[MegaPower]

I just found out the aperture cannot be fully open on wide end (i.e.16mm)

It does fully open on the tele end (i.e.50mm) and appears to be round.


Current owner of *16-50, can you verify this with me?
I own the lens for more than a year and I just notice this.

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Last edited by MegaPower; 03-11-2013 at 11:20 PM.

[MegaPower]

so i got panic and I found this.

Question for DA*16-50 owners - please check: Pentax SLR Talk Forum: Digital Photography Review

Can some more owner confirm this?

[vonBaloney]

That is normal for many (all?) wide zooms.

[aleonx3]

Mine is the same... at 50mm end... circle appears smaller and round; at the wide-end, the blades are not fully receded when looking thru the circle which seems smaller than the blade opening

[Adam]

It's probably because in this particular lens's case, the blades are what's helping keep the aperture rating constant.

[stanislav]

I have noticed the similar behaviour on Tamron 28-75 f2.8.

In this case the blades are providing the constant apperture, if this wasn't the case the apperture should be more open on the wide end.

Simplified the apperture is relative size of a lens with respect to the focal length when lens is focused in infinity. In that case the lens is simplified with a simple glas (one simple lens) with diameter which is focal_length / apperture. So a 50mm lens with apperture of 1 is corresponding to a single (simple) lens at 50mm distance from focal plane so that it projects the infinity on the focal plane sharp. Furthermore that lens has a diameter of 50mm. If apperture is say 2, on 50mm lens, then the corresponding simplified lens has a diameter of 25mm. Well I hope that i didn't mess things up even more with this explanation.

[MegaPower]

looks like it's the same on 16-45.
but most constant aperture telephoto lenses do not do this. (i.e. my FA*80-200)

[Usuqa]

I just found this thread so sorry for bringing it back up.

I just got this lens and I noticed even at 50mm my aperture blades are closed a little when wide open, is this normal?

[stevebrot]

It's probably because in this particular lens's case, the blades are what's helping keep the aperture rating constant.

What he said.


Steve

[jpyykonen]

Not long ago I got FA* 28-70/2.8 from robbiec at this forum (thanks again). I've noticed some way similar behavior in this lens but quite an opposite: the aperture blades are fully open at F2.8 at all focal lengths. I've got similar lens Sigma EX 28-70/2.8 where aperture blades are very noticeably closed at F2.8 at all focal lengths.

What made me curious was the thing that I noticed about 1 stop over exposure at F4 and smaller apertures. Then I went and tested this FA* with Sigma and noticed that the DoF (and bokeh) is very similar in FA* @F4 and Sigma @F2.8 with same focal lengths and shooting distances. And what is more interesting is that I noticed some slight differences in bokeh with FA* between F2.8 and F4 especially at wide end (at long end the differences were unnoticeable). There are also slight sharpness differences and at F2.8 the image has warm color cast (it changes more blue at F4 onwards).

So to me it seems like the lens opens too much when aperture ring is at F2.8 and it is actually F2.8 when aperture ring is turned to F4 (when aperture blades are noticeable inside the lens). So is this lens actually bit brighter than F2.8 when aperture is set to F2.8? Is this even possible? Any FA* 28-70/2.8 owners with similar behaviour?

[Usuqa]

I managed to fix mine.

Before


After


It was pretty simple.

This blog post describes it
RiceHigh's Pentax Blog: (Hardware) Exposure Adjustment of Pentax Lenses
although in the 16-50 f2.8 the adjustment screw's are not where the leaver is, but to the right more closer to the spring.. if you zoom the lens to 50mm the lens element will sink into the lens showing a metal rod running down into the lens the screws are at the top of this rod.

The reason I attempted this fix myself was that I got the lens second hand, so no warranty and I didn't see the point in having a fixed f3.5 lens that was men't to be f2.8.
I know there isn't much in it but the little extra shallow dof and light gathering (2/3 ev) can help at times.

Test image.


On a side note, during testing I found out my sigma 24-60mm f2.8 is brighter than it should be at 60mm f2.8 (by around 1/3-2/3ev) so while its a constant aperture zoom, the brightness does change.. I guessed I noticed it a while ago as my wider shoots with strobes seemed a bit darker than at other focal lengths.

[MortenV]

I managed to fix mine.

Before


After


It was pretty simple.

This blog post describes it
RiceHigh's Pentax Blog: (Hardware) Exposure Adjustment of Pentax Lenses
although in the 16-50 f2.8 the adjustment screw's are not where the leaver is, but to the right more closer to the spring.. if you zoom the lens to 50mm the lens element will sink into the lens showing a metal rod running down into the lens the screws are at the top of this rod.

The reason I attempted this fix myself was that I got the lens second hand, so no warranty and I didn't see the point in having a fixed f3.5 lens that was men't to be f2.8.
I know there isn't much in it but the little extra shallow dof and light gathering (2/3 ev) can help at times.

Test image.


On a side note, during testing I found out my sigma 24-60mm f2.8 is brighter than it should be at 60mm f2.8 (by around 1/3-2/3ev) so while its a constant aperture zoom, the brightness does change.. I guessed I noticed it a while ago as my wider shoots with strobes seemed a bit darker than at other focal lengths.

My DA* 16-50 got a strange behaviour. The aperture it reports to the camera is one full stop too low. When I compare it to my other lenses I can see that at F2.8 it let only the same amount of light passing through as another lens do at F4.0. The images look nice but the ISO values are at the double value as my other lenses at same aperture.
Did you have the same problem Usuqa?