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01-10-2010, 10:40 AM   #1
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Constant Aperture vs Variable Aperture

Hi there,

I am making some choices on some lens to replace my kitlens for better image quality on my K200D. However, after reviewing some questions, I have concluded to some lens choices and options but what's pulling me back from purchasing is the differences in the fixed and variable aperture offering on some lenses.

Take for example:
1. Tamron 17-50mm f/2.8
2. SIGMA 17-70MM F/2.8-4.5 DC MACRO

What advantages does fixed aperture has over variable aperture lenses?

01-10-2010, 11:04 AM   #2
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Itīs funny, because what you think as a fixed aperture is variable in fact, and vice versa! Since the aperture number is a fraction of the focal length, for a zoom lens to have a constant aperture number, the iris-diafragm is getting wider as you choose a longer focal length.

Saying that, the big advantage of a constant (low) aperture number (say 2.8) is that the lens can be used in low light easier also on the longer end.
01-10-2010, 11:05 AM   #3
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Say, you're shooting under (dim) artificial lighting and you want your exposure to be constant for a series of shots and so you shoot in manual mode (and wide open). If you change the focal length of your zoom, you can mess up your exposure.

The main problem with variable aperture lenses is that they're slow at the long end. The max aperture of the Sigma is around f/4 at 50mm, while in the case of the Tamron it's still f/2.8 at 50mm.

Just buy the Tamron and forget about it.
01-10-2010, 11:08 AM   #4
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Fixed aperture means the lens is capable of doing f/2.8 at all focal lengths - meaning you can get f/2.8 at both 17mm and at 50mm. Variable aperture means you only get the larger aperture (smaller f-number) at the wide end (17mm) of the zoom. At the long end (70mm), it can only do f/4.5 at best. Meaning it's not really much better than the kit lens for low light.

Aside from that, some find it more useful to not have the aperture change as they zoom, independent of what the actual maximum is. But that's actually just a minor difference compared to the verylarge one I just described: the former lens can shoot f/2.8 at 50mm, the latter cannot (probably either f/4 or f/4.5 max).

01-10-2010, 11:44 AM   #5
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Consider flash use with a variable aperture zoom. If, as you zoom, the aperture changes, your fill light ratio changes as well.
If you are a wedding shooter, this really can spoil the look of a photo album.
01-10-2010, 02:10 PM   #6
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QuoteOriginally posted by Wheatfield Quote
Consider flash use with a variable aperture zoom. If, as you zoom, the aperture changes, your fill light ratio changes as well.
If you are a wedding shooter, this really can spoil the look of a photo album.
This is true, and requires some checking, because it could give a 1 stop change in background.

The real advatage of fixed aperture was before TTL flash, and when you set the F Stop on your flash, because in those cases, if you zoomed with a variable aperture you screwed up the exposure totally.

TTL flash allowed zoom makers to have variable aperture and today's cameras and lenses change the aperture automatically as you zoom and share all information so at least the subject exposure is costant
01-10-2010, 02:46 PM   #7
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Always consider the 'weakest link' with regards to the variable aperture lenses.
At full stretch the Sigma can only shoot your 70mm images at f/4.5 or smaller. Will you be requiring any larger an aperture? At 50mm it may be f/4 - is that wide enough?
Hence the virtue of having fixed aperture - you know you can open up to f/2.8 regardless of your focal length setting.

If you are looking for a longer walkaround zoom and find 28mm wide enough for your purposes, you may want to consider Tamron's 28-75 f/2.8 fixed aperture zoom.
01-10-2010, 03:05 PM   #8
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I feel like some constant-aperture zooms are cheating the user out of some potential aperture at the wide end. The DA 17-70mm and the DA* 16-50mm both have aperture blades visible when the lens is wide open at the wide end. As you zoom with the aperture held open, they disappear at some point in the middle of the lens's zoom range. (The DA* 50-135, on the other hand, doesn't do this..perhaps internal zooms are a different animal.) Assuming the iris is still the limiting aperture at that point (and why wouldn't it be, if they're bothering to stick it out there?), that means that there is the potential for a wider aperture at the short ends of both of those lenses. Maybe the 17-70 could be a f/2.8-4 zoom, and the 16-50 could be a f/2-2.8 zoom. That makes me think that Sigma just decided to allow for variability in a similar optical design for their 17-70, and Pentax decided that there was value in manipulating the diaphragm to make a constant f-stop value.

