roger55 wrote:
Let me reword this...If I point my camera towards the moon for example ,I would think putting my lens at infinity . then the moon should be in focus. Not so, i have to "back"and refocus the lens. I don't remember having that issue with film cameras.
Many modern lenses focus "beyond infinity" by design. Especially autofocus lenses. In some cases, this is done to be able to compensate for changes in how the lens focuses at different temperatures.
In addition, some lenses simply aren't marked all that accurately.
Used to be that lenses had an "infinity stop". Particularly manual focus lenses. That stop was adjustable and needed to be calibrated using special optical testing gear. So it also may be a time and cost savings to lens makers, not having a hard infinity stop, but simply allowing the lens to focus past it a little bit.
Essentially, the lens makers feel that the automation of AF will make up for the manufacturing/cost shortcuts.
It's very probably similar cost and complexity reductions that have led to many zoom lenses today being "varifocal", which means that they don't maintain focus when the focal length is changed with the zoom. If you change the focal length, you have to remember to refocus the lens afterward. Early zooms were varifocal... as were some cheaper ones in later years. Higher quality zooms were "parfocal", which maintained accurate focus throughout their zoom range. For example, you could zoom in on a subject to focus accurately, then zoom out to get the composition you want and depend upon the lens to still be in focus. But true parfocal designs are more complex and require more exacting calibration.
For example, look at the cost of cinema zooms sometime... which are often highly developed parfocal designs. A Canon 70-200mm f/4L IS USM lens for their EOS cameras costs about $1100. A Canon CN-E 70-200mm T4.4 Compact Cine Zoom costs $5000 (other factors, such as power zoom, also add to the cost).
With modern autofocus, more and more zooms are varifocal to keep the cost down, since the AF will sort of "auto correct" any loss of focus (though it depends upon the focusing mode being used in many cameras). Some zooms are also only partially panfocal... They maintain focus through much of the range, but not when shooting really close.
It's much the same with an accurate infinity hard stop.... It adds to the cost to have one in a lens.... But is relatively unimportant with autofocus.