@ravan1011 wrote:Sorry if I'm being stupid, but I got a new lens, a tamron 70-300mm, f/4-5.6. I'm wondering if the infinity symbol means it has infinity focus or do I have to do something to get it to focus? I'm new to photography and this is probably me being stupid.
*Infinity focus for astrophotography.
Are you having trouble getting the lens to focus at far away subjects?
I am not familiar with that particular lens, but for a few different reasons, modern AF lenses do not have a hard stop exactly at the infinity position. Most will go slightly past infinity which can result in out of focus photos. You must use AF, or carefully manually focus before the hard stop, to get in focus photos.
I am not sure which version of the Tamron 70-300 you have.
This is a quote from the manual for the Tamron AF 70-300mm f/4-5.6 Di LD
"At infinity, make sure the image in the viewfinder appears sharp. The infinity position is made with certain allowances to insure proper focus under a variety of conditions."
This is a quote from a review on the Tamron 70-300 VC USD: "The focusing ring ends at soft stops at the close-focus and infinity focus points, and will focus past infinity."
@ravan1011 wrote:Sorry if I'm being stupid, but I got a new lens, a tamron 70-300mm, f/4-5.6. I'm wondering if the infinity symbol means it has infinity focus or do I have to do something to get it to focus? I'm new to photography and this is probably me being stupid.
*Infinity focus for astrophotography.
Welcome to the forum.
Infinity is a concept, not an actual distance. Most lenses do not have a hard stop at infinity and some will actual go beyond infinity in the sense that as you focus you can go from out of focus, then in focus, and if you keep turning in the same direction you go back out of focus.
The most accurate way to focus for sky photography is to use the LCD in LiveView and adjust to get point stars or focus on a distant object like a far tree or a mountain top and then switch to manual focus (if you were in AF).
For either approach swith to manual focus and then tape the focus ring with gaffer tape so you don't inadvertently shift focus.
Most folks use a wide angle lens for astrophotography unless they are into deep sky or the moon.
For your lens the concept is the same, but 1. you will have a narrow field of view and 2. unless you are trying to photgraph star trails around the North Star your shutter speed will need to be very short (to get point stars) - something on the order of 3-4 seconds. At that SS your ISO will probably need to be beyond the range of your camera.
Here is an example of point stars - 13 seconds with a 24mm lens, ISO 3200.
This is star trails - 20 4 minute exposures blended together. Typically you want 60+ minutes for good trails.
You can look for info via google comparing field of view for various focal lenghts and estimate how narrow your filed would be at even 70mm.
