Spot Metering

I am always on a quest to improve my photography. Recently I have been watching YouTube videos from the likes of Justin Lowery or Nick Carver. Young guys who cut their teeth on digital and found film; landscape photography in particular. Working in medium and large format and they put their experience and knowledge on video. The area I wanted to explore more is getting good exposures in difficult situations. Typically this involves some adaptation of Ansel Adams’ Zone System. 

This article is a summary of what I have learned in researching the topic and my limited application so far. One should not hold this as authoritative by any means but it might be a helpful resource to introduce the subject. I have attempted to synthesize some sensible summary from the various sources I have researched. At the end I list some additional resources.  

Zone System

Having read in the past about the Zone System as promoted by Ansel Adams it seemed inaccessible to those working with medium format film. (Ansel Adams used large format cameras that take a single sheet of film for each exposure.) Especially since the zone system as originally formulated encompassed not just exposure but also development of the negative. It relies on the idea of developing each negative in relation to the dynamic range one wants to express in the image on film. The idea of 'expose for the shadows and develop for the highlights' encompasses this idea. It was ideally suited and created for large format photography where each negative is developed separately. 

So in addition to wisdom of where to place the shadows or highlights in exposure 'Zones' one either lengthens or shortens the development time depending on which Zones the highlights land. If they land in Zone VIII for instance one might use normal developing time for the film and developer combination. If the highlights are compressed into Zone VII then one might lengthen the development time to darken the areas on the negative where highlights would be. (Remember on a negative the highlights of a scene are dark and darken more with more time spent in the developer.) This would be N+1 developing and might mean extending development time say 10% longer. (The amount of extra development time ideally is determined experimentally for the film and developer and your meter combination.) 

Similarly where the scene might place the highlights you want to yield detail into Zone IX one would reduce development (N-1) to bring those highlights into a useable range. (David Kachel has a beef with this development-time-based compression. He developed SLIMT (Selective Latent Image Manipulation Technique)  an extremely clever technique to bleach the latent image in the film before development to reduce contrast.) 

Needless to say this exposure-by-exposure based development is impossible on a roll of film unless all images share the same range. Additionally if one is not developing their own film it is essential you discard this part of the process or you will find yourself confused. Also modern film is reputed to offer more latitude than the film used by Ansel Adams when he developed the technique.

Another conundrum I faced in implementing the Zone System was how to meter the shadows and the highlights separately. A conventional exposure meter (I use a Sekonic L308S) shows exposure for say a 40 degree field of view and gives an exposure for the overall scene. It is exceedingly difficult to get a reflective reading for shadows or highlights alone for a distant scene. (I will give some tips later for how in some circumstance you might approximate this.)

In general the zone system defines a range of 11 exposure values (0, I, II, III, IV, V, VI, VII, VIII, IX, X) with 0 being the dark end of the range (pure black) and X being pure white. Zone V is where a light meter places the exposure. 

Zone Scale

A light meter has no magic in it. It merely indicates what the exposure should be to achieve what is called 18% gray for the light available to measure. For a traditional (non-spot meter) this will give an average for a wide area of a scene (a 40 degree cone in the case of my Sekonic meter). Don’t get hung up on the 18% figure and why this is halfway in the Zone range just accept that a meter is calibrated this way and that it will read for the light it ‘sees’ and produce an EV or exposure setting for the chosen film speed that is this halfway point or Zone V. Film is manufactured and tested using this same 18% gray reference. 

What this means is that a pure white object metered for Zone V will end up looking gray. It is why snowy scenes don’t turn out right when exposed per the meter reading, as most of the scene is white and the meter will tell you to expose it to look gray. You actually want the snow to end up in Zone VII or VIII to push it to the white end of the scale. This means you need to overexpose the scene by 2 or 3 stops. 

Only scenes that have a equal balance of white and black will turn out OK using an average metering of the scene. Even in this 'balanced' case the scene highlights and shadows may exceed the range of the film so that you get both blown highlights and blocked shadows. Your creative intent could have solved this problem if only you knew what was happening! (Alan Ross' article referenced at the end of this post gives some excellent examples of how an image changes depending on where the reading is placed in the Zone System.)  

