No
You can place a ring (eg, out of cardboard paper) at the back of a lens that cuts off the light from the f/5.6 to f/8 ring, and the PD AF system will stop working, no matter how bright the scene is, simply because the PD AF system will receive zero light. Marianne has actually done this experiment to illustrate this point.No
Stopping down the lens does not impact the focus function - it impacts the capturing function only.
If you use a lens, which maximum aperture is less than f:5.6 (or f:8) the AF module receives less light.
You are of course right, if you are using LiveView - then the AF-module receives zero light
Yes - of course - but that's the same as changing the maximum aperture to something less - not actually the same as "stopping down the lens" in normal context.You can place a ring (eg, out of cardboard paper) at the back of a lens that cuts off the light from the f/5.6 to f/8 ring, and the PD AF system will stop working, no matter how bright the scene is, simply because the PD AF system will receive zero light. Marianne has actually done this experiment to illustrate this point.No
Stopping down the lens does not impact the focus function - it impacts the capturing function only.
If you use a lens, which maximum aperture is less than f:5.6 (or f:8) the AF module receives less light.
You are of course right, if you are using LiveView - then the AF-module receives zero light
Yes - that's right - my misunderstanding - did just reread Marianne - and got it again
The AF-system is tuff understanding - but thanks to Marianne for her contribution to make it more understandable to less educated, non engineers like me
Yes/no... my understanding of the AF system is perhaps a bit different (and yes, I remember the very long thread by Marianne).
There are no f/2.8 AF points in Nikon (D)SLRs (nor to my knowledge in Sony or Pentax (D)SLRs). Only high-end Canon bodies have a small number of f/2.8 AF points.
For decades (literally, since the Minolta 7000 in 1985), PD AF systems in (D)SLRs almost universally had an official f/5.6 cutoff. For almost all of that time no first-party AF lens (only exception I am aware of is the Minolta 500 mm f/8 mirror lens), with a maximum aperture smaller than f/5.6 had been released. But there was always a little bit of play and lenses with maximum apertures between f/5.6 and f/8 (f/6.x through roughly f/7) could autofocus. Third-party manufacturers exploited this with various lenses with maximum f-stop between 6.x to 7.x. The earliest reference I can find for this is the Sigma 500 mm f/7.2 from 1990. (Nikon only added three f/x-6.3 lenses between 2014 and 2016).
I did not indicate that there is no upper limit. Where exactly that is will vary based upon where on the objective lens the individual images are taken from. I don't know that information, and I can't know without extensive testing of each individual system.
Nope. This works at the sensor plane because the sensor is recording light arriving from a wide range of incidence angles. The PD AF system is only receiving light from a relatively narrow range of incidence angles (where the incidence angle is proportional to the aperture 'rings').
Nope. The optical path to the AF sensor is quite different than the one to main sensor. Have a good look at this document (which starts with split prism focussing before covering PD AF because both use the same principles): http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf
Very interesting - thanks to both of you - and thanks for the link, have to read it more than twice to get it, but I will
The AF optical elements shown in that article are not correct.
I believe this agrees with what I said. Except that I used the word vignetting of the image circles in the transition range to indicate dimming of the image as opposed to masking/blocking of the AF sensor/points (which happens when beyond the transition range).
Never mind mentioning where you copied all four diagrams from. The third one especially, was quite a bit of work; it is the superposition of a photo of the AF sensor chip, with a photo of the actual images projected by the separator lenses - a virtual peek at the inside of a working AF module.
This figure was not intended to represent the D300 AF module. It was presented early in my thread, before the discussion of the D300 AF module was introduced, and only illustrates the general principles of PDAF optics.
Here is my explanation:
Also note that because the separator lenses gather the light from separate images (light paths before converging again into a single focused image), the light has an angular direction to it. This causes the images to shift positions on the AF sensor with changes of focus (in opposite directions). This displacement is the "phase" being sensed.
This image shows the separate images on the AF sensor, and when they shift so that the same image points overlay the same AF sensor/position the image is determined to be in focus. It also shows how horizontal and a vertical line sensors combine to make cross type sensors for the central AF points.
This last image shows the areas on the objective element where these separate images for the center AF points originated from, as viewed through the lenses aperture. i.e. aperture vignetting/masking (*Marianne calls it the exit pupil).
And in looking at the 1Dx mapping, if you consider that 20 of the 21 pts from the f/5.6 ring and the 20 pts from the f/4 ring could be seeing the same portion of the different images with the f/4 images being projected onto horizontal sensors and the f/5.6 onto vertical sensors; you can then understand how having a smaller max aperture can cause a focus point to shift from being a cross type to being a line type.
To tie it all back into the OP, because the process involves images cast onto the AF sensor I believe that if the lens' aperture is stopped down to something less than the AF module's aperture it would affect the depth of focus at the AF sensor. But because the apertures in question are smaller it would likely have little notable affect in showing focus shift.
My sincere apologies. I thought I gave appropriate credit and the original thread is linked to earlier in this thread.
Now we are at it, Marianne - do you mind if we are using contents from this terrific thread (including some of the discussions) when discussion AF in i.e. other fora - not necessarily net fora, as long as we refer to you?
Simply including a link to the thread is all that's needed.
Yes, put simply the field lenses are at the image plane.
Yes, light arrives at the field lenses from all portions of the main lens exit pupil.
OK.
No. Each separator lens only projects the portion of the image that is covered by its corresponding field lens. If there are three field lenses, then there are two separating baffles which prevent light from leaking to a separator lens from the other field lenses that it doesn't "belong" to.
What you're missing, is the function of the field lens. It enforces a one-to-one correspondence between points on the separator mask, and points on the main lens exit pupil. Because of this, the openings in the separator mask (where the separator lenses are), dictate the "patches" within the main lens exit pupil, which can project light to the separator lenses.
Original thread is full.
My English is not good enough to fully understand your question. But maybe this is a help to find the answer.
Thank you. I understand this already (I think).My English is not good enough to fully understand your question. But maybe this is a help to find the answer.
I understand that *if the field lenses were not in place the separator lenses would then see specific areas of the exit pupil correlating to aperture positions... but they would also each contain/project the same complete image as each point on a lens (the exit pupil) contains a complete image (which is what I was trying to communicate with the "there are only lenses/apertures before/after").
