Sony A700 Backfocus

Started Apr 16, 2008 | Discussions thread
Ma55l Contributing Member • Posts: 805
Re: Sony A700 Backfocus

I can help with the back focus. I did the repair on my 5D and it was simple. 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.

I waited 2 years to do the adjustment because I was timid and concerned about damaging the camera. That is just plain Malarkey and I don’t believe it voids the warranty. It is just an adjustment procedure and has no more effect on voiding your warranty than adjusting your F stop or changing lenses.

The bottom line for me was after doing this in a precision manner the camera astounded me. The sharpness of the 1.4 lens is beyond belief at 1.4 and gets better as you go to smaller apertures. Almost all the web sites that test the 50mm 1.4 including Photozine and others, report that the Minolta-Sony 1.4 has the highest resolution of all the 50mm lenses out there and that is wide opened at 1.4. But if you can't focus it correctly it is worthless. Go to Photozine and look at the resolution figures for yourself and note that the Minolta-Sony beats the Nikon.

Some will say, send it to Sony and don't be a fool as you will damage the camera and void your warranty. I am sure that Sony can damage the camera as well as shipping can cause damage. Both of those will outweigh the chance that you will cause permanent harm. That is unless you are the equivalent of a bull elephant in heat in a China shop. Anyway the repair personelle couldn't give a hoot if you adjusted it, as they get paid for the repair whether it is under warranty or not and they know the problem exists.

Roosevelt said we have nothing to fear but fear itself. If you do the adjustment yourself you will get it absolutely right. If you send it to Sony they may get it right but more likely will not get it exactly right. You can do a better job on this than they can because you can devote more time and effort to making it exactly right. After all it came misadjusted from the factory in the first place.

When focusing, while doing your tests, do not focus on things that go at an angle. Focus dead head on, on your subject. The reason is that the pick up area for the spot is larger than the square in your viewfinder and is not always exactly in registration. That will cause focus errors. Even leaves of a tree or something in the distance may give inaccurate results. So check by focusing on things between 4 to 6 feet away and on things that have good contrast.

But, there will be times that the focus will be slightly off but not like before. It is the nature of mechanical things which have tolerances built into them. In order for the lens to turn freely to focus, there is a little play so it doesn't bind. But that should be very minimal and occurs in all DSLR camera systems including Nikon and Canon.

When doing the test use good lighting so that camera shake does not enter. You might even use flash to more easily see the differences in focus accuracy because you can use a lower iso. Use aperature preferred setting at 1.4. Don’t use ADI flash.

After saying all that, the wosses out there will still shake in their boots and either put up with the poor focus or send it off to Sony. I suppose if it is under warranty you might want to send it in but beware that repair personal occasionally drop things and the shippers are not perfect.

By the way all your lenses will be in registration when the alignment is done properly. Some people have thought that only one or another lens had the proble. A dim lens (small aperture ~ F6.3) or wide angle will not notice the change that the 50mm 1.4 will. But when focus is dead on for the 1.4 or 1.7 - all your lenses will be dead on unless you have a defective lens with a loose element etc.

My father was noted for saying that a "faint heart never won nuthin". The older I get the smarter I have come to realize he was. Happiness awaits you as it happened for me after 2 years of thumping my fingers on the table trying to decide what to do about FF.

I hope this was helpful both to you and anyone else who has the problem.

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