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Metering with a 35mm camera for Medium Format shooting

Nick Masson

New member
Hey all,
I was wondering if anybody could help me figure out the conversion I would use if I meter with a 35mm camera for medium format shooting.

In particular, I will be using a NikonN90s to meter for a Mamiya 6. I plan to do this when trying to meter for long exposures, since the Nikon will read down to 30sec, and I can use a 50mm f/1.8 lens, whereas the Mamiya only goes down to 2sec and f/4.

So I want to use the Nikon with an 'L' focal length lens (probably 50mm) at f/ 'A' (probably 1.8), and want to know how to translate this to medium format 50/75mm etc.. Then I can compensate for reciprocity etc...


Thanks!
-NICK
 

Doug Kerr

Well-known member
Hi, Nick,

Hey all,
I was wondering if anybody could help me figure out the conversion I would use if I meter with a 35mm camera for medium format shooting.

Basically, the way a camera's automatic exposure system (or a free standing exposure meter) works is independent of format size (and focal length). (The way the f-number is defined is an important factor in making that so - that's the reason we specify apertures in terms of that metric.)

Thus, if the automatic exposure system of a 35-mm film camera, with the exposure index set to ISO 100, suggests an exposure of 1/30 second at f f/2.8 (or any equivalent), then with ISO 100 film in a MF or LF camera, an exposure of 1/30 sec at f/2.8 (or any equivalent) should be appropriate there.

There are of course small wrinkles owing to differences in lens transmission and such (not a function of format size). And there can be some complications if we are shooting at a large image magnification. And of course we always have the matter of different manufacturers' views of the best exposure metering "strategy", which again has nothing to do with format size.

Best regards,

Doug
 

Doug Kerr

Well-known member
Let me review briefly the reason for the situation I described.

The film responds to the physical property photometric exposure, H, which is the product of the exposure time and the illuminance on the film. For a given ISO sensitivity, the value of H needed to produce a certain result on the tonal scale is known (if we set aside the wrinkle of reciprocity failure, not a concern for modest exposure times).

We of course directly control the exposure time (the shutter speed).

The exposure meter (or camera automatic exposure system) measures the luminance of the scene. Our interest is in having that luminance produce on the film the needed illuminance that will, in combination with the exposure time, produce the proper H (considering the ISO sensitivity of the film involved).

The ratio between the illuminance on the film and the causing luminance of the scene is proportional to the square of the ratio of the diameter of the aperture to the distance from the lens to the film.

Here's why. The diameter of the aperture affects the area of the aperture, which affects the amount of light gathered from any little element of the scene (for a certain luminance of that element).

The distance from the lens to the film affects the linear size of the image of that little element; the square of that distance affects the area of this image element, the area of the film over which that gathered light is spread. The ratio of the amount of light to the area over which it lands is the illuminance.

Now, unless the subject is very close, the distance from the lens to the film is very nearly the same as the focal length of the lens. So (a) the ratio of the diameter of the aperture to the distance from the lens to the film is very nearly (b) the ratio of the diameter of the aperture to the focal length.

But the f-number is defined as just the reciprocal of that ratio.

Thus, under the stipulated conditions, it is the square of the f-number (its inverse, actually) that affects the ratio of the illuminance on the film to the luminance of the scene element.

And the "reckoning" of the exposure meter, or of the automatic exposure system, is predicated on just that. The focal length as such need not be addressed - that is taken care of by the definition of the f-number. And there is nothing about the format size itself that gets in the story at all.

Best regards,

Doug
 
Doug Kerr is right.

A 35mm camera and a medium format camera set to the same aperture, at the same shutter speed, exposing the same type of film, and looking at the same piece of subject matter, will deliver equivalent on-film results. Whether the actual exposure given will be correct is another matter.

The Nikon N90s is a 35mm camera using a 1.5:1 aspect ratio and the Mamiya 6 is a square shooter. Care should be taken to ensure the exposure data is not biassed by something the N90s sees but that the Mamiya 6 won't see.

And an ongoing concern is an inherent limitation of all through-the-lens metering systems. For example the correct exposure for a black dot on a white field is the same as that for a white dot on a black field but no reflected light measuring system, TTL included, gets this even approximately right. Only a knowledgeable user of a spot meter will find the correct numbers. Mercifully most of the world looks like broad masses of mid tone with some highlights and shadows averaging out nicely. The Nikon N90s meter gives a high percentage of useable exposures and the Mamiya 6 should be good to go with the same settings.
 

