How can I work around metering issues with adapted lenses on a Canon DSLR?

Question

I have a bunch of Contax lenses (C/Y mount) and a mount adapter with "dandelion" chips. This should allow me to use my lenses on any Canon DSLR using aperture mode and stopping down the lens. Thanks to the chips I also have focus confirmation.

However, I have metering problems. Wide open it's ok but when I close the lenses I get overexposure (this is aperture dependent).

I have performed tests on the 50D, 5D MII, and 600D. They all present this problem as if the metering was unable to work in low light (with the lenses stopped down). The problem is present for all the lenses I tested (50mm 1.7, 80mm 1.4, 135mm 2.8). Overexposure may reach 2.5 stops with the 80mm totally closed on a 5D while wide open the metering is ok.

Has anyone else also encountered this problem?

Is there an explanation (I mean, there is no coupling: the body does not know if the lens is wide open or stopped down) for how the metering can be off in one case and not in the other one?

Is there any known workaround (apart from a table giving the correction according to the f-number)?

I have made some measures and plots which seems to (at least partially) invalidate the jrista's explanation see the pdf any other ideas?

Answer 1

Canon cameras (as most DSLR cameras these days) meter and AF wide-open. There is a limit on the maximum aperture wherein metering and AF can perform properly, and a cutoff where they are unlikely to work at all. In modern Canon cameras, the metering sensor is up in the viewfinder housing, just above the eyepiece you look through. At narrower apertures (lets say smaller than f/8), the amount of light lost between the primary mirror, the focusing screen and the pentamirror/pentaprism is probably so much that the metering sensor has too little light to work with (well less than 50% of the light that originally entered the lens).

Metering sensors are usually rated in a range of exposure values. I think all of the cameras you listed are rated to work between 1 - 20EV and a certain temperature (roughly room temperature, although the exact value sometimes changes from camera to camera.) The 5D II might actually work from 0-20EV. Top-end professional grade cameras and newer mid-grade professional cameras can operate from -1 or even -2 EV up through 20 or more EV, and therefor have up to three stops more low-light metering capability than your average entry-level.

If you stop down your Contax lenses enough in poor enough scene lighting, you might be reducing the EV at the metering sensor below the minimum of its operating range. Since the metering sensor is designed to operate wide open, before an electronic aperture is stopped down for exposure, if you are working in weak light with a stopped-down manual aperture, you'll make it even harder for the metering sensor to operate correctly. You might simply try half-pressing the shutter with the aperture wide-open, meter, use the AE-Lock function to lock in exposure, stop down, and fully press the shutter button to take the photo at the previously metered settings.

Answer 2

What's happening is pretty simple. Canon cameras, when they see a lens that actually has electronics, assume that the lens has electronic control over the aperture, and apparently these lens adapters do nothing to dissuade them from that notion (as demonstrated by the Err 01 if you try to take a photo with the DOF preview button down).

Lenses with automatic control over the aperture always meter at their most open setting to ensure the most accurate metering. When you set your lens to F/16, the camera assumes that it has control over the aperture, and that the lens is, in fact, at its widest setting (e.g. f/2). When it computes the exposure duration, it then multiplies the amount of time it would use at f/2 by an appropriate amount to compute the exposure duration for the smaller aperture. Unfortunately, it was actually metering at the desired aperture, which means that your shots get progressively more blown out the farther you get from wide open.

To fix this problem, first, be sure your adapter is programmed with the correct maximum aperture for the lens. Then:

1. Set the camera to the desired aperture in AV mode.
2. Set the lens wide open.
3. Press the automatic exposure lock button (usually a *).
4. Stop down the lens to the desired aperture.
5. Take the shot.

You must do steps 4 and 5 before the camera times out, or else the exposure lock will go away, but otherwise, if you do this quickly enough, the exposure will be correct.

Answer 3

There are variations in Canon DSLR. I had Canon-XT and used non-EMF chipped adapter with very accurate exposure in AV mode. I just turn to the desired aperture setting, get AF confirmed, and take the picture.

For 5D, the story is a little different for the camera's brain is different from the lower level DSLR. I used EMF chipped adapter, and need to adjust the aperture both on camera and on MF lens to make it right; but, this make the process less enjoyable. One way to get around is to use Manual Mode (instead of AV) with non-EMF chip. You can either adjust shutter speed on camera, or adjust aperture on MF lens to get the exposure indicator on camera to the center. This way is much easier to work with accurate exposure. (I have limited MF lens, so this may because the specific lens or chip that works on my 5D. you need to try yours).

The so-call AV stop down metering does not work on 5D, or maybe other high level dslr.

Answer 4

However, I have metering problems. Wide open it's ok but when I close the lenses I get overexposure (this is aperture dependent).

With addition of adapter, the lens is not in the correct position on the body. Resulting in some light loss. Now, if you are stopping down the lens, loss of light is greater with each stop.

You might need to test all your lenses and figure out how much -/+ compensation you need based on each lens/aperture. If possible, get hand held meter to fine tune your metering.

Answer 5

For reasons as to why the exposure metering has difficulty with the converted lenses I'll refer you to my answer here

Now as for why you are having problems in AV mode and exposure coming out wrong... with those chips, are you manually inputting the aperture values or leaving that blank? Inputting the value may be confusing the camera into thinking the stopped down aperture is the open aperture, instead of assuming it is the closed one which it will (should) in AV mode without a returned value. I think inputting the aperture values on the chip may allow for more accurate metering in modes other than AV, but will confuse how AV mode works.

So, to work around the metering issue, make sure that the chip isn't returning any aperture value while you are using the camera in AV mode. It may make other modes work fine however. Also as others have mentioned, the more you stop the aperture, the less accurate the metering can be as there is so little light entering the metering system, so avoid stopping down too much for more accurate results. If you want to use small apertures, you can always meter wide open and then calculate how many stops you will be closing the aperture, and adjust the shutter time manually to compensate (decrease light by one stop, double shutter time)