Question about Ikon exposure

[italy74]

Hi guys

I read somewhere that even if you can choose 1/3 f-stops on the lens before shooting, the camera doesn't calculate it when metering, that is as if the inner exposure meter "feels" only whole f-stops, reporting them with the appropriate feedback (correct, over or underexposed).

Is that true or I misread? In the first case why you can choose such fine aperture if the camera doesn't/can't use it?

How does it work, exactly?

 

[Bob Michaels]

The meter reads the light off the film plane and makes no use of what f stop the lens is set to. Therefore it will meter "correctly" if the lens is actually set halfway between one of those 1/3 increments.

Remember this is a RF, not a SLR, so the lens stays stopped down to the shooting aperture.

 

[italy74]

Sorry Bob,
let me understand better, since I'm not sure about what you meant.
For what I know, in SLRs available light is read through the lens always set at its max aperture and then converted by the exposure meter into other combos of shutter/apertures.
What I "read" (not what you "wrote", of course) in your lines is:
1) A RF lens works already stopped down when you set it BEFORE shooting (differently from SLR)
2) you have to set the lens halfway the aperture tabs (and this is no problem)

what I really don't understand is why if the lens is already stopped down and the light is read onto the film plane, is not important which aperture is set the lens.
A larger aperture will give you a faster shutter time and viceversa, so the camera should be able to show you when the correct exposure is reached and at which time.
Sorry, but I can't get it, actually. I know it's stupid, but call my brain isn't working, today

 

[Keith]

I was just checking the Ikon specs on the Zeiss site and they say the Ikon shutter moves in 1/12 of an f stop increments in AE mode which means a subtle movement of aperture by a third of a stop or less will definitly change the shutter speed by whatever the metering system feels is required. If you have a lens that moves in third stops a half of one of those movements will change the shutter speed by about 1/6 of the value of a total f stop movement. This only applies in AE mode of course ... when manually metering the viewfinder readout will give you the setting that relates to the nearest available shutterspeed dial setting that matches the EV reading taken from the film plane ... the cameras metering brain will create a slight over or under exposure by going to the nearest available shutter speed value!

Does this makes sense! 🙂

 

[abenner]

to help you umderstand please know that the Ikon, unlike an SLR with electronics, does not "know" what aperture the lens is at. SLR's have electrics through the lens mount which communicates this info from lens to body and shows you on the LCD or in the finder. The Ikon or M7 or any leica does not have this. Its just plain TTL metering. The meter senses the light through the lens and sets the exposure. So while the camera doesn't know what f stop you're at, it will always come very close, within 1/12 of a stop, to the correct metered exposure.

 

[rxmd]

Hi Dino,

what I really don't understand is why if the lens is already stopped down and the light is read onto the film plane, is not important which aperture is set the lens.

The camera's built-in lightmeter knows nothing about apertures. All it does is selecting a shutter speed that will produce a correctly-exposed picture, with the amount of light that is falling in from the front. Even if you don't mount a lens at all, the light meter will still try to find a shutter speed that matches the amount of light that is falling in.

If you have a lens with a large aperture, there will be more light falling in. The camera thus sees more light and selects a faster shutter speed. In order to do this, it does not need to know precisely which aperture number is selected on the lens; the light meter is concerned only with light and shutter speed, not with f-stop numbers.

A larger aperture will give you a faster shutter time and viceversa, so the camera should be able to show you when the correct exposure is reached and at which time.

And the camera is indeed able to show you an exposure time which will produce a correctly-exposed picture. All the camera's light meter does is that it shows you exposure times. It does not show you f-stops, because the camera knows nothing about them, and it does not need to know this. All it needs is to see how much light is falling in.

So, for example, if you point your camera at a scene, the camera will look through the lens at its present setting and will tell you that for this particular scene, with the amount of light that comes in through the lens at this particular moment, you need to photograph at 1/60 second.

However, because the camera knows nothing about aperture and lenses, the camera is not able to tell you what aperture you need to set if you want to photograph at, say, 1/250 second. That's why there are no M-mount rangefinders with shutter speed priority automatic; there is no way to set the camera, say, to 1/60 of a second and have the light meter select aperture. That's the main drawback of a rangefinder vs. a SLR.

Philipp

 

[abenner]

off topic but slightly interesting is the Canonet which is a fixed lens 70's rangefinder which did have shutter priority rather than aperture priority. A different experience using it as you selected your speed and the camera would then meter and set the aperture. In order to accomplish this the meyer cell was on the face of the lens rather than inside the body. Thinking about that kind of helps to understand the current question.

 

[awilder]

The ZI meter reads in a similar fashon to my Hexar RF. The LED display won't always display a shutter speed change if you change the f/stop by a 1/3 but the meter is sensitive enough to make the change in the shutter in the smallest increment, I suspect as low as 1/10 stop. OTOH, my MP is very sensitive and even 1/3 stop change in f/stop will register some change in the LED.

 

[rxmd]

off topic but slightly interesting is the Canonet which is a fixed lens 70's rangefinder which did have shutter priority rather than aperture priority. A different experience using it as you selected your speed and the camera would then meter and set the aperture. In order to accomplish this the meyer cell was on the face of the lens rather than inside the body. Thinking about that kind of helps to understand the current question.

Well, for shutter vs. aperture priority it does not matter at all whether your metering cell is at the front of the body or inside it. The important difference between the Canonet and a Leica M or Zeiss Ikon is not where the cell is located. It's that in the Canonet there is a link between the lens' aperture and the camera. The Canonet has a much more complex aperture mechanism; the can set the lens' aperture from the meter readout. It's easy to do this in a compact because it doesn't have interchangeable lenses. The Canonet's automatic aperture works much similar to an SLR. In "A" mode the Canonet's aperture is always wide open until you shoot, and the camera sets the aperture value from the meter readout. (In manual mode the Canonet's meter doesn't work at all for whatever reason.)

The reason why the Canonet's metering cell is at the front (instead of through the lens) is different. If you want to put TTL metering into a leaf shutter camera, you add a lot of complexity for little gain:
- You want to meter from the center of the image. The center of the image is projected onto the middle of the film, so you have to read off the film. You can't meter while the shutter is closed, because no light falls on the film. So you have to meter while the shutter is open, i.e. while the camera is taking a shot. This is difficult to do; you need very sensitive metering cells and lots of electronics. (There's a reason why this came to the market late in the 1970s in the the Olympus OM-2.) Also you can't use this to get automatic aperture, because by the time the camera is taking a shot it's already too late.
- Or you might think of painting a white spot in the middle of the shutter blades and read off that. So you'd be metering inside the lens, really. Firstly, you'd still need a much more sensitive cell to read reflected rather than direct light. Secondly, it's difficult to get the metering pattern right, because the leaf shutter is close to the center of the lens where all light is passing through (it has to be to avoid vignetting at high shutter speeds). Thirdly, look at the Canonet; there is simply noo room for an in-lens metering cell interfering with the already-complex shutter and automatic aperture mechanics.
- Or you could add an extra shutter in the camera with a white spot in the middle. Obviously you don't need two shutters, so you might use a focal plane shutter only and remove the in-lens leaf shutter. That is complex, there are no off-the-shelf parts like with a leaf shutter, cameras get big and expensive, and you lose the fast flash capability which compact camera customers are interested in.
- But if you can't change the lens at all, why should you meter through the lens? A cell at the front with a similar metering angle is much simpler to do.

Leica chose not to do design the M-mount without a linkage from the camera to the lens aperture in the 1950s. Since then it has been impossible to do automatic aperture with M-mount cameras.

Philipp