I just made the switch to DSLR. Now that I can swap out my lens, I have been eyeing lenses, but I realized I don't understand how the camera decides to limit your max aperture. I got the d90 with the 18-105 3.5-5.6 kit lens. Most of the time I can't get below 5. How is this determined?
Posted Jun 5, 2010, 7:58 PM UTC
The camera doesn't limit it, its the lens. You will be able to use the aperture of 3.5 only at the 18mm end.
Aperture (f-stop) is the ratio of the focal length to the effective aperture diameter. Lenses with variable apertures are cheaper and lighter (less glass = less light) than lenses with fixed apertures. Cheaper lenses will have variable apertures but some better glass will give you f/4, f/2.8, etc throughout the entire focal range.
Conversely, lenses with fixed aperture like f/2.8 are muy expensive because it's nice not to have to worry about aperture changing when you zoom in or out.
That's not quite right. They're more expensive because there's a lot more engineering and much larger discs of glass involved to make a lens that collects that much (and constantly that much) light throughout a focal range.
Well I wasn't implying fixed aperture is the only reason. As you point out there are other reasons as well that contribute to the higher cost.
Different focal lengths have different max apertures due to the build of the lens.
At the widest focal length(18mm), the max aperture is 3.5 At the other end (105mm), max aperture is 5.6
It seems like you're using the lens zoomed out, which is why it doesn't go below 5.
EnnGeeOh said it, but I thought I'd add that the reason this happens is because the f-number is defined as the ratio between focal length of the lens and the diameter of the aperture opening. For example, if you're shooting with a 100mm lens and the aperture is 25mm wide, then that means you're at f/4. Therefore, if you increase your focal length (zoom in) you need to make the aperture wider in order to maintain the same f-stop. But some lenses aren't able to open wide enough to maintain the f-stop through the whole range, and you're left with a variable max aperture.
Then why are there fixed-aperture lenses with variable focal length? Wouldn't that imply there there is the capability for a greater aperture at smaller focal lengths, but it's unavailable for use?
Fixed-aperture lenses with variable focal lengths have a different physical design: the aperture blade assembly actually moves forward and backwards as you zoom in and out. This type of lens is more expensive to produce, thus the generally higher prices for these types of lenses.
aha
Huh?
here's one, i meant fixed maximum aperture
http://imaging.nikon.com/products/imaging/lineup/lens/af/zoom/af-s_70-200mmf_28g_vr2/index.htm
Ahh, I get it. Thanks!
A rule of thumb I use when purchasing lenses, is I never purchase variable aperture lenses. It's a bit more expensive, but fixed has better glass.
Not only that but fixed is easier to use, especially if you're using manual exposure mode. If you're at f/3.5 and 1/100 shutter speed, and then you zoom out, your aperture might go to f/5.6 but your shutter speed would stay the same, resulting in an under-exposed photo.
Which is very irritating.
Yes. This adds a whole extra element of confusion for new photographers: the people who are most likely going to be using variable aperture lenses because they are cheap kit lenses.
There are also some really good fixed-aperture lenses whose focus doesn't change as you zoom in or out. I believe the Canon 70-200mm f/2.8 and f/4L are two of these lenses.