Different people will give different answers, which is to be expected since personal preferences and experiences will affect choices. Nonetheless, there are reasons that most, if not all, large format photographers will cite.
I start from the viewpoint of a photographer who prefers black and white to colour; I’ve given my reasons elsewhere. When it comes to prints (which is to me the point of making an exposure in the first place) I can detect the difference between a photograph taken on film and a digital one. By this, I mean ones where the original capture was on film or digital rather than the means of output; as I scan and print my negatives using an inkjet printer, my prints are all digital. I simply prefer the “look” of an image that began on film. I could attempt to explain why I think the two sources give subtly different results in the print, but ultimately it’s down to my perception of the characteristics of the image. So, I start from a position of preferring to produce my photographs on film.
But why large format, rather than 35mm or roll film? Both are easier to use and carry around than a large format camera. It comes down to the limitation of smaller formats. I started serious photography using a 35mm camera, and after I improved enough to want to see my work larger I found that I simply couldn’t produce a satisfactory (to me) print from 35mm that was larger than about 8″x6″. What is about to follow is an extended discussion of resolution and the degrees of enlargement possible without too great a loss; but you can skip the parts in italics as being perhaps more than you need to know, especially as the limiting factor for me isn’t actually resolution per se anyway – there are other limitations on enlargement beyond simply the amount of recorded detail.
At this point, the laws of physics come into play. There are theoretical limits to the resolution of even a perfect lens, due to an effect called diffraction. Think of it as an effect whereby light cannot be focused to a point, but rather a disk whose diameter increases as you stop down the lens. The effect also depends on the wavelength of the light (colour, to most people!). How much effect does it have? Well, it’s partly subjective, and depends on your standards for what is sharp (as we get to finer and finer details, contrast drops, so it’s subjective to some extent) but if we take the middle wavelength (green) and a reasonable value for the subjective part, the maximum resolution of a perfect lens is 1500/aperture. This equates to about 94 lines per millimeter at f/16, dropping to 68 at one stop smaller. Some people would be extra critical and use 1000 rather than 1500, which drops the figures even more.
It’s generally accepted that the human eye can only resolve 6 lpm at best, but there is a but. A big one. We might not be able to see finer detail, but the eye is better at spotting differences, and tests have shown that even if we can’t see more detail, we are aware of differences from 30 lpm down. A print with that resolution (which happens to be that of a glossy darkroom print) will look better than the same print on a surface that only resolves 20 lpm, as Kodak found when they changed one of their papers from 30 to 20 lpm. People spotted the difference.
It’s not just the lens that doesn’t have infinite resolution. Film also has a limit on its resolution. In general, the faster the film, the lower the resolution. Films change, but as a representative figure the Kodak Darkroom Dataguide of 1988 gives the resolution of T Max 100 as 200 lpm and T Max 400 as 125 lpm. The discontinued and (at least by me) lamented Kodak Technical Pan had a value of 320 lpm.
Unfortunately, the resolution on the film isn’t the resolution of the lower of film and lens resolution. There is an empirical formula that, in its simplest form gives the resolution of the film RF as the solution to the equation
1/RF = 1/r1 + 1/r2
where r1 and r2 are the individual values of film and lens. There is a more accurate version of the Katz equation, but using it gives a lower value of the total resolution. The end result is that assuming a perfect lens, the on film resolution of T Max 100 would be 158 at f/2, 114 at f/5.6 and 63 at f/16.
Now lets assume that our enlarger has perfect lens. It will also be affected by diffraction, and at f/5.6 is going to have the same limit as the taking lens which at f/5.6 gives a value of 268, which is higher than we have on the film. Let’s assume then that we can discount this, and just look at the on film value. How much can we enlarge before we drop below even the lowest value of 6? With a perfect lens at f/2 and T Max 100 we arrive at 26 times enlargement. Drop the aperture to f/5.6 and we reduce the maximum to 19; and stop down to f/16 and we arrive at 15 times enlargement. This sounds like there isn’t a problem? Well, lets step back from the minimum and use 30 instead of 6 for the lpm we’re looking for, since it does make a visual difference in a side by side comparison. That basically results in a drop of maximum enlargement by a factor of 8; so at a taking aperture of f/5.6 we have just over a two times enlargement. Doesn’t sound quite so good, put that way. OK, 30 lpm may well be a bit high, but it still indicates that there are limits. And a 35mm negative is going to hit them at a small print size.
Resolution isn’t the whole story; film also has grain which intrudes into the image. Beyond a certain point, it becomes very visible. Some people like grain; I don’t, and this is about why I use large format, so let’s accept that it isn’t a good thing for me. Empirically, I find that 35mm doesn’t enlarge anywhere near enough for me. And that, by the way, brings in another personal preference – that prints should be 12″x16″ (A3) size as a minimum. The maximum print size I can make that satisfies me from a 35mm negative is about 6″x8″. I did once make a 10″x8″ I was happy with, but by “once” I do mean that literally. I generally work on a factor of 6 times enlargement as the maximum. It’s easy to work back from this to the minimum film size I’d be happy using. The smallest common size of large format film is 5″x4″, which at 4 times enlargement gives a 20″x16″ print – the same degree of enlargement that would give a 6″x4″ print from 35mm.
The other downside of granularity is that areas of even tone can take on a more speckled appearance as the degree of enlargement rises – a lack of tonality and a less smooth transition from one tone to another. Film may be described as analogue, but all the tones in a black and white negative are effectively made up of binary values – silver grain or no silver grain. It follows that the more you enlarge, the greater the chance of the analogue equivalent of pixelation.
That’s basically why I use large format, and would even if there were no other reasons to do so. But there are; and these may be the most important reasons for other people.
More to be added on
Helpful to composition
Process and involvement
Reduced need to worry about technicalities