Sunday, May 13, 2018
Action still photography
Modern digital cameras are now shooting video at 30 frames per second at a resolution that single still images can be picked out and used at a resolution suitable for large prints. The result will be the de-skilling of sports action photography. The evolution began with motor drives; instead of having the skill to anticipate and then perfectly time a single exposure photographers could press the button and capture 30 images at 10fps and one might capture just that critical moment when the ball crosses the line or strikes the racquet. Now, instead of looking through a series of still images, photographers can replay the video in slow motion, stop at the point where they see a good still and pick that fam in the video. While there is still some skill involved in composition, that involved in capturing the image in the first place is rendered moot.
Tuesday, February 2, 2016
Farewell, Olympus
My first camera (aged about 8) was plastic affair made by Kodak that took 120 film I think; a present from my aunt, Itta. I have no images of that camera to post though it's remarkably clear in my mind's eye.
Next was my first 35mm camera, a Halina Paulette Electric, my first with a light meter. The lens wasn't anything to write home about, and I remember the focusing ring being quite stiff. And I had to guess the distance to the subject.
At about age 16 I graduated to my first SLR, a black Nikkormat FTn with an f2 50mm lens. Raul Casares, who among other things, did photographic work for Gulf Oil in the UK, used Nikon and recommended it. It was heavy, but well built. It was after all, a Nikon. My first lens was a Soligor 90-230mm zoom. I also borrowed my father's Sunagor 200mm telephoto from time to time.
With the announcement of the OM-2 in 1975 (I can still see the brochure - a black OM-1 on a black background, really elegant, great marketing!) Olympus began to appeal to me (and my friend David Gould) as a camera brand. It was advertised as having a 20% brighter viewfinder, and being smaller, and lighter than any of its competitors.
David bought an Olympus OM-2 in about 1978, but it wasn't until I went to Imperial in 1989 that I got mine. Actually its wasn't an OM-2 but a second-hand black OM-1n with a 50mm f1.4 for about £125. In today's money that's an astounding $1,037 (what was I thinking?). I lent it to Andy North in 1980 who left it in his (unlocked) room from where it was stolen.
So I went back to Foxes and bought another OM-1n, this one silver. That camera still I have today, so it wasn't that bad a long term investment ($103/year over it's working life). It went to the Sahara (where sand got into the lens so the focusing was a bit crunchy for many years). The light meter gave up the ghost about 15 years ago but the shutter still works
In 2000, I discovered eBay and when the OM-1's meter gave up, I
bought two OM-2-SPs, one for color and one for black and white and
another 50mm f1.4. I think I paid less than $100 for each body.
Over the years I picked up some Zuiko lenses; 24mm, 28mm, 135mm, 35-105mm, 85-250mm zoom.
I loved these cameras mostly for nostalgic reasons (Finally, 35 years on, I had my own OM2). But I hated the battery drain; even switched off, the batteries would go flat in about 3 months. Given that I was taking pictures fairly infrequently, that meant that they were almost never ready when I needed them.
Since then, I have made three purchases in the digital realm; a Nikon Coolpix S200 which was fantastically small but had a fairly ropy lens.
Next was a Panasonic Lumix LX3 which was larger but with much better optics. The lens was a little limiting, 60mm at the long end, but it was much
sharper and didn't have the distortion that plagued the Nikon S200.
When the LX7 went on sale about 2 years ago, I sold the LX3 and bought an LX7.
The lens was a stop faster and now went to 90mm. This was a perfect zoom range for me; a good wide-angle (24mm is much wider than 28mm) to a portrait-length telephoto.
It is a really great little carry-around camera and I take it everywhere.
About 4 years ago I was thinking about getting a Sigma SD1 which had been rumored before its launch to be priced at just under $2,000. When it was announced, Sigma set the price at $10,000 so that was out of the question; instead I bought a second-hand Mamiya RZ67 for about $200 (and started this blog). But I still had a soft spot for Olympus and a hankering for the OM-D EM-5 which had been announced at about the same time.
