Fixing backfocus and frontfocus - DIY

Rashkae · 26th May 2008

Found this advice on a different forum (courtesy of user Ma55l), posting it here:

The way the focus works in all the Sony and Maxxum DSLR cameras is not from information on the main sensor but rather a second sensor on the floor of the mirror chamber. (see Sony explanation re a350 live view). The focusing sensor is mounted on a platform that can be adjusted up or down by turning some set screws. It is fed part of the light path by a second mirror mounted under the main mirror. So by adjusting the set screws you can raise or lower the platform. That brings the focus sensor closer or father away from the back of the lens. It is a simple and predictable system.

OK now you understand the focus scheme and what is not correct and needs to be adjusted. The three set screws can be reached thru the bottom of the camera. If you look at the tripod mount hole you will see a rubbery removable plate. Pry it up and remove it. Don’t touch the sticky stuff that will hold it in place when you replace it. Don’t’ scratch anything (achievable with care). The three set screws are under it.

They require a 1.5 mm Allen wrench to adjust. Those can be found in any hardware store. OK, so which way to turn them and how much? IMPORTANT: you should always turn all the screws the same rotational amount. That will keep the plate perpendicular to the light path. It is said that clockwise rotation will improve backfocus (BF) and most people state use 1/4 turn. If you have front focus (FF) use counterclockwise adjustment - but remember each the same angular amount.

You should use a 1.4 or 1.7 lens wide opened to get the smallest depth of field (DOF). In that way you can tell if the adjustment is correct as the image will be sharp or blurred. If not sharp go back and turn the screws clockwise for BF or counterclockwise if you have FF. When you have it adjusted correctly things will be very very sharp wide open.

It will take some more time if you want it to be precision. You will have to net the error in exactly over several tries. But it is worth the extra effort.

TME · 26th May 2008

But if you screw it up, u're really screwed... I rather send the lens back for SSC to solve it... at least they can put the focus sensor back to the factory condition... very scary fo DIY... and besides this works only for the lens that is BF and which might induce BF or FF in another lens that you had which had no problem to begin with?

Rashkae · 26th May 2008

Originally Posted by TME

But if you screw it up, u're really screwed... I rather send the lens back for SSC to solve it... at least they can put the focus sensor back to the factory condition... very scary fo DIY... and besides this works only for the lens that is BF and which might induce BF or FF in another lens that you had which had no problem to begin with?

Apparently NO. The BF/FF issues almost never have anything to do with the lens. It's most apparent at wide-angle or large-aperture lenses. What happens is the focus sensor says "Ok, yup, in focus!" when in reality the focal sensor is not perfectly equidistant to the sensor, so the focal point is not in the "correct" place resulting in BF/FF when the shot is taken. This issue then affects *all* lenses, but on small-aperture lenses with such a deep DOF it's usually not obvious.

TME · 26th May 2008

Originally Posted by Rashkae

Apparently NO. The BF/FF issues almost never have anything to do with the lens. It's most apparent at wide-angle or large-aperture lenses. What happens is the focus sensor says "Ok, yup, in focus!" when in reality the focal sensor is not perfectly equidistant to the sensor, so the focal point is not in the "correct" place resulting in BF/FF when the shot is taken. This issue then affects *all* lenses, but on small-aperture lenses with such a deep DOF it's usually not obvious.

Yes, that's possible but if you screw up the adjustment, you might just screw up the focusing of the camera re all lens?

Rashkae · 26th May 2008

Originally Posted by TME

Yes, that's possible but if you screw up the adjustment, you might just screw up the focusing of the camera re all lens?

Of course. So it requires patience.

TME · 26th May 2008

Originally Posted by Rashkae

Of course. So it requires patience.

Patience is one thing lar... but if at the beginning you screw up? I mean it's easy to say turn 1/4 turn right? But you need a reference point to measure 1/4 turn for each set screw and also each 1/4 is not going to be precisely the same... so sometimes the margin might be really very small between OOF and focused... not sure if patience can cure this? I think fastshot has done this before...

Rashkae · 26th May 2008

It's not that hard actually.

TME · 26th May 2008

Originally Posted by Rashkae

It's not that hard actually.

Scared lar... delicate job...

greenieadi · 27th May 2008

I just did it yesterday on my A700. I used a KM 50mm F1.7 and reconfirm with Tamron 17-50mm @50mm f2.8.

Initial test b4 adjustment shows my AF was slightly FF using the testchart procedure. Did the 1/4 anticlockwise turn. Then I did a real portrait test on my son.

It shows BF as I aim at his eyes and refocus b4 shutter release only to see his ears appears sharp.Repeat test with AF on his nose and face,eyes appears sharp.

So adjust clockwise 1/4 turn. Use the test chart to confirm and looks ok. Actually my 1/4 turn for all 3 screws are not equal at all but the best I could.

Retest close-up portrait again and all is well. Anyway my KM 50mm at f1.7 a bit soft to begin with and by f2.8 is razor sharp. The best part my Tamron is razor sharp at wide-open now.

