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Focusing movement

The cheapest devices have lenses that are supposed to be homeless. This means that the lens is fixed in a prefocused position on subjects about 2.5 m from the camera.


The hypothesis is that this is a typical situation for snapshots, and the elements slightly closer or further will seem reasonably pointed because of the depth of field.


This keeps costs low but is not ideal because of limitations. All manufactured camera lenses include some means of adjusting their forward or backward position to concentrate subjects closer or further away respectively.


Typically, the entire lens slowly moves a thumb or more across a handle. Focus is adjusted manually by turning the target barrel or using a motor controlled by an automatic focus sensor that detects when the image is enhanced.


In many cases, compact autofocus cameras show no distance marks on the lens. The camera lenses offering greater control show a range of the subject moving against a setting mark.


It is therefore possible to set the focus without looking into the viewfinder by estimating the distance of the subject or by measuring using a tape. Normally, this is not required, but it can be useful in very ill-informed situations.


All targets can be adjusted to focus on infinity for more distant subjects. For most targets, this amounts to more than a few dozen meters.


The closest distance between subjects varies for several reasons. Mechanically, moving the lens forward can be challenging. The longer the focal length, the greater the amount of physical movement needed to adjust the focus settings.


Close-up focusing may be purposely prevented because the lens is part of a camera with a separate direct viewfinder. It gets more and more inaccurate in framing your picture the more you work.


In some cases, the lens may not maintain the same high optical image resolution at a short distance. Normal lenses for large format cameras require more focus motion to cover a similar range of subject distances, due to their longer focal length.


The entire front portion of the camera moves independently of the back, both of which are joined by bellows. There is rarely a range scale on the camera objective or body;


Focus is performed by verifying the actual image on a glass screen on the floor at the rear of the camera.

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