Is this photo perfectly sharp? No, but to my eyes it's acceptably sharp--especially since I shot it with a 10omm Series E Nikkor lens at f/2.8 and 1/50 second at ISO 800. That's about as low as I'm willing to go. How about you?
These days practically anybody can get a nice, sharp, well-exposed photograph in broad daylight. Set the camera to auto-everything and the algorithms will take care of the rest. It's when the light gets dim that mastery, skill, and yes, sheer luck come to the fore.
Consider this: In low light--and by "low" I mean light that requires you to shoot at or near maximum aperture AND at a shutter speed below 1/60 second AND at an ISO above 400--you are dancing on the razor's edge of technical quality.
Let's start with the optical challenges. Most lenses are least sharp and contrasty when shot at maximum aperture, especially at the corners of the frame. Another issue is that you have less depth-of-field at maximum aperture, so your focus needs to be spot-on, yet low light is exactly the point at which most AF systems begin to struggle.
Next are the challenges related to camera and subject motion. If you're using a camera or lens that has image-stabilization you have the luxury of being able to shoot at a shutter speed at least two or three stops slower than otherwise. Unfortunately, many of such cameras and lenses have a maximum aperture of f/3.5 at best and f/5.6 at worst, so you're often forced to shoot at slower shutter speeds than you might otherwise. Slow maximum apertures reduce your depth-of-field problems while increasing problems with subject motion. It doesn't do much good if you can hold your camera steady at 1/8-second if your subject is in constant motion.
"No problem," you say, "I'll just crank up the ISO." Your success with the approach will, of course, depend on what type of camera you're using. If you're using a camera of recent vintage with a full-frame sensor you can use ISOs as high as 3200 and 6400 with relative impunity. If you're using a camera with a small sensor, the increase in image noise and loss of resolution can get grim even at ISO 800.
Just for comparision, here's a photo shot at the same ISO (800), but with a 35mm lens at f/2.8 and the shutter at 1/100 second. It's significantly sharper than the photo above. Does this make it a better photograph? I'll let you be the judge.
The final factor is this equation is you, hence the title of this post. How low a shutter speed can you shoot at hand-held and still get results you considerable acceptable with reasonable consistency? In my case it's 1/30 second with no image stabilization, two stops slower with it, and only as long as the lens has a focal length no more than 50mm. I'll shoot at maximum aperture if I have to, but even in low light I prefer to stop down a bit, especially if the maximum aperture is a generous f/1.4. As for ISO, 1600 is generally my upper limit. I find that once I go past that the image gets "fragile," with a lot less tolerance for color and contrast curve adjustments.
All of these self-imposed limits are towards the goal of producing what I would consider to be an acceptable 8x10-inch print--not truly and completely sharp, mind you, just acceptably sharp. Your standards might well be higher if you want larger prints or lower if you only need small JPEGs for screen display. Whatever your standards, don't assume thatbecause you've got great technique in bright light that it will automatically transfer to shooting handheld in low light. The only way to get good at it is to do it and see for yourself just how low you can go before you either have to use a tripod, add supplementary lighting, or give up.
So, just out of curiosity, my question for today is, how low can you go? How low a shutter speed? How high an ISO? How long a lens? And how reliably sharp are your results?