A question of compensation - August

[ap4a]

Originally Posted by hallsworthb

Since this runs contrary to all the advice I have previously received and also since it doesn't seem to make sense, perhaps you could help me understand.

It is contrary to previous advice, in that data lost in highlights is so terminal that the recommendation has been to shoot "Left of centre" on the histogram, since there is more chance of recovering data in shadows. (My experience reinforces that -a picture with a bright window and harsh shadows can only be recovered in the shadows).

It's not advocating blowing out your highlights. It's advocating shooting to the right whilst not blowing them out. So you still retain data in all regions of the image.

The bit that doesn't somehow seem to make sense in the essay is the assumption that the 4096 discrete tonal values are shared between the F-stops. That suggests that if I shoot at f2.8 I will get 2048 levels, whilst at F32 I will have 128levels. Or have I misread it? If I haven't misread it then frankly I find it hard to believe. The F stop is a mechanical thing, changing the amount of light that reaches the sensor. So the amount of light hitting it will increase / decrease, but why would that change the dynamic range (The frequency / colour) of the received light?

It has nothing to do with the aperture you set your lens to. It relates to the difference in light intensity in darkest and brightest regions of an image (the shadows and highlights). And their estimate is that there is a maximum of 5 - 6 stops of difference between these regions that can be recorded by a 12 bit sensor. Irrespective of what aperture, iso or shutter speed you use to expose the image.

The 4096 tonal values relate to the bit depth of the image - 12 bits is 2 to the power of 12, which equals 4096. What they say is that 50% of those tonal values lie in the brightest (un-blown) stop available from that 5 - 6 stop exposure range that it was possible to record. Whilst the darkest (un-clipped) stop available in the range will hold in the region of only 128 tonal values.

You can put this to the test by shooting a shot where the histogram is bunched up to the left, try without clipping it, and then increasing the exposure in Lightroom, ACR or DPP etc. You'll very quickly see a lot of noise and banding appear as a result of lack of data. Whereas if you shoot to the right, without clipping, and then decrease the exposure you will find that you have far more tonal data.

[hallsworthb]

Originally Posted by ap4a

It has nothing to do with the aperture you set your lens to.

there is a maximum of 5 - 6 stops of difference between these regions that can be recorded by a 12 bit sensor. Irrespective of what aperture, iso or shutter speed you use to expose the image

OK - I'm nearly there, and thank you so far

Just to satisfy a yearning in me for a last dot on the "i" and cross on the "t" - Please define a "stop" - I thought that was relating to "F" stop - The aperture set on the lens?

Bob

[chris-p]

Originally Posted by hallsworthb

Just to satisfy a yearning in me for a last dot on the "i" and cross on the "t" - Please define a "stop" - I thought that was relating to "F" stop - The aperture set on the lens?

It's half or twice the amount of light and it doesn't just refer to the aperture (although when people talk about "stopping down" they do mean the lens).

Aperture-wise f/4 lets twice the amount of light through that f/5.6 does. f/5.6 lets through twice the light of f/8 etc.
Shutter speeds and ISO also work the same. 1/500th of a second is exactly twice as much light as 1/1000th of a second. ISO200 is twice ISO100. ISO400 is twice ISO200 etc.

I wrote a tutorial on this (and I'm going to link to it twice in 1 day!) which you can read here

[hallsworthb]

OK - Thank you to Geoff; Forseti; ap4a and Chris-p and all others that agreed or disagreed

So to test my understanding -

If I take a photograph at (say) ISO 400; F5.6; at 1/250th second and I look at the histogram, then I can change any of them to a maximum of +/- "clicks", giving a spread of 5 or 6. ideally, and without killing the highlights, that will move the histogram to the right, to preserve as much information / retrievable image as possible.

The decisions I make about which to move include speed of subject; depth of field; noise; etc as normal

Finally (Sorry) If I include compensation, and am on "P", does the camera decide which to change? I assume it has criteria programmed , with ISO last to be moved.

I clearly need to get out more - with the camera.

Bob

[chris-p]

Originally Posted by hallsworthb

If I take a photograph at (say) ISO 400; F5.6; at 1/250th second and I look at the histogram, then I can change any of them to a maximum of +/- "clicks", giving a spread of 5 or 6. ideally, and without killing the highlights, that will move the histogram to the right, to preserve as much information / retrievable image as possible.
The decisions I make about which to move include speed of subject; depth of field; noise; etc as normal

Yes. Although I'm not sure what you mean by "clicks". If you're talking about the notches on the adjustment wheel, they usually adjust in 1/3 of a stop so 6 "clicks" is 2 full stops. The method you've described retains as much possible highlight and shadow detail.
Don't forget that you can adjust the settings you've got above and main the same overall exposure by compensating one adjustment with another.

For example you've said ISO 400, f/5.6 1/250th. If you wanted a larger aperture to get more depth of field you could adjust either the ISO or shutter speed to compensate. Say to ISO 200, f/4, 1/250. Or (adjusting the shutter speed) ISO 400, f/4, 1/500th.

Originally Posted by hallsworthb

Finally (Sorry) If I include compensation, and am on "P", does the camera decide which to change? I assume it has criteria programmed , with ISO last to be moved.

This will be specific to your camera and will be in the manual somewhere. The camera chooses what to change and it will have a preset range. It's called the program line.

In dim light it opens the lens up as far as possible and uses the highest "safe" ISO (as preset by Canon) to get as fast a shutter speed as possible. Once the shutter speed reaches 1/f (ie 1/4 at f/4) then the lens stops down as the shutter speed increases.

Then it tries to keep the aperture 1 stop smaller than wide open and the shutter speed faster than the reciprocal of the focal length (ie 1/200th at 200mm) and the ISO at the minimum setting it can. You may also have an adjustable auto ISO limit which you use to tell the camera it can use any ISO up to and including whatever you've set.

There are program line diagrams (although they don't account for auto ISO) here.

[hallsworthb]

Fantastic - Thank You Chris.

Regards
Bob