Exposure Metering and Filters

[Jon Lewis]

I’m sure metering for daguerreotypy is another one of those things you only get (somewhat) right after doing it for years and years but I thought I’d see what techniques people are using. Alan started a new page in the gallery for for meters. In there is a meter for measuring the UV index. I have often thought one would be convenient when printing cyanotypes in the sun in order to have a better measure of how long it’s going to take. Here in Santa Fe during the summer at noon it is fairly common to have a UV index of 11+. For cyanotypes this means a print is done in about 90 seconds versus 10-15 minutes in a UV light box. For daguerreotypes I’m thinking this will mean the possibility for quicker exposure and/or easily solarized skies. According to Robert Shlaer in his book Sights Once Seen (p.60), lenses have a hard time focusing UV light and he actually uses a UV filter to only allow in the blues and violets which, according to him, makes his image noticeably sharper. He then meters with a spot meter through a blue filter to simulate the daguerreotypes sensitivity. However, he indicates that his metering is only a guide that often gives way to experience. With his filtering he says his sunny 16 exposures are about 16 seconds (ISO 1/16 or 0.0625?).

So what sort of metering works for you? Are you using a photo light meter or UV meter or some kind of voodoo? Are you using filters and/or compensating for a UV focus point?

[CasedImage]

I use regular light meter as a guide to my experiences shooting, I always guesstimate based on my humble experience and the UV light meter is another one of those things that I try and factor in, how the plate went during sensitization being another. Most often I take light readings to tell me if what I am seeing matches with my memory of another time I was shooting. Daguerreotypes are orthochromatic and so are most sensitive too UV light, to me I wouldn’t want to filter it out at all. Kingslake in his book on lenses remarks how the early petzval lens had a favorable combination of crown and flint glass types for UV light, which is what I use.
I come from NZ where we have some of the highest uv rates in the world (and skin cancer), supposedly cause of our proximity to the hole in the ozone in Antartica. To me a 16 sec exposure on a bright sunny day even with Becquerel would seem a long exposure. I guess it depends on a lot of things in your set up, for me though I prefer the petzval lenses.
Some of the first Daguerreian lens weren’t achromats – they didn’t have the crown and flint glass combination so the colors focused at different focal lengths. Claudet invented the focometer, a wheel stepped back in numbered segments with patterns on it so when in the shot it showed the true focus as opposed to what the ground glass was showing. This meant though that you had to develop a test plate to find the focus… yes a lot of work.

www.CasedImage.com

[botticelli1972]

I also read that in Shalers’ book though I work in Becquerel and he in Mercury. I tried it and have found that the use of a UV filter adds about 2-3 stops to the exposure, though I did not noticed a change in image quality. I also tried a 80A blue filter thinking that it would send only a blue image to the plate. This resulted in the normal filter factor of 2 1/2 stops, but the image produced did appear to be "better" (though that is hard to quantify). I noticed this mostly on shooting a red brick building that otherwise didn’t record well.

My exposure guideline is adapted from one given by John Hurlock. For Becquerel I set the a digital light meter at ISO 100 and multiply the exposure by 70,000 (John recommends 80,000). I then add stops depending on the color of the scene, lots of vegetation requires two stops additional exposure, red brick building as mentioned above 3 stops extra, plus any filter factors if used. I also have to add a stop if I have been lazy at the polishing stage and have not gotten that "perfect polish." This gives a base Becquerel exposure of around 20 seconds at F/2.8 on a full sun day. I made a fairly good 4×5 sized exposure this weekend in Washington, DC full sun, lens stopped down to F/8 for depth of field, would have been 3 min. but I added two stops for the vegetation laden scene totaling 12 min for the exposure. I probably should have added another stop for good measure but I was in a hurry.

Larry Shutts

[Jon Lewis]

Thank you for you insights Larry and for your exposure method! I wonder if Becquerel development results in a different spectral sensitivity than mercury. I wouldn’t think so since up until the development the process is the same provided you don’t use bromine or any other accelerators. When I get some plates going somewhat consistently I’ll try to run a set of tests on a single scene using a UV blocking filter, a blue filter, and a filter that admits only UV if I can find one.

