The point about the problems of focus, front and back focus
The inspiration for this article come from reading an interesting study published by Roger Cicala CanonRumors.com as well as the numerous questions posed on this front-back focus by many fans.
The objectives of our expensive digital SLR cameras are manufactured so that their focus is as accurate as possible but always within a range acceptable, a tolerance of manufacture as any type of product manufactured in series.
Add more, perhaps many do not know, the same cameras that affect the problem because of existing tolerances in the sensor-target distance and flatness problems of perfect implant lens board. We exclude from the examination objectives defective origin (the 3 / 7% of the lenses produced approximately) for which there is nothing left to do but the request for warranty replacement.
This therefore means that more or less all the optics (and SLRs) have in some way the phenomena of displacement of the point of focus in the front (front focus) or posterior (back focus).
Of course the manufacturer is careful to include these small inaccuracies in a range that is acceptable, normally the Depth of Field shall compensate for these minor differences. However, for a set of circumstances that we will now see the problem is worsening the problem by making clear to the eye of the photographer appears to be annoying and unbearable feeling of softness and lack of definition.
- The problem of the front / back focus is most evident on optical high-brightness (f / 1.4 to 2) where the aperture value implies a low depth of field and correction of poor focus;
- The phenomenon is most apparent in telephoto lenses, also for the low compensation PdC
- Each lens and camera products may differ from one another, all brands are affected
- The combination of camera body-lens can therefore decrease or increase depending on the effect of the case in which differences are administered or cancel each other.
Better clarify this point. Suppose that X has an optical staratura of +2 points of fire (hypothetical value), that is, move slightly forward the point of focus (front focus), but within the tolerance range specified by the manufacturer for the given goal, say, both + -3 points of fire. The optics are so 'perfect' in terms of sales.
Now let's take a Y body, which in turn has its staratura, suppose also because of +2 points and then also here a slight front focus, but always within the manufacturing tolerances, we can say that this camera is 'perfect' are fully covered by specific construction techniques.
But what if the lens is mounted on the body X Y? In this case the +2 +2 is added to the optics of the camera providing a shift of +4 points out of range of fire and sending the complex goal-machine, the lack of clarity on the point of impairment of the focus now turns visible to the eye, the optic coupled X, Y is not the most perfect machine ...
Of course the opposite also happens, namely that a phase shift of +2 on the one hand magically be corrected by an offset of -2 on the other providing an ideal result, or we can have a mix of the two possibilities.
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