Metering for dynamic range

It's a cold hard fact, but in case you didn't know it - photography is not 'real'. We don't capture reality as it is with a camera. You might wonder what I'm on about, but let me explain further. Firstly, a camera sees in 2D, whereas we see in 3D. But also, our perception is a lot more different than a camera sees because we have far greater 'dynamic range'. We are able to register detail in shadows and highlights that a film or digital camera can only dream about.

ISO 50, f4 = average reading of 1/4 second, 5 stop dynamic range where shadows are at 2s and highlights are at 20th of a second

This point is mainly the reason for why we often get images back from the lab, that looked nothing like how we perceived the scene at the time. It takes skill and patience to be able to get to a point where you are confident you are going to capture what you saw, and you may need to use neutral density graduated filter to squeeze the the entire range of tones from highlights right down to shadow onto your film/sensor.

So I thought I'd explain a bit more about dynamic range.

Our eye is capable of seeing over a range of 24 f-stops. But a digital sensor or film can see roughly 3 to 6 f-stops depending on the medium (negative film has a higher latitide and can often record over a wider range than slide film can). So it's very common to want to record a scene that the camera simply cannot handle. You are either going to have an image with underexposed ground, or over exposed sky, or a bit of both.

Now, your camera simply meters everything and works out an average of 18% grey. For instance, if you point your camera at a black wall, the meter will make an exposure which will make the black wall grey. The same is conversely true for a white wall; your meter will make a white wall grey. This is why you need to add or subtract exposure compensation for situations when you know that the overall scene is too bright or dark.

So how do you get the right exposure?

Well the truth is that there is no such thing. It is purely about what you consider important to record, and what you are willing to sacrifice, but there are times when using an ND graduated filter will allow you to squeeze an entire scene which has a dynamic range greater than your film/sensor can record, and here's how I do it.

I have a rather nice little Sekonic meter which allows me to take many spot readings around my scene and from that I can see the entire dynamic range that the scene contains. For example, I will take a spot reading of the darkest part of the scene and then a spot reading of the highlights in the scene, and from that I will perhaps see that there is a 10 stop difference between these extremes. I know that my Fuji Velvia film won't handle this and that I really need to get the scene down to around 6 stops of a difference. If I put on a 3 stop ND graduated filter to compress the sky (highlights) down by 3 stops, I've reduced the entire dynamic range from 10 to 7 stops. Which is a bit more manageable, but I also have to determine what in the scene is going to appear 18% grey. Remember that when I meter the dark areas of the scene, the meter is telling me what the exposure would be if I wanted the dark areas to be grey. And the same is true when I meter the highlights. So I need to decide where in the middle of the range I want to be my mid tone. This is the bit that is subjective, but if I find for example some rocks in the scene that I think are 18% grey, I will decide that is my exposure point. I will then set this exposure manually (to stop the camera's meter from changing the exposure when I add on the grad), and take the shot.

With an SLR, it's a lot easier than this - sure it's nice to have a meter that shows you the entire range so you can figure out what difference in exposure latitude you need to make, but often, the camera will work out a nice average for you - if you have already placed a grad on the scene anway. So it's much more of a point and shoot approach to it, and if you are using a DSLR, you can check the exposure to see if you over did the grad - often for example, I tend to find that 3 stop grad is too much and 2 stops is more appropriate.

Placement of ND Grads on a Mamiya 7

I've been asked a few times now, how I manage to use graduated neutral density filters with my Mamiya 7II. So I thought it was about time I wrote an article about it to explain ND grads and how to place them on a range finder camera.

What the human eye sees, and what a camera see are two different things. The human eye is capable of capturing a dynamic range (brightness values from dark to light) that is wider than what a camera can capture. This is why often you will see a shot where the sky is burnt out while the ground is exposed correctly, or the sky is ok, but the ground is underexposed (almost black). Film and digital sensors cannot cope with such a difference in light values between the sky and ground, yet our eyes are able to handle this difference in contrast and make it appear to us as though the brightness is the same between the sky and the earth. In order to bring this contrast or 'latitude' down to a manageable level, so we can 'squeeze' the entire scene into the dynamic range that a camera can record, we use Neutral Density Filters. Neutral means that they do not affect the colour of the scene in any way, they simply darken down an area of the scene (typically the sky) so we can get a good exposure in camera. I use these all the time for landscape work.

With a rangefinder camera, you do not view your scene through the lens. In the case of the Mamiya 7II, the lens contains the shutter and so is permanently closed, until the moment of exposure. There is also no prism or mirror. Which means the camera is a lot more compact and more silent too. But the draw back is that you don't get to see what you'll get when you expose. Most range finders have a side window showing you an estimate of what you'll get with some dotted lines at the edges to compensate for different focal lengths.

Anyway, the problem with using ND filters on a range finder is that most folk think it's very hard to judge the correct placement in front of the lens.

Above are two Lee 0.9 (3-Stop) grad filters. The left hand one is a soft grad, while the right hand one is a hard grad filter (one of my most used grads). As you can see - the graduation of the hard grad is rather dramatic in the middle of the filter. Most folks think that placement needs to be precise, but to be honest with you - it doesn't. When you put something so close up in front of the lens, it becomes diffused. This means that the graduation effect of the hard grad becomes less pronounced. Unless you are really way off with your placement, you're not going to see a problem.

So how do I place the filter? Simple - I guess. If the scene I'm shooting has a sky that is using 1/3 of the area of the scene, then I place the filter roughly 1/3 of the way down. If the sky takes up 1/2 of the scene, then I simply place the filter half way. Etc, etc.

The other thing that I tend to do is take more than one shot of the same scene. If I'm uncertain about the filter placement, I'll take the scene a few times, each time with a subtle adjustment, moving it up or down by a few centimeters. But I've often found that it's been unnecessary.

My favourite ND Grads are made by Lee in the UK. I've tried others such as the Cokin system but found them not as effective. But they are considerably cheaper. As in everything - you get what you pay for.

One last thing, if you feel that placement on a range finder sounds problematic - what I love about such a system is the 'visualisation' process that I go through. I like to 'imagine' the final image, and not having direct feed back (via an LCD screen) is a benefit, also being able to imagine the scene in my head and place the filter accordingly, allows me to remain in the 'creative-mode'. Being confronted with real world feed back takes me out of this 'creative-mode' and into 'editor-mode' which is something that I feel kills my creative flow when out shooting scenes. So yes, it's a benefit to me rather than a hindrance.