I also wonder if it would be possible to add some sort of dye to the plates as they do in film to bring the sensitivity down into the longer wavelengths…

[jdanforth]

I just bought a UV A / UV B light meter from Talas. I’m going to keep metering using my Pentax digital spot meter but I’m also going to note the time, date, and UV reading to my exposure notes. Hopefully I’ll be able to use the new UV data to more accurately predict a good exposure.

[dagist]

If you live in an area with an average amount of UV in the atmosphere (nothing extreme that would necessitate compensations), here’s an easy method for accurately determining the proper exposure for a daguerreotype plate. My method gives you the starting point for an average-contrast scene that is being exposed onto a plate that has been properly sensitized to record a full range of tones for an average scene, similar to the way our eyes see. Certain colors will of course not reproduce the way our eyes see them (reds, oranges and yellows) but your overall exposure will be very close every time, giving you more consistent predictable results.

You can use any modern exposure meter (no need for any UV filtering). I take mostly incident light readings, but you can certainly use reflected readings also, as long as your meter is seeing a representative sample of the lights and darks the camera is seeing.

Set your meter’s ASA/ISO on 25, select the aperture that you plan on shooting at, and take your reading. Whatever the resultant time is, add 10 f/stops. It’s very easy to simply double the time – 10 times. If your meter tells you 1/4 second at your given aperture, then just count up 10 stops from there: 1/2 second, 1 second, 2, 4, 8, 16, 32, 1 minute, 2 minutes, and finally 4 minutes. Four minutes will be the correct exposure time.

The four minute exposure will be your starting point, from which any modifications can then be made depending on your specific circumstances.

Here’s a convenient time saving tip if you’re metering in bright sun. Get a second incident dome for your exposure meter (the Minolta meters have removable domes) and fill it with a cotton ball held in place with circular pieces of typing paper. Just the right amount of cotton and paper will reduce the amount of light reaching the sensor by exactly 10 f/stops, thus allowing you take readings without doing any conversions. Whatever the meter says, will be the correct exposure for your daguerreotype. Multiple sheets of typing paper make fine tuning the process to exactly 10 stops pretty easy. I call it my “Dag Dome.”

Cheers,

Rob McElroy

Buffalo, NY

[Andy Stockton]

Thanks Rob – This is very helpful. I have noticed others comment elsewhere about how much plate preparation can affect sensitivity. Are there tricks to compensating for that as well? Or do you just work to make your plates as consistent as possible?

jdanforth – Have you noticed any patterns in the 2 months you have had your UV meter? (Or has winter kept you indoors?)

[dagist]

Andy,

There are no “tricks” for exposure compensation as it relates to plate preparation that I have ever used, or needed to use. I have never made any compensations despite the many techniques and methods I have used to polish my plates over the years. My “Ten stops over Kodachrome 25” rule has been the same for the past ten years.

Plate preparation is “the key” to achieving a full range of tones on a daguerreotype plate. Poor plate preparation can inhibit a plate’s ability to attract mercury during development which can result in the plate’s inability to reproduce the tone that was exposed on it by the lens. The effect is, that you end up with the brightest white highlight becoming “grayed” because the mercury was prevented from fully filling in all the minute spaces (highlights are actually overlapping layers of mercury, not just a single layer).

This essentially lowers the contrast of the plate and makes it “seem” like it’s less sensitive, because the resulting image doesn’t seem bright enough. The solution is often to give the plate more exposure in order to make it “brighter,” when in fact it just needs a better polish that won’t inhibit the mercury attraction in the highlights. By increasing the exposure to make it “brighter” you then raise all of the mid-tones and end up with a plate that has a lot of middle grays and light grays, which often still has no real highlight because of the ineffective plate preparation. It does seem to look better with the lighter grays, thus leading one to believe it was underexposed initially, when all it needed was a better polish.

If you look at all the daguerreotypes on this site you will see that almost no two daguerreotypists have the same range of tones from black to white on their plates, yet most of them still look pleasing to the eye. That is because of the daguerreotype’s amazing ability to reproduce a very long tonal scale. Which parts of the tonal scale we choose to expose our plate in, is up to us. And that is where the concept of correct exposure can sometimes be misleading.

