Sigma UC 70-210mm f/4-5.6 giving weird meterings on Pentax K1000

[Caldas]

I've metered a scene with my sigma UC compact f/4-5.6 and it gives me two completely different meterings depending on the zoom level.

To test it out I compared that metering to my Pentax-m SMC 50mm f/1.4 and Pentax-m SMC 50mm f/2.

At 1/60 f/8 ISO100 with the f/1.4 the meter gives me more or less -1.

The f/2 gives me the same result.

But the sigma gives me

-1@70mm f/8 1/60 and

-1.5@135mm f/8 1/60

-2@210mm f/8 1/60.

Can you guys help me out with this one? Do I meter by what the camera is telling me or do I consider that the meter is off by -1 stop when using it at 210mm?

 

[compur]

It's a variable aperture zoom. That's what "f/4-5.6" means. As you zoom the lens to a longer position your set aperture gets smaller. There is at least a one-stop difference beween the shortest zoom position and the longest. And, most likely, the lens manufacturer "underestimated" that difference so there may be more like a 2-stop variance.

All that means is, when using a manual camera like the K1000, you need to re-meter and possibly re-set your exposure when you zoom the lens.

 

[Braineack]

He's shooting at f/8. Might be more to do with transmission.

Does the exposure actually change on zoom?

 

[Caldas]

When I zoom in the dial remains at f/8 but the meter on the pentax indicates that the exposure is now -2 instead of -1. So what you are saying is that although the lens says it is at f/8 it's actually at f/11? (that would make sense, I guess)

I thought the variable apperture only applied to 4-5.6

 

[timor]

I would not trust light meter built into K1000. Especially now, thirty years later. BTW, what is the year of production of your K1000 ? What battery it is using ?

 

[PWhite214]

f4-5.6 to f? is only at the low end of the zoom. By around 120mm or so, the maximum aperture should be f5.6 and zooming no longer has an effect. If the aperture is set to f5.6 or higher, zooming from 70-210 should not change the set aperture. I suspect something wrong with the lens.

Phil

 

[Caldas]

I'll shoot a proimage roll to check (I have no ideia what year the K1000 is, but I've bought a Pentax 645N for more precise work and I think I'll eventually buy a spotmeter for precise meterings)

(I'm a Pentax fanboy btw)

 

[compur]

With most variable aperture zooms the aperture numbers on the lens ring apply only when the lens is at its shortest zoom position. The aperture changes as you zoom regardless of the aperture setting on the lens. It's not that it only changes when the lens is "wide open." It changes at all f/stop settings.

When I zoom in the dial remains at f/8 but the meter on the pentax indicates that the exposure is now -2 instead of -1. So what you are saying is that although the lens says it is at f/8 it's actually at f/11?

Yes.

I thought the variable apperture only applied to 4-5.6

No. When you move the aperture ring from "wide open" to another setting the lens doesn't magically lose its trait of varying the aperture.

Lens manufacturers don't like to advertise this fact so they just leave the more optimistic numbers on the aperture ring and let you figure out the rest. And, with many modern lenses there is no aperture ring because they are set internally by the camera body.

When zooms were first manufactured they all had constant apertures which stayed the same as you zoomed. This meant that the lens had to be much bigger and heavier to accommodate this. It was believed at that time that no photographer would buy a zoom that had a variable aperture. But, as automatic exposure cameras became more popular they started making variable aperture zooms and they sold well because they were no problem if you had a camera with a TTL-metered auto mode.

PS - Many photographers believe that the variable aperture only applies when the lens is wide open and lens manufacturers make no effort to correct that misunderstanding.

And, since so many people shoot with TTL auto exposure modes these days, most never notice that the exposure settings change as they zoom.

PPS - This is also why zooms with a constant aperture, like, say, an 80-200mm f/2.8 are so big and expensive. Even an 80-200mm f/4 is a pretty big and pricey lens. It takes a lot of optical trickery to make a zoom that keeps the same aperture at all zoom settings.

 

[Caldas]

Thanks for the help!

 

[timor]

I'll shoot a proimage roll to check (I have no ideia what year the K1000 is, but I've bought a Pentax 645N for more precise work and I think I'll eventually buy a spotmeter for precise meterings)

(I'm a Pentax fanboy btw)

Me to. Fan of Pentax. Especially old Super Takumars. Spotmeter ? Yes. The latest digital Sekonics are good. But remember, it might take a while before you learn, how to use them effectively. Do you shoot colour or b&W ? Do you develop yourself ?

 

[Caldas]

I shoot both colour and B&W, I've also ordered some Velvias and Provias to play with slides. Unfortunately I haven't had the chance to develop myself but I've thought about it (for B&W at least), the big problem is getting all the tools to be able to do it.

 

[Caldas]

And the even bigger problem is actually getting film in Brazil, all I can find in my town are expensive B&W films (some stores charge 17 USD for a roll of 400TX!!) and some ****ty colour negatives. Oh, and the only 120 film I've been able to find are those god damned lomography films. I think I'll have to start ordering from B&H every now and then