All existing Nikon Z bodies focus with the lens stopped down (up to f/5.6), which avoids the potential effects of focus shift, the optical phenomena where the focal plane shifts at different apertures. This can cause OOF images when focus is acquired at a large aperture but the photo is taken at a smaller aperture. The downside to the Z's stopped-down AF is that it reduces the amount of light the AF system sees, which can affect AF performance in low-light and studio situations.
Nikon just announced FW 3.0 for the Z8, which introduces a new feature called "Maximum Aperture Live View", which allows you to optionally focus the lens at its wide-open aperture, improving low-light AF performance. The downside to this feature of course is the potential for focus shift, the issue that stopped-down focusing was designed to fix in the first place.
With the announcement of this new Z8 feature there has been some discussion about whether focus shift is really an issue that we should worry about on Nikon lenses. In this video I demonstrate a simple procedure you can use on any Z body to check if your Nikon lens suffers from focus shift and to what extent, providing useful information on whether you might want to use the new Z8 "Maximum Aperture Live View" feature.
.
Here is the procedure for those who prefer to read rather than watch:
1. Make sure the camera's "view mode" is set to "show effects of settings", so that aperture changes are reflected in the Live View preview. Set the mode to Aperture Priority.
2. Put your camera on a tripod and set up a well-lit AF subject close to your lens's minimum focus distance.
3. Set your lens to its wide-open aperture and use AF to focus on your AF subject
4. Switch the camera/lens from AF to MF mode. This enables the Z's phase focus indicator, where it will display a dot+arrows and red/green squares to indicate if your subject is in focus.
5. Without adjusting the focus, start stopping down the aperture and watch if the focus confirmation switches from a green box / solid dot to a red box / flashing arrows. If it does then this indicates the focal plane of the lens is shifting with aperture, ie it is exhibiting the effects of focus shift. You can stop when you reach f/5.6, since Nikon doesn't stop down the lens further for apertures smaller than f/5.6.
6. Repeat the procedure in step #5 but his time while viewing LV at maximum magnification, so you can visually see the effects of the focus shift on your target in real-time.
Focus shift is most apparent at the minimum focus distance, where DOF is the shallowest and less apparent at greater distances where the DOF may hide some or all of the effects of focus shift. To see if focus shift will affect shooting at your other common shooting distances, repeat the above procedure using focus targets at longer distances.
Please share your experiences in this thread, identifying which lens you tested and your results. Thanks!
It's good that Nikon is acknowledging the defects in their AF system, but shouldn't they just have the camera focus at the set aperture, at least in 1/3 stops from wide open f/1 to f/5.6?
It seems that they are not concerned about focus shift from f/5.6 to maybe f/9 for example. In the DSLR days I had problems with some f/5.6 lenses.
So, the question is, will Nikon introduce AF fine tune to only affect "Maximum Aperture Live View" because as I see it, AF fine tune should only be an issue (mostly) when in "Maximum Aperture Live View", not with the normal shooting mode. None of my Z lenses require AF fine tune, but they may need it if I am in "Maximum Aperture Live View" only. If I adjust for "Maximum Aperture Live View", will it affect the lens when in normal shooting mode?
EB-1 wrote:
It's good that Nikon is acknowledging the defects in their AF system, but shouldn't they just have the camera focus at the set aperture, at least in 1/3 stops from wide open f/1 to f/5.6?
It seems that they are not concerned about focus shift from f/5.6 to maybe f/9 for example. In the DSLR days I had problems with some f/5.6 lenses.
EBH
It's a trade-off between reasonable low-light AF performance and avoiding focus shift. I didn't mention it in the video but you can check lenses with minimum apertures of f/5.6 or smaller by using the DOF-preview button, although you'll have to rely solely on the phase-detect focus dot/arrows rather than viewing at magnification since Z bodies don't support magnified view while the DOF-preview button is engaged.
snapsy wrote:
Nikon just announced FW 3.0 for the Z8, which introduces a new feature called "Maximum Aperture Live View", which allows you to optionally focus the lens at its wide-open aperture, improving low-light AF performance. The downside to this feature of course is the potential for focus shift, the issue that stopped-down focusing was designed to fix in the first place.
Probably, most photographers requiring a fast(er) AF in low-light do take pictures at the maximum aperture and, consequently, focus shift, which is due to spherical aberration, isn't an issue. Moreover, high-quality lenses should not suffer from focus shift at "normal" distances. With these lenses, the use of "Maximum Aperture Live View" shouldn't ruin the picture if taken at closer aperture(s).
Focus shift depends on focusing distance; it's more evident at closer distances not only due to lower depth of field; actually, at close distances the spherical aberration could be not as corrected as at long distances. Most of modern lenses have floating elements, i.e., their optical scheme (and focal length as well) does change with focusing distance and this might affect the amount of spherical aberration. I bet the amount of spherical aberration of the 50/1.8 S you've used in the video does decrease at longer distances.
