Need help with exposure question

[john.margetts]

As I said above, Joe is on a hiding to nothing with his accuracy. The three variables for getting the exposure right are light, aperture and time - all three are under the photographer's control (or should be).

There are occasions when wrongly thinking ISO is a part of exposure prevents a photographer from producing a good picture. There are no occasions when a full understanding gets in the way.

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[AlanKlein]

As I said above, Joe is on a hiding to nothing with his accuracy. The three variables for getting the exposure right are light, aperture and time - all three are under the photographer's control (or should be).

There are occasions when wrongly thinking ISO is a part of exposure prevents a photographer from producing a good picture. There are no occasions when a full understanding gets in the way.

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You have no control of the light (natural). You do have control of the film emulsion speed (ASA or ISO) in a film camera or gain of the digital amplifiers (ISO) in a digital camera. The two other variables are aperture and shutter speed (time). All three are part of getting the proper exposure on the final result.

 

[480sparky]

The difference may also be strictly mechanical. Maybe the shutter runs a bit long at 1/200 and is actually 1/175. Or f/22 is actually closer to f/20.

 

[john.margetts]

You have no control of the light (natural). You do have control of the film emulsion speed (ASA or ISO) in a film camera or gain of the digital amplifiers (ISO) in a digital camera. The two other variables are aperture and shutter speed (time). All three are part of getting the proper exposure on the final result.

I have plenty of control over the light. If I am outdoors, I can choose the weather and the time of day. I have a flash gun. Indoors, I have even more control.

The incorrect idea of the 'exposure triangle' give a false idea that altering the three gives equivalent exposures and equivalent results. An example: set the camera at ISO 400 (could be any other ISO number). Determine that the optimum exposure is 1/100 at f/16. Leaving the ISO the same, you could change the exposure to 1/200 at f/11 or 1/400 at f/8. The results will be the same. Change the ISO to 25,000 and 1/100 at f/3.5 and the results will not be the same - the new exposure is not equivalent. With most sensors on the market, you are now in the realms of noise because you are significantly under-exposing and compensating with electronic gain.

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[AlanKlein]

You have no control of the light (natural). You do have control of the film emulsion speed (ASA or ISO) in a film camera or gain of the digital amplifiers (ISO) in a digital camera. The two other variables are aperture and shutter speed (time). All three are part of getting the proper exposure on the final result.

I have plenty of control over the light. If I am outdoors, I can choose the weather and the time of day. I have a flash gun. Indoors, I have even more control.

The incorrect idea of the 'exposure triangle' give a false idea that altering the three gives equivalent exposures and equivalent results. An example: set the camera at ISO 400 (could be any other ISO number). Determine that the optimum exposure is 1/100 at f/16. Leaving the ISO the same, you could change the exposure to 1/200 at f/11 or 1/400 at f/8. The results will be the same. Change the ISO to 25,000 and 1/100 at f/3.5 and the results will not be the same - the new exposure is not equivalent. With most sensors on the market, you are now in the realms of noise because you are significantly under-exposing and compensating with electronic gain.

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I agree you have some control of the light, more so inside with artifical lights. That's actualy a fourth variable not usually part of the setting especially when shooting outdoors and using ambient light. But ISO or film speed is a legitimate setting that you can change to affect the final picture. The point is if you leave the aperture or shutter the same, you have to adjust either the iso or shutter speed or both. All three effect each other.

Noise due to high ISO is just a problem just like changing speed of the film effects grain, aperture effects depth of field, or shutter speed effects blurriness. But these things are separate from getting the right exposure which require the three parts of shutter, aperture and ISO to interplay. The "exposure triangle" is a clear way of understanding how to set your camera. It's not a false idea.

 

[WayneF]

I have plenty of control over the light. If I am outdoors, I can choose the weather and the time of day. I have a flash gun. Indoors, I have even more control.

You two guys are funny (to be PC, I never said laughable ). How would you suggest taking a satisfactorily exposed picture without addressing the proper ISO to do it? (not speaking of relying on automation to do it for you).

The incorrect idea of the 'exposure triangle' give a false idea that altering the three gives equivalent exposures and equivalent results. An example: set the camera at ISO 400 (could be any other ISO number). Determine that the optimum exposure is 1/100 at f/16. Leaving the ISO the same, you could change the exposure to 1/200 at f/11 or 1/400 at f/8. The results will be the same. Change the ISO to 25,000 and 1/100 at f/3.5 and the results will not be the same - the new exposure is not equivalent. With most sensors on the market, you are now in the realms of noise because you are significantly under-exposing and compensating with electronic gain.

