[aslewis02]

I just purchased the Sigma 17-70 mm
Amazon.com: Sigma 884306 17-70mm F2.8-4 DC Macro OS HSM Lens: Camera & Photo

I set my camera, as usual, (using manual).. put it on 2.8, zoomed in and it changed to 4.0. Am I missing something? I am familiar with lenses and manual settings, so changing things isn't new. But if I set the aperture, why did the lens change it when I zoomed?

[JinDesu]

Because the lens' maximum aperture at full zoom (or around 50mm actually) is F4. By zooming in, you've changed the lens elements into a position where the size of the opening within the lens has become smaller with respect to the front element. That is the effect of a variable zoom.

[aslewis02]

I guess, I don't have this issue with my other zooms.. 24-70 and 70-200..

[Julie]

The f/2.8 is not constant- most cheaper zooms aren't.

There is nothing wrong with your lens or camera, at the wide end, you will be at f/2.8, but as you continue to zoom in, it automatically changes to f/4.

That's why the lens is called the F/2.8-4, there is also a constant F/2.8 version of the lens, if I'm not mistaken...

---------- Post added 02-17-14 at 06:31 PM ----------

I guess, I don't have this issue with my other zooms.. 24-70 and 70-200..

Your 24-70 and 70-200 are probably the constant f/2.8 versions.

They're not labeled f/2.8-4, are they?

[stevebrot]

put it on 2.8, zoomed in and it changed to 4.0. Am I missing something?

Yes, you are missing something and it is printed right on the lens. The hyphen ("-") between the 2.8 and 4 indicates a range of maximum aperture over the zoom range. As noted above, this is the expected behavior. Sorry about the sarcasm but variable vs. fixed aperture is part of the decision tree for an informed purchase. Variable maximum aperture lenses tend to be smaller and lighter and less expensive than their fixed maximum aperture stablemates with a small penalty to be paid in terms of long-end speed. If you want a fast fixed maximum aperture lens you will pay more in exchange for a larger and heavier optic.


Steve

[aslewis02]

simply misunderstood. sorry for the stupid question.

[mattt]

sorry for the stupid question.

There is no such thing.

[Tanzer]

A variable-aperture zoom is not necessarily a bad thing. They can save you a lot of weight and offer a reduced size, for the zoom range. Compact size is one good reason why many choose Pentax. An even more positive way to look at this, is that you get a broader zoom range than you otherwise would, for the same size, weight, or price.

I know stevebrot said this already, and his comments are always right-on IMO; I just felt compelled to repeat this point.

Many folks automatically view variable-aperture zooms with some contempt. But I prefer to say that a 17-70 f/2.8-f/4.0 is still "better" than a fixed aperture 17-70 f/4.0.

[stevebrot]

I know stevebrot said this already, and his comments are always right-on IMO;

ROFL!

In this case I am a little embarrassed as well.

I may have been correct, but sarcasm is really not a good thing.

In my personal experience, I have to admit being mildly surprised the first time I zoomed with the kit lens on the K10D and noticed that the camera was "smart" enough to "know" to zoom the f/stop (relative aperture) along with the focal length. I don't know what it does for non-AF variable aperture zooms where the focal length information is not shared with the camera.

My apologies to the OP.


Steve

---------- Post added 02-17-14 at 09:01 PM ----------

But I prefer to say that a 17-70 f/2.8-f/4.0 is still "better" than a fixed aperture 17-70 f/4.0.

My understanding is that the OP's new lens is a stand-out performer and probably the best value in K-mount in the "normal zoom" class. If/when I purchase a K-3, I will likely pair it with the new Sigma 17-70/2.8-4.0 Contemporary.


Steve

[VoiceOfReason]

It's a mistake I made when I first bought a Sigma 18-50 f/2.8-4.5. I thought it was an f/2.8 throughout its range for some reason.

[Bagga_Txips]

When it first happened to me, decades ago, I was surprised, thinking " How does it do that? WHY does it do that?"

Then I realised that it is merely because the hole is further away...

[Tanzer]

When it first happened to me, decades ago, I was surprised, thinking " How does it do that? WHY does it do that?"

Then I realised that it is merely because the hole is further away...

This is very insightful, I always wondered the same thing, but never really took the time to figure out why. That's a good explanation.

[wildman]

Am I missing something?

Maximum free aperture remains constant throughout zoom range, all else being equal, effective aperture changes with focal length.

[stevebrot]

Maximum free aperture remains constant throughout zoom range, all else being equal, effective aperture changes with focal length.

Or to use another common terminology, absolute aperture remains constant throughout the zoom range while relative aperture (f number) varies with the focal length.


Steve

[Bagga_Txips]

That is to say, the biggest hole stays the same size, but because it is further away, less light lands on the exposure area. (We need a single word to describe film frame/sensor as a single entity)