50/1.4G backfocus issue

[bgolds99]

Hey Guys,

My 50mm 1.4G is definitely back focusing. I've spent a few weeks using it and have come to the conclusion that i just can't live with it focusing like this. Of course it is especially a problem when using it wide open. It seems that when in live view, it is much more accurate with the contrast focus, but through the view finder the AFS is off significantly. Anyone know where i can take it around Newcastle or Sydney to have it recalibrated? And any idea how much it costs. I purchased it new, but its a grey import, so i ran the risk and it burned me.

Cheers for any help you can give me.

[kiwi]

Tried in camera micro adjust ?

[bgolds99]

D90 doesnt have the micro adjust ability. I've heard of fiddling with the screws in the mirror box to adjust the angles of the mirror slightly, but im worried it is going to throw my other primes out of whack.

[bgolds99]

Actually i just checked the definitions of back/front focusing, and it appears my lens is affected by front focusing. Did a quick test with my 35/1.8G as well and it seems to front focus about half the amount of the 50/1.4G... I've never noticed it with the 35 in the real world, but its definitely happening when shooting with the subject 1-2 feet away.

So, with 2 lenses front focusing a little, i decided to have a go at adjusting the screw in the mirror box. Has anyone done this before? How much do i turn the screw, and which way? I've done a reasonably thorough google search on this topic and it seems people arent really discussing the details of it.

[kiwi]

Not something I'd be doing

[William]

Not me either :eek:

[arthurking83]

Actually i just checked the definitions of back/front focusing, and it appears my lens is affected by front focusing. Did a quick test with my 35/1.8G as well and it seems to front focus about half the amount of the 50/1.4G... I've never noticed it with the 35 in the real world, but its definitely happening when shooting with the subject 1-2 feet away.

.............

So, with 2 lenses front focusing a little, i decided to have a go at adjusting the screw in the mirror box. Has anyone done this before? How much do i turn the screw, and which way? I've done a reasonably thorough google search on this topic and it seems people arent really discussing the details of it.


Here you have to separate issues to address.

First up, is the real world usage of the gear in question.
Sometimes .. maybe many times .. the issue of misfocusing can be different for different uses of the gear.

(we'll use an example of the 50mm lens referred to here) .. Say if you use the lens for close in tight crop images of a person, wide open and find that it's slightly backfocusing and hence missing some crispness in the eyes, you adjust the backfocus level to compensate for this purpose, but if the lens focuses fine when subject distances are further out.. say 20meters or more(ie., close to infinity) you may end up with less sharpness in those shooting situations.

So be very weary of adjusting the focusing ability of the camera/lens combination to suit one purpose at the expense of doing harm in other areas!

Adjusting the mirror stop screw is very easy, and unless you klutz it, you can't do any damage.. but you must take notes, only for the purpose of resetting things back if required.
The screw is a simple cam system, where it moves the relative position of the mirror, and hence refocuses the image differently for either the AF system(which is one screw) and then the MF system(which is another screw, and less of an issue if you stuff it up)
Technically speaking you can't stuff it up, even someone like me with no organisational skills and too much eagerness to delve into the guts of it all, without a plan, pen, paper or nuthin sensible like that.

Note camera's with Live View are so much easier to do this on, than those without(Lv).

I have diagrams for anyone wishing to try this, if they really want too. BUT.. and this is a major warning .. as the issue affects the lens you're having trouble with, and you haven't mentioned this previously.. in effect meaning that the issue is only for this lens and not any of the others, you will ruin the current working relationship with the other perfectly focusing lenses!!!

hence it's very strongly recommended that you do not do this(adjust the mirror stop) for AF.
if the problem is persistent and real, better that the camera and lens down to a local authorised repair centre and have them recalibrate the lens instead!!

What I did, was to adjust the manual focus mirror stop. I played with the AF screw as well, but of course because in another life I must have been a cat!

The AF screw is the deeper one, the screw that is more readily within reach is the manual focus adjustment screw.

You will need a very long allen/hex tool, I think it's about 1-1.5mm in size, quite hard to find.
I purchased a set of tools, just to use this one particular allen key, and have saved it in a very safe and convenient place, where no one will ever find it.. not even me.

LOL! I recently reorgainise my study, and I can't locate it yet, as the study is in more of a mess. So it'll take me a bit of time to find it.
I also have a link to a very accurate diagram, but again it's buried under a mountain of links in my bookmarks, so may take me a while to find this(if it still active).
I'd written my diagram down on a sheet of paper, and had the idea to one day transfer it onto my PC, but of course never got around to it! :D

.. have I mentioned that I'm quite disorganised but spontaneous? :p

On a slight tangent, but I think still pertinent, I found that the best cure for slightly misfocusing lenses, was to acquire a thirdparty focusing matte screen(I got a KatzEye) with a split prism.
The split prism was very accurate for confirming focus(as long as subject was centred or close to it), and as long as the lens was an AF-S type, ie. non screw driven. it was easier to give the lens a quick tweak to achieve a better focus, rather than rely on AF 100% of the time. Other benefits in having a much better focusing matte screen are a brighter viewfinder(in some cases) and better manual focus ability, for focusing on the periphery of the frame.

