Lens aperture slow to respond, thus overexposed pictures
Hello all!
I'm a long time lurker, but in the last few years I've not focused
seriously on photography, thus if what I'm asking about might have
already been discussed please point me in the right direction.
So, I have the following problem with a Pentax D-FA 100mm macro lens:
the aperture system on the lens is sluggish to respond and thus all
images are overexposed, especially on high f-stop numbers (i.e. 5.6,
8, 11 and onward).
Has anyone encountered this problem? How has one solved it? I'm
especially interested in do-it-yourself approaches, as I doubt I can
send the lens to be serviced (without costing more than the lens
itself)...
Here is how I've confirmed the issue is with the lens indeed and not the camera.
(A) First of all empirically, I've detached the lens and removed the
caps, then looked through it which by default is fully closed (f/20+).
Then I've manually pushed the aperture lever wide open and suddenly
let it go (just like the camera I presume does). The aperture blades
don't snap back to the fully closed position, but slowly crawl to that
position. Here "slowly crawl" is compared with how other lenses react
to a similar experiment; but in real time it takes perhaps half a
second or a bit less.
(B) I've taken raw photos (in manual mode at constant ISO and
exposure time, focused at infinity) of a close white surface (my
monitor) at various aperture values (f/2.8 through f/16 in one f-stop
increments) making sure the resulting image is not over- or
underexposed (by choosing a proper, but then constant, exposure time
that works for all tested aperture values). Then using the rawshack
tool I've identified the mean value of one of the green channels.
Then I've computed the ratio between two successive aperture levels
(starting with f/2.8) and the resulting values are 1.61, 1.96, 1.27,
1.04, 1.05. The theoretical values should be close to two. I've
confirmed this with a 35mm lens (in a similar fashion) whose values
are around two, with the exception of f/2.8 that with the 100mm lens
seems to be somewhat off. (I've tripled confirmed this by taking
photos without a lens, only with the semi-transparent body cap, but
varying the exposure time in one f-stop increments.)
Thanks all for the feedback,
Ciprian.
Seems to me that your lens needs some professional help, like an internal cleaning and lubrication.
Rick
On Jan 29, 2023, at 5:02 AM, Ciprian Craciun ciprian.craciun@gmail.com wrote:
Hello all!
I'm a long time lurker, but in the last few years I've not focused
seriously on photography, thus if what I'm asking about might have
already been discussed please point me in the right direction.
So, I have the following problem with a Pentax D-FA 100mm macro lens:
the aperture system on the lens is sluggish to respond and thus all
images are overexposed, especially on high f-stop numbers (i.e. 5.6,
8, 11 and onward).
Has anyone encountered this problem? How has one solved it? I'm
especially interested in do-it-yourself approaches, as I doubt I can
send the lens to be serviced (without costing more than the lens
itself)...
Here is how I've confirmed the issue is with the lens indeed and not the camera.
(A) First of all empirically, I've detached the lens and removed the
caps, then looked through it which by default is fully closed (f/20+).
Then I've manually pushed the aperture lever wide open and suddenly
let it go (just like the camera I presume does). The aperture blades
don't snap back to the fully closed position, but slowly crawl to that
position. Here "slowly crawl" is compared with how other lenses react
to a similar experiment; but in real time it takes perhaps half a
second or a bit less.
(B) I've taken raw photos (in manual mode at constant ISO and
exposure time, focused at infinity) of a close white surface (my
monitor) at various aperture values (f/2.8 through f/16 in one f-stop
increments) making sure the resulting image is not over- or
underexposed (by choosing a proper, but then constant, exposure time
that works for all tested aperture values). Then using the rawshack
tool I've identified the mean value of one of the green channels.
Then I've computed the ratio between two successive aperture levels
(starting with f/2.8) and the resulting values are 1.61, 1.96, 1.27,
1.04, 1.05. The theoretical values should be close to two. I've
confirmed this with a 35mm lens (in a similar fashion) whose values
are around two, with the exception of f/2.8 that with the 100mm lens
seems to be somewhat off. (I've tripled confirmed this by taking
photos without a lens, only with the semi-transparent body cap, but
varying the exposure time in one f-stop increments.)
Thanks all for the feedback,
Ciprian.
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Yes, the aperture mechanism is dirty/gummed up: the lens needs a CLA (clean, lubricate, adjust) service. The auto-aperture actuating lever should snap back instantly, somewhere around 10-20 milliseconds max. Half a second is almost literally forever in terms of these mechanisms.
G
On Jan 29, 2023, at 2:02 AM, Ciprian Craciun ciprian.craciun@gmail.com wrote:
...So, I have the following problem with a Pentax D-FA 100mm macro lens:
the aperture system on the lens is sluggish to respond and thus all
images are overexposed, especially on high f-stop numbers (i.e. 5.6,
8, 11 and onward).
...
(A) First of all empirically, I've detached the lens and removed the
caps, then looked through it which by default is fully closed (f/20+).
Then I've manually pushed the aperture lever wide open and suddenly
let it go (just like the camera I presume does). The aperture blades
don't snap back to the fully closed position, but slowly crawl to that
position. Here "slowly crawl" is compared with how other lenses react
to a similar experiment; but in real time it takes perhaps half a
second or a bit less.
...
On 1/29/2023 4:02 AM, Ciprian Craciun wrote:
Hello all!
I'm a long time lurker, but in the last few years I've not focused
seriously on photography, thus if what I'm asking about might have
already been discussed please point me in the right direction.
So, I have the following problem with a Pentax D-FA 100mm macro lens:
the aperture system on the lens is sluggish to respond and thus all
images are overexposed, especially on high f-stop numbers (i.e. 5.6,
8, 11 and onward).
Has anyone encountered this problem? How has one solved it? I'm
especially interested in do-it-yourself approaches, as I doubt I can
send the lens to be serviced (without costing more than the lens
itself)...
Lazy aperture syndrome. It's almost always caused by a small amount of
whatever lubricant they use in the mechanism migrating onto the aperture
blades making them sticky.
I've never torn down a lens and put it back together with any success,
but if it's what I think it is, the lens needs to be disassembled, the
blades need to be removed and cleaned, then the lens needs to be
reassembled and recollimated.
bill
Thank you all that have responded!
I'll try to see if there is stil a Pentax service in my country
(Romania), although I highly doubt it...
If you say that disassembling the lens on my own is out of question,
and if I don't reach (an affordable) service here, I'll have to use it
as it is...
I often use it at f/5.6, and from playing a bit with the lens
(actuating manually the aperture lever), it seems it became a bit
"quicker" to respond... :)
Thanks,
Ciprian.