How do AI Lenses Work?

asteroid_death · Jul 29, 2013

I use both AI and pre-AI lenses on my SLR. I've noticed photos taken using the pre-AI stop-down method tend to have better exposure than photos using the AI lens & meter together.

So I'm confused because if I go by the AI lens and meter on a sunny day, if I press the dof switch I get a darker exposure than what the meter reads. Depressing the dof switch makes the meter jump back up to the initial AI reading. Does the aperture not stop down all the way on an AI lens when the shutter is pressed?

What happens when I take a picture using the AI function as opposed to using the stop-down method on a pre-AI lens?

Can someone spell this out for me? Thanks!

waynecrider · Jul 29, 2013

It would be great if you told us what camera your shooting. Some meter open aperture and some require stop down metering. Generally the dof switch is to check the dof, with then a return to open aperture for composition, not to close the aperture to shoot after metering open aperture.

Dr Croubie · Jul 29, 2013

Not nikon-related per se, but here's my experience with stop-down metering.
I always thought it best to use stop-down metering, in case the aperture doesn't close properly or isn't well calibrated, using stop-down metering (I thought) would prove more accurate.

However, on a recent trip out, I was using my Mamiya 645AF with an old MF m645 lens.
In Av, I metered wide-open (f/2.8), and I also metered stop-down at f/11.
From the wide-open metering I counted stops down to f/11.
There was a discrepancy of about 2-3 stops between the two numbers.
Clicking each stop manually and metering each time, every click past f/5.6 only reduced the Av-calculated time by 1/2 a stop or so.
After development, the ones metered wide-open and exposed in M were perfect, the ones stop-down metered in Av were rather black (at least, the slides were, the Efke was recoverable).

So now I'm thinking the other way, that when the aperture is closed far enough the meter just isn't accurate any more because there's not enough light on the metering cell, and that could be what you're experiencing here, AI or not.
Shoot a few test-shots on Av stopped down, shoot a few wide-open, and a few wide-open-metered and manually-set by counting-stops. Or just (beg/borrow/steal/buy) a light-meter and check against that.

clayne · Jul 29, 2013

I doubt that is the case as it would mean the meter isn't accurate if the actual light dropped a couple of stops and we know that isn't the case. Even if the mechanical linkage for DOF preview were throwing off the meter (ie it doesn't take DOF preview into accout), it should result in overexposure not under.

removed-user-1 · Jul 29, 2013

The aperture mechanism should work exactly the same way for both AI and non-AI lenses. The only real difference between the two is in how the camera's meter "knows" what the maximum aperture of any given lens is. It really would be helpful to know which Nikon SLR you're using.

dorff · Jul 30, 2013

Dr Croubie said:
However, on a recent trip out, I was using my Mamiya 645AF with an old MF m645 lens.
In Av, I metered wide-open (f/2.8), and I also metered stop-down at f/11.
From the wide-open metering I counted stops down to f/11.
There was a discrepancy of about 2-3 stops between the two numbers.
Clicking each stop manually and metering each time, every click past f/5.6 only reduced the Av-calculated time by 1/2 a stop or so.
After development, the ones metered wide-open and exposed in M were perfect, the ones stop-down metered in Av were rather black (at least, the slides were, the Efke was recoverable).

So now I'm thinking the other way, that when the aperture is closed far enough the meter just isn't accurate any more because there's not enough light on the metering cell, and that could be what you're experiencing here, AI or not.

I have the same with my Mamiya 645 AF-D II. Its light meter in low light is where the problem lies. However, I have found that it stays more or less consistent up to a point, and then simply crashes two stops. I found a simple and serendipitous solution - use a manual focus body such as the Super or Pro / Pro TL if you are going to use manual focus 645 lenses a lot . That way the camera can measure at full aperture while the lens is set at the desired aperture. Much nicer way to work. In addition, you'll probably have a split screen to focus with.

When calculating for stopped down from full aperture meter readings, one gets a more accurate reading. It is important to account for reciprocity failure if the calculated exposure exceeds one second or so. It varies somewhat from film to film. Some, such as Acros and Provia, are virtually immune to reciprocity failure. Others are very much affected, though.

dorff · Jul 30, 2013

asteroid_death said:
I use both AI and pre-AI lenses on my SLR. I've noticed photos taken using the pre-AI stop-down method tend to have better exposure than photos using the AI lens & meter together.

