Help me understand M7 back exposure dial.

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parasko

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Hi all,

A newbie question...

Could somebody explain the two different functions of the back exposure dial. That is, there is the outer dial scale of -2 to +2 but also the inner dial with the ISO numbers.

I understand that film will be exposed at its 'normal' ISO when the DX mark (inner dial) is opposite the 0 mark on the outer dial. At this 'default' setting, the ISO number next to the 0 on the inner dial is 200.

If I wish to alter the exposure to under or over expose, what is the difference between altering either dials?

For instance, I loaded Tri-X at the default DX setting and then wanted to rate it at ISO 800 so I changed the inner ISO dial so that the ISO number next to the 0 is now 400 (rather than the default 200). Should I have instead changed the outer dial (ie scale of -2 to +2)?

(I know this sounds confusing but if you look on your M7 you'll know what I mean).

if somebody could explain this to me that would be great.

Apologies for the newbie question...😱
 
The inner dial is for exposure compensation, the outer dial is for manual iso setting. You can set the outer dial to any ISO you like, or you can set to DX for it to read your film DX code. You can then set the inner dial to anything between +2 and -2 for stops exposure compensation. This is typically done only on a temporary basis, ie if you only want to have the compensation for a few shots. If you intend to expose the whole roll at a specific ISO, it's usually easier to just set the outer dial to that ISO.
 
waileong said:
The inner dial is for exposure compensation, the outer dial is for manual iso setting. You can set the outer dial to any ISO you like, or you can set to DX for it to read your film DX code. You can then set the inner dial to anything between +2 and -2 for stops exposure compensation. This is typically done only on a temporary basis, ie if you only want to have the compensation for a few shots. If you intend to expose the whole roll at a specific ISO, it's usually easier to just set the outer dial to that ISO.
Click to expand...

Brilliant!

So in fact, I can set a different ISO setting with the inner dial but still also have the latitude to under or over expose by 2 stops using the outer exposure compensation dial.

Cheers! 🙂
 
waileong said:
The inner dial is for exposure compensation, the outer dial is for manual iso setting.
Click to expand...

Huh? On my M7 the outer dial is for exposure compensation and the inner dial is for iso setting, not the other way around as described!

Anyway, I have a related exposure question...

...If, for example, I am using 400 speed film but I use the OUTER dial to open up 2 stops just for one shot due to insufficient light, won't that one shot be overexposed when the film is exposed as per normal?
 
psychokiller said:
Huh? On my M7 the outer dial is for exposure compensation and the inner dial is for iso setting, not the other way around as described!

Anyway, I have a related exposure question...

...If, for example, I am using 400 speed film but I use the OUTER dial to open up 2 stops just for one shot due to insufficient light, won't that one shot be overexposed when the film is exposed as per normal?
Click to expand...

If you're using the meter (Auto mode) to set the shutter speed, then the resulting exposure will be correct (assuming ~18% reflectivity). If you add (+) compensation then, yes, the frame will be overexposed. The purpose of compensation is to compensate for non-18% reflectivity in the metered area. For example, say you're shooting portraits, one of a very light-skinned Caucasian, the other of a dark-skinned African. Both will be gray without compensation, positive in the former, negative in the latter.
 
hlockwood said:
The purpose of compensation is to compensate for non-18% reflectivity in the metered area. For example, say you're shooting portraits, one of a very light-skinned Caucasian, the other of a dark-skinned African. Both will be gray without compensation, positive in the former, negative in the latter.
Click to expand...

Are you able to explain this a bit further? I don't fully understand this. For the situation you describe, how you would you use the compensation dial?
In the situation you describe, currently I would point the camera at the individuals and note the 2 different meter readings. I would then point the camera to somewhere in the scene which was between these readings and take the shot.
 
First, you must remember that film is a NEGATIVE image. Therefore, if you want something to be lighter than 18% gray (the default setting for all light meters) you need to add more light to the exposure. This will cause the density of the negative to to be greater. That means that less light will pass through the negative and the POSITIVE (print) will be lighter. To make the image darker, you reduce the amount of light, causing the negative to be thinner, thus allowing more light to reach the paper. The print will then be darker.
 
John Rountree said:
That means that less light will pass through the negative and the POSITIVE (print) will be lighter. To make the image darker, you reduce the amount of light, causing the negative to be thinner, thus allowing more light to reach the paper. The print will then be darker.
Click to expand...

Yes of course. But in the example previously given, the suggestion was that you can use the exposure dial to compensate for a situation when both a dark and fair skinned person are together in the shot?

Like Psychokiller, I also don't understand how you can use the exposure dial to compensate for this.
 
Exposure compensation is easy... As others have already said, the internal meter "normally" will try and render the scene mid-toned. If the scene is overly bright (sand an snow are often given as examples) you need to tell the meter to over expose to stop getting grey snow. A more practical example might be shooting around dusk - if you you want to convey that feeling of the time of day, dial in about -1 EV, if you want it to look like a daylight scene, then dont

Basicaly, just remember you meter will make every thing look mid tone on average. Ask yourself is that the overall tonality I want? Is teh meter going to be fooled? If not work out if you want it darker -EV or brighter -EV, and by how much, Apply the correction and shoot.

IMPORTANT: Do remember to reset EV compensation back to zero 😀
 
psychokiller said:
Are you able to explain this a bit further? I don't fully understand this. For the situation you describe, how you would you use the compensation dial?
In the situation you describe, currently I would point the camera at the individuals and note the 2 different meter readings. I would then point the camera to somewhere in the scene which was between these readings and take the shot.
Click to expand...

No, no, no. My example was for two separate portraits! I tried to explain how you would compensate for a reading from a reflection meter, as on the M7, for subjects that were not ~18% reflective. If both the light-skinned and dark-skinned subjects are in the same portrait, the problem is also easily solved. One way is to find a nearby surface ("somewhere in the scene") that approximates ~18% reflectivity, lock the exposure, then recompose for the portrait. More reliable still would be to use an incident light meter. In this case, no matter what the reflectivity of the subject, you will get a nominally correct exposure.

Here is a suggestion: get yourself a Kodak Gray Card, available in most photo shops for a few dollars. The gray card has 18% reflectivity on one side and 90% reflectivity on the other. Go out with your M7 and meter various subjects. Then train your camera on the gray card and compare the readings. You'll find various surfaces that approximate ~18% reflectivity, such as grass, a brick wall, etc. Also meter your hand and compare to the gray card. A compensated reading from one's hand is often useful in getting the right exposure. For example, you may know from the gray card exercise that your hand reading requires a +1.5 compensation; dial that in, and your subject (under the same lighting conditions) will be properly exposed.
 
parasko said:
Yes of course. But in the example previously given, the suggestion was that you can use the exposure dial to compensate for a situation when both a dark and fair skinned person are together in the shot?

Like Psychokiller, I also don't understand how you can use the exposure dial to compensate for this.
Click to expand...

You can't. You misunderstood my post (or I didn't compose carefully enough.) Read my latest response to Psychokiller.
 
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