Once you understand this you can appreciate the value of the Zone System to manipulate the tonal range of an image. You can pick key areas of interest and if you can meter them accurately you can make them whiter, blacker, or grayer depending on what you want the image to convey. Of course to meter a specific area of interest requires we discuss the spot meter. 

Spot Metering

Here is a device that can read a one degree field of view and give an accurate metering of the light in just that area. The problem is they are exceedingly expensive new. Digital ones are over £500 and I would argue they are not that useful. Another option is a digital camera as suggested by Alex Burke. Finally old Pentax spot meters are available online with a price of around £200-300 for the digital model and £100-200 for the analog V model.  Here are my personal views on these three options based on very limited experience and some research.

Digital Spot Meters (Sekonic or Gossen)-These are available and no doubt accurate. Buying a new device means you know it works and can get it repaired when it doesn't. Important considerations. However having used a Pentax V meter I have come to think half the value of this meter is the 'analog computer' built into it. That is the dial that converts the ISO/ASA and EV reading to a whole host of f-stop and shutter speed pairs for that exposure. Digital Meters are not good for this. I find myself clicking up and down the range till I find what I am looking for. For some meters that display a linear line of EVs on the bottom of the display it is possible to use a linear scale for the Zones. This is illustrated below from the Alan Ross website linked to in the caption. Alternatively you could make an exposure dial using the instructions at the link at the end of the post. Use the digital spot meter in EV mode and refer to the dial.

Digital Spot Meter Zone Scale (Alan Ross graphic)

Digital Camera-- This option  is very seductive and Alex Burke has written quite a bit about this. Not only do most digital cameras have a spot meter (he uses a micro 4/3rds camera) but you also can see a histogram and evaluate the exposure overall which meters cannot do. He makes one important point that applied to me however. Alex points out that it should have f-stops down to f22 (or lower) to match what you would use on the film camera. The digital camera I had laying around (Fuji X10) only went as low as f8. This meant using higher shutter speeds instead of small f-stops so I was forced to run through the conversion from shutter speed to f-stops in my head and the inevitable errors in that process. Not great. Also digital cameras have a range of f-stops and shutter speeds not found on most film cameras. What to do when it says f7 and 1/44 sec? More chances for errors. If you carry a digital camera normally when you shoot film then there is no extra weight or cost so all to the good. (I do find a digital camera invaluable for night photography with long exposures as I can mock up the exposure and get an idea of the final image.)

Spot Meters-- I will refer to the Pentax units here (Ansel Adams used the Pentax Digital it is reported.) The Pentax V and the digital version are very similar. You read an EV value from a single one degree spot in the magnified field of view of the scene. For the Pentax V this is a conventional meter reading from 1 to 19 in 1/3 EV increments. For the Digital this is a digital EV number followed by one or two LEDs to show 1/3 EV values. In both cases you take the EV number and line it up on the analog dials to then get a selection of equivalent exposures for the EV number from the dials.  I bought the analog meter on the basis the electronics are simpler and it is more likely I could repair it myself  if needed. It is also cheaper.  The digital version uses LEDs so I suspect are easier to read in lower light situation or with dark backgrounds. The analog meter has a light that can be used to illuminate the low end of the scale when metering in dark conditions. The meter is bulky but using one for me was almost a religious experience.

Pentax V Exposure Dial

The illustration above show the great benefit in my mind of the Pentax-style meter. What I call the exposure dial (Pentax refers to it as the Calculation Dial) is a great piece of efficiency in the form of an 'analog computer'. In the days before digital electronics and digital computers engineers had to come up with clever ways to make calculations and represent data. The ASA is set on the right hand side of the dial. The white arrow at 6 o'clock is where the EV from the meter is set. 

Pentax V Viewfinder image.

In the image above the view inside the meter is shown. The white background is a magnified field of view of the subject. The small circle represents the 1 degree metering area. You aim the meter at the subject and place the area to be metered directly under this black circle. Pull the trigger and the meter indicates the EV for that spot. 

