Clarifying Over/Under Exposure and Stops

[Fulcanelli]

I have been researching this question and I am not finding the answer that matching my thinking process and I think I can better understand by engaging in dialog with the community.

I am struggling to grasp the concepts of over/under exposing film and adding/subtracting stops for 35mm film. I have been shooting digital for years but recently started shooting film again.

If I understand this, by lowering the ASA (say 400 to 200) I am essentially over exposing one stop. I do realize that depending on what I want regarding the exposure and detail in the dark areas that I can develop without adjusting the development or making adjustments through the developing times. That I don't need to delve into much right now as I first want to understand the under/over exposure and stops.

Can I also adjust the exposure with my speed and aperture settings also by lowering the shutter speed (without changing the aperture setting for the 'proper' exposure from the light meter) om essence exposing the film longer during the shot? And can I get the same effect by opening the aperture a stop or whatever without changing the speed? This would be without changing the ASA setting.

I would really appreciate help in getting this straightened out. Thanks

 

[Svein Eriksen]

Hope I understood your question correctly..

Basically, a stop is a stop is a stop regardless of if you lower ISO/ASA, open the aperture or adjust the shutter to stay open longer (or the opposite for under exposure).

In my opinion the best method to use depends on the circumstances and the options on your camera.

For example, if you want to "overexpose" a whole film then I'd recommend adjusting the ISO. However, if you have a camera that read the DX-coding on the film then you might or might not be able to override that depending on the features of the camera. If you can override, then use that. If not, you could use exposure compensation set to +1 to achieve the same. If exposure compensation isn't available either, then you have to manually compensate the (build in) meter with longer shutter speeds or open the aperture more than the meter indicate.

If you just want to increase exposure for one or a few shots on the film then leave the ISO as is and use exposure compensation or manual override the shutter speed or aperture.

 

[robshot]

Can I also adjust the exposure with my speed and aperture settings also by lowering the shutter speed (without changing the aperture setting for the 'proper' exposure from the light meter) om essence exposing the film longer during the shot? And can I get the same effect by opening the aperture a stop or whatever without changing the speed? This would be without changing the ASA setting.

Your understanding is correct. Using the exposure triangle variables of ISO-APERTURE-SHUTTER, altering any one of the three variables has the same affect on the scene exposure. There are only these three because that is all that we have the ability to control on a camera. Well, a fourth variable would be an ND filter placed on the lens, which would reduce the light independently of shutter, aperture, or ISO.

To keep proper exposure, if one variable is altered, then one or both of the other two must also be altered accordingly. They are intertwined.

With film, there are only two camera variables: shutter and aperture. Film ISO at the moment of exposure is not a variable, even if you override the camera's ISO setting, so think of that part of the triangle as fixed. And since film ISO is fixed, the only way to over/underexpose a given scene is in fact to alter the shutter speed or aperture.

On a digital sensor, however, ISO can be modified at the moment of exposure by increasing/decreasing the gain or signal amplification coming from the sensor. The loose equivalent to this with film is to increase or decrease development time in the chemicals, after the fact.

Hope this helps!

 

[Tomm111]

It seems you are over thinking this. To go from ISO 400 to 200, you either open the lens 1 stop or drop you shutter speed by half. To go from 400 to 800 you either close your lens 1 stop, or double you shutter speed. The first is over exposing, the latter under exposing. Negative films are very tollerant of over exposing, less tollerant of under exposing negative is very flexible this way. Transparency films (slide film) are the opposite, they can tolerate slight under exposure but not much over exposure. Their lattitude is much less than negative films.

Film speeds, it is best to start with box speeds, with transparency films it is mandatory. Some photographers like using a stop or so over exposure with negative films especially with b&w films. Tri-X, ISO 400, is often exposed at ISO200 or 320, one stop or a half stop. I personnally like transparency films under exposed by 1/2 stop, since I used to photograph a lot of stainless steel medical instruments. But to start off use box speed.

With digital photography I approach it the same as I do film, I use manual or maybe aperture priority, never auto ISO, a fixed ISO works better in my brain.

 

[TLGG]

Get a copy of Henry Horensteins book(s) on B&W photography. They should help.

 

[Overrank]

Can I also adjust the exposure with my speed and aperture settings also by lowering the shutter speed (without changing the aperture setting for the 'proper' exposure from the light meter) om essence exposing the film longer during the shot? And can I get the same effect by opening the aperture a stop or whatever without changing the speed? This would be without changing the ASA setting.

Your understanding is correct. Using the exposure triangle variables of ISO-APERTURE-SHUTTER, altering any one of the three variables has the same affect on the scene exposure.

Probably don’t use the words “exposure triangle” on the Open Talk Forum - life is too short

There are only these three because that is all that we have the ability to control on a camera. Well, a fourth variable would be an ND filter placed on the lens, which would reduce the light independently of shutter, aperture, or ISO.

To keep proper exposure, if one variable is altered, then one or both of the other two must also be altered accordingly. They are intertwined.

