Question about Nikon spot metering and AF lenses.

[Bonkers]

According to Nikon's description of the D2X, you have to have a lens with a CPU in it in order to link the spot meter to a given focus point.

Why is this? Why can I not use a manual focus lens and do this? What is it that the CPU in the lens does?

[Franglais]

According to Nikon's description of the D2X, you have to have a lens with a CPU in it in order to link the spot meter to a given focus point.

Why is this? Why can I not use a manual focus lens and do this? What is it that the CPU in the lens does?

Nobody answered. That means that nobody really knows about the D2X. Looking into my book on the D70 we get a few clues:

There have been many generations of lenses using the Nikon F mount, which has been around for 40 years. All of the changes concern the way the lens transmits information to the body. Originally this was just the maximum aperture of the lens (essential to be able to measure exposure with the lens wide-open, but now the information includes identity of the lens, it's current focal length, the distance that the lens is focussed on, and who know what else? I guess that some or all of these elements are used to calculate exposure and if the lens doesn't supply them then the camera doesn't want to play.

Charles

[Bonkers]

everything you're talking about is relevant to autofocus and to a some extent, exposure. The lens' CPU, as far as I know, is for autofocus purposes. What I'm talking about is selection of a focus point, i.e. a point in the rectangular image that the camera will seek to focus on. It would be the camera body that drives the lens (if in autofocus mode) to focus on that specified area, be it the focus point in the center or one of the focus points left of, right of, above or below center. The camera knows which focus point you want to focus on. From there, you should be able to manually focus the lens until the camera sees that focus point in focus. This, presumably, should not require a lens with a CPU in it.

So, what's the role of the CPU in having the camera focus on a given focus point, if you're not going to use autofocus?

Nobody answered. That means that nobody really knows about the D2X. Looking into my book on the D70 we get a few clues:

There have been many generations of lenses using the Nikon F mount, which has been around for 40 years. All of the changes concern the way the lens transmits information to the body. Originally this was just the maximum aperture of the lens (essential to be able to measure exposure with the lens wide-open, but now the information includes identity of the lens, it's current focal length, the distance that the lens is focussed on, and who know what else? I guess that some or all of these elements are used to calculate exposure and if the lens doesn't supply them then the camera doesn't want to play.

Charles

[another view]

So, what's the role of the CPU in having the camera focus on a given focus point, if you're not going to use autofocus?

It's a compatibility thing. For example, the F4 was (I'm pretty sure) the only camera to matrix meter with non-cpu lenses. Because they could do it with the F4, why couldn't they with the F5 and F6? If I was a cynic (OK, maybe a little...) I'd say Nikon is trying to get you to buy AF lenses. Problem is, there are a few cases of MF lenses not being available in AF (50 f1.2, 35 f1.4, etc). The CPU shouldn't have anything to do (far as I know) with whether or not a camera/lens combination can matrix meter, but it does.

Check your other post on the Viewfinder forum here. I got your "report this post" but maybe you meant to hit "reply to this post" - it's a common mistake, no big deal. Anyway, I explained a little more there too.

I haven't seen or used a D2X myself, but the D2H and D2Hs are probably the same in this respect - and might be easier to find at a local shop. You might want to bring in a non-CPU lens and try it to see.

[another view]

I just read somewhere else that this may not be the case. The info did not come directly from Nikon, so you might want to call them to be sure that this is in fact true.

It looks like the D2X can be programmed to Matrix meter with one non-CPU lens. Also, the F6 can be programmed to work with several non-CPU lenses this way. Whether or not this means that the spot meter can be tied to any of the AF sensors, I don't know - that wasn't discussed. I'm wondering why the D2X only gets to work with one non-CPU lens, how that works and all that stuff. So - the question gets a little harder to answer!

[Franglais]

everything you're talking about is relevant to autofocus and to a some extent, exposure. The lens' CPU, as far as I know, is for autofocus purposes. What I'm talking about is selection of a focus point, i.e. a point in the rectangular image that the camera will seek to focus on. It would be the camera body that drives the lens (if in autofocus mode) to focus on that specified area, be it the focus point in the center or one of the focus points left of, right of, above or below center. The camera knows which focus point you want to focus on. From there, you should be able to manually focus the lens until the camera sees that focus point in focus. This, presumably, should not require a lens with a CPU in it.

So, what's the role of the CPU in having the camera focus on a given focus point, if you're not going to use autofocus?

As far as I'm aware the CPU in the lens is all about exposure and nothing about focussing. There are several factors:

1. The transmission of the lenses current maximum aperture (which is what the camera is metering at). On many zoom lenses lenses this varies with focal length.

2. The lenses current focussing distance. This is used with flash to compensate a reflection of the flash in a mirror - if the camera detects that there is a very bright spot in the image that seems to be the flash then it will disregard it and base the flash duration on what seems to be right for the distance of the subject as indicated by the lenses focussing distance. It is also used for landscapes (where the lens is focussed at infinity) to give a slightly darker image to compensate for haze in the atmosphere.

Focussing is quite simple - the camera just focusses the lens until the image contrast is at its maximum at one of the focus detector zones. No need for a CPU to do that.

Charles

[another view]

As far as I'm aware the CPU in the lens is all about exposure and nothing about focussing.

The thing is, all AF lenses have CPU's (that I'm aware of) but only a couple of manual focus lenses have them (45 f2.8, 500 f4 P). It's just an easy generalization of which lenses have CPU's and which don't.

[Franglais]

The thing is, all AF lenses have CPU's (that I'm aware of) but only a couple of manual focus lenses have them (45 f2.8, 500 f4 P). It's just an easy generalization of which lenses have CPU's and which don't.

There have been two generations of CPU's. The original one didn't transmit lens focus distance. This was added with the "AF-D" series and carried through on the "G" series. Basically the camera will do matrix metering but not the 3D-Matrix compensation for when you have a mirror in the middle of the subject with flash.

This doesn't matter to me. I have a 28, 50 and 85 with non AF-D chips and I rarely use them with flash.

Charles