"Density Units" vs stops/zones/values?

[bhuij]

<p>Hey guys—<br>

I've been shooting, developing, and printing my own B&W film for years in several different formats, but I finally decided to actually calibrate for Zone System use. I'm starting with HP5+ and Rodinal, a combo I really like.<br>

I've been using this article, which describes a process of basically finding film base + fog, then comparing Zone I and Zone VIII exposures to dial in true film speed and optimal N development.<br>

http://www.zone2tone.co.uk/zone-system-film-testing.htm<br>

Everything makes sense and is explained really well, except the math at the end, just before the table where he gives you densitometer results with corresponding adjustments to make to EI and dev time.<br /><br />Everywhere I have found online, as well as this very article agrees that 0.1 "density units" is equal to 1/3 of a stop, or 1/3 of a zone, or 1/3 of a print value. By this math, assuming FB+F is an ideal "Zone 0" (no exposure to light, full development), a frame exposed at Zone I should be 0.3 density units above FB+F. This table says it should be 0.1 DU above FB+F, which doesn't make sense to me.<br>

Furthermore, again assuming 0.1 DU = 1/3 stop, a Zone VIII exposure should correspond to 2.4 DU above FB+F or 2.1 DU above Zone I. This article tells you to look for Zone VIII densities 1.2 DU above Zone I. It even goes so far as to say something about Zone VIII being "4 stops" denser than Zone I, which just doesn't make a darn bit of sense to me.<br>

Am I off my rocker or missing something here? My own results seem to back up my own interpretation of the math as it should work:<br>

I hypothesized based on research from around the internet that my EI would probably fall somewhere between 200 and 250 for HP5+, and that Rodinal 1:50 for 11 minutes at my agitation/temp/etc. would be a good starting point. On my test roll, I shot Zone I frames at EI 400, 320, 250, and 200, as well as Zone VIII frames. I also left a few frames blank for FB+F density readings.<br>

At EI 200, my Zone I exposure came out to 0.34 DU above FB+F, so right in the ballpark of where it should be according to my math, and close to right on for my hypothesis. At EI 200, my Zone VIII exposure came out to 2.15 DU above FB+F, indicating slight underdevelopment.<br>

However, if I use the target values of 0.1 and 1.2 as outlined by the article, then I should be shooting at something like EI 500 (just doesn't sit right with me to find true film speed faster than box speed), and 11 minutes is a gross overdevelopment. I've shot a lot of HP5+ at box speed, and I used to develop for 11 minutes in Rodinal 1:50. The results generally came back lacking in film speed and somewhat flat, so these results using the article's math seem all wrong to me.<br>

I'd love to get your feedback before shooting another test roll. I'm confident it's not my shutter or aperture throwing off my results by giving inaccurate or inconsistent exposure. I am however going to re check the frames from this first test roll with a different densitometer to see if that's part of my problem here.</p>

[steven_clark]

<p>Density is base-10 logarithmic. 1 density is a division of light by 10, exactly 10. 0.3 density is about a halving of light, approximately 2. I hope that clears some things up.</p>

<p>Also, film compresses the light range. A scene shot 1 shot brighter will not be 0.3 density darker on film. The contrast is retrieved during the printing process. Only slide film works approximately linearly, which is why it has enormous contrast if cross-processed and used as a negative film.</p>

[Bill C]

Hi, I looked through the linked article briefly. I think you may be misunderstanding the relationship between exposure and film density, and

the article doesn't seem to make this real clear.

 

I take your post to mean that you expect increasing your camera exposure by one f-stop should increase your film density by 0.30. But this

is not the typical result. It would happen only if you developed the film to a rather high contrast - a gamma value of 1.0. (Gamma is roughly

the film density change divided by the base 10 log exposure change.) Typically a normal film gamma is something closer to 0.55 or 0.60.

 

You said, "This article tells you to look for Zone VIII densities 1.2 DU above Zone I." What this is saying is that the author expects an

exposure increase of 7 f-stops to increase the developed film density by 1.2 units. This is a film density change of about 0.17 density per f-

stop exposure change, which is a film gamma of about 0.57. So what the author is suggesting is that you should develop your film to

produce an amount of contrast equivalent to a gamma of roughly 0.57.

 

One more comment - you question why the Zone I value is only 0.1 density unit above base +fog, rather than the 0.30 you expected (or the

0.17 suggested with normal film contrast). I don't know exactly how this value came about (I'm not a zone guy), but the initial film response

starts off more slowly, then tends to increase to the normal gamma, so I'd guess it's related to this.

