Technical question on lens apetures and apeture settings

[DoleWhipDVC]

I'm a bit confused by the difference between the stated apetures on a lens and the ability of a camera body to shoot at different apeture settings. If a lens (such as a zoom lens) states 4/5.6 I know those are the limits of the lens at the shortest and longest focal lengths. But if the camera body allows for adjutments outside of those limits, say you set the apeture at 2, does that mean the lens is not capable of achieving that setting? In other words, is it not possible to shoot apeture settings outside of the manufacturer's lens specs even if the camera allows different settings to be indicated? Perhaps a rather silly question but I know some of the experts out there have tricks..... Thanks to all who respond.

 

[seashoreCM]

If the lens is rated at X aperture say f/4 then you cannot shoot at a bigger aperture.

If the adjustment dial is on the camera body and links to the lens you choose to put on, then dialing an aperture outside the range of the lens does not expand (or contract) the actual aperture any more. In some cases you won't be able to turn the dial that far.

If you put a lens for a film camera (or for a digital camera with a different size sensor) on your digital camera body, you use the aperture setting as-is; you do not use the crop factor or any other extra calculations to come up with the aperture setting. Except some lenses may require a different scale for example a quarter turn of the dial is marked f/2.8 to f/22 but a quarter turn for a particular lens may correspond to f/4 to f/16.

 

[pjacobi]

http://en.wikipedia.org/wiki/Aperture#Maximum_and_minimum_apertures


-Paul

 

[handicap18]

There are no tricks. If the lens says 4/5.6 then you are correct about the widest aperture at the widest focal length. So if the lens is say an 18-50mm and its aperture range is 4/5.6. That means that at 18mm the widest the aperture will go is f/4. As you zoom up to 50mm that widest aperture will shrink down to f/5.6. Now, you can us smaller apertures, probably down to f/22 or f/32 depending on the lens. There is no trick to make a lens get larger than it will allow. Its impossible.

The most confusing thing about f/stops, is that they are fractions. People will get confused by the big numbers. If 1/4th is larger than 1/8th, then f/4 is larger than f/8.

If the widest your lens will go is f/4, then thats it. You can't get f/2 out of it. The camera body is limited to what the lens limits are.

Say you have a 50mm f/1.8 lens on your camera and you set the aperture to f/1.8. Then switch lenses to an 18-50mm and have it at 18mm and the widest aperture for that lens is f/4. The camera will automatically go to f/4 because there is no f/1.8 available.

If you have that 50mm f/1.8 lens on your camera and you set the aperture to f/8. Then you switch lenses to an 18-50mm. The aperture will stay at f/8, because that 18-50mm lens has that size aperture available.

I hope doesn't confuse you more.

 

[photo_chick]

The f in f/aperture is focal length. f/aperture = diameter of the diaphragm

Like said, you can't make the aperture physically larger than it is. DSLR's and electronic focus SLR's have no actual numbers for aperture on them, just the LED screens and what's shown is what the camera gets from the lens. So a camera LED should not ever display an aperture that the lens, if it is in proper working order, cannot achieve. Older manual lenses have the aperture ring on the lens, not on the camera.

 

[Groucho]

The question is a good one if you consider older variable-aperture zoom lenses with an aperture ring. For example, my telephoto zoom from my K1000 days was a Quantaray 70-210mm F4-F5.6. You can set the aperture ring to F4 no matter what where you zoom it.

Furthermore, it has an "A" setting on the aperture ring, which means that it transmit aperture information to the camera - so when I connect it to a DSLR, it meters perfectly, just like a new lens. However, it does not have the ability to relay focal length to the camera. All the camera knows is that it can go from F4 to F22 (and whatever focal length I give it for image stabilization.) So, yes, I can zoom it to 210mm and tell the camera to take the photo at F4, even though the lens is technically not capable.

I just did a quick test to see what would happen. I mounted the lens and went into Manual mode and took two identical photos, same shutter speed and ISO, but one at F4 and one at F5.6 with the lens at 210mm. The verdict? Looking at the histogram, the F4 one is brighter, but not by much. So yes, you can "cheat" and get a litle bit extra in this case, but it will depend on the lens and it won't be much and it will probably be underexposed anyway, as the camera is expecting a certain amount of light to come in, which is less than really will be. FWIW the same shot at "true" F4 is brighter than the 210mm set for F4, as you would expect.

 

[photo_chick]

I hadn't thought about it from the perspective of putting manual lenses on a newer camera. Pentax users get to have all the fun!

 

[DoleWhipDVC]

Thanks to everyone. This is what I suspected, but I wanted to be sure. Now I just have to wait until my new zoom lens get delivered from amazon....