Need help with exposure question

PJM

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I was out playing with my camera today checking out the effects of aperture on depth of field. I ended up with two photos with different exposures and I'm trying to understand why.

The camera is a Nikon D5600 with a 18-55 mm, f/3.5-5.6 lens.
Both photos were taken in auto exposure mode and metering mode = pattern.
Both were also taken in aperture priority mode.
For the first picture I set ISO = 100, f/4.5. The camera set the shutter speed to 1/640.

For the second picture I set ISO = 800, f/22. The camera set the shutter speed to 1/200.

Since the camera was in auto exposure mode I would expect the exposures to be more similar and am trying to understand why the second came out lighter.
 

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I would guess that slight variation is about 4/10 of an EV value different....might just have come down to the JPEG engine's processing of the overall capture. But definitely, the first shot looks a bit nicer to me. How close in time are the frames? A few seconds apart?
 
Another possibility is the minor framing variation (lighter version you titled the camera down a bit compared with the darker version). That's enough for the pattern metering to re-calc a minor difference.

Joe
 
Doing the maths, this is the absolute same exposure
From ISO100 to ISO 800:+ 3EV
From f4.5 to f22:- 4.67EV
From 1/640sec to 1/200sec:+ 1.67EV
0
[TBODY] [/TBODY]

So IMO there´s actually two things that could have happened:
  1. The light got brighter (sun cam out)
  2. There is a difference between f-stop (the nominal specs given by the manufacturer of the lens), and the so called t-stop (the real light transmission through your lens).
These will explain the difference in brightness, but not why your camera didn´t compensate. However, that does happen. In my experience cameras are not that consistent in their metering.
I didn´t use any automatic or semiautomatic exposure modes for years because I wasn´t happy with the results (I am/was a Canon shooter). That first changed when they introduced the 5D MkIII.
I think the difference in your case isn´t all that bad.
 
Doing the maths, this is the absolute same exposure
From ISO100 to ISO 800:+ 3EV
From f4.5 to f22:- 4.67EV
From 1/640sec to 1/200sec:+ 1.67EV
0
[TBODY] [/TBODY]
So IMO there´s actually two things that could have happened:
  1. The light got brighter (sun cam out)
  2. There is a difference between f-stop (the nominal specs given by the manufacturer of the lens), and the so called t-stop (the real light transmission through your lens).
These will explain the difference in brightness, but not why your camera didn´t compensate. However, that does happen. In my experience cameras are not that consistent in their metering.
I didn´t use any automatic or semiautomatic exposure modes for years because I wasn´t happy with the results (I am/was a Canon shooter). That first changed when they introduced the 5D MkIII.
I think the difference in your case isn´t all that bad.

Those are not the absolute same exposures. Doing the math there is a 3 stop difference between the two exposures. And so as Derrel suggested the variation could also be in the camera electronics as it compensated for the exposure difference.

Joe
 
Thanks for all the replies. While I understand the fundamentals of exposure there are lots of nuances that I still have to learn and this all helps.

To answer the first question, they were taken probably 15 seconds apart, enough time for me to change the settings.

So for next time, take more pictures (bracketing) and use a tripod.

Thanks again.
Pete
 
Doing the maths, this is the absolute same exposure
From ISO100 to ISO 800:+ 3EV
From f4.5 to f22:- 4.67EV
From 1/640sec to 1/200sec:+ 1.67EV
0
[TBODY] [/TBODY]
So IMO there´s actually two things that could have happened:
  1. The light got brighter (sun cam out)
  2. There is a difference between f-stop (the nominal specs given by the manufacturer of the lens), and the so called t-stop (the real light transmission through your lens).
These will explain the difference in brightness, but not why your camera didn´t compensate. However, that does happen. In my experience cameras are not that consistent in their metering.
I didn´t use any automatic or semiautomatic exposure modes for years because I wasn´t happy with the results (I am/was a Canon shooter). That first changed when they introduced the 5D MkIII.
I think the difference in your case isn´t all that bad.