[maris_rusis]

<p>If Zone 0 is the result of no exposure then what is one stop more exposure than zero? Impossible to answer! There are many camera settings that will give no measurable film density, 1/8000 second at f64 at night for example. One stop more than that is still not measurable. Definitely not Zone 1.</p>

[bhuij]

<p>Okay, so it sounds like my mistake is rooted in the erroneous assumption that a change of 0.3 density units on the film corresponds to one stop of exposure difference. Thank you all for your answers, they have been very helpful.<br>

It sounds like I should be aiming for an EI of closer to box speed (according to these tests, a little higher than box speed actually), which is surprising to me. I also appear to be overdeveloping pretty drastically. Do you think it's possible my development was so excessive it led to slightly denser readings even in the Zone I frames? If so, my EI would fall somewhere right around 400 and I would start by about halving my development for my next tests.<br>

Before I do any of that though, I want the opinion of a second densitometer...</p>

[Alan Marcus]

<p>About the density unit:<br>

In the early 1900’s Messieurs Hurter and Driffield started researching the relationship between exposure and development. They invented precision instruments to expose and measure how exposure caused film to blacken. This was before the calculator or computer, scientist used logarithmic notation as this method substituted addition for multiplication and subtraction for division. The slide rule is based on log manipulation. It was a natural to use log notation as the unit for the blackening of film.<br>

The number 2 can be written many way such as two – or 2 or II. We can also use log notation base 10. The number two in this notation is 10^0.30. This reads – 2 =2 10 elevated to the .3 power. <br>

I won’t go on with a lesson in logs but here are some key values that pertain to the blackening of film. <br>

We omit the base which is 10 and only write the exponent. This is because once the base used is identified; we can omit it as now it is redundant.<br>

2 = 0.30 in log notation. Since an f/stop is represents a doubling or halving of the exposure, that one f/stop equals a 2X change in film blacking. This 2X change is written in logarithmic notation as 0.30.<br>

Thus 1 f/stop = 0.30 density unit’s<br>

1/2 f/stop = 0.15 density unit’s<br>

1/3 f/stop = 0.10 density unit’s<br>

1/6 f/stop = 0.05 density unit’s<br>

2 f/stops = 0.60 density unit’s<br>

3 f/stops = 0.90 density unit’s <br>

4 f/stops = 1.20 density unit’s <br>

Etc.<br>

<br>

Now Hurter and Driffeld constructed graphs of the blackening of film. When graphed in log notation, the shape of the graph is deemed elegant plus it was believed that human hearing and vison both plotted followed a logarithmic curve. Thus the decibel and the film density unit is today plotted as a log value. <br>

<br>

That’s the molasses, now the sulfur. The plot is called an H&D curve. In theory, a film should double in density with every f/stop exposure increase. In other words, the density of film should elevate 0.3 density units per f/stop increase. It can but – if black & white film is allowed to do this, the results are too contrasty. For this reason films are generally made less contrasty.<br>

<br>

We measure contrast by measuring the upward slope of the film as it blackens with exposure. If this slope angle should equal 45⁰, a 1 f/stop change = 0.3 density units. Another name for this slope is gamma. If the angle is 45⁰, the gamma is 1. This is two contrasty, we typically adjusts this angle to about 36⁰. Now36 ÷ 45 = 0.8, we call this gamma 0.8. This is slope of a typical black and white negative film. Such a film does not elevate 0.3 density units per 1 f/stop change. This less contrasty film elevates 0.3 X .8 = 0.24. In other words a typical black and white film elevates only 0.24 density units per f/stop increase in exposure. <br>

<br>

Now I know this stuff is difficulty, I can’t tell you a lot in a paragraph or two, so you can email me and ask questions. <br>

<br>

This stuff is what I call gobbledygook. </p>

[bhuij]

<p>Thanks for the clarification. It sounds like from what I'm hearing, the ideal gamma to use when calibrating for zone system use is about 0.55 to 0.6. This supports the numbers found in the article (and in The Negative) for target density for Zone I and Zone VIII, so that's what I'll shoot for.<br /><br />Anyone here shoot HP5+ and find that 400 or 500 is the EI? That seems weird, most of what I have read indicates that EI usually falls somewhere around 1/2 of box speed depending on individual water chemistry, processing, etc.</p>

[glen_h]

<p>As well as I understand zone system (maybe not so well), the box speed should be N. If you want N-1, as some suggest, then you might go to half box speed. </p>

<p>Also, since printing paper can't do the full density range that film can, you have to compress some when printing, relative to the original source.</p>

 

[bhuij]

<p>Okay, I have done another round of test rolls (have not yet had a chance to compare my results to a different densitometer, but I still have those negs).</p>

<p>I did two rolls shot in the same environment. Both rolls included the same flat, out of focus subject filling the frame under the same lighting, with exposures for Zone I and Zone VIII bracketed around EI 400 (specifically, EI 400, 600, 300, and 200). I developed one roll in Rodinal 1:50 for 9 minutes, and one roll in HC-110(H) for 10 minutes.</p>