Recently B&H put the original OM-D E-M5 on sale. I went to dpreview.com and scrupulously studied the reviews; I looked particularly closely at the studio scene and compared the images made with the E-M5 with a wide variety of other cameras. I used the Nikon D810 as the 'reference' image - what the best digital imaging could produce - and my trusty LX7 as the baseline.
Of the many comparisons and alternatives the E-M5 seemed, to my eyes, to hold up extraordinarily well. The amount of detail it appeared to resolve was remarkable. Finally, it seemed, this was the digital camera I'd been waiting for.
So I bought one as a birthday present. My birthday was last week and with great anticipation I opened my present and began to take some test pictures. Last Friday I decided, I'm not quite sure why, to to an A-B comparison with my LX7. Really it was just a bit of a lark to see how much better the E-M5 was than what I had been used to. That turned out to be a bit of a shock.
These are crops from files taken with the OM-D and the LX7. It was dusk and the sun was setting.
The image above is the OM-D E-M5.
This is the same scene taken at the same time with the LX7.
These speak for themselves I think (If you click on each you can see a slightly larger version). The OM-D looks like someone has taken the Photoshop smudge tool to it - the pine needles have all been smeared into an amorphous green mass and the bark on the trees has no texture. The LX7 on the other hand preserves more detail despite being a smaller file.
B&H were of no help; reset all the camera settings to the factory defaults was their best advice (I shouldn't be surprised - it’s a high volume operation). Anyway, I did that but the results were no better. So my love-affair with Olympus came to an abrupt end. The camera was RMAd and sent back yesterday.
I had, at one point, thought about using my old OM series lenses on the OM-D but the micro 4/3 sensor made that awkward - my OM 24mm f2.8 would be an f5.6 standard prime and I'd have no wide-angle. That, combined with the outrageous cost of the adapter ($152), made this a bit of a non-starter. Since Olympus looks committed to the micro 4/3 format and unlikely to produce a full frame camera, any hope of using the old lenses looks forlorn.
Along the way, for no good reason, I'd bought a Nikon F5 - the company's last professional film camera. The lens (Nikon AF-S NIKKOR 24-85mm f/3.5-4.5G ED VR) I found reconditioned for about $350. It isn't professional grade, but DXOmark gave it pretty good rating compared to say the Sony FE Carl Zeiss Vario-Tessar T* 24-70mm F4 ZA OSS which costs about $1,200. The professional grade Nikon AF-S NIKKOR 24-70mm f/2.8E ED VR is faster and sharper, but at $2,400, is almost seven times what I paid for the 'kit' lens and until I win the lottery, unaffordable. (Of course there is the Tamron SP 24-70mm f2.8 which according to DXOmark is better than the equivalent Nikon and half the price, but it’s still outside my budget for the moment).
So my plan is to sell all my Olympus equipment (except perhaps the 1980 OM-1n) and eventually get something to use with the Nikkor 24-85mm, probably a second-hand or reconditioned D610 or D810. Since I don't have to buy a lens, that would be cheaper than a Sony A7R with the Vario-Tessar T* 24-70mm lensl; and it's a Nikon so built like a tank.
But that's some way off. This was about my breakup with Olympus. It's been a 40 year relationship but now it's over. And another valuable lesson learned was not to trust dpreview.com's reviews alone. Perhaps they mislabelled the file, but their test look very different from what I found. There is, ultimately, no substitute for shooting ones own test images.
 |
| Halina Paulette Electric, circa 1970 |
 |
| Nikkormat FTn, circa 1975 |
At about age 16 I graduated to my first SLR, a black Nikkormat FTn with an f2 50mm lens. Raul Casares, who among other things, did photographic work for Gulf Oil in the UK, used Nikon and recommended it. It was heavy, but well built. It was after all, a Nikon. My first lens was a Soligor 90-230mm zoom. I also borrowed my father's Sunagor 200mm telephoto from time to time.
