How Do You Know If Your Camera Is Making The Proper Exposure

How do you know if your camera is making the proper exposure? Do you know from “Chimping”, maybe the Histogram? I hear, “I look at the skin tone”, or “My camera is brand new”. I say you dont know if you haven’t tested it.  The great news is it is a simple test.

Back in the day of film the exposure index was used to determining if you had enough latitude to make a proper exposure. With color negative film you have 2 to 2 1/2 stops of exposure latitude, this means you could blow the exposure, and as long as it was within 2 1/2 stops the lab could fix the exposure. With digital this just isn’t the case, exposure latitude of a “cooked”  jpeg file is 1/2 of a stop.  Raw allows for more latitude with exposure, however if the highlights are gone there is no saving them, even in raw.

In digital, if you blow the highlights, the files are simply toast.  The file is ruined forever, and no amount of Photoshop will save it.  And this goes for RAW as well. The reason I want to talk about this, is, to ask you a question?  Do you know 100%  what your camera is doing with exposure?

Camera manufactures built millions of units, the camera is only a sum of all the parts.  Each part is assigned a ”plus” or ”minus” (+/-) tolerance or a “variance” number.  This variance will add up as a device is brought to life.  If you are lucky and buy a unit that has all the parts on the “+” side of the variance you might get what is called a “good copy”.  On the flip side, you might get one that is on the “-” side of the variance and it could be a “bad copy”.

Regardless of the camera’s make or model, the only thing that matters is knowing exactly how it’s performing.  Armed with this information you can make the proper adjustments to get constant exposure.  So you are asking yourself, how do I find out if my camera is exposing properly?  No problem, I will show you how.

The only tool you need is a reference card, in this case an 18 % grey card.  You can pick them up at you local camera shop for $5.00 – $10.00.  I own several of them and they are all the “Delta” version.  Some will say a grey card is for white balance, and I will say to you that you are wrong.  A grey card is for exposure. I won’t go into great detail about how it works, but I will say this, if you look at the center tic mark in your camera (most of the time it is the zero  [-2--1--0--1--2+]).  That center tic - zero – is exactly 18% grey. If you look at 18% grey on a histogram you will see it lives at dead center.

The histogram is broken in to 255 different shades of grey, from 0 (black) to 255, (solid white).  Dead in the middle is 128, or better known as, 18% grey.  This is also known as middle grey or the middle value.  So all you have to do is buy a grey card, take it outside mid day, find a evenly shaded area (even diffused shadow) place the grey card there and make a picture of it with the frame filled.

Next take the image into Photoshop or Lightroom and hover the histogram to read the value. In Lightroom middle value is 50% as the scale is not 0-255, it’s a 0-100 scale so 50% is 18% grey and is the exact same as 128 in a standard histogram. Armed with this information you will know if the camera is under or over exposing.

You can use the ISO to make correction adjustments. If the camera is underexposing 1/3 of a stop, just dial in 1/3 of a stop adjustment, the same goes for under exposure. It is really simple to correct. I had a Canon 7D recently that was off by 8/10′s of an f-stop. I use a Sekonic light meter most of the time so it is a simple fix, just make a custom offset to compensate.

Knowing what the camera is doing is 90% of the battle, the rest is making that simple correction.  On a side note, every lens sensor combo is different, so it is a good idea to test all of your gear to see what exactly it’s doing.  I have said it before and I’ll say it again. “Know Your Gear”, then making adjustments is as simple as a click of a knob.

Here are some images of a standard grey card and some test shots of Lightroom. My camera is almost perfect so I do not have to make any compensation adjustment.

Click here to see a large version.

Here is the back if a Delta Grey Card

Here is the captured image used for the test, so simple yet very revealing.