Bars are often dimly or dramatically lit, with lighting for bands, DJs or dance floors. These are designed to look good to the human eye, which is far more sensitive and adaptable to low light than most cameras. Bringing more light to the scene is possible, by using a flash for example, but this might destroy the ambiance by overwhelming the scene with light, so a bar photo requires managing other aspects of exposure -- shutter speed, camera sensitivity, lens aperture and focus.
Using Shutter Speed
Less light requires slower shutter speeds. This means motion may occur while you're taking the picture. While subject motion may enhance a sense of action in the final photograph, camera motion usually does not. A tripod is the ideal solution for camera stability, but this is usually impractical in a bar setting. A monopod is more portable but may still be and out-of budget solution. Supporting a camera underneath with your left hand avoids lens tipping and bracing your camera against solid surfaces such as door frames may add additional protection against camera shake. If your camera has an image stabilizer feature, turn it on to help compensate against camera movement.
Contemporary digital single-lens reflex cameras with extremely high light sensitivity are now available, making hand-held low-light photography easy. However, it, too, is a high-cost alternative. Setting your camera's ISO control to its highest rating allows for fastest shutter speeds in low light. The automatic ISO option on some cameras may not use the maximum value, so set this manually. Point-and-shoot cameras with limited controls often have a low-light program, so select this when taking pictures in a bar. The trade-off with high ISO is high noise, however this may add to the mood of the picture.
Wide Open Lenses
The aperture, or f-stop, of a lens is variable on many cameras. A smaller number corresponds to a larger opening, so an aperture setting of 1.8 permits more light to pass through the lens than a setting of 5.6. Zoom lenses, while great for composition work, have smaller maximum apertures than fixed lenses. As with shutter speed, point-and-shoot cameras adjust aperture as part of low-light programs. DSLRs often have these programs as well, so you have the option of changing settings yourself or allowing the camera to change these for you.
While focus has no appreciable effect on exposure, it has much to do with the success of your bar pictures. A complicating factor is that autofocus systems don't work as well in low light. Choose a single-point focus mode if you have the option. Your camera may have a focus assist beam, but on some cameras, this is done with the on-board flash, and it may be disruptive. Experiment with autofocus settings before heading to the bar.