The actual "triangle" concept makes no real sense to me, but it is established in literature, and does no harm, might even help beginners. The Three factors are very important though, and of course exposure is obviously the result of the three factors of aperture, shutter speed, and ISO. There is the implied concept of the ISO being chosen correctly to match the exposure of the light (the result of aperture and shutter speed) to the sensor sensitivity.

Yes, of course, a higher ISO can affect the picture, but not the matched exposure.

And of course the exact same is also true of aperture and shutter speed, affecting depth of field and motion blur (yes, can ALSO be very different pictures, but can still be a matched proper exposure). The trick is to select those of the three that are important for satisfactory result, and of course the proper match of the three for exposure.

From time immemorial (or at least the 150+ years since, say Matthew Brady), exposure has been about aperture, exposure duration, and sensor sensitivity to light. Digital may use a different technique (like we always wished boosting film speed did), but ISO still affects the histogram exposure in the same way it always did.

 

[480sparky]

Just to show the difference between the two 'equal' exposures:
A histogram of the 1/640 f/4.5 ISO 100 image:

And a histo of the 1/200 f/22 ISO 800 image:

The blue channel of the ISO 100 image has a lot of unexposed pixels and some blown-out red pixels, while the lighter ISO 800 image has fewer unexposed blue pixels and lots of blown-out red pixels.

While very similar in shape, you can see that although the ISO 100 has a median pixel brightness of 85 (compared to the 'lighter' f/22 image of 108). This explains why the ISO 800 images looks lighter.

But what's even more interesting is the ISO 800 image histrogram is noticably flat compared to the 100 image. Especially in the pixel counts in the 35-150 brightness range. This means the ISO 100 image contains more data. And more data may well affect how the demosaicing algorithm renders the final JPEG.

 

[john.margetts]

You two guys are funny (to be PC, I never said laughable ). How would you suggest taking a satisfactorily exposed picture without addressing the proper ISO to do it? (not speaking of relying on automation to do it for you).

Because I am not confusing exposure with brightness of final image. Your ISO setting affects the brightness of the final image, not the exposure. You can achieve the same thing after the shot is taken and the image file is downloaded to your computer. Lightroom has an "exposure" slider which will brighten or darken the image but clearly has no effect on what the camera has already done and so has nothing to do with the exposure.

 

[john.margetts]

My maths aren't different. ISO is not a determinent of exposure. In the above examples exposure is a function of shutter speed and f/stop. Look at the standard definition of exposure: In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. It matters; and in this case it could matter because the electronic processing the camera does to apply ISO brightening after the exposure could be one of the suspects. Don't let the triangle heads cloud your thinking.

Joe

You are quite correct, Joe, but you are on a hiding to nothing trying to get that accepted now that "everyone" knows about the exposure triangle. The curse of Youtube allowing everyone and his dog to post 'educational' videos.

I should have listened to myself!

 

[Derrel]

The difference may also be strictly mechanical. Maybe the shutter runs a bit long at 1/200 and is actually 1/175. Or f/22 is actually closer to f/20.

I doubt that the shutter timing is off, since that's electronically timed, but the lens aperture not being quite right--that's a resounding YES, perhaps. At small apertures, like f/13 to f/45 let's say, even a very tiny amount of incorrect diameter can be a very large percentage of error. At times, some diaphragms do not stop down to a regular,exactly-uniform shape, with one or two of the blades not quite forming the correct shape...it is possible that the slight over-exposure could be attributed to the diaphragm not stopping down to f/22, but to a slightly larger sized hole. On most Nikkor lenses, except for the new E-diaphragm models, the diaphragm actuation is all-mechanical. And as we know, mechanical systems are prone to slight inconsistencies.

 

[WayneF]

You two guys are funny (to be PC, I never said laughable ). How would you suggest taking a satisfactorily exposed picture without addressing the proper ISO to do it? (not speaking of relying on automation to do it for you).

Because I am not confusing exposure with brightness of final image. Your ISO setting affects the brightness of the final image, not the exposure. You can achieve the same thing after the shot is taken and the image file is downloaded to your computer. Lightroom has an "exposure" slider which will brighten or darken the image but clearly has no effect on what the camera has already done and so has nothing to do with the exposure.

So I guess you are saying you imagine your notion is somehow correct for digital, but that you are obviously wrong for film?

My own notion is about the histogram that I can see, and the usability of that image that I can see and access. Both film and digital satisfy the normal and accepted concept of exposure... for both, we just balance the three factors to produce the usable image.

But you failed to answer my question that you quoted. How do you take a properly exposed picture (the usable picture) without consideration of proper ISO? That must be very tedious, and interesting to watch..

 

[WayneF]

Correct me if I'm thinking wrong....
So I need to think of ISO differently from film. Changing ISO on the digital camera does not really change anything about the sensor. Whereas different ISO or ASA films were physically different, changing ISO on the digital camera is about how the camera interprets or processes the light captured by the sensor.

