from NSC is strange. If the explanation is from the technician, then ask to speak to a tech support guy. I had an issue previously and the way the technician explained only seemed to make me confused and not understand, so he asked me to hold and asked a tech support guy to come out, and the tech support guy explained step by step my problem, and I understood.

Also, if your D80 is still under warranty, you can send it in and ask them to do tests on it and say that there seems to be a focusing problem where images are not sharp. They would then test it out with print outs, and may do adjustments to your D80.

 

[lsisaxon]

Same here....

I have the same lens (not sure if D nor not D) and it was accurate on my D200.

Usually the bigger contributor to front or back focusing is the body. The optical path from lens to image sensor has to match the path from lens to AF sensor. Once these are matched then the focus should be accurate independent of the lens.

I thought so too, but I was wrong. The lens has a backlash correction factor as well. It seems like nowadays, the AF algorithm doesn't hunt anymore but rely on the backlash correction factor to back off after the focus point is obtained, so the lens calibration is important also.

If the path does not match then the amount of front/back focusing can vary from lens to lens.

Speaking from my own experience only, from tweaking the mirror stop position on my D1x to fix a back focusing problem, as well as tweaking the focusing screen position on my D200 to fix manual focus error problem..

To the TS, do you only have focusing error on this lens? Have sonducted the same test on all your lenses?

If the back or front focus is consistent across all the lenses, the I'd say it's a body calibration problem, otherwise if some are front, some are back then it's probably the calibration of individual lenses. There wasn't such a problem in the past because people don't 'grain' peep like people pixel peep nowadays.

http://www.adorama.com/catalog.tpl?article=010207&op=academy_new

PS. Shouldn't this be posted under Nikon subforum instead?

 

[zoossh]

It seems like nowadays, the AF algorithm doesn't hunt anymore but rely on the backlash correction factor to back off after the focus point is obtained, so the lens calibration is important also.

any layman explaination?

 

[lsisaxon]

any layman explaination?

It's as layman as I think I can get.

Let me try again..

In the past, once sharpness is detected, the body will command the AF motor to stop, but motor will not stop immediately, so the body has to turn the motor back and forth a little to get back the sharp point again. (hunting)

Now, the AF would rely on the lens information to say how long it will take to stop once a stop command is issued, so when sharpness is detected, the camera will turn the lens back by the specified amount and it will not check focus again, so in a way it might not be spot on but it's definitely faster. (backlash correction factor) So best way is to get more accurate focusing is to half-press and release a couple of times for it to AF again. :dunno:

Another possibility is that the AF sensor is already at the maximum level of precision but because people are able to pixel peep nowadays, it seems to be not accurate? ;p

 

[zoossh]

It's as layman as I think I can get.

Let me try again..

In the past, once sharpness is detected, the body will command the AF motor to stop, but motor will not stop immediately, so the body has to turn the motor back and forth a little to get back the sharp point again. (hunting)

Now, the AF would rely on the lens information to say how long it will take to stop once a stop command is issued, so when sharpness is detected, the camera will turn the lens back by the specified amount and it will not check focus again, so in a way it might not be spot on but it's definitely faster. (backlash correction factor) So best way is to get more accurate focusing is to half-press and release a couple of times for it to AF again. :dunno:

Another possibility is that the AF sensor is already at the maximum level of precision but because people are able to pixel peep nowadays, it seems to be not accurate? ;p

you mean by the time the AF sensor detected the maximum contrast and sharpness and send an order to the motor to decelerate and stop, the motor already overshoot by a certain distance, say maybe 1mm. that overshot distance is called a backlash. but the body can measure the distance and specify the motor to move back by the same distance (back off). the amount that the motor moves back can thus affect fine focus if it is not calibrated well enough. right?

 

[CT 3833]

I am glad to see so many interesting discussion. Thanks for your feedback and participation.

(1) I dont believe I have a focusing error because the front focus is almost consistent on every test shots.

(2) I do not think it could be my D80's problem as I do not have the same focusing problem on my 18-200mm.

(3) NSC tested my 50mm lens, and compared with their own 70-200mm shot using my D80, the charts indeed show that the 50mm is of front focus by 20 micron, whereas the focusing produced by the 70-200mm is spot on.

(4) NSC asked if it can be due to my hand shake and did I use a tripod, my point is, if it is due to hand shake, then none of the line would be sharp.

 

[zoossh]

but does it actually impact on your photos in normal screen size or in print out?