Behold, Olympus!
 |
| Olympus OM-1n, circa 1980 |
With the announcement of the OM-2 in 1975 (I can still see the brochure - a black OM-1 on a black background, really elegant, great marketing!) Olympus began to appeal to me (and my friend David Gould) as a camera brand. It was advertised as having a 20% brighter viewfinder, and being smaller, and lighter than any of its competitors.
David bought an Olympus OM-2 in about 1978, but it wasn't until I went to Imperial in 1989 that I got mine. Actually its wasn't an OM-2 but a second-hand black OM-1n with a 50mm f1.4 for about £125. In today's money that's an astounding $1,037 (what was I thinking?). I lent it to Andy North in 1980 who left it in his (unlocked) room from where it was stolen.
So I went back to Foxes and bought another OM-1n, this one silver. That camera still I have today, so it wasn't that bad a long term investment ($103/year over it's working life). It went to the Sahara (where sand got into the lens so the focusing was a bit crunchy for many years). The light meter gave up the ghost about 15 years ago but the shutter still works
 |
| Olympus OM-2sp, circa 2000 |
I loved these cameras mostly for nostalgic reasons (Finally, 35 years on, I had my own OM2). But I hated the battery drain; even switched off, the batteries would go flat in about 3 months. Given that I was taking pictures fairly infrequently, that meant that they were almost never ready when I needed them.
Going Digital
Since then, I have made three purchases in the digital realm; a Nikon Coolpix S200 which was fantastically small but had a fairly ropy lens.
 |
| Panasonic Lumix-LX3, circa 2010 |
 |
| Panasonic Lumix-LX7, circa 2013 |
When the LX7 went on sale about 2 years ago, I sold the LX3 and bought an LX7.
The lens was a stop faster and now went to 90mm. This was a perfect zoom range for me; a good wide-angle (24mm is much wider than 28mm) to a portrait-length telephoto.
It is a really great little carry-around camera and I take it everywhere.
More Pixels
About 4 years ago I was thinking about getting a Sigma SD1 which had been rumored before its launch to be priced at just under $2,000. When it was announced, Sigma set the price at $10,000 so that was out of the question; instead I bought a second-hand Mamiya RZ67 for about $200 (and started this blog). But I still had a soft spot for Olympus and a hankering for the OM-D EM-5 which had been announced at about the same time.
Recently B&H put the original OM-D E-M5 on sale. I went to dpreview.com and scrupulously studied the reviews; I looked particularly closely at the studio scene and compared the images made with the E-M5 with a wide variety of other cameras. I used the Nikon D810 as the 'reference' image - what the best digital imaging could produce - and my trusty LX7 as the baseline.
Of the many comparisons and alternatives the E-M5 seemed, to my eyes, to hold up extraordinarily well. The amount of detail it appeared to resolve was remarkable. Finally, it seemed, this was the digital camera I'd been waiting for.
So I bought one as a birthday present. My birthday was last week and with great anticipation I opened my present and began to take some test pictures. Last Friday I decided, I'm not quite sure why, to to an A-B comparison with my LX7. Really it was just a bit of a lark to see how much better the E-M5 was than what I had been used to. That turned out to be a bit of a shock.
Breakup
These are crops from files taken with the OM-D and the LX7. It was dusk and the sun was setting.
The image above is the OM-D E-M5.
This is the same scene taken at the same time with the LX7.
These speak for themselves I think (If you click on each you can see a slightly larger version). The OM-D looks like someone has taken the Photoshop smudge tool to it - the pine needles have all been smeared into an amorphous green mass and the bark on the trees has no texture. The LX7 on the other hand preserves more detail despite being a smaller file.
B&H were of no help; reset all the camera settings to the factory defaults was their best advice (I shouldn't be surprised - it’s a high volume operation). Anyway, I did that but the results were no better. So my love-affair with Olympus came to an abrupt end. The camera was RMAd and sent back yesterday.