Nick Masson

New member
Great, thank you for the info. non-dimensionalizing the f-number and having it hold across platforms was a great idea, whoever came up with it....

So, then if the shutter speed varies linearly with incident light intensity on the film, then to scale between f-numbers I would then just multiply the shutter speed by the ratio of the squares of the f-number? eg. if my Nikon reads 1/100sec at f/1.8, then I would shoot my Mamiya at (4^2/1.8^2)*1/100sec ~= 1/20sec
Is that correct?

Also, I am not too familiar with how the ISO film sensitivity rating system was developed. If I recall correctly, the relationship is linear-inverse between shutter speed and ISO... right? So ISO 50 --> ISO 200 corresponds in a 4x increase, thus from 1/100sec to 1/400sec. Wheras an 'f-stop' is a base two log relation? So this would be a 4x increase, but log_2(4)=2stop increase?

I had learned this all at some point, but never really put it in to practice, and want to double check before going out and shooting a few rolls...

Thanks!
-NICK
 

Doug Kerr

Well-known member
Hi, Nick,

Great, thank you for the info. non-dimensionalizing the f-number and having it hold across platforms was a great idea, whoever came up with it....
Indeed! The concept seems to have been introduced in 1867 by Sutton and Dawson, although their metric (apertal ratio) was the other way up (makes more sense, actually). Dallmeyer carried the concept forward in 1874. He also recognized the potential difference between the diameter of the entrance pupil (used in reckoning the f-number) and the physical diameter of the aperture stop.

So, then if the shutter speed varies linearly with incident light intensity on the film . . .
The required shutter speed (exposure time) varies inversely with the illuminance on the film.

[Intensity* is another property, not best used as a "homey" synonym for illuminance or luminance.]​

But your conclusion (next) is correct, so I think that was just a slip of the tongue. (All those reciprocals can be confusing!)

. . .then to scale between f-numbers I would then just multiply the shutter speed by the ratio of the squares of the f-number?
Yes. (The illuminance goes as the inverse of the square of the f-number.)

eg. if my Nikon reads 1/100sec at f/1.8, then I would shoot my Mamiya at (4^2/1.8^2)*1/100sec ~= 1/20sec
Is that correct?
For the Mamiya at f/4? Yes. [Hint: I don't like to go back to an earlier post to get parameters!]

Also, I am not too familiar with how the ISO film sensitivity rating system was developed. If I recall correctly, the relationship is linear-inverse between shutter speed and ISO... right?
Yes.

So ISO 50 --> ISO 200 corresponds in a 4x increase, thus from 1/100sec to 1/400sec.
Yes.

Wheras an 'f-stop' is a base two log relation? So this would be a 4x increase. . .
In sensitivity, not required shutter speed.

. . . but log_2(4)=2stop increase?
Yes.

I think you are on top of all this. But I will press you for editorial clarity and terminological precision!

*Luminous intensity is a metric of the emission from a point source, and is luminous flux per unit solid angle.​

Best regards,

Doug
 

Elena Sbrana

New member
Hi Nick,
agree with Doug on the general terms of converting your light measurements, but... now I am curious as to what makes you want to use a 35mm automatic metering system if you actually shoot large format. Light meters offer several metering options, and would be more accurate than using the 35mm.
Is there a special reason for using the camera automatic meter instead of an external light meter, what effects exacty are you trying to achieve?
 

Nick Masson

New member
Thank you Doug,

The required shutter speed (exposure time) varies inversely with the illuminance on the film.
[Intensity* is another property, not best used as a "homey" synonym for illuminance or luminance.

Indeed. I meant to say illuminance or irradiation. Intensity is a function, as you mention, of emitted radiation and substended solid angle, and not of concern to the number crunching above. Good to make me recall my time in heat transfer and radiation courses... : )

Elena, I am not trying to achieve a specific effect by using my Nikkor 35mm and meter. Rather, I have just invested quite a bit in my first medium format camera, and don't have the money to buy a hand-held light meter right now. I mainly just need a means to get a light-meter reading in low-light conditions when the Mamiya does not meter. Plus i'm working mainly with BW film so accuracy isn't quite as crucial.

Cheers,
-NICK
 
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