Questions about Backfocus

[wormy1]

Hi,

I have recently bought a 50mm 1.4.
At the shop I tested the lens with my A100 on the focus chart and everything seems OK (f2, 45 degree to the sheet). Yesterday I have used the lens in a shoping mall to shoot at the people on a performing stage, I have got many shots focus sharp at the backdrop instead of the people (I stood about 5 - 10 meters away).

I would really appreciate if anyone could help me on the following questions:
1. May I know if the backfocus phenomena can be affected by distance or lighting condition?
2. As I know, backfocus is because of mis-alignment of the sensor and its focal points. But how come it is related to some particular lens?
3. I have read some posts here about adjusting the 3 screws at the bottom of the camera. May I know if I do so, will that affect my other lens as well?

Thanks in advance.

[marktuma]

I don't think the focus chart at 45 degrees is reliable - it may show a defect, but it could be a different defect from what you are expecting. In my case I have deduced that the centre sensor is actually a little higher in the viewfinder frame than what the overlay shows - using the 45 degree test this would show as backfocus, even though it is not. A better (IMO) test is to take your 50mm and arrange a number of objects at similar but slightly varied distances next to each other, then using a tripod take test shots with the centre line of the lens as close to parallel to the point you are attempting to focus on as you can.

My conclusion from testing was that I have a slightly misaligned focus sensor, and a tendency to lean forward after focussing, thereby throwing the focal plane behind my subject.

Hope this helps a little!

Mark

[01af]

1. May I know if the backfocus phenomena can be affected by distance or lighting condition?

Yes, they can, and they are. In particular, using this silly focusing sheet for determining focus accuracy is completely pointless---except when you are particularly interested in the close-range focus accuracy. It won't tell you anything about focus accuracy at medium and long focus distances. If a lens, or actually a lens-and-camera combination, exhibits back-focus then it's worst at medium distances usually while at close distance as well as near infinity there often are no or virtually no problems.

Also the colour temperature of the light as well as the colour of the subject affects focusing.

2. As I know, backfocus is because of mis-alignment of the sensor and its focal points.

No. Actually it's way more complex than that.

3. I have read some posts here about adjusting the 3 screws at the bottom of the camera. May I know if I do so, will that affect my other lens as well?

Yes, it will. Still, with some luck and patience, you can arrive at a state where the focusing errors across your whole range of lenses will get minimized.

-- Olaf

[marktuma]

Slight hijack of the thread now, but I wondered this morning why we don't hear more about front-focus problems? If it's about random variations, tolerances, etc, surely it ought there ought to be an even distribution of front and back focus problems?

Markg

[David Kilpatrick]

Exactly the same thing has happened to me, this is what affect lots of my Cirque du Soleil 50mm shots though images from the 28mm f2 and 100mm f2.8 SF were perfectly focused. The 50mm f1.4 nearly always focused on the stage set curtains/construction, and not the performers. Yet I was not aware of this problem in the viewfinder. I was stopping down to f2.8 or f4, and some investigation revealed that this lens - like many traditional f1.4 designs - shifts its focus plane when you move from f1.4 to smaller apertures. At f1.4, part of the softness you see is the effect of a whole load of distances being focused at the same time - by the inner circle of the lens (f4) and progressively changing towards the full opening.

This means that even with perfectly adjusted AF (and my A700 did have a very slight BF tendency to start with) anything you focus at f1.4 may be poorly focused at f2.8, very badly focused at f4 and smaller, but by the time you get to f8, f11, f16 the depth of field hides the problem. When the test-bench is set up to calibrate the camera, a 50mm f1.7 is used, so presumably this lens does not suffer from much focus shift on stopping down. It is a bit frustrating that Sony chose to continue the 50mm f1.4, which may have a problem with accurate focus at apertures like f2, f2.8 and f4 - rather than making the less expensive and more reliable 50mm f1.7.

David

[wormy1]

Thanks for sharing your experience and knowledge with me.