I had, at one point, thought about using my old OM series lenses on the OM-D but the micro 4/3 sensor made that awkward - my OM 24mm f2.8 would be an f5.6 standard prime and I'd have no wide-angle. That, combined with the outrageous cost of the adapter ($152), made this a bit of a non-starter. Since Olympus looks committed to the micro 4/3 format and unlikely to produce a full frame camera, any hope of using the old lenses looks forlorn.
Along the way, for no good reason, I'd bought a Nikon F5 - the company's last professional film camera. The lens (Nikon AF-S NIKKOR 24-85mm f/3.5-4.5G ED VR) I found reconditioned for about $350. It isn't professional grade, but DXOmark gave it pretty good rating compared to say the Sony FE Carl Zeiss Vario-Tessar T* 24-70mm F4 ZA OSS which costs about $1,200. The professional grade Nikon AF-S NIKKOR 24-70mm f/2.8E ED VR is faster and sharper, but at $2,400, is almost seven times what I paid for the 'kit' lens and until I win the lottery, unaffordable. (Of course there is the Tamron SP 24-70mm f2.8 which according to DXOmark is better than the equivalent Nikon and half the price, but it’s still outside my budget for the moment).
So my plan is to sell all my Olympus equipment (except perhaps the 1980 OM-1n) and eventually get something to use with the Nikkor 24-85mm, probably a second-hand or reconditioned D610 or D810. Since I don't have to buy a lens, that would be cheaper than a Sony A7R with the Vario-Tessar T* 24-70mm lensl; and it's a Nikon so built like a tank.
But that's some way off. This was about my breakup with Olympus. It's been a 40 year relationship but now it's over. And another valuable lesson learned was not to trust dpreview.com's reviews alone. Perhaps they mislabelled the file, but their test look very different from what I found. There is, ultimately, no substitute for shooting ones own test images.
Sunday, June 14, 2015
Time flies
I looked back today at the last roll of 6x7 I developed - it was a year and 10 days ago! That's pathetic. I've been so engrossed in testing and calibration that I haven't taken any pictures; which is a pity because that's what this was all in aid of. Anyway, here's a roll I developed this evening. I quite like the boot and tire (top left). I may re-do the trucks (if they are still there) to lose the white car on the left (which isn't).
Tuesday, April 7, 2015
Odessy over
About 10 months ago I began thinking about how to calibrate my film, much as one might a printer or a display. Yesterday that journey reached its destination. I now have a process that generates a digital file that fairly accurately represents the incident light at exposure, and backs out the characteristics of the particular emulsion use to capture it.
After working on the problem on and off for a while I have now managed to think through how to use this information to 'linearize' the scanned response for any given film and any exposure.
Initially, my approach had been to measure the scanned output for each 'zone' and create a series of correction profiles for any given exposure. If, for example, an image had tones ranging from 4 stops under to 2 stops over middle grey, I'd create a profile for that specific exposure range - in other words for every combinations of black at one end and white at the other, I'd produce a profile for the particular exposure on that film.
Clear this was unwieldy. To start with it quickly filled up the scanner software with a huge number of profiles for just one film. Second, it took a long time to generate all the profiles. Each had to be scanned, the peaks found and put into into a spreadsheet from where the individual profiles were built. Third, it was limited to incremental zones (exposure stops) and didn't cater for exposure ranges that lay between stops. This wasn't so much of a a problem in the shadows but was critical in the highlights where densities are very close together. So I spent several months wondering if there was an better way. In the process I wrote a program for detecting peaks in a test image which helped a great deal, but it was still cumbersome.
Ideally I wanted to simply take the 'master' profile for the film and generate a correction curve for the portion of the emulsion that was used in any given exposure. That would avoid having to choose between two profiles neither of which exactly matched thee exposure I was scanning. The first step was to fit a 5th order polynomial to the film's response curve. The fit was excellent (R-sq = .9997) and allowed me to predict the emulsion density for any given level of exposure.
However what made the end-to-end generation of a correction profile difficult was the logarithmic mapping the scanner used to turn the densities into output levels. The profile of that mapping depended on its breadth and, importantly, how close to the white end of the range the image input range was set to.
