I'm a little confused with F-stops on telephoto lens

[John M]

How do you go about metering for a telephoto lens when the aperture changes as you zoom? I don't plan on zooming during the shots, but is an f-11 still an f-11 no matter what focal length your using on the lens? I've noticed some Canon EF lenses have a fixe 3.5 or 4 throughout the lens focal length, but I think the camera adjusts ISO and shutter as the focal length changes. This is for video use so the camera is in full manual. Maybe I'm over thinking this but Im confused nonetheless

[David Mullen ASC]

First of all, you mean a zoom lens, not a telephoto lens, which is a prime and cannot be zoomed.

 

Read this discussion:

http://www.velocityreviews.com/forums/t646693-zoom-lens-and-aperture.html

 

Bottom line is that expensive cinema zooms should not change exposure as you zoom (though there is still some minor exposure change near the far telephoto end on some zooms.) Cheaper still camera zooms will change exposure as you zoom since they were designed for taking stills in which case you wouldn't see a zoom happen live with a visible exposure shift.

[Tom Jensen]

Some zooms are faster at the wider end but the lens will be set at the f stop for the longer end so there is no light loss as you zoom. For instance, and this is just example that might not be true at all, a Canon 150-600 is a 5.6. At 150mm the stop might be a 4 but if the lens was set at a 4 and you zoomed in you would lose a stop of light because at the long end you need to be lit to a 5.6 even if the lens is physically opened on the barrel an additional stop. The stop is set at a 5.6 so it remains consistent through out the zoom. At least this is my understanding.

[John M]

So with a lens such as the Canon EF

17-40 f-4, which is a fixed F-4, if I set the focal length anywhere In that range my f- stop will be the same? I.e if I select f-11 then I'm truly getting f-11 anywhere on that lens. I have some old Nikon lenses that get darker as you zoom in. How do you meter and set your stop properly for that?

[David Mullen ASC]

If you are shooting movie film, you don't use lenses that change exposure as you zoom because you can't really meter for that. If you are shooting digital, you can at least see the exposure change and adjust to correct, but even then, you don't want to see an exposure shift happen during a zoom so you avoid those lenses. There are some ENG zooms that are pretty good until the last little bit of the far telephoto end of the zoom, so you learn to stop zooming just short of that point.

 

There's a reason why a high-end cinema zoom is 10X the price of a cheap still camera zoom.

[John M]

So how do you know what your stop is on a changing zoom? I have a Tamron 70-210 zoom thats a 3.5-5.6. I'm not zooming during the shot, but if I'm at the 100mm mark, how do I know what my stop number is? I assume that If I have F8 selected (on the barrel) I'm not really at F8? Unless the lens is wide open? Sorry if I'm making this more confusing than it is.

[Travis Gray]

So how do you know what your stop is on a changing zoom? I have a Tamron 70-210 zoom thats a 3.5-5.6. I'm not zooming during the shot, but if I'm at the 100mm mark, how do I know what my stop number is? I assume that If I have F8 selected (on the barrel) I'm not really at F8? Unless the lens is wide open? Sorry if I'm making this more confusing than it is.

 

If you're in still mode on a DSLR, open the lens up as far as you can zoomed out as wide as you can, and as you zoom in, the LCD will display what stop you're at as you zoom.

 

There are occasionally some reviews out there where people have documented the stops as well.

 

 

One thing I've noticed though, when I've played around with a variable aperture lens on my nikon D7000, even if I set the aperture at 5.6 (this particular lens' most open at full zoom), it still stops down as I go from wide to tele. I can't speak for Canon since I've only used fixed aperture lenses.

But since the mechanism on a still lens doesn't actually move until you click the shutter, it can display whatever number it needs to and then adjust accordingly upon shutter release. Since when you're in video mode, it has to set the aperture at a specific amount mechanically during live-view, it's stuck at that number throughout recording. And since the f-stop to focal length ratio (not scientific enough explanation) is always changing, unless the mechanism can keep up with that, you'll still have a variable f-stop during zooming. (this is assuming Canon works like Nikon)

[John M]

If you're in still mode on a DSLR, open the lens up as far as you can zoomed out as wide as you can, and as you zoom in, the LCD will display what stop you're at as you zoom.

 

There are occasionally some reviews out there where people have documented the stops as well.

 

 

One thing I've noticed though, when I've played around with a variable aperture lens on my nikon D7000, even if I set the aperture at 5.6 (this particular lens' most open at full zoom), it still stops down as I go from wide to tele. I can't speak for Canon since I've only used fixed aperture lenses.

But since the mechanism on a still lens doesn't actually move until you click the shutter, it can display whatever number it needs to and then adjust accordingly upon shutter release. Since when you're in video mode, it has to set the aperture at a specific amount mechanically during live-view, it's stuck at that number throughout recording. And since the f-stop to focal length ratio (not scientific enough explanation) is always changing, unless the mechanism can keep up with that, you'll still have a variable f-stop during zooming. (this is assuming Canon works like Nikon)

 

This is my dilema. A couple of my lenses are Nikkor and Tamron fitted to the canon via an adaptor. So basically the camera has no idea the lens is even attached. Its pure manual. No f-stop reporting

[Travis Gray]

This is my dilema. A couple of my lenses are Nikkor and Tamron fitted to the canon via an adaptor. So basically the camera has no idea the lens is even attached. Its pure manual. No f-stop reporting

 

Well then...

 

 

 

Look up reviews. Some lenses that Ken Rockwell (nikon reviews) have the info that he recorded, but I'm not sure how those adapters work. I have a F-mount to E-mount adapter and it's just a ring that spins from open to closed, but no real markings, and since every lens may be a bit different, the open or closed (I forget which since I don't use it that often) goes further than the actual stop point (to accommodate lenses with more play in the aperture setting). So, at that point, it's kind of a guestimation game. You'll know if you open it wide open and zoom out it'll be f/4 (or 3.5 or whatever), and still open zoomed all the way in it'll be f/5.6. That's when you may want to take a test image and then check the histogram to get an idea.

 

Or go into still mode, meter what you want, and leave the setting there. Then avoid zooms while shooting to avoid losing light.