Archive for the ‘Zone System and Metering’ Category
Sunday, September 16th, 2012
How Do Light Meters Work?
Most of us can understand that a reliable light meter, either in-camera or hand-held is pretty essential to getting good film or digital exposures – but just how they work might be a matter of some mystery or confusion!
When light meters were first made, they were pointed from camera position toward a scene, measuring a rather broad area, evaluating the average brightness of objects either emitting light or reflecting some amount of light back toward the camera. The assumption was that if you mixed all the lights and darks in an “average” scene into one brightness, that mixture would be some sort of middle gray.
It was decided that that middle gray represented a neutral color reflecting 18% of the light falling on it. Photoshop-wise – that works out to a black set at about 55% opacity.
In photography, this works out fine so long as your meter (or camera) is reading an equal mix of lights and darks, or you are metering something that is itself middle-gray in brightness.
• But what if you are photographing a white horse in the snow? The meter thinks it is looking at something gray – and thus will give you the correct exposure to make that horse and snow GRAY! The solution is to give MORE exposure to your film or image sensor so that the scene is given enough light to look like a textured white in the image.
With film, this is about 2.5 to 3 stops more light than your meter reading.
With digital, this may be only 1.5 to 2 stops more light than the meter reading.
• Conversely, if you are photographing something dark in a dark surround, the opposite approach comes into play. The meter will tell you how to expose to make that DARK scene a middle gray! You then need to give LESS exposure to force that scene to look dark.
With film: a 2 to 3 stop reduced exposure will run you from a textured dark to nearly black.
With digital: a 1.5 to 2.5 stop reduction will typically run you from a textured dark to nearly black.
(Pssst: that’s pretty much the real basics of the Zone System! Hint – that 18% “middle-gray” is Zone V…!)
A word of caution: Some, if not many, of the modern in-camera meters with computer analytics can come up with completely unpredictable exposures. First, they seem to all be calibrated towards acceptable exposures for color slides or digital – so if you are working with a bw negative film (and to some extent color neg) you are almost certain to get a good looking contact sheet with terrible shadow detail. Second, computer analytics are not the same thing as thinking! The most sophisticated metering system in existence has no idea of WHAT it is analyzing! If you want to be in control of image tonality, you need to have an understanding partnership with your metering system and equipment.
Friday, March 9th, 2012
As I discussed in a previous blog, effective application of the Zone System (ZS) really requires the use of a reliable hand-held spot meter. There are basically two kinds of spot meters, those with analog dials for calculating exposure and those with digital displays. While both can afford effective results, the ones with analog dials make it FAR easier to evaluate exposure (and development, with film) than meters with LCD read-outs.
It is important to keep in mind that light meters (computer-evaluative meters in modern cameras are unpredictable) are designed to yield an exposure to render the metered area middle gray (Zone V). If the subject is light, you need to MAKE it light by giving more exposure than the meter suggests. This is called PLACING the subject on a higher Zone. Dark subjects are just the opposite.
In the illustration above, figures AA and CC were made automatically using the built-in meter in a Canon 5D MkII. The camera was set on Program Exposure and the meter set on Averaging. The background in figures AA and BB was a white card, and the egg in figures CC and DD was on a black card. Left to it’s own devices, the camera assumed the subjects were both gray and rendered them that way. Using Manual Exposure, I took a reading on the white card, PLACED it on Zone VII-2/3 knowing from my test (previous blog) that the card would then be rendered near-white, and the figurine would retain excellent detail. With the egg on the black card, my intent was to keep the egg from being overexposed, so I took a reading on the top of the egg and PLACED it on Zone VII-1/3, so I would be assured of recording some texture in the shell.
I have three spot meters: two Pentax Digital meters (these have analog dials) and a MeteredLight Pocket Spot. All three meters are matched to each other, so it doesn’t matter which I use.
The illustration above shows how a meter can help plan how to deal with subjects ranging from easy to extreme.
Figure A on the left shows a simple still-life with the spot reading’s EV values ranging from 10-2/3 on the brightest part of the glass, to 5 on the front face of the bowl. Just a bit under six stops range, and this fits my camera’s dynamic range almost perfectly.