Realizing the mapping was a log function was a breakthrough and allowed me to define a function that accurately captured Epson's mapping. Armed with this function I could now accurately predict, for any given range of exposure in the image, where the scanner would put a given zone on an 8 bit intensity scale (0-255). Knowing what the output would be I could compare it to what it should be; even increments for each zone.
Once a film has been tested and measured (once only) the master profile function and the log mapping function are used together to generate a new correction profile - on demand - for each new negative to be scanned.
The process works like this; an image is previewed on the scanner. The density values of the darkest and brightest points are noted, defining the tonal range in the emulsion that has been used. Using the log mapping from the film's master profile, the scan levels this would generate are calculated for 15 equal steps from the darkest to the lightest values. These are then used to create the correction curve which is written to the Windows registry. The scanner software is restarted (it only reads from the registry at start up) and the newly generated correction profile can then be applied to the image.
This is one example taken from the underexposed end (-6 to + 2). The top strip show the scan without correction.
%2B%23%2B76-209%2B%23004.jpg)
The next show the same strip with the correction applied.
%2B%23%2B76-209%2B%23003.jpg)
This experiment was done at the low end of the tone curve where the effect is most noticeable. Middle grey without the correction is at 102 which is about a zones below where is should be at 128 After applying the correction it appears at 126, almost exactly in the right position.
Some might suggest that photography isn't a science and this is way too clinical. And that's probably quite right; after applying this approach to a portrait recently, I preferred the version that I'd done without the aid of all this manipulation.
So why do it? Partly it was the challenge of seeing whether I could indeed generate an exact tonal representation of the scene by correcting for the characteristics of the film. And it may be completely pointless without taking into consideration the different way the eye registers brightness at different light levels; and most people don't see the world around them in black and white anyway, so a black and white photograph is already a distortion, the creation of something one doesn't normally see. To a photograph is as much about the affective response as it is about the exact replication of the original scene.
Nevertheless, in the process I've learnt a bit about properties of TMX100 (which is a really wonderful film - I really hope Kodak Alaris doesn't stop making it). It's also a more reliable (or at least constant) starting point for any manipulation done by eye. I know that if I over or under expose, I won't be having to deal with the way the film has altered the tonal balance differently in the high registers from the low ones. Even if I seldom use it, at least I now know that if I want to, I can faithfully reproduce the light levels in the original scene in a digital file.
It's the odd-shaped spanner sitting in the tool box, the one you bought for a particular job; you may never use it again but it's nice to know it's there.
 |
| Kodak TMX 100 for 7 stops either side of middle grey |
Initially, my approach had been to measure the scanned output for each 'zone' and create a series of correction profiles for any given exposure. If, for example, an image had tones ranging from 4 stops under to 2 stops over middle grey, I'd create a profile for that specific exposure range - in other words for every combinations of black at one end and white at the other, I'd produce a profile for the particular exposure on that film.
Clear this was unwieldy. To start with it quickly filled up the scanner software with a huge number of profiles for just one film. Second, it took a long time to generate all the profiles. Each had to be scanned, the peaks found and put into into a spreadsheet from where the individual profiles were built. Third, it was limited to incremental zones (exposure stops) and didn't cater for exposure ranges that lay between stops. This wasn't so much of a a problem in the shadows but was critical in the highlights where densities are very close together. So I spent several months wondering if there was an better way. In the process I wrote a program for detecting peaks in a test image which helped a great deal, but it was still cumbersome.
Ideally I wanted to simply take the 'master' profile for the film and generate a correction curve for the portion of the emulsion that was used in any given exposure. That would avoid having to choose between two profiles neither of which exactly matched thee exposure I was scanning. The first step was to fit a 5th order polynomial to the film's response curve. The fit was excellent (R-sq = .9997) and allowed me to predict the emulsion density for any given level of exposure.
However what made the end-to-end generation of a correction profile difficult was the logarithmic mapping the scanner used to turn the densities into output levels. The profile of that mapping depended on its breadth and, importantly, how close to the white end of the range the image input range was set to.