 

[scaredcloud]

you mean by the time the AF sensor detected the maximum contrast and sharpness and send an order to the motor to decelerate and stop, the motor already overshoot by a certain distance, say maybe 1mm. that overshot distance is called a backlash. but the body can measure the distance and specify the motor to move back by the same distance (back off). the amount that the motor moves back can thus affect fine focus if it is not calibrated well enough. right?

backlash is actually related to gear engagement, where clearance comes into picture. Depends on the application and accuracy, gear tooth thickness variation is guided by standards, and the tolerance could sometimes limit to a 10micron state.
When gear is in forward / reverse rotation, at the point of stationary, the position will never be exact, and since tooth thickness variation, each point is different, hence the backlash correction. And each individual set will be different.
When motor in stepper form, when no of steps to move is the command, however, no of steps will be correct but clearance shift is different, hence calibration is needed.

 

[CT 3833]

Hi,
I drop by Lord's today, after showing them the front focus chart printed by NSC, they replaced the 50mm lens with no questions asked. Unfortunately I do not have the D80 with me now(actually borrowed from my friend, mine was sold some time ago). I will wait till the D80 is available before I could test the new lens.

Will keep you all posted.

I take this opportunity to mention Lord's good and reliable service.

Zoossh,
I started teting front focus error because I noticed some of my photos from the 50mm were not as sharp. So yes it does impact my photos.

By the way, noticed another person having similar problem with his F1.4. And being given the same reason by the local NSC there.

http://dcfp.nikonians.com/dcfp/dcbo...orum=146&topic_id=121003&mesg_id=121003&page=

 

[psychobiologist]

if you are working by that distance you have mentioned which is probably the min focus distance of this lens, at 45cm, the depth of field at f1.8 is only an acceptable half a centimetre.


you cannot compare with the 70-200mm which focusses min 1.5m

when you start playing with a depth of field of half a centimetre or less, chances is that there might be AF in accuracies. how about you repeat half pressing the shutter to see if the focus changes? i used to play with the 50mm f1.8 and at min focus distance, if i keep half pressing, there is slight focus changes indecisively.

 

[gooseberry]

As I mentioned in another thread, the test chart you've used - the lines are so close together, you can't rule out the fact that the camera just focused on the line(s) further in front of the X (the focusing sensor area is larger than you think it is).

Google for D70 focus test chart to get a pdf of a much better test chart.

 

[zoossh]

maybe try out 1m away, 2cm lines apart. that may be more practical for shooting maybe a face, distance between eyes and nose, and where wrong focusing may mean the eyes became not sharp. i think this is a very reasonable expectation that the lens should performed correctly in this situation,

 

[zoossh]

chances is that there might be AF in accuracies

just for curiosity, is this where camera bodies with more (and newer better) AF sensors will perform better and more reliably? sorry for asking such a newbie question, but still feel i'm unsure of a lot of things.

 

[lastboltnut]

Hi Isisaxon,

I am not sure about the cam AF mechanism....but base on mechanical defition, I think scaredcloud is right. By mechanical definition, backlash means the error when the gear move in one directionthen reverse. For example at a location "0" the motor moves the gear 1deg Clockwise (CW), and then the motor moves it back 1 deg, but actually the gear in not in location "0" as there is a backlash error.....thus a backlash error only comes in if the focus hunts.....if the focus only move in one direction, there won't be any backlash problem....

The overshooting of the focus point, like cam ask motor to stop, but motor did not stop on time is a control problem.... like the gain of the control is not correct...

I am just trying to discuss in mechanical point of view...Hope someone can shed more light to this issue regarding the Nikon AF mechanism.

It's as layman as I think I can get.

Let me try again..

In the past, once sharpness is detected, the body will command the AF motor to stop, but motor will not stop immediately, so the body has to turn the motor back and forth a little to get back the sharp point again. (hunting)

Now, the AF would rely on the lens information to say how long it will take to stop once a stop command is issued, so when sharpness is detected, the camera will turn the lens back by the specified amount and it will not check focus again, so in a way it might not be spot on but it's definitely faster. (backlash correction factor) So best way is to get more accurate focusing is to half-press and release a couple of times for it to AF again. :dunno:

Another possibility is that the AF sensor is already at the maximum level of precision but because people are able to pixel peep nowadays, it seems to be not accurate? ;p

backlash is actually related to gear engagement, where clearance comes into picture. Depends on the application and accuracy, gear tooth thickness variation is guided by standards, and the tolerance could sometimes limit to a 10micron state.
When gear is in forward / reverse rotation, at the point of stationary, the position will never be exact, and since tooth thickness variation, each point is different, hence the backlash correction. And each individual set will be different.
When motor in stepper form, when no of steps to move is the command, however, no of steps will be correct but clearance shift is different, hence calibration is needed.

 

[zoossh]

am i right in say so far that all what you guys are discussing in terms of how much focusing moves front and back are determined by how much a wheel moves inside moves in terms of the circumferential distance (which moves in intervals of the teeth on the wheel)?

getting a bit too technical for me. but still want to clarify.