Pete

Digital ISO is an amplification, but it changes anything that we can get out of the sensor or the raw file or the JPG file.
The only way to access the non-amplified data is to set ISO low to native value. I suppose that could be a misguided goal, but you still have to set ISO to achieve it.
That might be a technical issue, but to US users, it is just semantics.
I don't see any reason to confuse the newbies with it. They have a lot more to deal with early on. Like learning to get aperture, shutter speed and ISO matched right.

 

[Ysarex]

My maths aren't different. ISO is not a determinent of exposure. In the above examples exposure is a function of shutter speed and f/stop. Look at the standard definition of exposure: In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. It matters; and in this case it could matter because the electronic processing the camera does to apply ISO brightening after the exposure could be one of the suspects. Don't let the triangle heads cloud your thinking.

Joe

You are quite correct, Joe, but you are on a hiding to nothing trying to get that accepted now that "everyone" knows about the exposure triangle. The curse of Youtube allowing everyone and his dog to post 'educational' videos.

I've been at work all day teaching a proper understanding of exposure to my students -- seems I missed all the fun.

Yes I know, ignorance abounds and Youtube is it's name. I describe Youtube to my students as a sewer. You're in the sewer on your hands and knees up to your elbows and of course can't see through the crap. In the next hundred yards the bottom of the sewer is covered with millions of pebbles. Strewn amongst those pebbles are a handful of priceless precious stones. Oh please don't tell me you watched that exposure triangle crap! It's going to be a long 100 yards!

Joe

 

[Ysarex]

I have plenty of control over the light. If I am outdoors, I can choose the weather and the time of day. I have a flash gun. Indoors, I have even more control.

You two guys are funny (to be PC, I never said laughable ). How would you suggest taking a satisfactorily exposed picture without addressing the proper ISO to do it? (not speaking of relying on automation to do it for you).

The incorrect idea of the 'exposure triangle' give a false idea that altering the three gives equivalent exposures and equivalent results. An example: set the camera at ISO 400 (could be any other ISO number). Determine that the optimum exposure is 1/100 at f/16. Leaving the ISO the same, you could change the exposure to 1/200 at f/11 or 1/400 at f/8. The results will be the same. Change the ISO to 25,000 and 1/100 at f/3.5 and the results will not be the same - the new exposure is not equivalent. With most sensors on the market, you are now in the realms of noise because you are significantly under-exposing and compensating with electronic gain.

The actual "triangle" concept makes no real sense to me, but it is established in literature, and does no harm, might even help beginners.

It confuses beginners by confusing cause and effect. That's never a good idea.

The Three factors are very important though, and of course exposure is obviously the result of the three factors of aperture, shutter speed, and ISO. There is the implied concept of the ISO being chosen correctly to match the exposure of the light (the result of aperture and shutter speed) to the sensor sensitivity.

Yes, of course, a higher ISO can affect the picture, but not the matched exposure.

And of course the exact same is also true of aperture and shutter speed, affecting depth of field and motion blur (yes, can ALSO be very different pictures, but can still be a matched proper exposure). The trick is to select those of the three that are important for satisfactory result, and of course the proper match of the three for exposure.

From time immemorial (or at least the 150+ years since, say Matthew Brady), exposure has been about aperture, exposure duration, and sensor sensitivity to light.

No. During that past period of time -- since long before you came along -- exposure was well understood and clearly defined. From Ilfords The Manual of Photography first published in 1890: "When a photograph is taken, light from the various areas of the subject falls on corresponding areas of the film for a set time. The effect produced on the emulsion is, within limits, proportional to the product of the illuminance E and the exposure time t. We express this by the equation
H = Et
Before international standardization of symbols, the equation was E = It (E was exposure, I was illuminance) and this usage is sometimes still found. The SI unit for illuminance is the lux (lx). Hence the exposure is measured in lux seconds (lx s)."

Here's the definition from Wikipedia that I posted earlier that agrees with the definition above and that is correct: "In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance."

Underline ISO in that definition and repost it.

Joe

Digital may use a different technique (like we always wished boosting film speed did), but ISO still affects the histogram exposure in the same way it always did.

 

[AlanKlein]

Joe, We understand that exposure is how many photons falls on the film or digital sensor. The the aperture and shutter speed control the photon quantity. But that's only of interest to engineers and camera designers. For average photographers, who are learning how to set proper exposure on their cameras, they have to input ISO as one of the variables to get the final picture to look exposed correctly. If ISO (ASA) wasn;t important, then you would (not) have that control on a light meter. IF you set ISO wrong, you'll get an over or under exposed picture.

Beyond that we've entered the world of word games. So I'm out of here.