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Forum: Photographic Technique
10-01-2018, 04:04 PM
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Getting focus "centred" is relevant when you have a 50/50 dof distribution. But if your expected dof is greater behind the focus point than in front, why do you want to get the focus "centred" ? I may be misreading your meaning here but if by "centred" you mean the numbers above and below the focus point on the test chart are equally blurred then you are wrong. If you do that then you are trying to adjust the dof not the focus point.
ie. assuming 10 foot distance to the focus chart, at f1.8 the 3" markers above and below the "focus here" point should be equally sharp. if not then you have front or back focus. At f8 you would expect the 13" in front and the 17" behind to be equally sharp for a correctly focussing lens. To use FA adjustment to make the 15" points equally sharp would be wrong.
Correct. FA does nothing to LV focus
Exactly the point I have been making all along.
The path the light travels through the lens changes when the aperture is stopped down. The convergence of the light paths (focus point) at 1.4 may not be the same point as where they converge at f8. This has been mentioned before.
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Forum: Photographic Technique
10-01-2018, 03:11 PM
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Don't know what you mean. I don't have a problem with my subjects.....
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Forum: Photographic Technique
10-01-2018, 05:59 AM
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You must not allow the camera to refocus at all if you are trying to diagnose focus shift. To do so would defeat the object of the test
Your proposed method will still not deal with the issue of dof distribution
Why do you suspect focus shift may be an issue ?
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Forum: Photographic Technique
10-01-2018, 04:31 AM
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Bruce. The point of focus shift is that the optics have a different focal point when the lens is wide open say 1.4 compared to say f8 FOR THE SAME POSITION OF THE FOCUS RING. Thus a lens focussed wide open but the image captured at f8 will not have the correct point of focus in the resultant image. As has been said this only applies to fast lenses and in most cases is negligible. It also gets disguised by the dof
My point to you about methodology is the fact that distribution of dof changes as you stop down in addition to the range of dof. So I would suggest that simply looking at your test picture is not an accurate method of determining if focus shift has happened at all. You claim to need an extra AF-FA at f8 but I suggest this may simply be the dof distribution change you are seeing
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Forum: Photographic Technique
10-01-2018, 12:24 AM
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Bruce, I have heard of focus shift. But one thing needs to be clarified. I don't know how far you were from your focus chart when doing these tests but I assume 10 feet. At f1.8 the DOF would be 49% in front of subject and 51% behind. At f8 that changes to 44% and 56% respectively. This change in DOF would give the appearance of the focus point moving back (back-focus). How are you eliminating this effect from your calculations ?
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