When you are shooting a zoom lens that is F2.8 all the way through the range of the lens, your camera will focus fast due to focusing at the widest aperture available of F2.8. So if I am shooting a lens that is say 18-55mm F3.5 to 5.6 does the camera focus at 3.5 all the way through the zoom range or does it change as you zoom out and the F-stop changes to f5.6 with the longer zoom? I just want to get this clear in my mind.
Need someone to explain something to me.
- Thread starter grandpaw
- Start date
I realize that the maximum opening of the lens changes for that mm but what I am wondering is does the camera still focus at the widest f-stop of the lens or does the camera focus on the widest opening available at the mm that you are zoomed to?
Will your camera focus at F3.5 regardless what position you are zoomed to or will it change as you zoom out.
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It will focus......... as long as the subject is at or beyond the minimum focus distance the lens is capable of, regardless of the aperture setting.
I appreciate both answers but that is not what I am trying to determine.The faster the lens the quicker and more accurate the camera will focus due to having more light to see with. What I am trying to find out is, does the camera focus slower as you zoom out and the max aperture gets smaller on the lens or does it always use the widest f-stop on the lens regardless of how far you have zoomed out and changed the maximum f-stop at the given mm of the lens that it is zoomed out to?
No offense, but I do not believe this is correct. Most zoom lenses have mechanical limitations, for instance barrels when zooming in, that keep the aperture blades from being able to use the widest position regardless of zoom length. In other words, a Tamron 150-600mm f/5-6.3 (had one in my hands yesterday, which is why I use it as an example) will have an aperture of f/5 when focusing at 150mm and an aperture of f/6.3 when focusing at 600mm. Less light is available at the long end, and the focus gets slower.
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So you're asking if the AF module 'slows down' due to the lens being zoomed out to it's longest focal length and it is at f/5.6, as opposed to being zoomed in to minimum Fl and it's at f/3.5?
Under some low-light situations, yes. But that depends on the light level, the contrast in the scene and the AF module in the given camera.
It just seems to me that if the maximum aperture gets smaller there will be less light coming in through the lens regardless how much light is on the scene, and if there is less light it would not focus as well. I am not arguing with anyone, I am just trying to understand how it works.
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It's not just the aperture that affects the performance of the AF module. It's how much light there is to start with. Obviously, there's a LOT more light when shooting polar bears in the snow in broad daylight compared to black cats in coal mines at midnight.
Variable aperture zooms usually have less light coming in at longer focal range. This does not mean the aperture blades are closing, it just means the actual change of lens elements arrangement is allowing less light to come in even with the aperture open all the way.
So in a real life low light situation, the slower lens (5.6 compared to 2.8) should take more time to achieve focus or might even mis-focus.
So in a real life low light situation, the slower lens (5.6 compared to 2.8) should take more time to achieve focus or might even mis-focus.
Thanks Marcel, that is what I was thinking but wasn't sure.
Moving from zoom to a similar decrease in light, this time from focus distance:
Macro lenses are notorious for extending way out during close focus and the amount of light captured at the sensor/viewfinder goes down, a lot. But the f number (as advertised / spec'ed) stays the same due to wrinkle in the definition.
Macro lenses are notorious for extending way out during close focus and the amount of light captured at the sensor/viewfinder goes down, a lot. But the f number (as advertised / spec'ed) stays the same due to wrinkle in the definition.
I was hoping someone would comment on your post because I have no idea what it means.
Let's say you are using a 105mm f/2.8 macro lens. You'd expect to always be able to shoot at f/2.8 for everything (if you choose to do so), but that isn't always the case. Depending on how close you focus, the widest aperture sometimes becomes f/3.5.
My post was a bit off topic, just that a zoom is often sold with a specification that acknowledges the f stop changes as a function of zoom.
A macro lens also changes it's effective f stop as focus distance changes but focal length stays constant. Macro lenses are sold with just one f stop advertised. (and the EU and US Federal Trade Commission have not problem with this marketing)
Weird I know, but legal given the scientific definition of f stop.
A macro lens also changes it's effective f stop as focus distance changes but focal length stays constant. Macro lenses are sold with just one f stop advertised. (and the EU and US Federal Trade Commission have not problem with this marketing)
Weird I know, but legal given the scientific definition of f stop.
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