Anyway thanks bro Rashkae.

fastshot · 27th May 2008

The adjustment for the 3 screws are not difficult. Just proceed slowly and have a good look through the holes with a torch before starting. I do suggest making the adjustments 1/8 turn at a time, then test carefully before making further adjustments. It is possible to adjust the screws in different directions if you find that some of the focus sensors behave different from the others. This may be due to the "platform" being slightly tilted. If you try this, do write down the adjustments at each step, so you can reverse the adjustments if necessary.

ninelives · 28th May 2008

where are the 3 screws? can someone photo a picture?

stjustin · 28th May 2008

bro can show us? later kena do wrong scared la...ps for your time.tkx in advance

fastshot · 28th May 2008

See this link
http://www.pbase.com/pganzel/sony_a1...cus_adjustment

There are also a lot of information posted in the past at www.dpreview.com.

If you're not mechanically adapt, perhaps you shouldn't try this. Just send the camera back to the service center. They may not charge you for this adjustment

Drudkh · 28th May 2008

leave the job to the experts if you do not know what you're doing.

26th May 2008	#1
Rashkae Senior Member Join Date: Nov 2005 Posts: 8,313	Fixing backfocus and frontfocus - DIY Found this advice on a different forum (courtesy of user Ma55l), posting it here: The way the focus works in all the Sony and Maxxum DSLR cameras is not from information on the main sensor but rather a second sensor on the floor of the mirror chamber. (see Sony explanation re a350 live view). The focusing sensor is mounted on a platform that can be adjusted up or down by turning some set screws. It is fed part of the light path by a second mirror mounted under the main mirror. So by adjusting the set screws you can raise or lower the platform. That brings the focus sensor closer or father away from the back of the lens. It is a simple and predictable system. OK now you understand the focus scheme and what is not correct and needs to be adjusted. The three set screws can be reached thru the bottom of the camera. If you look at the tripod mount hole you will see a rubbery removable plate. Pry it up and remove it. Don’t touch the sticky stuff that will hold it in place when you replace it. Don’t’ scratch anything (achievable with care). The three set screws are under it. They require a 1.5 mm Allen wrench to adjust. Those can be found in any hardware store. OK, so which way to turn them and how much? IMPORTANT: you should always turn all the screws the same rotational amount. That will keep the plate perpendicular to the light path. It is said that clockwise rotation will improve backfocus (BF) and most people state use 1/4 turn. If you have front focus (FF) use counterclockwise adjustment - but remember each the same angular amount. You should use a 1.4 or 1.7 lens wide opened to get the smallest depth of field (DOF). In that way you can tell if the adjustment is correct as the image will be sharp or blurred. If not sharp go back and turn the screws clockwise for BF or counterclockwise if you have FF. When you have it adjusted correctly things will be very very sharp wide open. It will take some more time if you want it to be precision. You will have to net the error in exactly over several tries. But it is worth the extra effort. __________________ Alpha

26th May 2008	#2
TME Senior Member Join Date: Jan 2002 Location: Clementi Posts: 6,188	Re: Fixing backfocus and frontfocus - DIY But if you screw it up, u're really screwed... I rather send the lens back for SSC to solve it... at least they can put the focus sensor back to the factory condition... very scary fo DIY... and besides this works only for the lens that is BF and which might induce BF or FF in another lens that you had which had no problem to begin with?

26th May 2008	#7
Rashkae Senior Member Join Date: Nov 2005 Posts: 8,313	Re: Fixing backfocus and frontfocus - DIY It's not that hard actually. __________________ Alpha

27th May 2008	#9
greenieadi Member Join Date: Mar 2006 Posts: 381	Re: Fixing backfocus and frontfocus - DIY I just did it yesterday on my A700. I used a KM 50mm F1.7 and reconfirm with Tamron 17-50mm @50mm f2.8. Initial test b4 adjustment shows my AF was slightly FF using the testchart procedure. Did the 1/4 anticlockwise turn. Then I did a real portrait test on my son. It shows BF as I aim at his eyes and refocus b4 shutter release only to see his ears appears sharp.Repeat test with AF on his nose and face,eyes appears sharp. So adjust clockwise 1/4 turn. Use the test chart to confirm and looks ok. Actually my 1/4 turn for all 3 screws are not equal at all but the best I could. Retest close-up portrait again and all is well. Anyway my KM 50mm at f1.7 a bit soft to begin with and by f2.8 is razor sharp. The best part my Tamron is razor sharp at wide-open now. Anyway thanks bro Rashkae.

27th May 2008	#10
fastshot Member Join Date: Mar 2002 Location: 3rd Mile Stone Posts: 374	Re: Fixing backfocus and frontfocus - DIY The adjustment for the 3 screws are not difficult. Just proceed slowly and have a good look through the holes with a torch before starting. I do suggest making the adjustments 1/8 turn at a time, then test carefully before making further adjustments. It is possible to adjust the screws in different directions if you find that some of the focus sensors behave different from the others. This may be due to the "platform" being slightly tilted. If you try this, do write down the adjustments at each step, so you can reverse the adjustments if necessary.

28th May 2008	#11
ninelives Senior Member Join Date: Jan 2002 Location: BB Posts: 2,622	Re: Fixing backfocus and frontfocus - DIY where are the 3 screws? can someone photo a picture? __________________ Common citizens have common sense. Common sense is not restricted by the law

28th May 2008	#12
stjustin Member Join Date: Aug 2006 Posts: 142	Re: Fixing backfocus and frontfocus - DIY bro can show us? later kena do wrong scared la...ps for your time.tkx in advance

28th May 2008	#14
Drudkh Senior Member Join Date: Mar 2004 Posts: 6,019	Re: Fixing backfocus and frontfocus - DIY leave the job to the experts if you do not know what you're doing.