It's no secret that I'm a big fan of Ilford's Pan-F+ film. Last year I decided to answer for myself the one question that seems to dog this emulsion: is it critical that you develop it soon after exposure? In other words, will the latent image deteriorate if you don't get it processed quickly? Many people seem to believe so; it's a piece of advice you frequently hear when Pan-F is mentioned. I admit to saying so myself.
Indeed, This is what Ilford says: "Important Note: Once exposed, process PAN F Plus as soon as practical – we recommend within 3 months." Compare that to what they say in the FP-4 datasheet: "Once exposed, process FP4 Plus as soon as practical." So people give this advice, but based on what data? I'm sure Ilford has the numbers, but short of that, what does it mean for me? Are three months truly safe? What does one month look like, or six months? One way to find out: test it.
Methodology
I exposed a fresh, factory-loaded 36 exposure roll of 35mm Pan-F to the same scene at the same exposure. I then snipped a few frames and developed that film the same day. I put the remaining film in a secure location, on my darkroom shelf, keeping real-world conditions. I did another snip test two weeks later, and then at the one month, six weeks, two months, four months, eight months, and finally one-year intervals. You can see by the specific dates my breaks were not perfect. It's not always possible to get to the darkroom when one desires, but I kept it close enough.
The exposure.
TTV
Backed up by my trusty spot meter, I placed the grey siding on Zone VI.
The developer was Rodinal, developed at my standard dilution and time of 1:100 for 10 minutes at 70º F. That has been my go-to combo since my school days. I have some thoughts about that in the context of these tests that I will share further down. I measured the same five areas from each batch of negatives on a densitometer and recorded the data. I also printed a selection of negatives on Ilford Multigrade RC.
Data
The bottom line in blue is the film base density plus fog. Over the year, it stayed consistent, with a slight lift over the final months. This lift may reflect some age-related fog, but it's very minimal. The green bar represents a zone III shadow density; note how it starts to dip at about the ten-week mark. The magenta bar shows a zone IV density; it too displays a dip at the ten-week point. The orange bar represents a zone VI density. It shows a more gradual dip. The top bar in red is a highlight density in the zone VIII range. It stays stable. Highlight densities are a good indication of development consistency. If this varied, I would blame it on some error in time, temperature, or dilution.
Conclusions
I think what I found lines up pretty well with Ilford's recommendation. There is a measurable difference in shadow area density at about three months, almost equal to a one f-stop change in the low values. So, How critical is all of this? Is latent image degradation the cause? If your roll of Pan-F fell to the bottom of your bag and you've just discovered it after a year, should you throw it away? Should you wail in despair at ever pulling decent prints from your negatives? Please don't do any of that. In the end, what matters is how the photo or scan looks to you. I printed a selection from my test negatives, and I can barely detect a difference between the prints made from negatives developed one year apart. Indeed, when I first reviewed the negatives before taking density readings, I thought they looked nearly identical, and all quite printable.
Start
6 months
9 months
12 months
A brief sidebar
This shift of density in the low values is very much like what happens when you push film. There, you deliberately underexpose and develop the film for longer than the "normal" time. These shifts expand the tonal separation in the mid to high values and compress the shadow areas, increasing the overall contrast, among other things. Many people, 35mm shooters, in particular, prefer this more graphic look. In many images, the content is carried in the mid and high values, while the shadows define shapes and texture. The simplification of tone from a "push" can be quite desirable, suppressing distracting detail and directing attention to the picture's key elements or subject. Yeah, yeah, so why say all of this? These are subjective decisions, but you may find that your sad, aged film isn't so bad after all and that actually, you prefer how it looks.
Back to the question at hand, is latent image degradation the cause? Quite possibly, but other factors could be at work here. My choice of a highly dilute developer may not have been the best one. That bottle of Rodinal has aged one year as well. It's keeping properties may be the stuff of legend, but it doesn't last forever. At such high a dilution, any change in developer activity could have an adverse effect where you need it most, in the shadow areas. So here is a developer takeaway: If you have a roll that has waited patiently for three months or more, choose a developer more suited to holding shadow detail, a developer like Ilford DD-X would be a great choice.
One last takeaway: Shoot Pan-F, it's great. Develop it when you can, but don't stress yourself. You may find yourself pleasantly surprised.
If you do a similar test yourself, I'd love to see your results.