Results 1 to 4 of 4

Thread: Preflashing Test Results

  1. #1
    Senior Member rydolan's Avatar
    Join Date
    Jan 2009
    Location
    Connecticut, USA
    Posts
    106

    Preflashing Test Results

    This evening I conducted a preflash test on the two papers I currently have: Arista Grade 2 Semi Matte and Adorama Multigrade Glossy.
    To preflash, I use a 7.5w frosted bulb placed inside a soup can with a 1/4" aperture (Joe's design).
    The can is suspended 35" above the table.

    The Adorama Multigrade paper first showed the slightest signs of going off white at 6 seconds.
    The Arista Grade 2 paper went at 14 seconds.

    My question is, what exactly does this tell me about these papers? (I have recent experience using the Adorama paper, and have never used the Arista Grade 2 btw). Does this mean the Adorama paper is more than twice as fast as the Arista, or does it have more to do with the begining of each paper's curve? Or neither?
    I dont know how to do an actual speed test without a densiometer (or with one for that matter) so I have only been able to make rough guesses as to the speed of the Adorama paper and then adjust it with some experience shooting it. (Preflashed I rate it at ISO 15) And what I am calling "ISO 15" is really just what I've arrived at after overexposing shots when I rated it at 12 and underexposing shots when I rated it at 20.

    The whole reason I did a preflash test, is because A) I finally bought some Grade 2 paper, and B) I noticed that the preflash time I have been using for the Adorama paper actually gives a Way darker change than "just off white." Tonight's test showed that I had been preflashing the paper for 2-3 seconds too long.

    Again, I'm just wondering if there is anything I can learn about these papers based on the results I got tonight.

    Thanks, Ryan

  2. #2

    Preflashing Test Results

    G'day Ryan

    An interesting post but I'd caution you to not get too technical.

    Pre-flahing is done to reduce contrast. Different papers require different flashing times and different photographers require different effects.

    You've already learnt more than you need to go and start making exposures.

    So, go and make exposures.

  3. #3
    Senior Member rydolan's Avatar
    Join Date
    Jan 2009
    Location
    Connecticut, USA
    Posts
    106

    Preflashing Test Results

    Ray, I appreciate the comments. My questions however, are purely for the purpose of increasing my overall understanding of photographic materials and processes, and thankfully, have never interfered with the enjoyment I get from simply taking pictures and learning from their results. The technical stuff is just another facet of the interest.

    What I was really looking to gain from my post was to better understand why the two papers could have such a significantly different response to the preflashing. If it is simply because different brands/grades of paper just react differently, then that's just as useful to me as knowing a more technical reason if there were one.

  4. #4

    Preflashing Test Results

    Ray's suggestion is a good one, but for the curious (like me), I would assume that the difference in results is mainly due to the difference in spectral sensitivity between the two types of papers.

    As you might know, multigrade paper has two emulsion components, one UV/blue sensitive for low contrast and one green sensitive for high contrast, while graded papers have one emulsion that's mainly UV/blue sensitive. Additionally, the relative sensitivities of the two types of emulsions are different. The multigrade emulsion uses color sensitizing dyes to expand its sensitivity into the green wavelengths, which the graded paper lacks. These dyes also have the side effect of increasing the effective speed of the MG emulsions.

    Pure silver chloride B/W emulsions are only UV sensitive (and thus very slow, requiring strong UV light sources to expose), while other silver halide emulsions like silver bromide (like most modern printing papers, employing both silver chloride and bromide, along with other halides) extend into the visible blue parts of the spectrum and hence are faster. The addition of color sensitizing dyes along with silver halides serves to extend the overall emulsion's sensitivity toward the red, panchromatic film emulsions being examples of this, these film emulsions being faster than paper emulsions.

    Another factor in all of this is that your light source, being incandescent, has a broad spectrum of light output, but not as much in the UV and blue parts of the spectrum, more in the green, yellow to red.

    The net result is that the MG emulsion is more sensitive to the spectrum of light from the incandescent bulb than is the graded paper. But you can't necessarily expect the same relative results between the two papers in daylight, since their respective sensitivities to UV/blue light will be different. It's best to do tests of the papers' working exposure index under the lighting conditions you expect to use them with.

    Regarding preflashing in general and how it works, we should be reminded that each zone in the zone system (from I to IX) represents a logarithmic change of base 2, such that adding a preflash exposure equivalent to, say, zone II will only add 1/2^(IX-II) = 1/2^7 = 1/128 of a stop more exposure to the zone IX (highlight) exposure, meaning that preflashing essentially does not effect the highlights at all, just adds visible exposure to the lower zones, thereby reducing the overall brightness range captured in the negative.

    ~Joe

    PS: Just to add that, were I to attempt to initially calibrate a new paper that I've never used before, I'd first find its base ISO without preflashing, by setting up a daylit scene and have my darkroom chemicals ready to go, then do a series of meter readings and exposures at various settings, them develop the results and see which setting gives you the optimal exposure (optimal meaning the midtones of the scene are represented the best). Once you've found the base ISO of the paper for daylight, then you can do preflash tests with various timer settings on your light source, and see which preflash exposure gives you a faint gray tone to an otherwise unexposed paper. When I did this with my existing preflash light source I did the tests in increments of two second exposures (i.e. 2,4,6,8,10, etc.), all exposed on one strip of paper using a round window cut in a black piece of paper as a mask. The results after development were a series of round gray circles on the paper of varying densities. I chose the circle that was the first one to become visible below paper white.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •