How to characterise an emulsion at home

[RogerHyam]

I've just made my seventh emulsion. Tonight I'll wash and pour a few test plates.

I'm getting to the stage in my emulsion making when I need a repeatable way to tell the difference between batches. I'm wondering the best way to do this given that I live in Edinburgh (55 degrees North) and light through the year is very variable. I'm thinking of a standard test target lit with compact florescent bulbs. But is there another way? Could I inkjet print a step wedge target for example? Can I use my scanner as a densitometer?

Any suggestions gratefully received.

 

[Lachlan Young]

I'd suggest getting hold of a Stouffer 21-step wedge (possibly the calibrated one) for plotting a characteristic curve and something like a Colour Checker for testing practical colour sensitivity. You can use a scanner as a densitometer and mathematically convert luminosity units into density units - though keep an eye out for densitometers, they're often not that expensive for a good Xrite reflective/ transmissive one - avoid the simpler ones sold for offset printing. I'd also avoid the use of CFL - unless you get Kino Flo lamps and ballasts (expensive!) - discontinuous spectrum output and odd colour spikes are well known issues. High CRI (ideally high TLCI) daylight balanced LED or tungsten will give you your best chance of daylight balanced sources. Or you could use a regular flash to illuminate the target.

 

[dwross]

I second the use of a Stouffer step wedge. The 21 step version is equal to 1/2 stop increments. A 31-step does 1/3. You don't need a color checker until you start making ortho or pano emulsions. They can be expensive. For your purposes you can just print out one from the internet. The only thing that matters with in-house testing is consistency. Much better to spend your money on a good light. I use an LED panel -- just a small one. Bi-color is best with a top temperature range of at least 5600K. Keep the light intensity and color constant between similar tests, as well as distance and angle, and don't set up near a window that will add variable light to your test. Testing is useful and fun, but try not to go down the rabbit hole of spurious precision .

 

[RogerHyam]

Thanks for this.

I'm confused a little about light sources. Normal LEDs (I presume even daylight balanced ones) emit very little UV. I have used CFL bulbs topped up with UV specific LEDs for wet-plate to mimic an actinic afternoon. I get similar exposures with my colour blind dry plates when I use this set up. But the colour balance is obviously crazy.

LED daylight balanced lighting would be OK for truly panchromatic film but if the emulsion has loads of sensitivity in the UV then you won't get any idea of how it will perform outside from the indoor test?

But the path is made by walking. I have an LED video light. I'll go and expose some plates and see what happens with different light sources.

I love the term "spurious precision". I'm a more a roughly right guy myself.

 

[dwross]

With my own testing, I draw a distinction between a standardized set of tests that are invariable throughout the year and outdoor photography throughout the year, with all the inherent variations. Mere mortals can't afford the lab equipment that precisely mimics all possible outdoor conditions. You just need to take good notes for a year. Cross-reference the standardized test results with what you get outside. Record the time of year, time of day, altitude, latitude, UV level for the day (there are websites that track UV). If tracking UV is a bit much (and I don't do it much anymore), then just make an observation of the weather. My categories are bright light (10am to 2 pm), bright light before 10 am and after 2 pm, two hours after sunrise and two hours before sunset, high bright clouds, heavy clouds, and the same for shade. In a year, you'll find yourself hardly even needing a light meter to make good exposures.

One more thing: If your emulsion-making protocol is very consistent, you'll rarely need to do an indoor test until you decide to change your recipe. Save your time and energy for photography and good notes outside!

 

[RogerHyam]

Save your time and energy for photography and good notes outside!

We are on about 8 hrs of theoretical daylight just now and falling. Not that you can tell through the thick cloud. Today the sun was 15 degrees above the horizon at noon (I just looked that up - explains why exposures are soooo long just now even when the sun is out). Work makes it tricky to be out doors in daylight at all during the week.

So plenty of dark time to mess indoors! I'm thinking of getting more seriously into still life work ...

You are right though. What I need is a test setup that is easy to setup and assess. I'm just wanting to check the emulsion has worked and if it is much different from the last. I already do a fog test by coating a five 1" slips of glass and developing them for different times. I completely fog a sixth slip to check I get a good strong dmax. I used to take a photo out the window to guess the speed but it is too dark recently.

 

[radiant]

Roger, I understand you. Living in Finland I know your problem

I think you can use scanners for analysing density, here is one analysis using scanner: https://www.scotty-elmslie.com/uplo...tical_guide_to_film_characteristic_curves.pdf

I'm waiting my stouffer wedges. I don't think you need a calibrated wedge if you decide purchase one. BTW: fotoimpex is selling stouffer wedges!