Howdy JW, Thomas!
How long did it take? Several years to get a good grasp. But 5 years from now, we will all be 5 years older anyway. May as well have learned something new, eh?
Why? I did it for a living. Technical excellence is required when running a photo lab - and also as a photographer. (well it should be, I processed enough -- other stuff -- to know that's not always the case.

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Does it make a difference? Absolutely, positively maybe! Is one better than the other. YES! sometimes. When it is it is and when it's not it's not. really.
Subjective?? Many times I would give a big no on that one. Highlight AND shadow details either there or they are not. Able to reproduce within the limitations of the end product - it's a go or a fail most of the time. Very narrow margins of error for really good reproduction. Depending on what your trying to achieve, what is difficult with combination film developer A is a piece of cake with combo B.
With final products being ART however, its up to you - are you happy with it? Then its perfect. And also subjective to each viewers discretion. But the artist makes the call in the final product.
Does the developer choice make a difference? One may render the highlights as needed or envisioned, and one may not - when the mids and shadows are where you want them.
If you learn to "see" like hp5+ and D76 with such and such a paper - then everything is already couched in those terms before you make the image. So one combo will care of you if your inner vision is already tuned to those results
If however you see the image as you would like it, and then have to figure out how to get from what you visualize to what you want on the paper, you may find yourself thinking of the paper first, and then working backwards to the film, developer combo - and then to the final shutter speed and aperture and camera movements to get it from your brain to paper.
Both methods are valid, but one has more variables and can produce a wider range of effects. The only limiting factor is not so much the availability of materials, but the knowledge of applying them to be able to create a desired effect. I had to be able to take an art directors ideas, and turn them into what they wanted. They were not coming to me for "my style" They already had the ideas. They brought them to me, showed me examples of work they liked, and I had to duplicate or pull off the request to match THEIR vision. Which can be quite a challenge some times. To have only one set of materials would have resulted in a failure to deliver much of the time.
An example would a scene with fog or mist may feel wet, fluid, moist, or it may just be a dead resolution cutting flat grey. The highlights can be subtle and feel real, or devoid if texture. If you wanted one of those traits and got the other, then maybe it could have been done better by using a different developer or film type. The developer can make that much of a difference, on some subjects. But that does not mean you cannot get that effect with X vs. Y. it just means that when You print for the highlights, the shadows and mids may not be where you really wish them. And you may have to resort to dodging, burning, masking, etc. to get that print to "feel" a certain way when processed with developer A, that may have been a straight easy print with developer B. I spent a lot of time "fixing" peoples prints that would have been easier to have simply shot and processed differently. ( not to mention cheaper! ) HP5+ and D76 wont give the same results as pan f and D76, or fp4 and <insert name of developer here>.
Sure certain artists / photographers only shoot one film / developer. It's OK. They may use if for their "trademark look" or whatever. But nothing wrong with learning several tools either. Some images just don't fit into a "trademark look mold", imo.
-- Imagine Classic Radio Announcer voice, subbing for a DJ on a classic rock station. "That was Motley Crue and his Orchestra performing "Shout at the Devil" opus 14 in G minor... " -- Or-- maybe it's better to just forget that image..
One film and developer is certainly not a "beginner approach". In my experience, beginners tend to hack around at everything for while, and master little or nothing, getting a taste of everything, and trying to emulate the work they admire. They tend to purchase the same equipment as the artists they wish to emulate, hoping to find that magic lens or camera, or film -- ( Hey, If I have the same kind of Pen or typewriter used by Hemmingway, I will then be able to write like Hemmingway! ) ... then they get tired, ask for advice or read great advice on places like APUG - and learn good shooting and darkroom practices, and rapidly improve.
JW - the idea that much of what you do is "not worth a tinkers dam" is likey far from reality. Not ONE of us came from our momma's womb knowing squat about any of this. There is a learning curve for every endeavor, and everyone has to go through it. Where anyone is on the curve, has no bearing on its value - or the value of photography for them. Are you having fun? Lovin it? Then it serves you well. And is not in any way invalid.
I remember well when I first started out. I had more fun during those years than the more disciplined years after I was doing this for a living. Matter of fact, I got burnt out as hell doin this for a living. So just enjoy it and have a ball. Take photos - make prints. It will come. It will come faster and easier if you get consistent.
Thomas, It sounds like you are set - you know what you want and how to achieve it, which is a great accomplishment in and of itself. Many would benefit to do it just as simply and consistently as you.
But as long as you use one film and developer - your limitations are are set by the manufacturer and based on the emulsion and developer - that much beyond the imposed design specifications - do not function as well as some other combination may be designed to work perfectly at.
So if one wants to use one developer - D76 and Tri-X for example, that's a superb choice. I am just saying that having a bit more in your tool chest that you know how to use opens even more possibilities, it does not take away anything from the artist that he or she already possess.
Having a tool chest chock full of every film and developer ever made, and not knowing ANY of them well -- is not going be be better than knowing one well.
Poorly chosen or used, they are all equally capable of delivering poor results. ( I had a "silver recovery pile" that proved that point. ) Well used, most can do what is required, in most circumstances, but there will be places that they will come up short. Pan F is not a good sub for Tri-X and visa versa.
As an extreme example, don't use tri-x as a lith film, and anyone who ever used tech pan as a continuous tone film knows what a PITA that was to get a low enough contrast that worked as expected. Superb when done correctly - but it took the correct developer and a bit of work to do right. D76 just was not going to cut it. Does that make D76 a bad developer? No. Its great. One of my favorites. Consistent, long lasting, good grain - a real gem for sure. If I had to settle on only one, it would be a top contender for sure.
Film speeds? Meh. Its just a number. It matters not one bit. What matters is the amount of light reaching the emulsion, creating a latent image that the developer converts into an image that has the correct contrast, density and curve to print in the selected paper in the desired fashion. What numbers one uses are just a reference for THEM. It may or may not match what others use. If someone uses my numbers, they may get great results, or be under or over exposed for their developer, agitation, water, temp, ( and what their thermometer tells them so they THINK they are at such an such a temp ) etc. Just use what WORKS, the rest if fluff. Substance and results over flash.
As I said, superb thread. I like the simplicity of using one film dev for certain things. But know that there are times that an image just really demands something different, to be able to get it on paper in the manner I see it.
Warmest regards,
Blaine