This article was freely available on the internet but I can no longer find a link to it. I hope it's ok to post a copy of the text here.

Ilford ISO400 film: The same but different
Ilford claims to have improved its high technology ISO400 film
Article by Geoffrey Crawley (September 2000)

Whether ASA, ANSI, ISO, EI or whatever, the number 400 has marked a pivotal point in the film speed hierarchy in both b&w and colour. Below it and you are in the safe high quality zone. Above it is the outer region of the super speeds. It is fifty years since the first true ISO400 materials, Kodak Tri-X and Ilford's (uprated) HP3, appeared.

So in basic sensitivity, advances over the past half century, in b&w at least, have not been that spectacular. The ultra fast 3200 emulsions of today use EI rather than the stricter ISO standards to substantiate their claims. What has indeed improved is image quality, notably fineness of grain and, with it, resolving power. As regards tonal quality, the granularity improvements mean that this is retained at much higher degrees of enlargement before obvious image break-up by the grain pattern.

Much is made in promotional material of the advanced silver halide crystal technologies such as Ilford Delta, Kodak T-grain, and the like. Their greater efficiency in trapping incident photons arriving through the camera lens is now well-known. But it appears that many photographers still prefer the more traditional emulsions made up of a span of mainly hexagonal halide crystal sizes. The result, with Kodak and Ilford, is the provision of two lines of b&w materials: Plus X Pan and Tri-X Pan, and Pan F , FP4 and HP5 , respectively.

In a traditional emulsion, latitude is given by a mix of halide crystal sizes. Each crystal is a photo-conductive unit. So the larger ones are 'faster' and the smaller ones, presenting a smaller area to the imaging light are 'slower'. Sometimes the slower emulsion is coated separately from the faster, making a double-coated or bi-pack emulsion.

Uniform in size
In a modern high-tech emulsion, the halide crystals are much more uniform in size, and present a larger surface area to the light, upping the speed/grain size efficiency. But since the speed mix is more restricted, the tendency had been for these films to have less latitude, most noticeably in the under-exposure region, than traditional films, despite their more even image grain. Photographers accustomed to the long curve toes of films such as Tri-X and HP5, giving under-exposure latitude and push process possibilities, complained that the high-tech equivalents, T-Max 400 and 400 Delta, were over-rated in speed.

With the launch of Delta 3200 last year, Ilford came up with a solution to the problem, evolved from 100 Delta. The same principle is applied in the new Delta 400 reviewed here. In fact it harks back to traditional know-how but in an updated guise. Delta 3200 is a bi-pack, and each of the two layers is made up of two emulsions, four in all, of which three are new. The high-tech halide crystals have a silver iodide rich core around which is the bromide shell and sensitisers. The interfacing of the iodide rich core with the bromide shell creates a larger number of possible latent image centres in the shell to trap the incident light. So there is enhanced sensitivity and pushability.

The new Delta 400 is also a twin-layer or bi-pack, but with only one emulsion in each layer. The top layer has the medium speed emulsion from Delta 3200 and the bottom layer has its slow component itself in fact the fast component emulsion from 100 Delta. Light that over-exposes the faster top layer is accurately recorded by he slower bottom layer. The result, as with bi-packs of long, long ago, is exposure latitude and pushability.

A word about the nomenclature over which some confusion could arise. The existing Ilford 400 speed Delta film carries the name 400 Delta Professional on the box. The new, much improved film carries the name Delta 400 Professional, in line with Delta 3200 Professional. This leaves the 100 material as 100 Delta Professional. Is that clear?

The new material's performance was checked in comparison with 400 Delta and HP5 Plus. The developer used was the standard ID-11, D-76 type. Other developers will give their own particular bias to speed, sharpness and grain, but the materials will maintain the same relationship in such properties as shown in these results.

The sensitometric tests at once showed the lie of the land. The earlier Delta material could be rated 200, 400, 800 or 1600. The new allows 400, 800, 1600, and 3200 using Ilfotec DD. The sensitometry confirmed that Delta 400 is faster than 400 Delta, and also marginally so than HP5 . The increase is very much in the characteristic curve's toe contrast, higher shadow or dark tone reproduction densities for the same overall negative contrast or G-bar.

The improvement is quite dramatic and comparison with HP5 , which has much the same toe rendition, shows how far behind the earlier 400 Delta comes, finer grained though its structure may be. The new film has a noticeably lower base/fog level than its ancestor. Processed to 400, 800, and 1600 gave Dmins of 0.38, 0.4 and 0.42, compared to 0.45, 0.48, and 0.54. HP5 gave 0.4, 0.42, and 0.44 again proving the quality of the popular traditional material. As always, push-processing did not elicit deeper tone detail with any of the three, only increased the density, and so the printability of existing detail.

The speeds achieved by push-processing a film are given EI, Exposure Index ratings, not ISO ratings. A film has one ISO speed and only one, arrived at by a rigorous standard exposure/processing procedure. In practice, with suitable developers and/or extended (non-standard) development procedures a film may be exposed as if it were of a higher rating. It is those ratings that are known as EIs.

With the three films, it was found that the EI800 and EI1600 extended development recommendations were most effective with Delta 400 and HP5 , 400 Delta lagging behind. This was only to be expected as extending development only brings up further what is already there with standard development and 400 Delta, as noted earlier, was already behind the other two.

Comparative prints
Looking at the comparative prints illustrating this review, the term 'dramatic improvement' for the new Delta 400 may seem over-stated. The reason is this. A film with inherent high toe contrast will, when pushed, also give higher mid-tone and highlight contrast. As a result, its negatives may often require a softer grade of printing paper. In its turn that means that some but not all of the shadow contrast increase is lost. The test scene was deliberately chosen to introduce this factor. On a low contrast or diffusely lit subject, the shadow density improvement on pushing would appear much greater in a print.

This is why a good 'speed-increasing' developer restrains the highlights while bringing up the shadow detail. A similar effect can be obtained by diluting a normal developer, so tending to exhaust it in the highlights, restraining their growth, while the shadow detail grows.

Nevertheless, the new 400 certainly outclasses the previous both in basic effective speed and in pushability and the quality then obtainable. But HP5 runs it very close. What other advantages are there then to justify using Delta 400 over HP5 ? The grain of 400 Delta is just finer than HP5 and that of the new Delta 400 is finer than both of them when developed to its basic ISO400.

When pushed the advantage is reduced, as might be expected, though Delta 400 remains just that little smoother than HP5 , with 400 Delta noticeably grainier. However, it should be emphasised that granularity is hardly likely to be a problem with any of the three. The same applies to sharpness and resolution. The compact, even and sharp grain of the Deltas just wins out over HP5 and in this respect there is little to choose between them. The better toe, shadow contrast of new Delta 400 at the same overall contrast as 400 Delta will give a better lit feel to the image, which may translate subjectively to better sharpness.

Similarly the best method perhaps to characterise the improvement in Delta 400, used at its basic speed, is a general qualitative one. This is coupled to a much enhanced push capability. Those who have played hard to get with the former Delta film at the 400 speed, staying with HP5 , may well now find themselves tempted to change over to the new version. Aficionados of 400 Delta will certainly want to transpose, with the brand name, to Delta 400.

This speed of film has become the work-horse of b&w photography. With modern high-speed shutters it can deal with bright light and still give hand-held exposures in poor light and for adverse lighting it can be push-processed. The new Delta 400 fulfils those job specifications admirably and better than the earlier film.

First published in BJP 20 September 2000