[bgolds99]

OK, so time for an update. Kiwi and Bernie, adjusting the screw in the mirror box is really no big deal, and as Arthur mentioned, it's completely reversible.

So before doing anything i set up a test chart with my camera on a tripod and went through all of my prime lenses to see what the state of each was. Tamron 90/2.8 was pretty much spot on, although a little irregular in its focusing. All resulting tests for the Tammy were very close to the desired focus point. The 35/1.8G had a slight front focus issue at a focus distance of around 45cm. The 50/1.4G had a slightly bigger problem, but still not enough to explain what i've been noticing with regular use. ANyway, i adjusted the Auto focus screw slightly, just to bring the two nikkors in line (the Tamron is still well on target).

Now, I put the 50/1.4G back on and decided to have a little shoot around the house just to test it a little further in practice. The problem appears to be when focusing at a distance.... the further away the focus, the more the lens front focuses. To test this, i set up a few household items along a corridor in the house and fired a few shots off. Confirmed. The 35/1.8G does not behave like this at all.

What the hell do i do about this? Is it something only Nikon will be able to fix?

The main reason i purchased this 50mm was to use it as a walk around 'normal' prime, if/when i upgrade to FF, and in the meanwhile use it for medium close range people shots. This focus issue pretty much deems it useless as a walk around normal prime on FF. I'm guessing my only option is to send it off to NIkon. Any idea how much this type of service costs?

[arthurking83]

One of the reasons I recommended not to do this(the screw adjustment) is that you've adjusted the lens at the incorrect subject focus distance.

You should have had the test chart at a more appropriate working distance.. say 2meters or more away when you 'checked focus'.

All you've done in doing the testing checking and mucking about is allow for what's referred to as focus shift.

Wide aperture lenses have what's referred to as focus shift.

If you understand how to do the test, what you should have looked for in the testing, was how the lens was focusing, at this 45cm distance, over a wide range of apertures.

Have a read up on how focus shift works, but what it basically amounts too, is that as you stop down, the DOF gets naturally deeper, but if you know what to look for, you will notice that the actual point of focus is receding further back too.
While it natural to assume that this sharper point of focus is a product of deepening DOF, there is and will always be only one point of focus(or more accurately plane of focus).
The problem with looking for a plane of focus is that the plane may be curved and so you may think that there is a soemthing wrong with the lens being soft at the edges or whatever.

Without a full understanding of how to test, what to look for and so on, all you end up doing is chase your tail in futile endeavours for perfection!

I would say that the lens is most likely operating correctly, and that if you send the camera and lens to Nikon to fix it, they will return it with a comment that the lens is within spec.

You asked the question of "what the hell do I do about this" .. and my recommendation is to re adjust the camera so that the camera focus better at the slightly further distances(but this is optional).

Take many shots, in may situations, under different conditions. Learn what the lens is capable of producing .. or not.
If need be, adjust yourself to suit the gear.

This may well be easy for me to say, but I do so, with a small amount of experience(and that's with the Tammy 28-75), and otherwise great lens, only let down by front focusing at f/2.8 at 75mm @ close up(ish) focus distances.
My 'work around' is simple and in general 99% effective.
The focus ring on the lens has a very small amount of play .. about 0.1-0.5mm of slack, which would normally be something to be weary of, or concern about.. but in this instance the amount of slack is the perfect match to the amount of rear bias focus play that I need to get the focus back to dead on! :th3:
As I use the AF-On method to focus(and why I've been recommending anyone with this feature on their camera to also do so, for a long time), but also to uncouple focus lock from the exposure.
So what I end up doing: focus, flick focus ring, shoot. Sounds tedious or cumbersome, but in fact it's not!
What I have to remember is that I'm at 75mm, and not 60mm or so.
All in all, it works a treat, and I've not bothered to get it to Tamron to have it calibrated or serviced or whatever.

In your case the answer is simple. reset the camera back to where it was working at this medium close range people shots

What you almost certainly noticed in the hallway test shooting is that the inherently deeper DOF of the 35mm at roughly similar aperture values.

Something that you really should be aware of too is lens variation.
Another tech not to be aware of too, is that there are camera to lens tolerances that will also affect the relationship between a camera and a lens, and where a lens works a bit strange on one camera, but will work perfectly fine on another.

Apart from this, I can't really offer any other 'solutions' remembering that I've already mentioned the third party focusing screen matte option too.
If this is something you may want to explore, (from experience)don't even think about the cheapo $30 solutions on ebay!!