Film Lattitude

[bill_taylor2]

<p>Short and sweet, I spent a few hours looking at log exposure and density charts. Did some estimations and came up with this. Obviously it is all my opinion, you are free to reinterpret as you please. In particular, how I derive the exposure offset, and using 1.5 stops of "toe" is purely arbitrary. I just like the idea of having a little bit of (theoretically) usable detail in slide highlights and negative shadows.</p>

 

[mauro_franic]

<p>Nice work.</p>

<p>I did a quick check and the results don't seem correct. TMX has much higher DR than 10 stops. Also TMY2 has significantly less DR than TMX (not the other way around).</p>

<p>The approach may be correct but the graphs on the data sheets may only be accurate enough to illustrate the shape - not to extrapolate DR.</p>

[Bill C]

<p>Hi Bill, I see a lot of work has gone into your chart. Just to be clear, did you make this from the manufacturer's published data sheets, or did you perform your own tests? (I'm guessing the first). If so, you might be misinterpreting what is drawn on the graph as the maximum "latitude", where it is really just the limit of what someone happened to graph. If you don't see the top of the graph leveling off, then you've not actually encountered the limit of the film. (note: for reversal films, I think that you probably DID see this leveling, and so the latitude data is correct.)</p>

<p>The reality, for negative films (at least the handful of films with which I am familiar), is that the "luminance recording range" is quite a bit greater than what is commonly graphed. (I think this terminology, "...recording range", is less ambiguous).</p>

<p>Way back in 2005, I gave some references supporting this, on the last page of this thread:<br>

http://www.photo.net/digital-camera-forum/00BL1x</p>

[bill_taylor2]

<p>Bill C, you may be right about the graphs. But since all I have are what the manufacturers publish, I can't just assume too much else. Ideally the density curves should follow an elongated S curve, and the measurements for useful range come from the upper and lower knees. But if the charts don't show the knee, how can I assume where I think it ought to be?</p>

[mauro_franic]

<p>I don't think you can make any valid assumptions. Testing each film is the only approach I would vouch for.</p>

<p>Great job and compilation though.</p>

[Dave Luttmann]

<p>The deal breaker here for me, was reading the list and finding Fuji Astia having a fair bit less DR than Velvia 100. I can tell you right now, that isn't the case...not by a long shot.</p>

[Bill C]

<p>Hi again. I can appreciate where you're coming from. It just wasn't clear what your original source of data was. And I think that plenty of people misinterpret what happens where the graph ends. I've had quite a bit of experience in testing certain films, including extensive sensitometric tests, so I well appreciate that pictorial neg film response goes on and on. This is a good learning experience, though, isn't it?</p>

<p>I hate to be the one who turns stuff "loose" on the web, but I'm going to attach a graph, pulled from one of the aticles I had previously referenced. It shows essentially the full characteristic curve of the TMax 100 of about 20 years ago. You might find it interesting, even though it's not very relevant to normal photography.</p><div></div>

[steve_levine]

<p><strong><em>""You might find it interesting, even though it's not very relevant to normal photography.""</em></strong></p>

<p>It is certainly relevant to normal B&W film photography. It shows the relationship between "gamma", and development times. And if you are clever, you can figure out that longer development times, always increases contrast.</p>

[Alan Marcus]

<p>First, the graphs are by tradition drawn on special graph paper using logarithmic notation. This method was chosen because it presents an "S" curve that is considered "pleasing to the eye". Secondly, this method was derived in the late 1800's and early 1900's. Interpolation of the data requires lots of math. The engineers and scientist of that era used logarithmic notation because multiplication and division is performed by substituting addition and subtraction. You need not know much about logarithmic notation to work the graphs however, one key point is: The values listed are exponents, the base which is 10 is omitted.</p>

<p>The key values:<br>

0.05 = 1/6 f/stop<br>

0.10 = 1/3 f/stop<br>

0.20 = 2/3 f/stop<br>

0.30 = 1 f/stop<br>

0.60 = 2 f/stop<br>

0.90 = 3 f/stop<br>

1.20 = 4 f/stop<br>

Ect.</p>

<p>In other words, each 0.30 increase is 1 f/stop because this value = 2 and 1 f/stop is a doubling or halving of the light energy allowed to play on the film. Thus 1.00 = 3 2/3 f/stops and 1.30 = 4 1/3 f/stops etc.</p>

