Meter reading from ground glass

Gossen advertises its Luna-Pro S light meter as having a Flexible

Fiber Optics Probe accessory that "can be used to take through the

lens readings from a large format camera's groundglass" -- shouldn't

this work off of any ground glass? -- i.e., including medium format?

And has anyone tried this? Is it good? Is there a better meter for

reading off of ground glass?

 

Thanks,

 

John Ha

Good point. Firstly the probe..I've only ever read of a probe that goes on the inside of the glass plate to recieve the light at first hand so-to-speak. Your wording of 'probe' reinforces my thought that this is what you may have?

 

Otherwise I understand your point in general. I have toyed with my M645 and Sekonic but have found the physical scale of the light meter and ground glass screen such that its impossible to *really* spot meter. On the whole I found it quite difficult and got many odd readings, which put me off further trials.

 

Bri

 

Sounds like it should be easy but it doesn't seem to pan out that way.

you cannot accurately meter off the viewing side of a groundglass. fresnel lenses do weird things to light intensity -- they usually are brighter at the center, tansmission rates vary depending on glass mfr., transmission rates vary according to focal lenngth (remember, a fresnel is itself a lens with a focal length), etc. some large format cameras are set up to allow you to meter from the backside of the glass, i.e. inside the bellows. this is a rough equivalent of ttl/OTF metering. this can be very accurate, especially if you are concerned about light falloff due to bellows extension, don't trust the stated transmission rates of your lens, enjoy finicky procedures generally, etc. in my experience, however, this technique is no more accurate -- and 100 times more cumbersome -- than simply taking spot reflectance readings of your scene and applying a bellows extension factor to the result (bellows extension is usually only an issue in macro photography). in any event, there is no ay to use a probe on the typical MF camera because there is no access to the underside of the ground glass. nice thought, but save the bus fare.

I have a Calcu-light XP meter with an accessory fiber optic probe

for ground glass metering. The probe is a short fiber bundle

about 2 mm in diameter. One end has an attachment to fix it to

the meter so that end of the bundle points at the naked light

sensor (i.e. the incident dome and reflective-metering lens are

removed). The other end is in a plastic housing with a flat front

face.

 

You can use the probe for straightforward reflective metering, but

it absorbs two stops of light compared to the standard lens, and

it's harder to see where it's pointing, so I can't see any reason to

do this. The standard use is against the outside of the ground

glass to measure the contrast between different parts of the

scene, or if you do a simple calibration for your ground glass, to

determine the exposure.

 

You have to shield the ground glass, or stray light from the back

of the camera will give a false reading. Also, there is a

theoretically the problem that the fiber bundle has a limited

acceptance angle and won't measure properly in the corners of

the ground glass. My widest lens in 4x5 is a 127mm and I've

had no problem, so I can't comment. Both of these might cause

problems for MF use.

 

There is no reason why you shouldn't use such a probe for MF

work, although the probe will obviously cover a larger area of any

given composition. My Kowa's screens have a built in

condensor to aid viewing, so I can't use my probe effectively with

it, but I have used the meter in reflective mode just pointed at the

screen to work out correction factors for macro situations.

 

Incidentally, the XP has a useful low-light sensitivity of -7 EV (100

ASA, f1.4) which helps to make this a practical meter for ground

glass measurement. With a slow lens, plenty of extension and a

dim screen this can be important.