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From: Tom
Newsgroups: rec.photo.misc
Subject: Re: about AI, AIs, AF, P
Date: Thu, 29 Feb 1996 20:38:30 -0500
Organization: McMaster University, Hamilton, Ontario, Canada (NewServer)
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I apologize in advance for the length of this post...


On Thu, 29 Feb 1996, d.galensky wrote:

>
> there is no 500/4 AI-P lens. the 500/4 is AIs.

There are at least 2 AI-P-type Nikkors, (the other is a 1200-1700 f5.6)


> its usual designation is just plain "P" which implies AIs

AI and AI-s is usually left off the name of the lens (ie: 500mm f/4 P
IF-ED) but it is listed as a "AI-P-type Nikkor" (Nikon Corps. own words,
honest!)


>
> ... the FG, but it requires AIs lenses to work in program mode

The FG _definitely_ works in program mode with AI and AI'd F lenses. Mine
certainly does and so do ones belonging to people I know.


> (for future reference, it doesn't even have a shutter
> priority mode.)

Yes your right, I was talking about program mode. Sorry. (sometimes what
I'm thinking just doesn't make it to my fingers...)


> there **is** a difference.

Yes there is, there is no warning if you don't set the f-stop beyond
f11. Other than that, the FG doesn't care, the smallest aperture is
simply limited to what is set on the lens.


> ... i suspect the FA is the
> same based on correspondence i've received from FA users.

My FA works in both program and shutter priority mode with AI and AI'd F
lenses. However, it reverts to center-weighted metering, and as the
focal-length signaling pin is absent on the pre AI-s lenses, it will not
shift into "Hi-program" (but you can add a simple modification for this).
It also does not display the selected f-stop because it does not know
what the maximum aperture on the lens is. The finder display that should
shows the f-stop simply indicates "---" but it definitely will select an
aperture - shutter combination linearly between 'wide open - 1/30 sec'
and 'the set minimum - 1/4000 sec' for correct exposure.


>
> i'm not a lens designer, but it's my understanding
> that the 's' improvement had to do with the linearity
> of the stopping down of the diaphragm

This is true. The AI-s lens series introduced a linear stopdown _wrt the
f-stop_ such that one full stop always corresponded to the same amount of
movement on the stop-down pin. Note that this means that the actual
diaphram didnot move linearly as the aperture diameter had to decrease
as the inverse square. Also introduced at the same time was a notch in
the mount that signaled that this was an AI-s lens and a ridge inside the
mount that signaled 2 things, the position of the ridge signaled the
maximum aperture and the depth of this ridge signaled wether the lens
was a telephoto or a normal/wide angle.


> ... (as would
> be necessary from a computer-controlled aperture
> setting in program mode, or shutter priority on
> cameras supporting that feature.) ...

This is not true for the FA and FG, (but true for the N2xxx series and
later). The FA and FG used a system that was first used by Minolta on the
XD-11 (I'm not sure if it was developed by them). When the exposure
sequence is tripped, the aperture starts closing without any
other mechanical action (the mirror stays down). The metering system
monitors the incoming light, and when it has determined that the
aperture has closed the required amount, and electro-magnet is tripped
which stops the actuating pin. Due to mechanical backlash and
oscillations, a fixed delay occurs before the metering system takes a
final measurement. This reading is compared to the calculated value for
the selected shutter speed (wether by you or the program) and
a slight adjustment is made to the shutter speed to ensure that the
exposure is correct. This technique allowed the use of older lenses whos
aperture stop down action varied from lens to lens. As more mechanical
error would occur with these, particularly at smaller apertures, the
selected shutter speed could be as far off as a full stop (in my
experience, it was always rarely more than 1/2 stop off).

The N2000 series and all latter ones used the previously mentioned linear
aperture movement. If the program decided that to set the lens to f8 on a
f2.8 lens, the camera had to close the aperture by 3 stops. Instead of
monitoring the light until 3 stops were reach, the actuating
pin would move a set distance. The rest of the sequence is the same, a
stop down reading is made and the shutter speed is adjusted to compensate
for mechanical errors. This latter system is simpler and more accurate at
setting the desired aperture than the previous, and is not dependent on
changing light conditions. It is also faster, resulting in a shorter time
delay between pressing the button and the actual start of the exposure.
This is quite noticeable if you shoot action, especially if you compare say,
an FE with an FA like I do (part of the delay on the FA was the AMP
metering system).

One should note that because the diaphram actuating pin was not
originally designed for this purpose (as opposed to say, the
Canon FD mount), and they made these lenses backwards compatible with
older camera bodies, the current crop of Nikon AF cameras still
make a final stop down measurement and the shutter speed is adjusted to
compensate for mechanical error. (the shutter speed is thus never
exactly that selected by you or the program for "proper" operation)

The FA would sometimes under-expose, which was often attributed to the
early matrix meter (then call AMP). However, I believe this was due to the
fact the the stop down reading is made on a dim screen, which means that
light that leaks through the viewfinder eyepiece contributed more
significantly to the overall light level, especially if you wore glasses.
Thus shooting slow film (ie K64) in bright sunlight and using small
f-stops often resulted in an underexposed slide. I would like to
qualify that this is a personal experience which I tested, and did
occur other peoples FA's. This is also why a eyepiece shutter was
included on the FA, as this is very noticeable if you shoot on a tripod
and your face is not blocking the light that is entering the eyepeice. This
was not as much a problem with the FG or latter cameras, and was most
likely attributted to better internal baffling of the finder system.



> again, AIs is not the same as AI. all AIs lenses have
> the AI property, but no one calls an AIs lens an AI lens,
> just as no one calls an AF lens an AI lens, or a P
> lens an AI lens, even though they too have the feature.
> it would be like calling a five speed transmission a
> four speed because it had the same bottom four gears.
> if you call a used camera dealer and purchase an AI lens,
> an AI lens is what you'll get. not an AIs lens, not
> a P lens, not an AF lens.

Humm, I don't recall saying you could get a AI-s or P lens if you had an
AI lens. I guess I should have been clearer and stated that AI-s lenses
had all the features of an AI lens, as did the P lenses (ie backwards
compatibility) but the AF lenses donot have all the features of the
original F lenses (no longer backwards compatible) although there is a
modification that can be done.

I apologize if my statements were not as clear and concise as they should
have been, obviously I'm not very "nit picky", just overly (overtly?)
sarcastic...(always been my downfall)

Tom