I use primarily manual focus lenses on a D3 and a D3X. The D3X seems to front focus 4-5cm at 2M when I use the focus assist . Using just the screen seems to match the focus assist readout. Testing the AF system with the Lens Align tool shows its right on.
I understand that the AF system works independent from the screen ....but how does the focus assist work?
Assuming you are talking about the electronic rangefinder, I've found the zone through which it shows the in-focus green dot is too wide for fast lenses. Although I have no idea how it works, on my particular D3 I made the discovery that the junction between the ">" and "o" indicators, where they are basically alternating back and forth, gives sharp focus. Given that, I find the electronic rangefinder too cumbersome to use and in most cases would rather use focus bracketing and take a couple more frames.
It is called the ELECTRONIC RANGEFINDER on a Nikon. I am pretty sure that the AF system works independent from the prism viewfinder. What I can t find a discussion of how the ELECTRONIC VIEWFINDER works? My guess is it works off the AF system ..which means I have a more difficult problem to solve.
Ive noticed, that rather wide range on my D300 using that electronic rangefinder also. I wonder if you focused it by the illuminated dot, then put it in LV to check the result, maybe by approaching the dot by focus from the left or right, you might determine which approach is the most accurate? I dont have an answer for you .
Ok, the rangefinder is part of the AF system. The AF sensors sit behind the mirror. What happens after that is that the image is split and some processing is done using phase frequency analysis and the rest, which tells the AF how much the image is out of focus and whether the target it is behind or in front of the current point of focus. This is what is used to light up the >o< in the viewfinder.
The viewfinder, what you see in it, is completely separate. If the viewfinder is not in the correct position then when you look through it images will appear in/out of focus at different points to what the AF system sees.
As for the AF-assist, all it does is provide a light pattern that makes it easy for the AF system to do the math. If there is low light and/or low contrast then there is insufficient differences when the frequency analysis is performed, therefore the AF system would have trouble working. The AF-assist beam is a special grid pattern that increases the contrast to help with that.
It also depends on what apertures you use and other conditions and what subject you are attempting to shoot. If you have lots of high contrast points close to each other but at different distances (say within 4-5cm...) then the AF sensor will grab any of them as the AF sensor does not know which one you want to photograph. It cannot read minds, it only looks for points of contrast within the AF point. If there are lots around, it just picks one and that's it. And if you shoot wide open and your DOF is very shallow, then it will most likely pick the wrong one.
Show some samples and the EXIF with it and then you can get more help. Otherwise it is all theoretical.
If anyone from Nikon reads this my suggestions are:
1. custom function to adjust the sensitivity of the electronic rangefinder.
2. Ability to calibrate the electronic rangefinder for non-CPU lenses.
3. Ability to use electronic rangefinder in liveview mode (using contrast AF principles).
In the mean time the best advice for people using fast MF lenses on demanding sensors such as D3X is to use liveview, magnify the display and focus by eye.
Thanks for the input. The important information was as expected ...the ERF works off of the sensor that drives the AF system. The AF system is easy to test and to calibrate (on a D3/D3X) for a CPU lens. Not so for a non CPU lens...but both my D3 and D3X require no adjustment..so the AF system is working correctly. I believe I can conclude that the ERF must be accurate as well.
As with any of these questions ..they need to be a context...subject matter, EV range,lens type and speed etc. I am at the limits of the system. I shoot primarily street shooting....people moving or with some activity. Normally I am in low EV light and often need to shoot at F1.4 to get a shutter speed of 1/250 . I use primarily the zeiss zf lenses and some converted Leica R glass. I rarely use flash and my title should have referenced the ERF verse focus assist.
Live view seems useless in my context and a work around for a compromised focusing system.
I have found the center focus point distracting and have turned it off. Hopefully I can improve my technique and skill enough to get reliable focus points.