So I'm confused because if I go by the AI lens and meter on a sunny day, if I press the dof switch I get a darker exposure than what the meter reads. Depressing the dof switch makes the meter jump back up to the initial AI reading. Does the aperture not stop down all the way on an AI lens when the shutter is pressed?

What happens when I take a picture using the AI function as opposed to using the stop-down method on a pre-AI lens?

Can someone spell this out for me? Thanks!

Is this with every lens, or just a single one? You should make sure that the movement of aperture blades is completely free, and that the diaphragm closes instantly and uniformly when the tab is released. Sometimes, the blades can get oil or dirt or even rust on them, and then they either don't close completely, or they take longer to close than the stroke time of the camera. It may not immediately be apparent when you stop down with the DOF button, as you are not looking at the lens, but through it.

Another possibility is that the light meter is slightly out of whack. What camera is it? It could also be first signs of a battery running down, but that seems unlikely. Try new batteries if nothing else resolves it.

E. von Hoegh · Jul 30, 2013

asteroid_death said:
I use both AI and pre-AI lenses on my SLR. I've noticed photos taken using the pre-AI stop-down method tend to have better exposure than photos using the AI lens & meter together.

So I'm confused because if I go by the AI lens and meter on a sunny day, if I press the dof switch I get a darker exposure than what the meter reads. Depressing the dof switch makes the meter jump back up to the initial AI reading. Does the aperture not stop down all the way on an AI lens when the shutter is pressed?

What happens when I take a picture using the AI function as opposed to using the stop-down method on a pre-AI lens?

Can someone spell this out for me? Thanks!

As long as your camera is functioning properly, there should be no difference.
What may be happening is that when using the stopdown method you are simply taking more time and care, thus getting better puictures.

mweintraub · Jul 30, 2013

I'd be concerned that you're mounting an non-AI lens on a body that has a tab and it can damage it.

asteroid_death · Jul 30, 2013

This one is a Nikon FE and I always flip the tab up when using non-ai lenses on it. Also just put in two new batteries.

I know how to shoot but I'm a relative novice when it comes to the mechanics of this particular camera. The 'needle system' tends to throw me off. I am more accustomed to a standard + - meter. It just seems like I am able to get a move accurate reading using the stop-down method than I am going off the meter.

Anyway, I just put a roll through, half with my 50mm AI-s and half with my 28mm Pre-AI. Same subject, high contrast lighting. Hopefully I will be able to determine what the meter is thinking. Only issue is the film I used was expired ultramax 400 from 2009. I shot at different speeds, 50-800, and wrote down every fstop, shutter, and asa rating and checked every meter reading against my DSLR, my FM2 and my OM1. Hopefully this will tell me if my FE is busted - or if I shouldn't do tests on expired film.

Slightly off topic, but I've been looking for a reasonably priced battery-less reflected/incident light meter. I see the Seconic L-398a is in my price range. I'm am wondering if I should buy one new or just get a used one off ebay. I see they've been making them for a long time. How long do Selenium cells last? And can they be replaced?

Thanks for the responses!

E. von Hoegh · Jul 30, 2013

asteroid_death said:
This one is a Nikon FE and I always flip the tab up when using non-ai lenses on it. Also just put in two new batteries.

I know how to shoot but I'm a relative novice when it comes to the mechanics of this particular camera. The 'needle system' tends to throw me off. I am more accustomed to a standard + - meter. It just seems like I am able to get a move accurate reading using the stop-down method than I am going off the meter.

Anyway, I just put a roll through, half with my 50mm AI-s and half with my 28mm Pre-AI. Same subject, high contrast lighting. Hopefully I will be able to determine what the meter is thinking. Only issue is the film I used was expired ultramax 400 from 2009. I shot at different speeds, 50-800, and wrote down every fstop, shutter, and asa rating and checked every meter reading against my DSLR, my FM2 and my OM1. Hopefully this will tell me if my FE is busted - or if I shouldn't do tests on expired film.

Slightly off topic, but I've been looking for a reasonably priced battery-less reflected/incident light meter. I see the Seconic L-398a is in my price range. I'm am wondering if I should buy one new or just get a used one off ebay. I see they've been making them for a long time. How long do Selenium cells last? And can they be replaced?

Thanks for the responses!

I use a Weston Master III made about 1955, in fact I have it with me today. It is still accurate, and it's as-new twin in the original box is accurate too.

