How to use internal meter on Nikon pre-AI with variable aperture zoom ?

[Maxime de la Tour]

Hi, I have a third party 60-300mm ƒ/4 - 5.6 zoom in Nikon F AI-s mount. I have it mounted on a pre-AI body (Nikkormat FTn), with which it is compatible thanks to the "rabbit ears" on the aperture ring.
But it seems this would be a problem with the internal meter :
I can set it to ƒ/4 and zoom to 300mm, but then it means the max aperture is ƒ/5.6, however the aperture ring still tells the meter it's ƒ/4. Will my images be underexposed ? Must I correct manually by adding a stop of exposure ? And would it be different on a AI or AI-s body ?
Thanks in advance

 

[RLangham]

Hi, I have a third party 60-300mm ƒ/4 - 5.6 zoom in Nikon F AI-s mount. I have it mounted on a pre-AI body (Nikkormat FTn), with which it is compatible thanks to the "rabbit ears" on the aperture ring.
But it seems this would be a problem with the internal meter :
I can set it to ƒ/4 and zoom to 300mm, but then it means the max aperture is ƒ/5.6, however the aperture ring still tells the meter it's ƒ/4. Will my images be underexposed ? Must I correct manually by adding a stop of exposure ? And would it be different on a AI or AI-s body ?
Thanks in advance

It probably depends on the zoom but I would say, based on looking at my two pre-AI bodies, including an FTn like yours, you would definitely need to compensate manually if you want a guaranteed precisely correct exposure. I would also make sure that there is nothing about the lens that can damage your camera. Almost every Nikon body can mount a large percentage of Nikon lenses, but every single one has a few that can be useless or harmful if mounted. (The most compatible is the Nikkorex F which can mount any Nikon lens barring the ones without manual focusing and aperture rings, and those that require MLU)

I would say that If you're at the narrow end of the lens's zoom range, you might not be far off if you centered the needle then decreased shutter speed by one stop or opened up the lens, assuming it's not wide open as in your example.

Now, the interesting thing is that the lens is actually compensating a little. In the example you mention, the camera is metering through the lens, and at 60mm it's seeing through an f/4 entrance pupil and at 300mm it's seeing through a f/5.6 entrance pupil. Now, it doesn't necessarily matter exactly what aperture the camera thinks it is, as long as it's pretty close, because the amount of light it's receiving is determined by the entrance pupil, which is a function of the size of the diaphragm and the magnification of the elements in front of it. It may meter almost accurately without compensation in most cases.

Now, there will be some problems. Some camera do want to know the max and min apertures of your lens, because of some limitations of the metering system I don't quite understand... Minolta cameras from this same period just sense how many stops down from wide open the lens is set at, and calculate exposure from that information. The Nikkormat FTn on the other hand does "want" to know the actual value of the aperture. I think there's a linearity problem, since the Minoltas calculate exposure mechanically using strings and pulses and the Nikons calculate it electronically with variable resistors built in to the aperture ring, the film speed ring and the shutter speed ring. So there are likely to be small miscalculations if the camera is wrong about the functional aperture of the lens. I would compensate by +1 stop at 300mm and maybe a half stop if possible in the middle, but you may find that the actual error is smaller than that and wish to not bother adjusting.

I don't know how these lenses behave on AI bodies but honestly a lot of zoom lenses don't compensate in a mathematically perfect way and just rely on the exposure latitude of the film.

 

[Ariston]

Are you able to remove or bypass the rabbit ears and simply use stop down metering?

 

[Pieter12]

Unless I misunderstand how things work, the maximum aperture is the only one that is affected by the zoom focal length. Since the camera meter is reading at maximum aperture, that exposure should be correct. The numbers don't matter. But the best thing to do is run a roll of film through it, shooting an out-of focus grey card at different focal lengths and apertures. That will indicate if your meter is thrown off by the variable maximum aperture. Better yet, shoot reversal (slide) film that has less latitude.

 

[Chan Tran]

You don't have to do anything. The exposure will be correct except that the f/4 is actually f/5.6.

 

[RLangham]

Unless I misunderstand how things work, the maximum aperture is the only one that is affected by the zoom focal length.

That is not correct. The aperture remains the same size at any given position of the ring; only the functional magnification of the lens elements in front of it and the overall focal length of the lens (the two have a complex relationship) are altered when you zoom. If a zoom lens has a one stop variation in its max aperture it has the same variation in all aperture stops. I.e. 22 will be 32 regardless of whether the markings on the ring indicate this or not.

 

[RLangham]

You don't have to do anything. The exposure will be correct except that the f/4 is actually f/5.6.

I still say there will be corner cases where the exposure will be noticeably off but for the most part yeah that's true.

 

[Chan Tran]

Unless I misunderstand how things work, the maximum aperture is the only one that is affected by the zoom focal length. Since the camera meter is reading at maximum aperture, that exposure should be correct. The numbers don't matter. But the best thing to do is run a roll of film through it, shooting an out-of focus grey card at different focal lengths and apertures. That will indicate if your meter is thrown off by the variable maximum aperture. Better yet, shoot reversal (slide) film that has less latitude.

You are used to modern cameras. The variable aperture affects all aperture on a manual lens like the OP has. With new camera the camera can control the aperture and thus it opens the lens wider so the effective aperture doesn't change. It also indicates to correct effective aperture.
With the OP setup the aperture always changes as he zoom and the meter would ask him to set a slower shutter speed but the aperture ring stay at the same place.

 

[Pieter12]

You are used to modern cameras. The variable aperture affects all aperture on a manual lens like the OP has. With new camera the camera can control the aperture and thus it opens the lens wider so the effective aperture doesn't change. It also indicates to correct effective aperture.
With the OP setup the aperture always changes as he zoom and the meter would ask him to set a slower shutter speed but the aperture ring stay at the same place.

