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Thread: Stop down metering

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    Stop down metering

    I've come to the conclusion that I simply don't understand stop down metering. If I want to shoot say, my Hassy 110 f/2 on the Contax 645 platform via the Mam-1 adapter, why do my shots come out as underexposed when shooting in Aperture priority mode. Is this a metering issue with the camera.. is it because f/5.6 with a Contax lens is not the same as with a Hassy lens. If I were to use exposure compensation to adjust for this should it be consistent throughout the aperture range, i.e. if +2 works at f/5.6 will it work at f/11.

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    Re: Stop down metering

    David, just to clarify:

    The camera should be set to Av metering mode then it should read the light that hits the sensor accurately as the lens is stopped down. HOWEVER, many metering systems get "confused" after f5.6 since there is too little light for an accurate meter reading, so it's usually best to meter open then count down manually to smaller apertures. Also, some cameras default to a spot reading when unrecognizable lenses are mounted and this can give greatly varied exposures in high contrast light. (I have to set Mamiya to it's average metering pattern when I mount a 3rd party lens or it defaults to spot in the auto-matrix mode.)


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    Jack
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    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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    Re: Stop down metering

    So basically, meter wide open in Av mode, then switch to manual mode with appropriate adjustments in shutter speed and aperture.

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    Re: Stop down metering

    Hi David,

    I use the Hassy 110 on the Contax all the time.

    The Contax Av metering is based on taking light readings at wide open aperture and adjusting its exposure time for the desired aperture that it can read from its lenses. With the Mam adapter, it cannot see your desired aperture on the Hassy 110. So, in order to get proper exposure, you must meter wide open, stop down and then manually set exposure. If you are only stopping down two stops or less, you do not have to switch to manual mode, but use the exposure compensation dial instead.

    The exception to the above is when using TTL flash. The in-camera flash meter will stop down a Contax lens to the desired aperture, take a pre-flash at that aperture, and then set exposure accordingly. So, with the 110, you would first stop down to the desired aperture (and keep it there), trigger the pre-flash exposure, and then you are ready to shoot immediately. This will nail the flash exposure pretty much every time. As long as you do not close the pre-flash lever, the shutter speed will remain active in its memory so that you can re-use it until either you change aperture or the lighting changes where you must use the pre-flash meter again. The TTL pre-flash mechanism is one of the best and unique features of the Contax 645. It is both fast and accurate.

    David

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    Re: Stop down metering

    I forgot to mention another thing, which will save you time in ambient light shooting ....

    As Jack says, the camera metering system is not very good at F/5.6 and easily gets confused unless it is really bright. So, you do not really have to meter wide open, but instead meter at F/4. Then, you can use compensation dial to adjust for your shooting aperture, up to F/8. This saves the hassle of switching to manual exposure mode. Of course, if you are shooting slower than F/8, then you must indeed switch to manual mode. However, in that case, I find a pocket size light meter a better alternative and then just shoot in manual mode.

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    Re: Stop down metering

    The response of some types of light sensors varies greatly with angle of incidence. Light that comes from the center of the lens might read 1.0 while the same amount of light from the side reads 0.5. As a result there is a fudge factor in the ROM for each lens that tells the camera how to adjust the reading. In our example, at wide open, the reading might be 0.8 of actual; the ROM would then supply this number (of 1/0.8x factor) for use with this type of sensor. Without a ROM but with aperture cams the camera can use a reasonable internal table, not as accurate but probably good enough. Without aperture linkage the camera can only assume it's say an 80/2.8 set to wide open and meter for that.

    When you stop down a lens you change the proportion of light at different angles of incidence on the light sensor, biasing towards straight-on. As a result, the sensor overreads and without compensation the camera will underexpose. Without aperture linkage you can't get this compensation since the camera doesn't know what aperture the lens is set to and what its exposure fudge factor is.

    Other types of sensors are less finicky. I found the 645AFDII works perfectly well with the 120/4N Macro, just turn the aperture ring to stop down and shoot.


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    Re: Stop down metering

    Jack, David, Jan... thanks for the excellent explanations. I now understand the "why" which is what I was after.

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