The L398 is one of the best selenium meters ever made, absolutely first class. If you get a used one make certain there is a return policy and check it against a known accurate meter as soon as you get it.

vsyrek1945 · Jul 30, 2013

AI vs. Pre-AI lenses on Nikon FE

asteroid_death said:
This one is a Nikon FE and I always flip the tab up when using non-ai lenses on it. Also just put in two new batteries.

< < < snipped > > >

Anyway, I just put a roll through, half with my 50mm AI-s and half with my 28mm Pre-AI. Same subject, high contrast lighting. Hopefully I will be able to determine what the meter is thinking. Only issue is the film I used was expired ultramax 400 from 2009. I shot at different speeds, 50-800, and wrote down every fstop, shutter, and asa rating and checked every meter reading against my DSLR, my FM2 and my OM1. Hopefully this will tell me if my FE is busted - or if I shouldn't do tests on expired film.

< < < snipped > > >

Thanks for the responses!

If memory serves, the FE has a 75-25 center-weighted meter, that is a 75% bias to the viewfinder area of the microprism/split image rangefinder circle and 25% to the rest of the viewfinder. The difference in field of view between your 50mm and 28mm lenses may account for the different exposures, especially if you're subject distance is the same for a high-contrast scene.

One good way to verify your FE's meter is to focus each lens at the northern sky on a clear day with the camera in metered manual exposure mode; under the Sunny f16 rule, with each lens set to f16 the meter should call for a shutter speed of 1/the film speed you've set. You can check the meter at f16, f8, and f2.8 for both lenses at whatever range of film speeds you need to verify its accuracy. Since the sky will be evenly lit across the field of view for both your normal and wide-angle lenses, your FE's meter readings should be identical for each lens.

A reminder: Nikon lenses are built with their diaphragms held stopped down to minimum aperture when unmounted. The camera's stop-down lever opposes this spring tension to open the lens to its widest f-stop when the lens is mounted on a camera, and allows the diaphragm spring to close the aperture down to the selected stop when the shutter release is pressed. If one of the lenses has a sluggish diaphragm, it should theoretically overexpose, particularly at higher numbered f-stops.

Hope this helps - I used the Sunny f16 Rule to verify meter accuracy of several cameras that came my way and later went up for bids on eBay, and never had a complaint from my customers.

Regards,
Vince

asteroid_death · Jul 30, 2013

Thanks everyone for the responses - big help!

Maris · Jul 30, 2013

I had a Pentax which was sometimes wrong in stop down metering at small apertures. But the camera was is perfect working order. The basis of the problem was light getting through the eyepiece which exerts more and more influence as the light through the lens gets dimmer.

vsyrek1945 · Jul 30, 2013

Maris said:
I had a Pentax which was sometimes wrong in stop down metering at small apertures. But the camera was is perfect working order. The basis of the problem was light getting through the eyepiece which exerts more and more influence as the light through the lens gets dimmer.

Good point - this also applies to the FE, as well as any SLR whose meter circuits read the light on the focusing screen.

Xmas · Jul 31, 2013

I thought with a FE you had to meter and fire the shutter in stop down mode with a non AI lens?

Am I wrong and are you doing that?

PhotoJim · Jul 31, 2013

Flip the tab up and do stop-down metering with your AI lens, and then flip it back up and use AI - then you'll be comparing apples to apples.

Chan Tran · Jul 31, 2013

vsyrek1945 said:
If memory serves, the FE has a 75-25 center-weighted meter, that is a 75% bias to the viewfinder area of the microprism/split image rangefinder circle and 25% to the rest of the viewfinder.

Regards,
Vince

The FE is center weighted with 60% weight at the 12mm circle in the viewfinder and 40% to the rest of the screen. This is Nikon traditional. The F3 is 80/20 and the newer cameras are 75/25.

vsyrek1945 · Jul 31, 2013

Chan Tran said:
The FE is center weighted with 60% weight at the 12mm circle in the viewfinder and 40% to the rest of the screen. This is Nikon traditional. The F3 is 80/20 and the newer cameras are 75/25.

Thanks for setting me straight. I knew Nikon went to 75/25 somewhere along the way.

Vince

John_Nikon_F · Jul 31, 2013

They did that in 1988 with the N8008. That said, the F4 is the last camera that I know of with the traditional 60/40 pattern. Said camera came out after the N8008/F-801.

Re: stop-down metering with the FE and, its ugly brother, the EL2, you have to make sure that the DOF preview is in use when firing the shutter on A mode. Same with an F3 or an F4 (or modified F5/F6). In manual mode, you can release the DOF preview before firing the shutter.

-J

How do AI Lenses Work?

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