More on the relationship between ISO and exposure compensation

A little while ago I wrote a post HERE about ISO and exposure compensation. I explained that whilst I could see the practical results in the two cameras I tested (Nikon D200 and Nikon 40) and could see that the results differed between the two cameras, I did not know what the theoretical answer was to the question I asked, nor why the two cameras should behave differently.

Recently I posed the question to Ctein, and here is his answer:

Dear David,

On the basic level, here’s what happens:

— When you dial in exposure compensation, all you do is change the
exposure. The ‘sensitivity’ of the sensor array isn’t altered.

— when you change the ISO setting, the ‘sensitivity’ of the array is
altered by changing the amount of amplification that gets applied to
the signal that’s read out of the array. The actual physical
sensitivity of the array doesn’t change– it still collects the same
number of electrons for the same amount of incoming light.

The key thing here is that even in RAW mode, you’re never actually
looking at the charge collected by the sensor. You’re always looking
at an amplified signal derived from that charge. Exactly how this
plays out in any particular camera depends upon the camera’s
electronics and software. In effect, what kind of electronic
“characteristic curves” the designers decided to build into the
camera. That’s likely why you see different results with different

pax / Ctein
— Ctein’s Online Gallery
— Digital Restorations

Which zoom telephoto lens?

Testing the Nikon 55-200mm f4-5.6 AF-S VR DX

Before it was stolen out of my luggage at an airport when it was left behind with other luggage on a flight two years ago, I had a 180mm fixed focal length Nikon lens that was very sharp. Recently I decided to replace it and considered the heavy and expensive 70-200mm VR lens; the heavy 80-200mm AF-S lens (that does not seem to be advertised new anywhere now); the 80-200mm AF (non ‘S’ version); the 18-200mm VR, and the 70-300mm.

The 70-300mm stretches further but 200mm was enough for what I wanted, so if weight and cost were not an issue, the 70-200mm would be top of the tree – but it is heavy.

I tried all of them except the non ‘S’ version of the 80-200mm and decided that I just had to use the 55-200mm for a while because the images in the LCD didn’t look noticeably different from the 70-200mm.

At less than £200 in the UK the 55-200mm is £1,000 cheaper than the 70-200mm and a lot, lot lighter. The weight difference is over a kilogram (335g compared to 1470g).

Both are VR lenses and for anyone who has not used A VR lens, I advise trying it if for nothing more than the experience of focusing with it turned off, and then switching it on.

VR (vibration reduction) uses a system of gimbals built into the lens that steady it against the movement of your hands.

The way the image steadies is remarkable. Without VR the image sits like a rectangle vibrating and weaving about, and then ‘click’ the VR on, and it suddenly stops.

Of course the 55-200mm is nowhere near as strongly built as the 70-200mm, but maybe I can coddle it.

I haven’t done enough shooting to decide how sharp the 55-200mm is, but here is a shot taken with the lens mounted on my Nikon D40.

The shot was taken out of a motel window, with the pole about 200 feet away, at 800ISO 1/250sec and f7.1 – full frame and a 100% crop around the pole.



And here is a crop of 2% of the area of the frame of a grab shot portrait shot at 1/400sec f5.6 at ISO1600


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