<p> The "s" shape graph describes the photographic characteristics of the film for a given development. The density values of the toe and shoulder are not directly proportionate to the light exposing energy. At the toe the emulsion will be sluggish as to its reaction while at the shoulder it reaction is more laid-back. Only the straight-line gives a true representation of how the film reacts to changing light energy. In other words, the toe and shoulder compress the tonality whereas the straight-line yields a proportionate change with regard to changes to the subject's brightness. </p>

<p>If we examine lots of good images with expansive dynamic range we find that the we do use and need the toe and the shoulder. We also look at the steepness (gradient) of the straight-line. This the indicator as to contrast. A long shallow straight-line means low contrast whereas a steep angle upward tells us the film is contrasty. We measure this angle and find the average slope angle, the tan (trigonometry) is the gamma , a measure of contrast. If the angle is 45 degrees the tan is 1 and the gamma is 1. Most films have a gamma of 0.80 meaning the angle is about 38 degrees. We select paper grades based on the gamma or a more modern counterpart called Contrast Index.</p>

<p> The key here is if the gamma is 1 then the film's response to a doubling of the light energy will be 1 x 0.30 = 0.30 meaning a 1/stop change gives 0.30 density increase. Whereas a typical film with a gamma of 0.80 responds 0.30 x 0.80 = 0.24. Meaning for each f/stop increase we only get a 0.24 density increase. </p>

<p> It takes a lot of study to make sense of these H and D curves. Named in honor of Ferdinand Hurter 1844-1898 Swiss/English and V.C. Driffield 1848-1915 English. Founders of this science of sensitometry published 1890. </p>

[Farkle-Mpls]

<p>I think this is a very interesting thread. Could I trouble someone to explain WHY prolonged development increases contrast? Chemically speaking, what's happening which causes this (increase in contrast)? Flip side, I infer I get better tonality with shorter development times?</p>

[larrydressler]

<p>Carl That also depends on the film and developer you use. Short answer is extending your development time makes the more exposed areas to build up more density where it may allow some increase in the shadow areas but not if the film never recorded it so you get more light and dark with compressed middle tones.<br>

As I said this is not always true as some films and developers play better together and do not act this way all the time.<br>

But for the normal I hope you got my jist.</p>

[Alan Marcus]

<p>First, contrast is a variation in tonality. An image has high contrast when only the extremes of the tonal range are recorded i.e. the intermediate tones are missing. Low contrast usually means the image has shadow and intermediate detail with missing highlights, or highlights and intermediate with the shadow missing. A normal contrast image consists of all thee - shadow - intermediate - highlight tones.</p>

<p>If film receives a gross under or over exposure the results will be flat meaning the contrast will be low. With overexposure, much of the subject's density is shifted to the shoulder region and these tones will be compressed. If we underexpose, the image will be flat because important tones in the highlights and shadows will not have the needed separation.</p>

<p>To achieve an image with the maximum tonal range (dynamic range) we must expose and develop so that as some densities fall on the toe and shoulder region of the H & D curve while most fall on the straight-line portion. Keep in mind that the developer is a reducing agent able to liberate metals from their salt. The light sensitive goodies are crystals that are salts of silver. These silver crystals are under a chemical inertia to remain stable. When exposed to light chemical reaction results that triggers the developer's ability to split (reduce) the crystal and liberate the metal (silver).</p>

<p>If the film remains in the developer for a prolonged time, the developer begins to act upon crystals that have received only moderate exposure. This is due to a breakdown of the chemical reaction that signals that the crystal is developable vs. not developable. Prolonged development time yields a higher contrast because crystals in the toe region that have received little or no exposure continue under chemical inertia and resist being reduced. However, increased action in the center (straight-line) region and the shoulder region means these crystals will be reduced with a resulting block up, i.e. a loss of intermediate tones. The contrast climes because the intermediate tones blend together with the tones of the shoulder resulting in a loss of scale. </p>

<p>If development continues, most all the crystals will be reduced and the resulting image will be highly or completely fogged. </p>

[Farkle-Mpls]

<p>Larry and Alan -- thank you! I need to read some more books on this as I think it's a fascinating topic. Plus, since I spend a reasonable amount of time flipping a development tank over and over, I should strive to understand more about the process and appropriate variations. Thanks again.</p>

<p>I have the Ansel Adams set of books. I need to read them. I hope there is some information regarding this topic within those pages.</p>