Because I wanted to make sure the highlights were not burned out, I took the 10•• reading for the reflection and placed it just less than Zone VIII. Here is where I let the meter do the work for me. At a glance I can see my Exposure Index, an array of equivalent exposure settings, and how EV values fit in the scale from dark to light. This meter has a “window” that shows three stops above and below middle gray, so I can see the range from Zone II at the left and Zone VIII on the right. I turned the EV ring until the 10•• was positioned a bit below the Zone VIII extreme. All I had to do now was choose the particular exposure combo the meter rings displayed. On the top of the barrel I could see that one option would be 1/30 at f 3.2, but my lens wasn’t that fast, and I wanted more depth of field anyway, so following the exposures around the right side of the barrel, I could see that an equivalent exposure was ½” at f 13. Click!
Next I decided to work with a broader composition, so used a wider focal length to include a window scene along with the fruit. Figure B. The light had not changed, so using the same exposure as before, the tonalities of the bowl of fruit and surrounded areas recorded the same, but the window was completely blown out. The only way to deal with a scene like this is to use HDR techniques: make a number of exposures holding detail in the shadows and on up to the brightest high tones, then use a computer program to combine them. The ZS and a spot meter can take the guess work out of this, too.
Taking a reading on the front of the dark bowl, it still reads EV 5, so if I place that on Zone II I know it will have tonality. I see that an exposure to hold the shadows is still ½ at F 13. I now measured the brightest value outside the window and note that the sky measured EV 15. I want the sky to render well below near-white, so I’ll take that EV 15 and turn the EV ring so the 15 is placed just below Zone VII. I don’t want to change my f-stop, so I see that 1/80” will safely hold tonality in the sky.
So, a series of exposures ranging at least from ½” to 1/80” will hold all the tones in this extreme scene. Assembled in Photoshop CS5, the full range is held, figure C. This looks a bit dull because of the compression of such a wide range of tones, so figure D shows the combo with the mid-tones brightened up a bit.
Sadly, Pentax seems to no longer be making spot meters, though they can often be found on-line. So far as I know, the only spot meter still being made with an analog dial is the Metered Light Pocket Spot.( http://meteredlight.blogspot.com/ ) It is only 2×2-1/4” in size and has a ZS-ready dial, engraved from Zone I to Zone X. For the illustration, I put a couple of pieces of magenta tape over the scale, leaving a “window” equivalent to the scale of the Canon 5D.
I have had a lot of students with the do-everything Sekonic meter pictured above. It is a great meter, but a bit awkward to easily use the ZS. I made a simple little accessory card that makes it a lot easier to evaluate Zone placement and exposure. The example above shows a shadow exposure of 1/60 at f 5.6, and a highlight exposure of 1/60 at f 64. If I want to place the highlight on Zone VII-1/2, I put the VII-VIII on the card over the f64 mark and see that the reading under the Zone V on the card is f22-23, so that’s my exposure at 1/60”! I also see that my shadow now falls between Zone O and Zone I, so that area and subjects up to a stop or so brighter will not be recorded.
Bottom line. There are many reasons to avail yourself of the automated features in today’s sophisticated cameras. Computerized metering modes can work wonders in fast-changing lighting situations or if your subject is evenly lit and the sun is behind your shoulders. Autofocus can save the day with moving subjects, and auto-bracketing can help explore the nuance of exposure shifts. But if your camera is on a tripod or if you have a moment to reflect upon your subject, you may well find that with just a few seconds spent with spot meter and thought will give you more successful first-exposures than letting the camera make the decisions. It’s a snap!
P.S. In black-and-white, the “extreme” scene discussed above could be easily recorded with one exposure, on negative film. ;>)
Tuesday, February 21st, 2012
In a word: YES!
The Zone System (ZS) can be an integral and important part of any digital photographer’s workflow because it allows you to plan and predict an image’s tonal values rather than letting the camera make the decision.
The computerized metering systems in modern cameras are really amazing, and a lot of the time they will give you practical exposures, but in difficult or extreme lighting situations, the scale of the subject’s brightness is simply greater than the camera’s technology can handle.
The Zone System:
- Lets you be aware of whether, or how much, the scene brightness exceeds your camera’s limits
- Lets you make an intelligent decision about how to expose when the tones/contrast in a scene are “bigger” than what your camera can capture
- Helps you avoid blown-out highlights
- Lets you know how much exposure range you need for successful HDR captures
The Zone System 101
To use the Zone System effectively in the digital world, you need to understand a few of its basic principles:
The ZS was originally conceived by Ansel Adams and fellow photo instructor Fred Archer as a tool to give photographers working with black-and-white negative film (no digital back then!) the ability to plan and control the effects of exposure and development. They created a “scale” of tones from black to white and assigned each one a number, with “I” being almost pure black and “IX” or “X” being nearly white or white. Zone “V” is middle gray, and each “Zone” is one stop lighter or darker than its neighbor.