Realizing the mapping was a log function was a breakthrough and allowed me to define a function that accurately captured Epson's mapping. Armed with this function I could now accurately predict, for any given range of exposure in the image, where the scanner would put a given zone on an 8 bit intensity scale (0-255). Knowing what the output would be I could compare it to what it should be; even increments for each zone.
Once a film has been tested and measured (once only) the master profile function and the log mapping function are used together to generate a new correction profile - on demand - for each new negative to be scanned.
The process works like this; an image is previewed on the scanner. The density values of the darkest and brightest points are noted, defining the tonal range in the emulsion that has been used. Using the log mapping from the film's master profile, the scan levels this would generate are calculated for 15 equal steps from the darkest to the lightest values. These are then used to create the correction curve which is written to the Windows registry. The scanner software is restarted (it only reads from the registry at start up) and the newly generated correction profile can then be applied to the image.
This is one example taken from the underexposed end (-6 to + 2). The top strip show the scan without correction.
The next show the same strip with the correction applied.
This experiment was done at the low end of the tone curve where the effect is most noticeable. Middle grey without the correction is at 102 which is about a zones below where is should be at 128 After applying the correction it appears at 126, almost exactly in the right position.
Some might suggest that photography isn't a science and this is way too clinical. And that's probably quite right; after applying this approach to a portrait recently, I preferred the version that I'd done without the aid of all this manipulation.
So why do it? Partly it was the challenge of seeing whether I could indeed generate an exact tonal representation of the scene by correcting for the characteristics of the film. And it may be completely pointless without taking into consideration the different way the eye registers brightness at different light levels; and most people don't see the world around them in black and white anyway, so a black and white photograph is already a distortion, the creation of something one doesn't normally see. To a photograph is as much about the affective response as it is about the exact replication of the original scene.
Nevertheless, in the process I've learnt a bit about properties of TMX100 (which is a really wonderful film - I really hope Kodak Alaris doesn't stop making it). It's also a more reliable (or at least constant) starting point for any manipulation done by eye. I know that if I over or under expose, I won't be having to deal with the way the film has altered the tonal balance differently in the high registers from the low ones. Even if I seldom use it, at least I now know that if I want to, I can faithfully reproduce the light levels in the original scene in a digital file.
It's the odd-shaped spanner sitting in the tool box, the one you bought for a particular job; you may never use it again but it's nice to know it's there.
Saturday, February 7, 2015
News flash
Why do all this? Well, my LX7 only stops down to f8 so I can't test the exposure below f8; and my Schneider 150m is f5.6 full open so I'd like to be able to use more stops than just f5.6 and f8. Which means I need to up the flash intensity and stop down the lens from an exposure I've tested with the LX7.
Armed with this handy cheat sheet, I set about testing my theory...
My first thoguhts was 'did I load the holders green side up'. If so how many holders would I ned to check?
Then I remembered something about flash synchronization. And the answer was I had not set the shutter to 'X' so it was triggering the flash before it opened (which was what you needed to do with flash bulbs and presumably powder before them). I went back and took four more using the my flash power cheat sheet at different exposures (with the shutter now set to X!) - and they were indistinguishable in terms of exposure! And since I was doing a portrait for a friend the day after, that was a worthwhile experiment.
Sunday, December 14, 2014
Dilute Xtol keeps fairly well
.jpg)
Sunday, June 1, 2014
Calibration
Most of the work I've been doing recently is about process
standardization and reproducibility (quality in the operations
management lexicon). Last year I systematized the time/temperature calculation creating a web-based app that uses data on the film, developer, dilution and temperature to calculate the appropriate development time. Hit and miss is (thankfully) a thing of the past. More recently I've been focusing on calibrating the film, much as one calibrates a display or a printer.
Each emulsion responds to light in a unique way. Thankfully Kodak's process control is such that I haven't yet found differences between batches (although I'm not sure the films I've shot weren't actually from the same batch).
