Question About Metering With Variable Aperture Zoom Lenses

[bwcolor]

I'm posting in this forum because I don't have any variable zooms except in SLR.

How do I use an incident meter with a variable aperture zoom lens? Specifically, I have a C/Y mount Zeiss 100-300mm that changes aperture as it zooms. I know that the maximum aperture decreases as I get towards 300mm, but I don't know how to translate this from external meter to the range of apertures on the lens when at the long end of the zoom.

 

[Ronald M]

They maintain the set aperture if it the same or smaller than the widest opening at the longest focal.

If you set it to max aperture at wide and zoom to long length, the camera will show the max available at the selected length.

In short, translate from incident ,meter to camera as long as the camera shows it to be available.

 

[sevo]

I'm posting in this forum because I don't have any variable zooms except in SLR.

How do I use an incident meter with a variable aperture zoom lens?

There haven't been many zooms for non-SLR interchangeable lens cameras - indeed I can't think of any other than the one zoom for the Contax G2, if we exclude the recent EVIL cameras and video.

You'll have to go with the manufacturers tables if there are any in the manual - which they often are not. In the early days of zooms, when external metering was still common, few zooms had variable aperture (the very first one, the Zoomar, already was constant aperture), and by the time all their cameras had internal meters and variable zooms became very widespread, most makers did not include tables any more.

In theory, speed might progress by some bigger than linear rule, proportional to focal length - but it is hard to tell which exponent should be applied, and you'll be close enough if you assume linear rule.

 

[tlitody]

F4.5 to F5.6 is a tad over 1/2 stop. Well you know what it is is at 100 and you know what it is at 300. And if you set lens to 200 you can assume that its roughly half of a half stop differnece so only 1/4 stop. If you are closer to 300 then assume 5.6 and if you are closer to 100 assume F4.5 and if you are dead on 200 then is a 1/4 stop really going to make a significant difference? Very little I think so its really not worth worrying about except when you are dead on 200 and even then its marginal. But in that case go for the over expsoure as you will still get the shadow detail. And if you want to be lazy just always assume the smaller aperture and base exposure on f5.6 cos at the worst you'll only ever get 1/2 stop over exposure by basing time on f5.6 instead of f4.5. Except if you are using positive film in which case base exposure time on f4.5 so that you get slight underexposure instead.

[edit] note I made mistake in original and have corrected it above

 

[sevo]

They maintain the set aperture if it the same or smaller than the widest opening at the longest focal.

Few do - it (as well as constant speed) was common while internal metering was still rare, but zooms did not really take over until internal metering had become standard and obsoleted these complex mechanics. The same action returned camera side as apertures became generally electronically controlled, provided that you set the aperture on-camera rather than on the lens. But don't expect the bulk of classic manual focus zooms to act like the one or any metal age manual focus camera to act like the other.

 

[rxmd]

I'm posting in this forum because I don't have any variable zooms except in SLR.

How do I use an incident meter with a variable aperture zoom lens? Specifically, I have a C/Y mount Zeiss 100-300mm that changes aperture as it zooms. I know that the maximum aperture decreases as I get towards 300mm, but I don't know how to translate this from external meter to the range of apertures on the lens when at the long end of the zoom.

Your lens is a f/4.5-5.6, yes? In that case I wouldn't worry too much to begin with, as tiltody has pointed out.

If you actually need to know the aperture precisely, you could mount the lens on a DSLR that knows nothing about the actual aperture, set the lens to, say, f/8 at the short end and take a couple of pictures of a gray card at various focal lengths. That way you can make a translation table for your particular lens. See if it's more or less linear - it probably isn't - and if it isn't, try to see at which focal length it gets close to the maximum, to give you a rule of thumb for translating the meter readout.

 

[tlitody]

Also, the apertures are true for infinity focus. But rarely do you focus at infinity. As you focus closer the lens extension causes effective reduction in aperture size so that 1/2 stop over exposure will actually be less than a half stop over exposure the closer your focus. So it is actually a good thing to base exposure time on the f5.6 setting as it stops any under exposure due to lens extension.

note that when using in camera metering the adjustment for lens extension is automatically catered for by the meter but I guess you figured that.

 

[ampguy]

Here is what Thom Hogan has for the Nikon 70-300 AF-S lens:


Focal length impacts aperture as follows:

70mm f/4.5
100mm f/4.8
135mm f/5
200mm f/5.3
slightly over 200mm and up f/5.6

Yours will likely be different, but perhaps someone who uses the lens on a body with ttl meter can come up with similar data for your lens.