I could easily expose three plates at one, two, and four minutes each, and individually each plate may look pleasing/acceptable to the eye; a testament to the daguerreotype’s uncanny ability to look great with a broad range of exposure options.

I don’t know if I answered your question or not, but always remember the daguerreian mantra which is, “Everything affects everything.” Iodine, bromine and mercury times, as well as mercury temperature, are also vital components to understanding the correct exposure for a particular scene or subject.

Cheers,

Rob McElroy

Buffalo, NY

[Andy Stockton]

You have answered my question about exposure “tricks” and as usual have given me much to think about. I appreciate your taking the time to share your many years of experience in such depth. You have a very clear way of explaining things.

It makes me want to get busy and make some plates! Several others have already chided me to read less and get my hands dirty more, I will renew my efforts in that direction.

I did have one more question though if you haven’t run out of patience yet. I live in an area of high humidity and have read a number of things that mention both temperature and humidity as critical factors. Is that your experience? If so, how do you compensate?

Thanks.

[photolytic]

Please read my “Frozen in Time” article in the resources section of the forum.

Both heat and humidity accelerate the destruction of the latent image of Dags.

If excess heat and humidity are present, store your sensitized and exposed plates in Ziploc plastic bags with lots of desiccant and work fast. If you use artificial means of cooling (AC, ice) you need to be especially cautious when you open the bags and expose your cool plates to more humid air. The slightest amount of condensation on the surface of a cold plate will rapidly erase or degrade the latent image. Allow the plates to warm up to the dew point before opening the bag.

Normally the winter months are little more forgiving but you still need to be cautious about moisture exposure. When temps reach 0C you have almost unlimited time (i.e., a week) before the latent image fades. At 10C the image will last about a day. At 20C the image will last several hours. At 30C, you have 15-30 minutes to work with. At 40C, you’d better have that mercury bath warmed up in advance.

Normally for Becquerel plates time is a less important factor but keeping the plates relatively cool until the image starts to show is important. Once the image begins to appear the silver atoms in the latent image have clumped together and grown more resistant to attack by water and other chemicals that might be present on the surface of the plate, giving you more time for completing the development process.

[dagist]

Andy,

Heed John’s advice above and always be conscious of the humidity and temperature. The slightest amount of condensation (droplets so small you can’t see them) will destroy all or part of the image, resulting in all kinds of image degrading effects. The same thing will happen if you breathe warm air on a plate after exposure. Always try NOT to bring a plate from a cool environment into a warm environment where the potential for condensation can occur.

In the ideal daguerreotype processing environment, the temperature does not vary (other than slowly cooling) from sensitization to exposure to development, eliminating any potential condensation problems. But, conditions are often far from ideal. Outdoor daguerreotypes during the winter can be a challenge even with low humidity because of the risk of condensation when you bring the plate from the cold camera into the warm processing environment.

Don’t shy away from making images during humid weather though, as sometimes you can still make great images. Your success-percentage will just be lower because of the potential for unforeseen condensation conditions. I made a 17-minute exposure on a whole-plate during a light rain a few years ago that turned out to be one of my more fascinating daguerreotypes. So, yes, you can make daguerreotypes in the rain.

Practice, practice, practice is the only way to achieve perfection. No matter how much you understand the process from books, everything that can go wrong, WILL go wrong, so you must continually experiment with your own equipment and techniques. What works for me, might not work for you.

Of all the daguerreotypists practicing today, none of us do everything the same. In fact we do very little the same as one another. We all have different equipment, techniques, sensitizing and processing times, development temperatures, and gilding methods. Even the silver surface itself is different (and reacts differently), depending on the chemicals and techniques used by the electroplater. And then there are the daguerreotypists who use clad plates, and some of them electroplate (galvanize) a thin layer of silver on top of the clad silver.

All of these factors (and many more) contribute to the unpredictability of the process, thus demanding the need to establish your own working methods with the plates, sensitizing boxes, mercury chamber, camera, and polishing method that works for you.

Regards,

Rob McElroy

Buffalo, NY

[Andy Stockton]

Thanks to you both for your detailed responses. They will be read by many, now and in the future I think. I have resolved to spend little time reading this weekend and to make more practical steps on the path to that first plate.

[Author]

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