This came up the other day on the photo discussion list at work, where one of the guys pointed out one of the many millions of flame wars on the subject and stated that there had to be one correct answer to the question of how the 1 over focal length rule (meaning if you have a 50mm lens, you need to have your shutter at least at 1/50) applies to digital SLRs with crop-factor.
My response? "No, there is no correct answer"
Now, you should recognize that a percentage of my posts here come from when I start writing a reply on some forum to a thread and then realize that I might as well just post it on a blog. And I found that once I started to list off the caveats and my personal opinions on the subject, I had a lot of ground to cover...
The long popularity of the 8-perf 35mm film format for cameras and the accumulated compromises involved in the construction of the early 35mm rangefinders and SLRs has left us with considerable mental baggage that simply doesn't work anymore. My daddy taught me the 1 / focal length rule when I got my 50mm lens and my Canon TX, and also that the 50mm was the normal lens and I still sometimes need to work things back to the 35mm focal length. And so, in my earlier days of photography, I'd slavishly adjust shutter speeds downwards until I'd hit 1/60th and then I'd put the flash on if that wasn't enough light. I wasn't thinking, so I've got tons of shots that could have been greatly improved by superior technique.
But this 8-perf 35mm brain-drain is hard to shake. I still sometimes end up needing to back-convert something to 35mm terms. This is really easy with P&S digital cameras, all of which have sensors that are marginally bigger than a grain of sand and so none of them really likes to market the small numbers that the camera lens really has (7.4-44.4mm) in favor of 35mm camera terms (35mm-210mm) or even just the super-meaningless 6x optical zoom figure. Either way, people don't talk about actual focal length with P&S cameras.
Oh, and while we're on the subject, a 50mm is actually a little telephoto for the fairly arbitrarily designated "normal" lens in the 8-perf 35mm format. And any time you start talking about different frame sizes, your conversion starts to have caveats depending on if you want to talk about the horizontal or vertical field of view difference and movements.
So, the 1 over focal length rule has tons of exceptions and caveats and slavishly following them is just going to prevent you from reaching a proper understanding of how things work. I have a friend and a few acquaintances who have some problems holding the camera super-steady, so they tend to need a higher shutter speed. I have an RB67, which is vaguely intended as a studio camera that was only marginally designed for off-tripod usage. My 50mm lens on the RB67 is actually about the same field of view as a 25mm lens on a more normal SLR... so what shutter speed should I use? And my G7 has a image stabilization feature in the lens that lets me take astonishingly long exposures as long as I hold myself roughly stationary. And I also tend to make prints at least as big as an 11x14 and view things on a 21 inch CRT, which is a different restriction on image quality than people who only print 5x7s and 8x10s and view them on a smaller screen.
And, really, photography is sufficiently personal that you can't just treat things that any one photographer tells you as unshakable rules. Just because I've got some tack-sharp shots taken at 1/15th of a second on my RB67 with a 50mm lens or at 1/4th of a second on my G7 at 7.4mm doesn't have any bearing on what you can do. At the same time, at these enlargements, following the rule of thumb can often times still net you images that are not very sharp.
With my RB67, I tend to shoot at a faster shutter speed than the rule would suggest, even at the expense of a narrow aperture or higher ISO speeds. I have a high regard for the level of sharpness of my RB67 lenses while fairly wide open and tend to keep my shutter pegged around 1/125th unless forced to, largely because I like to preserve the ability to blow up my images large. Even though I can produce perfectly acceptable images at fairly low shutter speeds, largely because of the leaf shuttered lenses that make handheld exposures on the Pentax 67 much harder to get sharp. So, in other words, I'm running one or two stops behind what the rule would suggest without compensating for the crop factor of the lens.
But I did say that I can shoot with slower shutter speeds if forced. I'm perfectly able to shoot at shutter speeds that the rule would suggest if you compensated for the crop factor of the lens, it's just that I'm shooting the RB67 because it feels good in the hands and not because I expect to be able to blow up pictures huge. And sometimes I get lucky! A lot of this is dependent on how you hold a camera. I find that I can get a lot of camera stabilization by using various brace positions while holding the camera... but it also depends on what sort of angle I'm trying to capture.
I have some shots that are astonishingly sharp given that I was shooting a 300mm IS lens (with a 1.6x crop factor) at a shutter speed of .3 seconds, which is largely because there's a lot of bulk to the whole setup. And, like I said, with a leaf shuttered lens on my RB67, I can get some fairly sharp shots at low shutter speeds because there's one less thing thumping around in there. People with leaf-shuttered rangefinder and TLR cameras have an even easier time. On my G7, which has a leaf shutter and IS, I find myself able to shoot much slower than the rule, adjusted for crop factor, would suggest, on a routine basis.
By this, you might assume that I'm saying that you just need to have a mental table where you know that you are within some number of stops away from the rule. But even that's not true.
The rule really falls apart when things start to move. I may be able to hold myself still at 1/8th on my G7 and the background will be acceptably sharp, but if somebody drives by, the car is going to be a streak, no matter how still I am and how good the IS system is.
The rule also ignores things like depth of field and lens sharpness. Most lenses are going to be a little soft while shot wide open. You will probably get a sharper image when you stop a lens down, up to the diffraction limit. You may take an image where the point of optimum focus is sharp, but you really wished you had stopped down to get more of the scene in focus. In both of these cases, it may be better for you to risk a little bit of blur in return for sharpness in other areas.
In other words, when you actually get the idea of shutter speed, you won't think of it in terms of a rule, you will just know what the right shutter speed is. And, while it may be similar to what happens if you apply the 1 / focal length rule with a fairly long list of modifiers and caveats, you won't need to sit down and work things out numerically, you'll just realize that you've moved the shutter control to the right spot.
I realized, in retrospect, that I was approaching burnout point on just about everything over the past two months. This is after a extra-long weekend to decompress and drive 1795 miles doing it. I've got a bunch of film from these excursions to develop and process, so that'll be fun. Except that I've got loaded down weekends for the next few weeks.
I shock myself with exactly how much fragmentary content -- even if it's just a subject for a future posting -- I can accumulate. If you consider that I've been posting a lot of photo geekery and not a lot of stories about the places I've gone to and things I've done and models I've shot, you might get an idea of exactly how long the backlog is.
I'm still closer to having the new blogging engine for the site set up, although my schedule's not going to permit me to really get it together until after my next set of travels. But version 2.0 of this site is taking shape on my development server, so it's not going to be long now.