Hi,
Manually or automatically focussing a lens in any make of SLR is done so that you/it stops when the subject is in focus. So focus shift doesn't matter. It may not be where it's expected (mathematically speaking) but it's there (focused) all the same...
But a CRF needs the focus to stay where it's expected as the same helix is used throughout the focus range of the lens. It can't change pitch for the last few degrees of turning the lens.
Or rather it couldif it was like a zoom lens and moves several bits it different rates as the thing is twisted or turned but think how complex and large that would make it.
Regards, David
(bolded) Sentence one ... sure. Sentence two ... huh?
SLR lenses in any SLR made since 1961 or so are focused wide open. They only stop down to taking aperture at the time of exposure. So focus shift is
just as critical for a lens in an SLR as it is for a RF focusing system, there's no difference in normal operation. The one benefit that an SLR provides is that since you're able to see what the lens is doing with the TTL focusing system, you can manually stop down the lens, let your eye accommodate to the darker focusing screen, and make an adjustment if one is needed. You can't do this with an RF because you cannot see through the lens (Visoflex and other TTL add-ons excepted).
All this other talk about focusing helixes and zoom lenses is irrelevant.
SLR lenses are usually even more complex than M-mount RF lenses because:
- they have the auto diaphragm mechanism
- they have couplings to the TTL metering system
- they have to clear the swinging mirror behind them
- they mostly have autofocus mechanisms in them since the 1980s
- they mostly have to be designed for zero focus shift because since you're focusing at the wide open setting and taking at the stopped down setting, and most people NEVER use the DoF preview properly to ascertain critical focus placement, if they didn't you'd have tons of blurry photos.
This makes SLR lenses bulky too. But most (excepting the guaranty of no-focus-shift) of these complexities are tolerant of high variance in manufacture, which makes them pretty inexpensive.
With RF lenses, you're focusing the lens through an external mechanism and most people tend to use only two or three apertures for the vast majority of their photos. You become accustomed over time to focusing the lens using the rangefinder and understanding how the focus shift affects sharpness, and compensating for it by adjusting the focus setting. The added cost in constructing an RF lens come from the precise mechanical-optical calibration and higher manufacturing tolerances needed to drive the focusing mechanism accurately ... this is essential to giving you a focusing baseline reference. You cannot check what the focusing mechanism is doing with a DOF preview at all, so you have to assume that it is giving you that correct baseline and you can compensate for the specific lens characteristics easily. If it didn't, you wouldn't be able to get sharp photos, regardless of whether the lens has focus shift or not.
Only the electronic TTL camera, the one that lets you see in real time the actual image on the capture media at the taking aperture with the benefit of amping up the display brightness to clearly visible levels, allows truly pinpoint critical focus capability without a lot of extra effort. With this class of cameras, focus shift is irrelevant to obtaining critically accurate focus because you can actually see where the lens is focused with ease. So eTTL cameras can
theoretically reduce the cost of lens manufacture compared to both SLR and RF cameras ... if they weren't called upon to add a bazillion other features to both lenses and bodies that only they are capable of as well. :-\
G