So how does one get a proper exposure with such a lens, since neither the camera's meter nor a hand-held meter would compensate. Does the manufacturer supply a table of equivalent f-stops at different focal lengths? I understand that the maximum aperture in this case can change by a full stop, enough to ruin a reversal film exposure. Bracketing might help, but would be useless in an action setting.

 

[reddesert]

It will work fine. The only thing you have to avoid is metering at one focal length and zooming to another without rechecking the meter (applicable on any manual camera). The Nikkormat FTn senses the difference between the maximum aperture and the set aperture, so that it knows how much the lens will stop down from the open aperture when you press the shutter.

When you mount a lens, the FTn has to be "indexed" by turning the aperture to fully-open, so that it knows the position of the prong at the fully-open position, and it can tell how far the prong moved from that full-open position. The Nikon F with FTn prism works the same way. Earlier versions of F metering prisms had a more awkward procedure for setting this maximum aperture.

Where variable aperture zooms become a problem is when you are trying to do non-TTL flash metering, since the absolute value of the f-stop matters.
(Edit to add: It's also a problem if you use a non-TTL ambient light meter and forget that the value of the aperture is different from the marking on the ring.)

 

[RLangham]

It will work fine. The only thing you have to avoid is metering at one focal length and zooming to another without rechecking the meter (applicable on any manual camera). The Nikkormat FTn senses the difference between the maximum aperture and the set aperture, so that it knows how much the lens will stop down from the open aperture when you press the shutter.

When you mount a lens, the FTn has to be "indexed" by turning the aperture to fully-open, so that it knows the position of the prong at the fully-open position, and it can tell how far the prong moved from that full-open position. The Nikon F with FTn prism works the same way. Earlier versions of F metering prisms had a more awkward procedure for setting this maximum aperture.

Where variable aperture zooms become a problem is when you are trying to do non-TTL flash metering, since the absolute value of the f-stop matters.

Wait, is that why they need to be indexed? I thought it was trying to ascertain the exact aperture. That would make sense, though. It's just a shame they couldn't make it like the Minolta system where the the indexing isn't necessary.

 

[Maxime de la Tour]

Thank you all for your constructive answers. I did some testing, and no correction is needed because, as long as you stay at the focal length you metered at, the light loss is effective at all apertures including fully open. So there is no need for stop-down metering or special compensation at the narrow end, it all works seemlessly.
The ultimate test will come when I put a roll through it though.

 

[Chan Tran]

So how does one get a proper exposure with such a lens, since neither the camera's meter nor a hand-held meter would compensate. Does the manufacturer supply a table of equivalent f-stops at different focal lengths? I understand that the maximum aperture in this case can change by a full stop, enough to ruin a reversal film exposure. Bracketing might help, but would be useless in an action setting.

If you use a handheld meter you will have to make compensation depending on the focal length setting. However, if you use the camera meter you don't have to. For example when you set the lens at f/4 and zoom to 300mm the effective aperture is actually f/5.6 but since the camera doesn't really care what aperture you set at. The meter only needs to know how many stops will the lens stop down during the actual exposure (in this case is none as the aperture ring is at maximum aperture), How bright the image is on the focusing screen (in this case the image is 1 stop dimmer than f/4 and the meter doesn't know that it's f/4 or f/5.6 but it's know how bright the image is and during the actual exposure there will be the same amount of light reaching the film as the aperture ring is at max.) I would know what shutter speed you set at and thus it knows your exposure is correct , over or under.

 

[swchris]

It will work fine. The only thing you have to avoid is metering at one focal length and zooming to another without rechecking the meter (applicable on any manual camera). The Nikkormat FTn senses the difference between the maximum aperture and the set aperture, so that it knows how much the lens will stop down from the open aperture when you press the shutter.

Yes. The internal meter will see "less light" when you zoom to the longer f. So it'll tell you how to adjust your aperture/time.

 

[reddesert]

Wait, is that why they need to be indexed? I thought it was trying to ascertain the exact aperture. That would make sense, though. It's just a shame they couldn't make it like the Minolta system where the the indexing isn't necessary.

The position of the prong does indicate the absolute value of the aperture (straight up is always f/5.6), and the body (or meter prism) shows the value of the maximum aperture when indexed. But the meter only needs to know the difference between full-open and the setting of the aperture ring (for M and A modes). Possibly Nikon thought that metering based on the difference rather than the absolute position was more reliable.

When the F meter was designed, open-aperture TTL metering that worked well at all was a major advance. The F came out in 1959, the non-TTL meter prism in 1962, and the TTL meter prism in 1965, but as far as I can tell, all of the lenses had the coupling prong from the beginning.

 

[Pieter12]

Wait, is that why they need to be indexed? I thought it was trying to ascertain the exact aperture. That would make sense, though. It's just a shame they couldn't make it like the Minolta system where the the indexing isn't necessary.

Nikon developed the AI (Auto Indexing) system that no longer relied on the prongs to register the f-stop, and did not need to be indexed. I don't know if that was before, after the same time as Minolta.

 

[RLangham]

Nikon developed the AI (Auto Indexing) system that no longer relied on the prongs to register the f-stop, and did not need to be indexed. I don't know if that was before, after the same time as Minolta.

A little after, I think. That would be seventies and the srT 101 came out in the late 60's, early enough to have a long and exemplary service record in Viet Nam. The failing of the Minolta system is of course that there's no way to show the aperture in the viewfinder without a Judas window as on the srT 202 and its derivatives.