Kodak Tri-X vs Ilford HP5+ Film

Introduction

Spending some time with Google shows that there are numerous comparisons between Kodak Tri-X (400TX) and Ilford’s HP5+ film. Are they the same? Are they different? Is one better than the other? On and On and On. Let’s take as objective look as possible between the two emulsions and see what the deal is here.

Cut to the chase

If you don’t want to read any further, and just want a fast answer, then here it is: Kodak Tri-X and Ilford HP5+ are so close to each other that I can say that they are totally, completely, one hundred percent interchangeable. This means that you can shoot and process them exactly the same way in the same chemicals for the same development time. The end result will be close enough that you can’t tell the difference.

The evaluation

We are going to evaluate the two emulsions on two criteria: Tonal range, and granularity.

Tonal range

To evaluate the tonal range, we’ll shoot an 18% grey card on each emulsion, and shoot it from 7 stops under to 11 stops over normal exposure in full stop increments with a studio strobe. The exposure will be set via a Sekonic light meter incident reading and shot through a T-Stop rated lens, with the light meter reading within 0.1 stops of the actual amount of light hitting the grey card. To evaluate the density values for each stop of light, the emulsion will then be digitized with a DSLR using a studio strobe through the same T-Stop rated lens.

From there, the raw captures are evaluated and the average sample value from a 256×256 square in the middle of the scanned frame is calculated. This is done for each emulsion. This will give us a good idea of the density level of the emulsion for a given exposure value.

This is done exactly the same way for each emulsion. The camera position relative to the gray card does not change between each emulsion, and the focal point does not change between each emulsion.

Granularity

For granularity, this is actually pretty straight forward. Look at the scan of the correctly exposed 18% grey card for each emulsion in Adobe Lightroom at 1:1. The scans are just over 4200 dots per inch, which is more than enough resolution to actually digitize individual grains.

Development

To ensure that we’re as close as possible for each emulsion, they’re both developed in Kodak D76 1:1 at 20 degrees Celsius +-0.1 degree in the same daylight tank at the same time for 13:00 with 1 fast inversion every 15 seconds. There was a several minute pre-soak at 20 degrees of the tank/emulsions to get everything up to temperature. A 1:4 vinegar/water stop bath was used to stop development. Both emulsions were fixed in Kodak Fixer for 10:00 with constant agitation.

Concessions

Obviously, this is not up to scientific standards, however, it is within the tolerances that I can bring to bear with the equipment available to me, and I feel that my tolerances are tight enough to use with a reasonable amount of certainty in the results.

Results

Below are the results for each item being evaluated.

Tonal Range

Here is the chart of the two emulsions.

400TX_vs_HP5

When looking at this, there’s a couple of things to remember: It’s not the actual values of each density step that matter because those will vary a bit due to variations in the the power of the strobe firing during the exposures, variations of power of the strobe firing during the scanning, and how many specks of dust and fibers there are on the emulsion in the scanned sample area, which will affect the average calculated sample value. In fact, I’ve repeated this test twice exactly the same way and have even done multiple scanning passes of each emulsion for each test and gotten different but similar results for every single density step. This is the nature of the medium. There’s a lot of moving parts and things that can affect the outcome.

The key takeaway here is the shape of the curve for each emulsion. I’ve included a combined curve that is the average of all the scanning passes of both tests for both emulsions with each end slightly extended beyond sampled values.

In short, both emulsions have the same tone curve and tonal range if developed in the same developer at the same temperature, for the same amount of time and same agitation.

Granularity

OK, what about the grain? I’ll let the image below speak for itself. You can right click on it and download the full image to look at it at full size if you want to look at it really close.

400TX_vs_HP5_grain

So, what are we looking at? A comparison of each emulsion scanned in at 4200+ dpi side by side at 1:1 in Adobe Lightroom. The grain structure is readily evident, and frankly, to me, the two emulsions are close enough in their granularity that at sane enlargement levels, they’re nearly if not completely indistinguishable.

Conclusion

With black and white film, the tonal range and granularity are really only the two things that matter, and as I said in the cut to the chase section, if Tri-X and HP5+ are shot and developed the same way, they’re interchangeable in terms of tonal range and granularity.

The Myth of Film Push Processing

“I shot it at 1600 and pushed it 2 stops! It looks great!”

I’m sure we’ve all heard this. It’s largely a myth. All films are within a third of a stop of the manufacturer’s published ISO rating in most developers.

This is extremely true of C-41 color films because the processing is standardized and the films are manufactured to conform to that processing environment. Given that, we’re not going to talk about color films. If it’s rated for ISO 100, shoot it at 100. If it’s a 400 speed film, shoot it at 400. Any lab that is worth anything can give you a wonderful scan of that film.

For black and white films, this is also largely true based on how the ISO standard prescribes determining the speed of the film which is explained very simply:

Film speeds are based on the exposure required to give a log density of 0.10 above film base plus fog (fb+f).

Keeping it simple

Terminology aside, we can say the speed of the film is determined by the amount of exposure needed to get a certain fixed amount of density above film base plus fog. For the ISO standard, that fixed density is 0.10 log density above film base plus fog. You can use a different fixed density level and you can use a different density scale and that’s totally valid except that it’s not to the ISO standard.

Where the confusion comes in is film density in the shadows or minimum density areas of the emulsion develops at different rates depending on the film developer used. The developer used along with developer temperature and time spent developing affects how much density is developed.

Push that film!

Not so fast. You see, depending on the size of the silver grains in the emulsion, you do in fact need a minimum amount of light to hit each grain so that it will develop anything at all. Believe it or not, that minimum amount of light is amazingly close to the ISO rating of the film.

So what does push processing actually do?

Simply put, push processing develops more density for the silver crystals that did get enough light to develop anything. What this does is make the density difference large enough that you can see it and gives the impression that the film is more sensitive than it actually is. In Adobe Lightroom speak, it’s the equivalent of going into the Develop Module and pulling the shadows up without changing the black level.

It does not make the film more sensitive. Once you go below that minimum exposure level, no amount of processing is going to bring more density into the emulsion. All you’ll do is develop more film fog.

So how come some films still look great pushed?

That’s because normally exposed and developed film typically has its film base plus fog at 6 stops below middle grey. That makes for some awesome blacks. If you switched over to the Exposure Index method of exposing, and under exposed the film by a stop, the film base plus fog effectively moves up by one stop to 5 stops below middle grey. This still results in totally acceptable blacks for most exposures. If you under expose by 2 stops (exposing a 400 speed film at EI 1600) the film base plus fog moves up to 4 stops below middle grey. This results in your blacks starting to look dark grey and a little muddy. This is where the push processing come in. More development time adds more density to what shadows you do have which makes the shadow areas have more contrast which to a degree counteracts the raised film base plus fog. There is a point of diminishing returns because once again, there is a minimum amount of light you need in order to develop anything above film base plus fog.

So that’s it in a nutshell. Pushing film is a myth. If you switch to a different exposure standard (like Exposure Index), then you can get a little more performance out of your film, but at the end of the day, the worst thing you can do is under expose your film.