In the digital age, image contrast can be easily increased post-capture, but there is no practical means of reducing image contrast in a single capture. Pre-exposure can enhance shadow tonality, but this requires the ability to double-expose, and High Dynamic Range techniques (HDR) require three or more exposures for best results.
Sophisticated “evaluative” metering modes in modern cameras can handle many complicated shooting situations, but if the contrast of the scene exceeds the recording scale of the camera, something’s gotta give. This is where the ZS can help.
Step One: You Need a Reliable Hand-Held Spot Meter
A 1° measuring spot lets you measure important areas precisely and with ease. Using a camera’s “spot” metering mode is not always practical: the size of the spot depends on the focal length of the lens and generally requires a lot of button-pushing and pointing the camera this way and that – an exercise in frustration and wasted time.
Without a spot meter, you may know that you will lose tonality at one end of the scale or the other, but you have no way of knowing which, or by how much, at least until examining an after-the-fact histogram. By then, your scene might be gone!
Step Two: Your Spot Meter and the Zone System
By design, a spot meter will give you an exposure to make the measured area middle gray. This gray is called Zone V. If you measure snow in sun, the meter will give you the exposure to make the snow Zone V gray. If you measure a black speaker grille, it will give you the exposure to make that grille Zone V gray. If you want the snow to look white (not paper white but a very light gray) you need to PLACE it on a higher Zone. If you give one stop more than the meter says, you are placing the snow on Zone VI, two stops more than the basic meter reading places that value on Zone VII, and so on. As for the speaker grille, it is just the opposite. You would need to expose the grille two or three stops LESS than indicated in order to make it look dark. This would PLACE the grille on Zone III or II respectively. You can only PLACE one value. Everything else, then, FALLS in natural relation to the placed value.
Step Three: Know Your Limits
In order to plan a ZS approach to exposure, you need to know what tonal range your camera can and can’t handle. The composite image below shows nine images made with the Canon 5D set on MANUAL exposure. The target was a Kodak Gray Card with white and black patches that I made many years ago for testing the tonal range of slide film. I set my Pentax Digital Spot Meter to the same ISO as the camera, took a reading of the gray card and exposed according to the meter (a Zone V middle gray). I made four darker exposures one stop (Zone) apart and four lighter one stop apart.
At three stops under the Zone V exposure, I had made the gray card almost as dark as the black patch, so that told me the camera could hold some tonality for a subject on Zone II. On the bright end, the gray at two stops brighter than Zone V was still a noticeable light gray compared to the white patch, but at three stops (Zone VIII) the gray had turned as white as the white patch. That told me that my upper limit for recording highlight tonality is about Zone VII-1/2, or five and a half stops total range.
Step Four: Measure Your Subject Highlights
In general, with digital (and color transparency film), images look their best when the highlights are not blown out. If a photo opportunity is fleeting or moving, and I only have one chance for a shot, I will take a quick spot reading of an important high value, maybe a white dress or bright cloud, and give it an exposure of about 2-1/2 stops brighter than the basic meter reading (Zone V). This PLACES that dress on Zone VII-1/2. Having already done the test above, I KNOW that it will be very bright, but not “blown out”!
With digital, there is nothing evil about some subtle bracketing, so if you have the opportunity, go ahead and give some + and – exposures. With some practice and your spot meter, though, you’ll be surprised how often you get it right the first time! You might not need step five!
Step Five: Measure Your Subject Range
If you suspect the range of brightness in the scene is significantly beyond the range of your camera, AND you have the opportunity to make multiple exposures of a stationary subject, you can use HDR techniques. The ZS can be a big help here, too. Rather than making random plus-and-minus exposures to cover subject brightness (dynamic range), you can quickly measure exactly how much range you need to cover.
Let’s say I have a tree in the foreground in deep shade with some textured charring on its trunk. In the same composition I have bright sun glaring off some pale boulders.
With just two spot readings I can determine the range and how to deal with it. First I’d read the charred trunk. Based on my test above, I would know that if I gave it a Zone II exposure it would still have tonality. That determines the shadow exposure. Then I’d read the bright rocks. Let’s say they measured 8-1/2 stops brighter than the trunk. That is 3 full stops brighter than what my camera can record. So I now know that AFTER my first shadow exposure, I need to make 5 or 6 more exposures each at ½ or 2/3 faster shutter speed than the exposure before, until the last exposure is at least 3 stops darker than the first. Then it’s time to let Photoshop CS5 and/or other preferred software combine the exposures.
