Autufocus errors

[nikonian]

Why do so many photographers think that many of the lens`s they have purchased show an error, either front focus or rear focus errors? With talk of returning them to the makers for recalibration.

[Sue Allen]

They probably read too many internet forums,,,

[silversnapper1]

It's the blame culture.

Blame anybody other than yourself.

[Cathus]

maybe they do have errors.

[KeithT]

I think a lot of people who blame front and back focus on very fast lenses are those who use them at the widest apertures without considering the effect of a shallow depth of field. At f1.4 for instance, the focus point can me measured in fractions of a centimeter and the slightest movement can throw the focus out. Shooting hand held at f1.4 takes a lot of skill and good technique to get focus spot on. Rather than blame themselves they blame the lens.

[matt wilson]

some indeed do.I have the ability to fine tune on a 50 d .Most where fine but the 100 to 400 L was not .Minor tweaking and it's now spot on.Certainly no need to return it though.

[Forseti]

So - 3 for the plaintiff and only one for the defence. The fact is that most lenses suffer from either front or back focusing errors - some worse than others obviously. KeithT touched on the issue without realising it of course and that is that a focus point can be measured in a fraction of centimetre. Um - something wrong there....old units and new units. Anyway, with mass produced lenses the manufacturers obviously have to work to tolerances with more more expensive lenses having much finer tolerances as well as overall build quality. This of course is not to mention operating conditions e.g. hot and cold climates in which the lens characteristics can change. Did someone mention Canon grey lenses? You also have to take into consideration the camera itself of course, so in short, the sheer number of camera/lens combinations make it essentially impossible to get a lens to focus with 100% accuracy. For most amateur users this is seldom noticeable although in extreme cases it can be. For this reason, Canon (along with others) on their more expensive bodies offer a facility to save a calibration for any particular lens used on that body. The differences between calibrations can come as quite a shock in some instances and certain entrepreneurs have made a successful business from manufacturing specific devices to assist in this calibration process - LensAlign to name but one.

So no - one cannot always blame the user because try as they might if they've got a lens that is utterly out of calibration with the camera in use, they will never be able to achieve spot on focus.

Edit - Matt posted whilst I was typing and has provided a good example.........unless he's half blind of course and you're going to blame him LOL.

[nikonian]

Originally Posted by KeithT

I think a lot of people who blame front and back focus on very fast lenses are those who use them at the widest apertures without considering the effect of a shallow depth of field. At f1.4 for instance, the focus point can me measured in fractions of a centimeter and the slightest movement can throw the focus out. Shooting hand held at f1.4 takes a lot of skill and good technique to get focus spot on. Rather than blame themselves they blame the lens.

Exactly. Having made a gizmo for checking focus errors then testing all of my lens (15)none showing an error.

[nikonian]

Originally Posted by matt wilson

some indeed do.I have the ability to fine tune on a 50 d .Most where fine but the 100 to 400 L was not .Minor tweaking and it's now spot on.Certainly no need to return it though.

Hi, What is this tweaking I read about . Body or lens. Take a 300mm lens with an average fully open aperture of F4.5 and a minimum focusing distance of 2 meters. This would give you a DOF of about 30mm. This is useless as it will mask any faulty lens focusing.

[SieSmith]

Originally Posted by nikonian

Hi, What is this tweaking I read about . Body or lens. Take a 300mm lens with an average fully open aperture of F4.5 and a minimum focusing distance of 2 meters. This would give you a DOF of about 30mm. This is useless as it will mask any faulty lens focusing.

Seems to be more like 6 or 7mm if focussed at 2m (using DoF calculator in following link).

http://www.cambridgeincolour.com/tut...h-of-field.htm

This will show up very well if focus position is even slightly off.