Now, there are probably good reasons why Pentax didn't allow those wide-normal zooms to go all-out at the wide end. Perhaps it would be too hard to make use of that extra aperture in a way that gives acceptable performance. After all...16/2 on a zoom sounds crazy, especially when the corners are already suffering on that lens at 2.8. Rather than complicating the optical design, they just avoided the extreme case at the wide end. Or maybe it was to please buyers who value a constant aperture, even if that means they're giving it up on the wide end. (No reason you couldn't make the DA 55-300 a constant f/5.8 aperture by limiting the iris at the wide end...but who wants a constant f/5.8?).

But regardless of their reason, they obviously made a conscious engineering choice to "force" constant aperture. If they had just kept the iris blades out of the way through the whole range, the zooms mentioned above would both be variable. That just goes to show that variable aperture can be the more natural state for a zoom lens, because you've got to cheat a little to achieve constancy.

01-10-2010, 03:14 PM   #9
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QuoteOriginally posted by aerodave Quote
I feel like some constant-aperture zooms are cheating the user out of some potential aperture at the wide end.
You can test this by taking shots of circles of confusion. If the aperture is slightly closed at the wide end, their shape should become more polygonal. EDIT: I just tried this with SMC-A 35-105 f/3.5. This is exactly what happened here. 35-105 is cheating!

Last edited by asdf; 01-10-2010 at 03:27 PM.
01-10-2010, 05:09 PM   #10
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QuoteOriginally posted by aerodave Quote
I feel like some constant-aperture zooms are cheating the user out of some potential aperture at the wide end. The DA 17-70mm and the DA* 16-50mm both have aperture blades visible when the lens is wide open at the wide end. As you zoom with the aperture held open, they disappear at some point in the middle of the lens's zoom range.
I think you are getting confused with the difference between what you can see and what the relationship of the elements is.

When zooming, there are lots of things moving, and it may be that while the aperture blades are more visible they do not interfere with the light path,. Watch next time as you zoom and see what happens internally.

Also note that at some point,. the aperture is restricted by the diameter of the front element, especially at the longer lengths, to make this faster, would require bigger diameter elements.

But, you are also partially right. your front element is what 50-55 mm in diameter on the 17-70? (not the filter ring but the element) at 17mm F2.8 only needs to be about 6 mm in diameter
01-11-2010, 06:53 AM   #11
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Thanks for the clear and brief explaination guys for clearing my confusion on the "fixed" and "variable" aperture that occurs within a lens. I didn't really knew about certain aperture available at certain focal length. Looks like I have more to learn from you guys.
01-11-2010, 06:58 AM   #12
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QuoteOriginally posted by Netsoft Quote
Thanks for the clear and brief explaination guys for clearing my confusion on the "fixed" and "variable" aperture that occurs within a lens. I didn't really knew about certain aperture available at certain focal length. Looks like I have more to learn from you guys.
It's great that you are eager to learn!

Also something to keep in mind, the Sigma 17-70 you showed in your post above is being replaced with a newer version so it's probably not the best time to buy that lens right now!
01-11-2010, 07:21 AM   #13
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QuoteOriginally posted by Buschmaster Quote
It's great that you are eager to learn!

Also something to keep in mind, the Sigma 17-70 you showed in your post above is being replaced with a newer version so it's probably not the best time to buy that lens right now!
That depends upon the reasons for buying the lens and the reasons sigma is "updating it"

it may not matter to the OP that the lens is being updated, and it may also mean one of 2 other things with respect to price.

If the lens is popular and sigma is late developing the PK mount (as thay have been with some other lenses) then the price could go up.

If they deliver promptly the PK mount or the lens is not all that popular you may get it for a bargin.

Also note, not all updated versions are optically superior. I think a lot of people might agree this is the case with the SIGMA APO 70-200 F2.8 EX. While later models offered rear element coating for dealing with reflection off the sensor (DG version) macro focusing and HSM in subsequent releases, the origonal may be the best as a true zoom lens.
01-11-2010, 07:29 AM   #14
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QuoteOriginally posted by Lowell Goudge Quote
That depends upon the reasons for buying the lens and the reasons sigma is "updating it"

it may not matter to the OP that the lens is being updated, and it may also mean one of 2 other things with respect to price.

If the lens is popular and sigma is late developing the PK mount (as thay have been with some other lenses) then the price could go up.

If they deliver promptly the PK mount or the lens is not all that popular you may get it for a bargin.

Also note, not all updated versions are optically superior. I think a lot of people might agree this is the case with the SIGMA APO 70-200 F2.8 EX. While later models offered rear element coating for dealing with reflection off the sensor (DG version) macro focusing and HSM in subsequent releases, the origonal may be the best as a true zoom lens.
I figured the price would go down since there will still be a 17-70 available, and it would hardly be seen as a worse version, so many people will elect for the new 17-70, most likely driving the price of the current 17-70 down, right?

Purely speculation I suppose.
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