Those are not the absolute same exposures. Doing the math there is a 3 stop difference between the two exposures. And so as Derrel suggested the variation could also be in the camera electronics as it compensated for the exposure difference.

Joe
Joe, you know I value your knowledge, but I wonder why your maths are different. So I recalculated and even took my camera to be on the safe side:

Let´s start with ISO
100 0.00
125 0.33
160 0.67
200 1.00
250 1.33
320 1.67
400 2.00
500 2.33
640 2.67
800 3.00
Next Aperture :
5 2.67
5,6 2.33
6,3 2.00
7,1 1.67
8,0 1.33
9 1.00
10 0.67
11 0.33
13 0.00
14 -0.33
16 -0.67
18 -1.00
20 -1.33
22 -1.67
and finally Shutter Speed
1/500 -1.33
1/400 -1.00
1/320 -0.67
1/250 -0.33
1/200 0.00
 
Doing the maths, this is the absolute same exposure
From ISO100 to ISO 800:+ 3EV
From f4.5 to f22:- 4.67EV
From 1/640sec to 1/200sec:+ 1.67EV
0
[TBODY] [/TBODY]
So IMO there´s actually two things that could have happened:
  1. The light got brighter (sun cam out)
  2. There is a difference between f-stop (the nominal specs given by the manufacturer of the lens), and the so called t-stop (the real light transmission through your lens).
These will explain the difference in brightness, but not why your camera didn´t compensate. However, that does happen. In my experience cameras are not that consistent in their metering.
I didn´t use any automatic or semiautomatic exposure modes for years because I wasn´t happy with the results (I am/was a Canon shooter). That first changed when they introduced the 5D MkIII.
I think the difference in your case isn´t all that bad.

Those are not the absolute same exposures. Doing the math there is a 3 stop difference between the two exposures. And so as Derrel suggested the variation could also be in the camera electronics as it compensated for the exposure difference.

Joe
Joe, you know I value your knowledge, but I wonder why your maths are different. So I recalculated and even took my camera to be on the safe side:

Let´s start with ISO
100 0.00
125 0.33
160 0.67
200 1.00
250 1.33
320 1.67
400 2.00
500 2.33
640 2.67
800 3.00
Next Aperture :
5 2.67
5,6 2.33
6,3 2.00
7,1 1.67
8,0 1.33
9 1.00
10 0.67
11 0.33
13 0.00
14 -0.33
16 -0.67
18 -1.00
20 -1.33
22 -1.67
and finally Shutter Speed
1/500 -1.33
1/400 -1.00
1/320 -0.67
1/250 -0.33
1/200 0.00

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
 
Doing the maths, this is the absolute same exposure
From ISO100 to ISO 800:+ 3EV
From f4.5 to f22:- 4.67EV
From 1/640sec to 1/200sec:+ 1.67EV
0
[TBODY] [/TBODY]
So IMO there´s actually two things that could have happened:
  1. The light got brighter (sun cam out)
  2. There is a difference between f-stop (the nominal specs given by the manufacturer of the lens), and the so called t-stop (the real light transmission through your lens).
These will explain the difference in brightness, but not why your camera didn´t compensate. However, that does happen. In my experience cameras are not that consistent in their metering.
I didn´t use any automatic or semiautomatic exposure modes for years because I wasn´t happy with the results (I am/was a Canon shooter). That first changed when they introduced the 5D MkIII.
I think the difference in your case isn´t all that bad.

Those are not the absolute same exposures. Doing the math there is a 3 stop difference between the two exposures. And so as Derrel suggested the variation could also be in the camera electronics as it compensated for the exposure difference.

