Please educate me on stop down metering.

Started Dec 28, 2013 | Questions thread
Island Golfer Veteran Member • Posts: 3,127
Re: Please educate me on stop down metering.
1

Normally your camera will meter the light using the widest aperture possible for the lens. (ie: the aperture diaphragm will be fully open - f/1.8 or f/3.5 or whatever the lens supports) This lets you look through the viewfinder with the maximum amount of light coming in. When you take a photo the camera quickly stops the lens down to your selected aperture just before opening the shutter. For this to work, lenses rely on electronically-controlled aperture diaphragms, or mechanical ones operated by the camera body via a cam or lever. As soon as the shutter has closed again and the picture has been taken, the lens diaphragm pops back to the widest setting once more.

There is a good explanation of "stop-down metering" (albeit for Canon equipment; but the principle is the same for any camera) here: http://photonotes.org/articles/eos-manual-lenses/

What is stop-down metering?

Normally your EOS camera will meter the light using the widest aperture possible for the lens. (ie: the aperture diaphragm will be fully open - f/1.8 or f/3.5 or whatever the lens supports) This lets you look through the viewfinder with the maximum amount of light coming in. When you take a photo the camera quickly stops the lens down to your selected aperture just before opening the shutter. Canon EF lenses rely on electronically-controlled aperture diaphragms, not mechanical ones operated by the camera body via a cam or lever. As soon as the shutter has closed again and the picture has been taken, the lens diaphragm pops back to the widest setting once more.

There are two exceptions to this mode of operation. First, if your camera has depth of field preview and you have it enabled, the camera will stop down the lens aperture to whatever you’ve set it to be. And second, if you have a lens installed that doesn’t have an electronically-controlled aperture diaphragm the camera will operate in stop-down metering mode - it’ll meter whatever the lens is currently stopped down to. Hence “stop-down metering.”

All Canon EF lenses contain a computer chip and various electrical connectors that allow the lens computer to communicate with the camera’s computer. Most third-party lenses for EOS also have EF-mount compatible electronics. However, if you buy a lens designed for another camera system and attach it to your EOS camera using a lens mount adapter you’re obviously not going to get the full electronic control you’d get with a true EF lens. In particular, if there are no compatible electronics then neither autofocus nor electronically-controlled apertures will work.

But my EOS camera has no aperture lever!

That’s correct. Canon EOS camera bodies, unlike older mechanical camera bodies, do not have a mechanical lever to stop down the lens aperture when the photo is taken. That’s why you need a lens capable of maintaining your selected aperture setting at all times.

Some lenses have manual and auto settings that you may need to adjust. Others have small pins on the lens mounting ring that need to be held in at all times - check to see if your adapter ring keeps the pin in place.

If you have the manual for your lens see if there’s a section on stop-down metering. What you don’t want is the lens operating under the assumption that a mechanical lever inside the camera body will stop down the lens aperture at time of exposure.

Old and new stop-down metering styles.

The earliest Canon EOS cameras and the later models handle stop-down metering in two different ways. It’s easy to tell which style your camera uses.

Turn your camera on and remove any lens that may be attached to it. With no lens attached to the bayonet press the shutter release down halfway and look at the aperture setting in the viewfinder or top-deck panel.

If the display reads “1.0” (or any number other than “00”) then you have the old stop-down metering style. Only a few ancient EOS film cameras use this method.

If the display reads “00” then you have the new stop-down metering style. Most EOS film cameras and all EOS digital cameras support this.

How to use the really old-style stop-down metering style (display shows something other than “00”).

If you attach a non-EF lens to your older Canon EOS film camera, the camera notices that the lens doesn’t have a working computer and goes into stop-down metering mode. It nonetheless displays a full range of apertures, which you can set from 1.0 to 32.

Set the camera’s aperture setting to 1.0 and leave it there.

Do not set the camera’s aperture value to match that of the lens. In fact, I don’t know why the camera lets you change the aperture setting at all, since it only screws things up by overexposing the image.

If your lens has an adjustable aperture (usually an aperture ring on the barrel) you must do the adjustment on the lens itself, not the camera. This will obviously vary the amount of light entering the camera. The camera reads it and meters from that accordingly.

Now, since the camera isn’t capable of adjusting the aperture setting on the lens it can’t work in P (program), Tv (shutter speed priority) or PIC (icon) modes with such a lens, but it’ll work just fine in Av (aperture priority) and M (manual) modes.

In Av mode you set the lens aperture using the lens aperture ring and the camera’s aperture setting to 1.0 and then camera will set a shutter speed automatically. In M mode you set the aperture using the lens aperture ring and then set the shutter speed on the camera yourself.

One other note - some older EOS cameras have the annoying inability to remember aperture settings when you switch from one mode to another. Some models, such as the EOS 620/650 or 10/10s, automatically switch the aperture value to 5.6 whenever you go into Av and M modes. Since the aperture value must be set to 1.0 when using stop-down metering and a manual lens you have to dial the aperture back every single time you enter either mode. This is particularly annoying in M mode, because since these cameras lack a rear command dial you have to hold the partial metering button whilst rotating the main dial. Oh, well.

How to use the new-style stop-down metering style (display shows “00”).

If you attach a non-EF lens to your newer Canon EOS camera, the camera notices that the lens doesn’t have a working computer and goes into stop-down metering mode, displaying the aperture value 00. This means that the camera knows that the aperture setting is not under its control and will not let you set the aperture electronically. This metering style makes a lot more sense than the old method.

If your lens has an adjustable aperture (usually an aperture ring on the lens barrel) adjust it now. This will obviously vary the amount of light entering the camera. The camera reads it and meters from that accordingly.

Now, since the camera isn’t capable of adjusting the aperture setting on the lens it can’t work in the Tv mode with such a lens, but it’ll work just fine in the other “creative” modes - P (program), Av (aperture priority) and M (manual). It doesn’t really make sense to use the lens in any of the PIC (icon) modes.

Set the aperture using the lens aperture ring and the camera will set the correct shutter speed in all other modes except M, where you’ll have to set the shutter speed yourself. Generally, Av mode is probably the most convenient.

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