To begin I create a negative with bands of different exposure raging from seven stops under exposed to seven stops over. This is about the dynamic range of TMX-100. I develop the film and scan it with a linear tone curve, setting the black and white ends of the output to the darkest and lightest bands on the negative.
I then use GIMP to measure the level of grey the scan has generated for each level of illumination on the negative. From this I generate a series of tone curves (49 in all) for different ranges of emulsion density to compensate for the emulsion's characteristics at different exposures.
For example, one curve might correspond to a negative in which the shadows were 3 stops below middle gray and the highlights 5 stops above middle gray. Rather than labeling them in stops over and under, I label each curve for the range of measured emulsion densities. So in this example, the tone curve would be the one that captures emulsion densities from 143 to 232. (These are all out of 255, the 8 bit tonal representation in the digital image).Doing so allows me to choose the appropriate curve simply by looking at the histogram of the each negative.
The red line on the chart on the right shows the density of the TMX-100 emulsion for different EV levels. The response is clearly non-linear. At light levels above middle gray, (EV>0) there are only small differences in density; at low light levels the differences in density increase at each step until 5 stops under when they decline sharply.
This suggests first that underexposed areas, the shadows, will be hard to deal with unless the response curve is exactly matched to the exposure; and it reenforces the dictum - "expose for the shadows, develop for the highlights", which helps ensure one doesn't have to deal with the non-linear behavior of the emulsion for very low light levels (-5 EV and below).
The curved blue line shows the intensity of the scanned image for each exposure level. Because the negative was scanned with a gamma of 1.5 the line curves upwards rather than being straight.
Using Microsoft Excel, the tone curves are written into directly into the Windows registry keys for the Epson scanner; when the scanning software is opened the tone curve drop-down box is populated with the generated tone curves. Using them is fairly simple. Looking at the histogram for each negative, the levels for the darkest and lightest points are determined - since each tone curve covers a different range of emulsion densities this measurement determines which generated tone curve to use.
Each emulsion responds to light in a unique way. Thankfully Kodak's process control is such that I haven't yet found differences between batches (although I'm not sure the films I've shot weren't actually from the same batch).
To begin I create a negative with bands of different exposure raging from seven stops under exposed to seven stops over. This is about the dynamic range of TMX-100. I develop the film and scan it with a linear tone curve, setting the black and white ends of the output to the darkest and lightest bands on the negative.
I then use GIMP to measure the level of grey the scan has generated for each level of illumination on the negative. From this I generate a series of tone curves (49 in all) for different ranges of emulsion density to compensate for the emulsion's characteristics at different exposures.
For example, one curve might correspond to a negative in which the shadows were 3 stops below middle gray and the highlights 5 stops above middle gray. Rather than labeling them in stops over and under, I label each curve for the range of measured emulsion densities. So in this example, the tone curve would be the one that captures emulsion densities from 143 to 232. (These are all out of 255, the 8 bit tonal representation in the digital image).Doing so allows me to choose the appropriate curve simply by looking at the histogram of the each negative.
TMX-100 measurements
The red line on the chart on the right shows the density of the TMX-100 emulsion for different EV levels. The response is clearly non-linear. At light levels above middle gray, (EV>0) there are only small differences in density; at low light levels the differences in density increase at each step until 5 stops under when they decline sharply.
This suggests first that underexposed areas, the shadows, will be hard to deal with unless the response curve is exactly matched to the exposure; and it reenforces the dictum - "expose for the shadows, develop for the highlights", which helps ensure one doesn't have to deal with the non-linear behavior of the emulsion for very low light levels (-5 EV and below).
The curved blue line shows the intensity of the scanned image for each exposure level. Because the negative was scanned with a gamma of 1.5 the line curves upwards rather than being straight.
Using Microsoft Excel, the tone curves are written into directly into the Windows registry keys for the Epson scanner; when the scanning software is opened the tone curve drop-down box is populated with the generated tone curves. Using them is fairly simple. Looking at the histogram for each negative, the levels for the darkest and lightest points are determined - since each tone curve covers a different range of emulsion densities this measurement determines which generated tone curve to use.
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