Joe
Joe, you know I value your knowledge, but I wonder why your maths are different. So I recalculated and even took my camera to be on the safe side:

Let´s start with ISO
100 0.00
125 0.33
160 0.67
200 1.00
250 1.33
320 1.67
400 2.00
500 2.33
640 2.67
800 3.00
Next Aperture :
5 2.67
5,6 2.33
6,3 2.00
7,1 1.67
8,0 1.33
9 1.00
10 0.67
11 0.33
13 0.00
14 -0.33
16 -0.67
18 -1.00
20 -1.33
22 -1.67
and finally Shutter Speed
1/500 -1.33
1/400 -1.00
1/320 -0.67
1/250 -0.33
1/200 0.00

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
Thanks for the clarification, of course you are techically correct ;). I remember we had a similar discussion a while ago. I wonder what would be a better term. I tend to use the word brightness, but in this case it would definitely be wrong.
 
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
 
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
That is correct.
 
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 looked at the EXIF information. In the 800 ISO shot, the Gain Control setting, it lists Low Gain Up. In the 100 ISO shot, we have Gain Control: None.

So...at 800 ISO, the JPEG processing was done with added Gain control added into the processing mixture.

Interestingly, additionally I see that in the ISO 100 shot, which was made at f/4.5, you have a slight bit of selective focus effect occurring: the foreground bush and its lovely red berries are rendered in crisp focus, but the background of the woods is clearly,and obviously, slightly de-focused. In the 800 ISO shot, made at f/22, the background is more subtly out of focus...it's not really blurry, but it's in-between sharp and de-focused.

Back in the 1980's, when Nikon _invented_ multi-segment light metering and scene analysis, the Nikon FA was fitted with an "8-bit microprocessor", as Nikon boasted, and the FA camera had a memory bank of over 100,000 actual scenes that had been photographed, measure,metered,and analyzed. Things like 3-D, color-aware light metering and dynamic range optimization have advanced markedly since those days if the FA and multi-segment light metering and exposure control. Today the camera not only meters the scene, but processes the final out of camera JPEG image!. One thing I notice is that, in the photo that has the greater depth of field, the f/22 shot done at ISO 800, there is more brightness in the slightly shadowed areas on the fir trees in the background. In other words, because the background is slightly more-recognizable, and more in-focus, I suspect that the JPEG processing has been biased to add a slight bit of brightness to the background due to 1) ISO 800 use and 2)slightly more in-focus, and thus deserving of being "seen". 3) Gain [ brightening-up] being added to the processing

In the f/4.5 shot, which is likely with the lens wide-open at the used 35mm focal length setting on the 18-55 lens, the camera's processing is favoring the foreground exposure, and the background brightness is what one gets with NO gain control added, a pretty straight-up scene analysis and image processing effort. The 100 ISO in use would likely be indicative of a well-lighted scene.

When one starts looking into how Nikon meters with new cameras, they have a color-aware, and distance-aware, "3-D RGB Color Matrix Metering" that uses at the low end cameras, 1,005 sensor measurements, up to many thousands of measurements on higher-priced camera models. The entire scene, the distances, the focused distance, the ISO,. the geographic location, and the time clock...all of those things play a part in the metering and scene analysis. The clock you say? Combined with the city/time zone location the user inputs, it tells the camera night from day; at 12 midnight in Seattle, a large, bright orb in a darker field is either the moon, or a street lamp; at 12 Noon in Seattle a large, bright orb seen against a darker field is...the Sun in the sky.

Reading the colors? White sand? Gray sand? Brown sand? The color-sensing ability of the metering can determine the colors of things AND determine their degree of reflectivity. The camera can then expose, and process, and thus determine how the scene might best be rendered as an out of camera JPEG.

Your shots are different in time by enough that some slight cloud cover over the sun _could_ possibly have altered actual scene brightness, but my experience with Nikon matrix metering is that is is quite affected by larger bright areas; center-weighted Nikon metering is less-affected by bright areas; this is where the slight framing variations Ysarex mentioned could come into play; still, we do not have enough information to really draw conclusive, valid conclusions about why the two shots are different, but my vote goes to ISO levels, 100 vs 800, and no gain versus Low gain Up on the 800 ISO shot, thus brightening it up.

Sorry for the long post on this, but hey...
 
Last edited:
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
A distinction without a difference. ISO is one of the three variables that have to be set to adjust for proper exposure on the final image.
 

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