<p>My film speed for the Rodinal roll came out to either 400 or 600, with Zone I frame density readings (after subtracting FB+F) of 0.12 (for EI 400) and 0.09 (for EI 600). This tells me EI 500 would be perfect at this dev time. The corresponding Zone VIII frames showed "net" densities of 1.69 and 1.55 respectively, indicating that I'm still at least 15% or 20% overdeveloping.<br /><br />HC-110 was a much stronger developer for both shadows and highlights, which is consistent with conventional wisdom. I was testing it to compare grain and sharpness to Rodinal and see if I like one or the other better for this particular film. With HC-110, EI 600 at Zone I showed a density of 0.12, so that's about right. The corresponding Zone VIII frame was way too dense, clocking in at 1.88 DU. So I'm severely overdeveloped here.<br /> <br />This is freaking me out. I've never heard of anybody finding a true film speed of 1/2 stop faster than box speed. I'm to the point that I want to now check my meter calibration and my shutter speed accuracy for the camera.</p>

<p>I'm pretty confident it isn't my temperature or agitation scheme giving me much shorter than expected dev times (or much faster than expected film speeds), but just so I can get a second set of eyes on it, I'll list them here:<br /><br />I'm developing at 68°F in a steel tank with 30 seconds of initial agitation (1 inversion per second) and 5 seconds of agitation (3 inversions) every 1 minute thereafter. It's probable that my soup temperature is creeping up a degree or two over the course of the dev time since my apartment is at an ambient temperature of about 75°F and I'm not dunking my tank in 68° water or anything like that after I pour the developer in. All subsequent chemicals (water, stop bath, fixer, water, orbit, a 7 minute rinse, and then photo flo, in that order) are basically room temperature since I don't bother cooling them down before use, but at worst that should only affect grain structure, not film speed or contrast.</p>

<p>Am I way overagitating or something? If you got these results would you trust them or suspect something was wrong?</p>

<p>Here is a link to my actual data from the two rolls if anyone cares to look:<br>

https://docs.google.com/spreadsheets/d/1F2_e3eAQWbpCiiDOTC3gDLAYWR6BfNlIkMYm6tWNG-M/edit?usp=sharing</p>

[glen_h]

<blockquote>

<p>This is freaking me out. I've never heard of anybody finding a true film speed of 1/2 stop faster than box speed. I'm to the point that I want to now check my meter calibration and my shutter speed accuracy for the camera.<br>

</p>

</blockquote>

<p>Interesting question. Box speeds round to the nearest 1/3 stop. If you do the actual ISO test, using the same developer as ISO, I suppose I am still not surprised to be a half stop off. <br>

<br>

For processing systems like E6 and C41, ISO uses them as specified. <br>

<br>

For black and white, you have to choose a developer and concentration. Then find the time that satisfies the ISO testing, and determine the ISO value. Plenty of room for uncertainty.</p>

[paul_hoyt]

<p>When Adams and Fred Archer devised the Zone System back in the 1940’s , they made the Zone I the anchor; 0.1 above film-base plus fog. The other Zone densities “fall” based upon the amount of development the film receives. In the early versions of the Basic Photo Series there was a very good graph that plotted Zones and density based upon development. He showed how increased development would raise Zones IV through VII and how decreased development would compact Zone IV through VII. He based the development on Zone V being 0.9 density units for Normal development. For N+1, Zone IV would have a density of 0.9. For N-1, Zone VI would have a density of 0.9. NOTE: In the early series there were 9 Zones; later the Zone System was revised for 10 Zones.<br>

<br>

In later versions of the Basic Photo Series the density value for Zone V was reduced, I suppose it had to do with the changes in film and paper and the amount of silver in the emulsions. Adams also suggested using either Zone V or Zone VIII for Zone testing and gave densities for the two Zones as a guide for Normal development. He also gave density suggestions for condenser or diffusion printing.<br>

<br>

When I performed my Zone standardization, after finding the ISO for Zone I, I exposed my negatives for Zone V and Zone VIII; developed them together, fix, dry, and read the densities. I found the developing time that placed both Zone V and Zone VIII in the range Adams suggested for my diffusion enlarger. I then did the same for N+1. Exposed negatives for Zone IV and Zone VII, developed them together until the Zone IV exposed negative had the same density as my Normal Zone V, and the Zone VII exposed negative had the same density as my Normal Zone VIII. To find N-1 development I exposed negatives for Zone VI and Zone IX. Reduced the development until Zone VI = Zone V, and Zone IX = Zone VIII.</p>

[bhuij]

<p>Yeah, I think my problem boiled down to two things:</p>

<p>1. Bad density readings. I was using Vuescan's density reading feature with my CoolScan 4000. Either I'm using it wrong or it's just not very accurate. I went instead to a tried-and-true Minolta Spotmeter F with a Ziess 50mm f/1.4 reverse mounted to it with a PVC Pipe contraption, reading tiny areas of each frame on my light table. Results were more consistent and made way more sense.<br>

<br />2. Severe overdevelopment. Not sure if it's my water chemistry, agitation, temperature, or some combination thereof, but my Zone VIII densities were significantly too thick, suggesting way too much development. Every time I toned it down my Zone I densities started suggesting more and more realistic true film speeds.</p>

<p>I believe I have now settled on EI 250 and HC-110(H) for 6:45 as my N for this film. Now I just need to find N+1 and N+2, then repeat my calibrations for FP4+ and Rodinal 1:50 :D</p>

<p>Thanks for everyone's help.</p>