DSLR f.stop aperture
I have just bought a new Nikon D3100. The camera has 1080p HD movie mode and I'm currently experimenting with depth of field created by choosing a small f.stop. In some conditions my camera restricts me from choosing the lowest f.stop even in full manual mode, how do I allow my camera to use the widest aperture possible? Sometimes it allows me to go down to 3.5 but sometimes I can only choose 5.
Your level of zoom on your lens will restrict you from stopping down too far.
You will be able to select the widest aperture (lowest number) if you do not zoom your lens.
This is about your lens not the camera. The lens you have is what is called a variable aperture zoom lens. It has the largest aperture (F3.5) at widest setting (18mm). As you zoom out the widest aperture gets smaller so that when zoomed all the way out you are at F5.6.
The only way to change this is to buy a new lens. One that has a constant aperture throughout the zoom range. These are not cheap though. Constant aperture zoom lenses will cost more than your camera did.
You probably have a zoom with a maximum aperture that changes with the focal length.
The 18-55mm f/3.5-5.6 is such a lens. Max aperture at 18mm is 3.5 but max aperture at 55mm is 5.6.
There are zooms with the same max aperture at both ends of the zoom's focal length but they are more expensive.
The above answers are correct.
To use selective depth of field, you may need to buy either a very fast prime lens like a AF-S 50 mm f/1.4 or older AF 85 mm f/1.8 and use the 85 mm in the manual focus mode (both cost under $500)
If you need a zoom lens, the 24-70 mm f/2.8 or 70-200 mm f/2.8 will do what you want, but they are rather expensive.
It appears no one has yet explained why this happens. When a lens is zoomed in it constricts the amount of light that can reach the sensor because of the barrel length of the lens.
David is correct. The only type of lens that doesn't do this is a prime - meaning one focal length.
That f/stop will always be the same.
I would also check the ISO settings. In the past I have had the same issue, and I have changed the ISO to something higher and then I was able to increase my f-stop.
First of all, make sure that you can actually view and edit Full HD videos on your PC. If not, choose HD.
When your 18-55 is at 18mm, you can go to f/3.5, but at 55mm, the widest you can go is f/5.6. This is with all lenses (there are some zooms with constant aperture too!)
Here's a section of tutorials - http://www.the-dslr-photographer.com/2009/11/tutorials.html
The lens you have is a 'ramped zoom'; that is, its max aperture varies from 3.5 to 5.6 according to the zoom length. That is the only way that Nikon (or any other manufacturer) can make a reasonably priced 'kit' lens that comes with the camera. Just use the smallest number available for max aperture and thus minimum depth of field.