The exposure dial is then turned to set the arrow at the EV value read from the meter. In the case shown in the Exposure Dial figure above the exposure dial shows an EV of 4 at this point (different from the meter which shows an EV of 6 and 2/3.) 

The exposure dial then shows around the top arc f-stops on the inside ring of numbers and shutter speeds on the outside ring. In this picture the exposures for an EV of 4 are ...

• f5.6 2 sec
• f8 at 4 sec
• f11 at 8 sec
• f16 at 15 sec
• f22 at 30 sec
• etc...

All of these are visible at once. 

This meter also features at the bottom of the scale with the EV scale what is called the IRE scale which is a scale used for metering television exposures. This scale is unnecessary for our purposes and this is where one can place the Zone Scale to make the meter more useful.

Pentax V EV and TV  IRE Scale

Above shows the IRE scale as found on the meter.

Pentax V EV with Zone System Scale

Here is a zone scale applied over the IRE scale. Now you can set the metered EV for say a shadow over to Zone III and then read the f-stop and shutter speed for the top of the dial. I find this very helpful in visualizing the range of an exposure. Similarly one can meter to get the EV for a highlight and see which zone it lands in or move the EV to the desired Zone to make sure a highlight this not blown out or that a highlight detail is exposed to get the right amount of detail.

 The Spot Meter and What I Got From It

I took the meter out on a short weekend camping trip back to the Llyn Peninsula in NW Wales. We were due to have some spectacularly sunny weather which is great for camping and maybe not so much for photography. Since there was to be so much light and I expected scenes with high dynamic range as a result, I left the Velvia at home and brought Portra 160 for my Fuji GSW690 camera and Portra 400 for my Fuji G617. The G617 has a center filter that cuts the effective film speed by one stop so I would shoot that at 200 ASA. This means they are practically the same film speed so if I forget to reset my meter ASA dial I should still get a good exposure.

The first thing I learned while metering around sunset the first evening was I should have brought the Velvia. The meter showed the full range of the scene was inside of the 5 stop range of Velvia! I measured the cliff shadow and the brightest part of the sky and it all fit in 5 stops. With the magenta and blues in the scene I really wished I had that Velvia color thing working for me. I had to hope the subtle color of Portra would work.

As shown below the exposure was good. The inset shows the shadow detail I metered at Zone III along the dark cliff. The sun had set from behind me by now. I had some trouble in ColorNeg with these shadows blocking that I thought was initially a scanner problem but it turned out the negative was sensitive to a feature called Black Point Tails. I lowered this setting and got all the detail back that was apparent on the negative. 

For reference however I will try and remember that for scenes like this of limited range I should place this shadow in Zone IV since the highlights were not beyond Zone VII for this scene. If I had used Velvia that would not be a good idea.

Beach scene after sunset. Metered cliff shadows for zone III.

Inset of above photo to show cliff exposure. (Ignore dust and fibers)

Next thing I noticed was the confidence I had about shadows and highlights. I had the sense I was not going to be surprised by anything. Only one photo out of 28 was badly exposed and that was one with a pre-dawn 15 second exposure and I forgot to bring any reciprocity information. I shot it at 15 seconds and later learned it should have been 35 seconds. A full stop underexposed. 

This blog entry from "Life is ANALOG is Life" is the most convincing I think of the value of a spot meter and it extends the value beyond a means to practice a useful subset of the Zone System. The article gives a half dozen examples of how dramatically photos benefited from the use of a spot meter.

Zoning In

Before I went out to make these photos I pasted a zone system scale over the IRE (for TV video metering) scale on the dial face. (Like the one below)

With the Zone V aligned to the EV index mark I could now place the EV spot measurement into the desired zone. For instance in the photo above, the shadow of the cliff where we still want detail in the photo is spot metered at EV 2 it is placed in Zone III where some detail will be resolved. Zone VIII would have detail to an EV of 7. If your highlights are metered at EV  7 or lower no problem. If they are higher you might be relying on the film dynamic range. In this case Portra or black and white film are probably OK. If you are shooting transparency like Velvia then highlights above EV7 for this example will likely be blown out.  (There is a different way to meter for transparencies.)

