Exposure

[MrB]

I hope no-one minds me asking one of those daft questions that (probably} serves no practical purpose but, as on the TV programme QI, I would be quite interested to know the answer.

When the camera is in a mode in which the exposure is set automatically, does it set the actual values shown on the LCD screen (e.g. 1/125, f16, etc), or is it really more precise (e.g. 1/129, f15.7, etc.) and simply displaying the nearest well-known increments?

Philip

[Colin Southern]

I hope no-one minds me asking one of those daft questions that (probably} serves no practical purpose but, as on the TV programme QI, I would be quite interested to know the answer.

When the camera is in a mode in which the exposure is set automatically, does it set the actual values shown on the LCD screen (e.g. 1/125, f16, etc), or is it really more precise (e.g. 1/129, f15.7, etc.) and simply displaying the nearest well-known increments?

Philip

That's not a daft question - it's quite an interesting one actually!

Unfortunately, I don't have a clue, but it's still an interesting quetion!

[Dave Humphries]

Hi Philip,

It does depend which EXIF viewer you use

My guess is;
It tries to set 1/125s and f/16, but if you look at the EXIF data, you may (probably depends on camera model) either see those exact figures, or especially for shutter speed, a seemingly 'more accurate' figure.

I suspect the latter arises because there's a bit of software in the camera counting clock pulses while the shutter is open and that is independent of the shutter timer

e.g. From a Canon 7D:

Exposure Time (1 / Shutter Speed) = 1/500 second ===> 0.002 second
Lens F-Number / F-Stop = 2/1 ===> ƒ/2
ISO Speed Ratings = 200
Original Date/Time = 2011:07:24 16:21:31

Shutter Speed Value (APEX) = 8965784/1000000
Shutter Speed (Exposure Time) = 1/500 second

Aperture Value (APEX) = 2/1
Aperture = ƒ/2

Flash = Flash did not fire, compulsory flash mode
Focal Length = 35/1 mm ===> 35 mm

From my Nikon D5000:

Exposure Time (1 / Shutter Speed) = 1/500 second ===> 0.002 second
Lens F-Number / F-Stop = 11/1 ===> ƒ/11
ISO Speed Ratings = 800
Original Date/Time = 2011:06:26 16:35:16

Shutter Speed Value (APEX) = 8965784/1000000
Shutter Speed (Exposure Time) = 1/500 second

Aperture Value (APEX) = 6918863/1000000
Aperture = ƒ/11

Flash = Flash did not fire
Focal Length = 105/1 mm ===> 105 mm

and from a Canon Rebel T1i:

Exposure Time (1 / Shutter Speed) = 1/200 second ===> 0.005 second
Lens F-Number / F-Stop = 28/5 ===> ƒ/5.6
ISO Speed Ratings = 400
Original Date/Time = 2011:07:29 05:43:27

Shutter Speed Value (APEX) = 61/8
Shutter Speed (Exposure Time) = 1/197.4 second

Aperture Value (APEX) = 5/1
Aperture = ƒ/5.66

Flash = Flash did not fire, compulsory flash mode
Focal Length = 300/1 mm ===> 300 mm

Note how the APEX values differ between some models of same manufacture, but agree, for some items even across manufacturers.

It all seems a bit random to me and nothing worth worrying about - but it still isn't a daft question

To my mind, it makes more sense for the values set to be nominal 1/3 or 1/2 stop steps, depending upon how the camera is set up - there's no advantage to trying to be more accurate here.
However, as part of the manufacture or service process, if there is a means in each camera where they can set the master clock frequency (of the computer chip running the camera) to a known standard, then take a picture and view the EXIF data to determine if any offset is required for mechanical tolerances of construction to get it accurate (because it was independently measured), that has to be useful during calibration (or so the engineer in me believes).

Clearly I don't have (much of) a clue either

Cheers,

[MrB]

Dave - you have certainly found evidence that appears to offer an answer to the question.

... if you look at the EXIF data, you may (probably depends on camera model) either see those exact figures, or especially for shutter speed, a seemingly 'more accurate' figure. I suspect the latter arises because there's a bit of software in the camera counting clock pulses while the shutter is open and that is independent of the shutter timer.

From the middle set of each of your examples (especially the Rebel) we can see that the camera is setting more precise values of shutter speed and aperture than those reported on the info screen -

... and from a Canon Rebel T1i:
Shutter Speed (Exposure Time) = 1/197.4 second
Aperture = ƒ/5.66

Also, I like your ideas about the calibration of the camera - there might be some practical use linked to the answer, which does make the question seem not quite so pointless after all! -

However, as part of the manufacture or service process, if there is a means in each camera where they can set the master clock frequency (of the computer chip running the camera) to a known standard, then take a picture and view the EXIF data to determine if any offset is required for mechanical tolerances of construction to get it accurate (because it was independently measured), that has to be useful during calibration (or so the engineer in me believes).

It is clear to me that your last comment -

Clearly I don't have (much of) a clue either

is not true.

Thank you for finding clues, for your great ideas, and for your effort in producing a QI contribution!

Philip

PS - How did you get the detailed Exif data (my software, Corel, shows only the usual 1/3 step values).

[Dave Humphries]

PS - How did you get the detailed Exif data (my software, Corel, shows only the usual 1/3 step values).

I got those from the EXIF plugin for Firefox web browser, I think this is Alan Raskin's.

Once located (URL above) and installed, you right click an image and there is a new "View Image EXIF Data" option.

Cheers,