Background:
The NEX camera knows what native SEL lens is installed and the software knows where to set the minimum shutter speed depending on the selected shooting mode and the focal length of the lens. But when a manual focus lens is installed via an adapter, the camera does not know the focal length information and I think it will often set the shutter speed at a default of 1/60 sec minimum for hand held SCENE shooting modes - no matter what lens is installed. This speed will obviously automatically increase (and the ISO will automatically decrease) in very bright environments.
Sometimes it is better to go manual exposure and set the shutter speed yourself, but setting your camera to the fastest shutter speed and highest ISO is not always possible, necessary, or desirable. I was taught a simple guide from the old film SLR days... to use the focal length of the lens as the "minimum" hand-held shutter speed. Knowing the minimum hand held shutter speed for each specific focal length is very useful for practical and creative reasons.
Guide for old SLR:
• If I'm using 28mm focal length, minimum shutter speed is 1/28 sec (but no such thing, so closest is 1/30 sec shutter speed). With a steady hand, I will get sharp images with a 28mm lens as long as I don't go slower than 1/30 sec.
• If I'm using 210mm focal length, my minimum handhold speed is 1/210 sec (but no such thing so closest is 1/250 sec shutter speed)... any slower, I risk camera shake and can ruin the sharpness of my image even if it is in focus.
Going below the 'minimum shutter speed' is tripod territory. Of course a shutter speed higher than the minimum is better for hand held photography but this is not always possible because you may want a small aperture to increase your DOF - or maybe the scene is too dark for higher speeds.
Also sometimes you might want to do a 'panning' shot. A panning shot is when you follow a fast moving subject to freeze its action but at the same time you want to blur the surrounding background to depict motion in your composition. If your shutter is too fast, the background will not be blurred. If your shutter is too slow, your subject will be blurred as well.
So it is always good to know what your minimum hand held speed is... and this old SLR guide is easy to remember.
Guide for Sony NEX
Because the focal length of a legacy lens is changed when installed on a NEX camera via an adapter, this guide is no longer applicable - although the principle remains the same. We know that the true focal length of an adapted legacy lens is multiplied by a factor of x1.5 so we have to make a slight adjustment to this guide.
• 28mm in your Vivitar zoom is now a 42mm on your NEX, so your minimum shutter speed is 1/42 sec (use 1/40 sec).
• 210mm in your Vivitar zoom is now a 315mm on your NEX, you your minimum shutter speed is 1/315 sec (use 1/320 sec)
You will notice that if you compare these speeds to the old SLR guide, that the speed is just one click up on the NEX camera. So all you need to remember is to use the old SLR guide but select the next higher shutter speed on your NEX by one click. Easy to remember for any kind of adapted lens, zoom or prime!
Once you begin to use this guide in real life, it becomes instinct and you begin to make these adjustments on the field without even thinking about it. It is a very satisfying feeling when you get it right.