The other thing apparent from the picture above is the range of f-stops and shutter speeds available at a glance. For me I might like to shoot at f22 and that shows 30 seconds on the dial above (don't forget to add reciprocity on top of this). But I can also find f16 at 15 seconds, or f11 8 seconds etc. All at a glance without a lot of thinking.

I did some reading online about spot meters and a modified zone system (where development times are not altered according to highlights) useful for roll black and white and color film. There is a general consensus for color negative film (especially Kodak Portra) and black and white film. These films are characterized by a wide exposure latitude. Portra in particular is tolerant of overexposure (more so than underexposure). Here one meters for the shadows and places the EV for an area of the scene one expects to keep some detail in Zone III or Zone II on the meter. Then let the film take care of the highlights. If the sky is particularly bright and pushed beyond zone VIII then one should consider a graduated ND filter to bring this down. The exception here might be a waterfall scene where keeping detail in the white water might dictate this spot EV is placed at zone VIII and let the shadows and other highlights land where they may and hope the latitude of the film covers it.

For transparency film one tends to want to take care not to overexpose (the opposite of negative film) the image. It is generally accepted that transparency film has a exposure range of about 5 stops. In this case meter for the highlights and set the EV to Zone VII (Zone VII is the upper end of the exposure range for transparency film which generally is expected to have a range of 5 stops which is 2 stops on either side of zone V). The shadows will land where they will. Too much range in the scene may make the shadows block up and show no detail. Again a graduated ND filter might help dependent on the subject matter.

In the end you have finite tool with film and have to know and work with its limitations.

It should also be pointed out that where to place a shadow or highlight or area of interest is really determined by how one wants to convey the final image. I give examples of what to do above but only by way of example, they are not hard and fast rules. Knowing you can manipulate where shadows and highlights or points of interest land on the exposure scale is where the art and flexibility of the system come in. 

Using the Zone System without a Spot Meter

I promised there were cases where a conventional meter might be used to implement a simple Zone System. I ran across this situation while hiking to St Cybi’s Well on the Llyn Peninsula. I only had my Fuji GSW690 and my Sekonic 308S meter. 

The well I wanted to photograph had white tone walls in bright sunlight and a deep shadow covering half of the well. There was a single fern on the edge of the well. Cursing myself for not bringing my spot meter I reasoned that there were only two exposure values that mattered; the wall and the shadow. Each were large enough and close enough to know I could measure each unambiguously and I would know where the shadow and highlights were. The camera was loaded with Portra 160 so I would look to the shadows first. 

This reading I took aiming the meter only at the shadow  and got a shutter speed say at 125th sec for handheld and an f-stop of f8. Now this would place the shadow in Zone V if used this setting. But I want the shadow to be blacker and not the 18% gray of Zone V. So I have to move this to Zone III or two stops darker. I can advance my meter one stop at a time and ignore the shutter speed to f11 then f16 or just know that 2 stops underexposed is f16 at 125th sec. If I want to I can also meter the white wall at 125th second shutter and count out where it falls in the zone system to understand if the highlights will fit in the range of the film. 

St Cybi's Well

One can often get a hold on highlights easily enough when sky is present in the image as it is easily metered. By placing this is Zone VII or VIII then one increases the exposure by either 2 or 3 stops respectively and hope the shadows will land in zone III or higher.  These are obviously not universal techniques but they can be used where highlights and shadows can be metered or estimated and a spot meter is not available. 

Additional Resources. 

I encourage you to read these as they all have something interesting to say on the subject of spot meters and the Zone System.

• ...life is - ANALOG - is life... --Pentax Spotmeter V : the analog spotmeter
• Alan Ross-- Ansel Adams’ Zone System – Darling Or Dinosaur?
• Ray Larose--ZONE METERING WITH THE PENTAX V SPOT METER
• Alan Brock--How to Meter Using the Zone System
• Large Format photography Info--How to Build and Use a Zone Dial