With film, there are only two camera variables: shutter and aperture. Film ISO at the moment of exposure is not a variable, even if you override the camera's ISO setting, so think of that part of the triangle as fixed. And since film ISO is fixed, the only way to over/underexpose a given scene is in fact to alter the shutter speed or aperture.

The ISO/ASA setting on a film camera ONLY adjusts the meter setting, so if you change the ISO from 400 to 200 then you will effectively either increase the shutter open time or the aperture (or both) if you are using auto exposure (A, S of P) modes.

On Open Talk you will be told that ISO is not part of exposure, which is defined (by the ISO) in terms on photon flux and time IIRC. “ISO” affects the image lightness which isn’t part of the exposure. But life is too short for that

On a digital sensor, however, ISO can be modified at the moment of exposure by increasing/decreasing the gain or signal amplification coming from the sensor. The loose equivalent to this with film is to increase or decrease development time in the chemicals, after the fact.

Hope this helps!

 

[robshot]

Overrank wrote:
On Open Talk you will be told that ISO is not part of exposure, which is defined (by the ISO) in terms on photon flux and time IIRC. “ISO” affects the image lightness which isn’t part of the exposure. But life is too short for that

If the OP were asking about the size and density of silver halides in the emulsion layers, then we could go into that topic. My experience in helping others with photography is that the core concept of an exposure triangle, which has existed for ages, often helps to better understand fundamental photography principles.

 

[A Marcus]

Photo film consists of a transparent support coated with light sensitive compounds of silver. This light sensitive coat can be manipulated via changes to its recipe. Thus, it is possible to make films with different sensitivity to light and different contrasts. The expression “exposure” refers to amount of light energy allowed to play on the film when a picture is snapped.

There are multiple things that will change the conditions of the exposure.

1. The brightness of the ambient light
2. Utilizing artificial light sources
3. Selection of a film that has high, moderate, or low sensitivity to light
4. Variations as to how the film is developed in the dark room
5. Adjust diameter of the cameras’ aperture controls the energy of exposure
6. Adjust the duration of exposure (shutter speed) exposure is accumulative
7. Modifying exposure by a mix of any of the above

Traditionally exposure changes are made in 2X increments (doubling or halving) of the exposing light energy. The term “stop” comes from a method to adjust the diameter of the aperture to control exposure. In 1858 John Waterhouse invented “Waterhouse stop”, thin metal slides that were inserted in a slit in the lens barrel. Each slide had a different size hole labeled in the f-number system. This device allowed a measured amount of light to transverse the lens thus controlling the exposure by stopping a portion of the light energy.

The answers to your question: Exposure is the way we control the amount of light energy playing on the film or digital sensor as the picture is taken. Since exposure is cumulative, we can adjust the time of the exposure. We can adjust the amount of light that plays on the film via changes to the aperture diameter. We can adjust the amount of light that plays on the subject via time of day, introduce changes by use of filters, Select the sensitivity of photographic media. Alter the chemicals and time in solution during the developing cycle, Change the sensitivity settings of the digital camera. A combination of most any of the above.

Let me add, photography is an art and a science. There is no right or wrong exposure as the results are always in the eye of the beholder.

 

[Overrank]

Overrank wrote:
On Open Talk you will be told that ISO is not part of exposure, which is defined (by the ISO) in terms on photon flux and time IIRC. “ISO” affects the image lightness which isn’t part of the exposure. But life is too short for that

If the OP were asking about the size and density of silver halides in the emulsion layers, then we could go into that topic. My experience in helping others with photography is that the core concept of an exposure triangle, which has existed for ages, often helps to better understand fundamental photography principles.

When I used film before digital you only ever had the shutter speed and aperture - the film speed was fixed when you loaded the film. The idea that you can “change” the ISO on a per shot basis was an alien concept (I’m aware that this is the case with sheet film, but not really with roll film). From what I’ve seen the notion of an exposure triangle is a digital concept ( https://amateurphotographer.com/latest/photo-news/the-exposure-triangle-issue/ ). Changing ISO on-camera with film only really changes the meter settings.

IMO getting the correct exposure with film is massively overblown (excuse the pun) - it’s just a question of loading the right film for the conditions and twiddling the knobs until the meter is centred (or even better use autoexposure). But I’m probably in a minority thinking this.

 

[flektogon]

Yes, you understand the over/under exposure issue correctly. And, if you have a camera with the auto-exposure, the simplest way to change the exposure is to change the sensitivity setting on your camera. Well, this applies only to the cameras which doesn't use DX coding. Changing the sensitivity setting to 2x/0.5x ISO/ASA value, or increasing/decreasing sensitivity by 3˚DIN, you will under/overexpose your picture exactly by one stop. Otherwise use the exposure compensation setting (-1EV/+1EV, or so).

But use this correction only when it has a sense. Like shooting snowy/sandy (bright) areas calls for a (around 1 stop) overexposure, while for the shooting very dark/black subjects a 1 stop underexposure is recommended. This is not needed when shooting with the digital cameras, as you can obtain the same (if not better) results by PP.

