As Lee said.
Thanks for your input Lee. If the perspex or opal glass have no specs with them, how likely are they to have a colour cast to them ? (I.e. not neutral?).
Lee what do you think about my suggestion to use a piece of white card as I explained in my last post above?
The difficulty with the piece of card is lighting it evenly. You're working with 1/3 to 1/2 stop steps in the wedge, and for usable results the card has to be lit very evenly and consistently over the entire area that the lens can see. That's more difficult to achieve than even backlighting for a diffusion material over a smaller area behind the step wedge.
There is a 'danger' that perspex or opal/milk glass will have some colour cast, and I don't know what's locally available in Oz. It might be less of a problem than you think, and is likely immaterial if you're shooting B&W film. If it appears anywhere near 'white' to the eye when you hold it up to the light, you're well within bounds for B&W. There are diffusion gels for theatre or photograpy that are color neutral. Check for a local theatre or photo studio supplier. Google "theatre supplies sydney australia". Lee and Rosco make suitable materials that are inexpensive. The supply store should have small swatch books for you to use in making a selection.
If you can't find a lens hood, just shoot through any size dark box that you can find/make to put between camera and the step wedge.
Peter. Even without sunlight hitting the lens there is bound to be a minor contribution from indirect light, so doing the test with the same configuration I intend to use the camera would be important , especially since this test is designed to compensate for contrast reduction due to flare.
a) shooting through the window, which will add its own glare and reflections (and which seems to be the plan unless I'm misreading)
b) have the step wedge out in the sun as well, which would add glare and reflections off the face of the step wedge.
If you're shooting with the target in the sun, you may want to use a reflective step wedge. If it doesn't cover the range you need in one go, you can make bracketed shots to introduce the wanted greater or lesser densities.
I'm not really trying to be difficult, it's just that there are contingencies that we don't always take into account, and it's best (and in the long run easiest) to eliminate error up front rather than puzzling over where it came from later.
Thanks Lee. Fortunately I'll be shooting outside with the white card in the sun and the transmission step wedge in the shade taped to a piece of glass. We usually have lots of sun in Sydney, but today (Sunday) has been overcast and a bit wet and I only have time to do this hobby on the weekends !
This Sunday arvo was the only free time I had to get back to this testing, but alas the cloud cover kept shifting. As a consolation, I spent the time photographing and documenting the test setup I had assembled.
I developed one of the five rolls tonight (for 11mins). Before I develop the other ones I'm a bit concerned about two things
1. I used a 31 step Tx wedge. I did one development for 11mins and steps 24 through 31 (Tx densities 2.35 to 3.05) are all at the density of the film base+fog. This suggests my exposure might have been too long. Have I lost too much dynamic range in this test ? I exposed HP5+ using a nominal ASA of 200 (I know its 400, but I usually use it at 200 and your instructions say "You can use the manufacturer’s recommended film speed, since the actual exposure is not critical as long as it is within 1 stop."). I metered a grey card in the same direct sunlight as my white cardboard with a spot meter. My Mamiya C330 then required an exposure compensation ox x2.5 (1 1/3 stops) when shooting so close up, so I then put the ASA to 80 to address that). This resulted in me using f8 @ (1/250)s for all frames on all rolls.
2. I use XTOL 1+2 dilution which results in longer dev times. What set of dev times should I use instead of the the nominal ones you list (spanning 4-16min)?
With increasing dev time, (esp up at 16mins in the example excel spreadsheet) those steps do increase marginally in density on the negs. So I think I will just need to extend my dev times out to perhaps 25 minutes.
I forgot that increasing the dev time does have a minor effect on the shadow density but primarily it is the exposure times which lock in the shadow density. Therefore I haven't wasted those steps on my Tx wedge, as most of them will play a part at higher dev times.
So now to estimate my remaining dev times.
From the example excel spreadsheet I fitted the points (1,4),(2,5.5),(3,8),(4,11),(5,16)=(n,t) to the general exponential curve t = m x EXP (a x n) :
t = 2.7968 x EXP(0.3466 x n)
with Rē = 0.9992
I then fixed a, passed it through my own point (2,11min) and solved for a new m, giving me
t = 5.5 x EXP (0.3466 x n)
I then fixed a, passed it through my own point (2,11) and solved for a new m, giving me
t = 5.5 x EXP (0.3466 x n)
This gives me new points of (1,7.8),(2,11),(3,15.6),(4,22),(5,31.1)=(n,t)
The nominal dev time here for my choice of film, dilution, temp(20degC) and EI (200) is about 13mins.
I think my proposed dev times seem too long. So I'll recalculate this exponential equation with a two point fit to solve for both m and a after I do the next dev at say about 20mins.