For the most part, I would say time & money are the main two constraints.
Have you ever wondered what it would be like to be
Annie Leibovitz, for example - and know that you have got 500,000+ to play with, for example, to produce one or two images!
For her, I reckon money isn't really the issue most of the time (which is actually quite scary because then you don't have any excuses), it's time - because she may only have the subject for a few minutes.
No constraints = No excuses... which is actually a scary thought.
There are only a handful of people in this position world wide. And those people are in that position, not because they are necessarily and exclusively great artistic photographers, but because they are great managers and organisers and leaders who have demonstrated time and time again that they can deliver the Big Money Shoot under intense pressure.
Fortunately for most of us, we are not in that position. Instead, photography is not our profession, but our passion and we are not driven by time and money and accountability constraints, but by other constraints.
I agree with Jim that constraints of one kind of another, including self-imposed constraints (one lens photography, photography within the boundaries of a self-created project) can be a great driver to produce better work.
This is because as human beings we have the tendency to take the easy option, make the obvious shots, go for the low hanging fruit. Tough constraints remove the obvious options and prevent us from being lazy. Suddenly there are no obvious shots. Oh no, what am I to do!
Look, think, stretch, create.
Constraints can help us by forcing us to dig deeper.
Where I somewhat disagree with Jim is that casually wandering around can't yield good returns.
I think (for some people at least, it can). It's all about getting into the zone of concentration and observation.
I find that working in this way produces images that I could never have planned or imagined or worked out in advance. Unexpected opportunities present: they may be something as simple as a crack in a wall, a serendipitous patch of dirt you can turn into an image, a fleeting arrangement of shadows - impossible to predict, but which you can take advantage of if you can get into that hallowed state of focused observation. "In the zone", some people call it.
I think both approaches have their place.
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2024: Awarded Royal Photographic Society LRPS Distinction
Photo of the day:
https://www.whisperingcat.co.uk/wp/photo-of-the-day-2025/
Website:
https://www.whisperingcat.co.uk/wp/
DPReview gallery:
https://www.dpreview.com/galleries/0286305481
Flickr:
http://www.flickr.com/photos/davidmillier/ (very old!)