Bottom Line: Keep in Mind that the Zone System is Not Dogma!
Its application should be considered as something deeply personal. If you simply have a hunch that you like a certain shadow two stops darker than a basic meter reading (Zone V) rather than three, do it that way! It’s really a lot like cooking. If YOU like YOUR veggies al dente, don’t “expose” them to so much heat that they are over done! Bon appétit!
Sunday, November 14th, 2010
In a recent newsletter, I mentioned wanting to do some testing – something I feel I ought to do regularly. One of Ansel’s favorite quotes (attributed to Pasteur) was “chance favors the prepared mind.” Having started his creative life as a musician, he brought that musician’s discipline to his photography – the equivalent of a regimen of playing scales.
The first thing to test periodically is your light meter. Except for one old Nikon, none of my cameras have meters in them – so my meter(s) are the foundation of every exposure I make. The only requirement I have of a meter is that it be linear – that is – if I expose a gray or white card in dim light, bright light , medium and dim again and expose exactly as the meter indicates, each exposure should yield the same density on film. If the densities match closely, my meter is good, but if one exposure doesn’t match the others, the meter needs to go to the shop. This test is well worth the time and cost of a roll or few sheets of film!
Incidentally, this is a great way to test your meter’s color response. Instead of photographing subjects of different brightnesses, photograph a neutral card then do it through strong-colored filters – you’ll likely see reds underexpose and blues overexpose!
Once I’ve verified my meter’s linearity, I re-test for film speed and development time. I’ve switched to a new processing timer in the last year or two and changed some timer calibrations (see New Darkroom Timer, below) so it’s time for a double-check. I’m also going to compare my current “standard” film, TMax 100 with Ilford’s Delta 100 and FP4+. A friend recently showed me a test he’d done with TM100 and FP4, and the tonal renditions were quite different, so I want to take a look for myself. He also had some interesting results comparing Kodak Xtol and Ilford’s ID11 – so I think I’ll look into that, too.
All this testing doesn’t really need to take a lot of time – I don’t worry about making “art.” I can expose two sets of film and develop in separate developers – or expose a whole roll of film the same, cut the roll in half and develop separately. I can expose three different films the same and develop appropriately in the same developer and see the differences between the films themselves. I do use a test target and a densitometer (got it on eBay for $100!) and that saves a lot of time – for me – but it’s not essential by any means.
When I’m done with all this I’ll feel more up-to-date – prepared for the “chance” that may come my way.
Tuesday, June 23rd, 2009
According to Ted Orland’s poster Photographic Truths: “No two light meters agree.” Sadly, that does seem to be pretty much the truth – unless you do something about it.
For years I had a pair of supposedly “matched” Pentax digital spot meters that were never closer than 1/3 of a stop from each other – so I had to remember which meter I used for film tests and which one I had in the field. One of these meters had an accident and got sent off to its maker for a rebuild – and came back 2/3 of a stop away from where it had been, now 1/3 higher than the meter it had been lower than! So I sent the other meter off to its maker and after two months got it back about the same as when I had sent it. Which meter was right?
Tuesday, June 23rd, 2009
Ever since I was back at Ansel’s in the late ’70′s, I had been faithfully calibrating my high values at Zone VIII, targeting a density of about 1.25 above filmbase-fog.
Since 2004 or so, however, I’ve been thinking more and more that it makes more sense to calibrate to Zone IX, with a target density of about 1.45.
True, Zone VIII is supposed to be the high end of the “textural range”, but then Zone II-1/2 to III is generally considered the low end, and we don’t use THAT as a film speed point.
For me, Zone IX prints on a “medium contrast” paper as not quite a paper white. THIS is the end of “normal” photographic scale and I think it’s ultimately the most useful calibration point, in the same manner that we use Zone I for film speed (0.07 to 0.10 above Fb-F).
The Zone IX calibration point calls for more modest changes in development time and/or dilution to get useful Plus and Minus development. There also is less tendency for Minus development to “block-up” values which fall higher than Zone IX.
If you’d like to see a larger version of these theoretical curves showing why I like this idea, or if they don’t display on your browser, email me and I’ll send a pdf pdq!