--
Regards,
Peter

 

[Fulcanelli]

Thanks. I am only interested in manual settings and am wanting to get the concept straight. I think reading that film and digital are opposite (though at the moment I do not recall to what exactly this is referring and that is not important to me) - that seed of doubt entered my mind. I am not new to photography by any means. I quit film decades ago and only got back into shooting in 2004 with a digital camera. I exclusively shot aperture priority and let my camera make the basic exposure decision - and I then might modify it, mostly with exposure compensation. That I basically have the concept is all that I needed, and getting it multiple times is what I need. I understand the basic concept of the variables in those three settings I was warned not to mention here.

Thanks for the help. This is exactly what I needed.

 

[Fulcanelli]

Thanks. I have received enough confirmation that I have the basic idea down and that is all I needed. I shot film a long time ago and got active again with photography in 2004 - but digital this time and set my camera to aperture priority and let the camera do the rest so I could toss out my slide rule. I got lazy. Thanks

 

[flektogon]

Well, you are welcome, and I am happy that I helped. Regarding the opposite EV corrections in film & digital cameras, it has to be some misconception, because the light gathering physical laws are the same. The "needs" for the EV correction is derived from the fact that the cameras (both, the film and digital) have calibrated their exposure meters such a way, that the final (positive) picture has on the average brightness equivalent to 18% gray (on a scale from 0 ~ black and 100 ~ white). And this is not the best for certain pictures, like those with a lot of snow (they would appear grayish).

--
Regards,
Peter

 

[Tom Axford]

I have been researching this question and I am not finding the answer that matching my thinking process and I think I can better understand by engaging in dialog with the community.

I am struggling to grasp the concepts of over/under exposing film and adding/subtracting stops for 35mm film. I have been shooting digital for years but recently started shooting film again.

If I understand this, by lowering the ASA (say 400 to 200) I am essentially over exposing one stop. I do realize that depending on what I want regarding the exposure and detail in the dark areas that I can develop without adjusting the development or making adjustments through the developing times. That I don't need to delve into much right now as I first want to understand the under/over exposure and stops.

Can I also adjust the exposure with my speed and aperture settings also by lowering the shutter speed (without changing the aperture setting for the 'proper' exposure from the light meter) om essence exposing the film longer during the shot? And can I get the same effect by opening the aperture a stop or whatever without changing the speed? This would be without changing the ASA setting.

I would really appreciate help in getting this straightened out. Thanks

A major difference between film cameras and digital cameras is the meaning of the ISO setting (previously ASA for films).

On a digital camera, you can set the ISO to whatever value you choose (within the limits of that camera). If you keep the exposure fixed (e.g. 1/250s at f/8) and increase the ISO setting on the camera, the image gets lighter and eventually will become "overexposed", i.e. pure white.

On a film camera, the ISO setting controls the light meter only. If you are in manual mode and keep the exposure fixed, changing the ISO dial has no effect on the image. The image captured by the film depends on the characteristics of that film (and how it is developed).

Every film comes with a recommended ISO setting for the light meter on the camera. At this setting, the meter will give the recommended exposure to obtain a "good" image on the film (assuming it is developed in the recommended way).

 

[Overrank]

Every film comes with a recommended ISO setting for the light meter on the camera. At this setting, the meter will give the recommended exposure to obtain a "good" image on the film (assuming it is developed in the recommended way).

Some films (Kodak Pro Image 100 being a prime example) don’t have a ISO but instead have an Exposure Index (EI) rating. Kodak have created a sheet to explain the difference ( https://125px.com/docs/techpubs/kodak/cis185-1996_11.pdf )

“ISO speed ratings are determined under one set of very specific conditions. Sometimes those conditions, while highly standardized, do not correlate as closely as desired to real world conditions.”

It then lists a number of reasons why ISO may not be useful as a film speed measure. Ideally you should test a particular batch of film to determine the correct EI to shoot it at (for example I normally shoot Pro Image at an EI of 160 because I find the box EI of 100 tends to overexpose a little, but YMMV)

 

[Tom Axford]

I avoided going into so much detail because I thought it might be more confusing than enlightening to the OP.

 

[Overrank]

I avoided going into so much detail because I thought it might be more confusing than enlightening to the OP.

It seems mostly to be motion picture films (so Vision3 250D is EI 250 ). I have a feeling that ProImage is really a ISO 200 film with an EI rating of 100 to allow its use in warm climates. I suppose what I’m saying is if you’re going to use a lot of one particular film it pays to do some test shots and work out for yourself what the best speed is.

 

[Tom Axford]

I avoided going into so much detail because I thought it might be more confusing than enlightening to the OP.

It seems mostly to be motion picture films (so Vision3 250D is EI 250 ). I have a feeling that ProImage is really a ISO 200 film with an EI rating of 100 to allow its use in warm climates. I suppose what I’m saying is if you’re going to use a lot of one particular film it pays to do some test shots and work out for yourself what the best speed is.

I agree completely, and be aware of the differences between positive and negative films, b&w and colour, etc.

 

[Fulcanelli]

Thanks. Makes perfect sense.