strange effect with 135mm lens

[sebastel]

inspired by some movie, i recently reactivated my long resting spotmatic. with an S.-M.-C. takumar 135mm f/3.5 that i had obtained long time ago (but for some reason never had used on the SP yet), i noticed something strange.

a small area along the lower long edge of the film (when held in normal landscape orientation) is gradually underexposed.
one explanation for this effect would be the mirror not retracting completely. the funny thing is - this occurrs only with the 135mm, but not with the other lenses (a 55mm f/1.8 and a 28mm f/3.5, both super-takumars).

i checked by setting the shutter to "B", opening the back door and releasing the shutter while the 135mm was attached. nothing unusual - the mirror lies flat to the top and does not obstruct the light falling in.

any ideas what could be the reason?

for your reference, here is an example picture. in this case, the unwanted effect nicely adds to the subject, but it is certainly not something i can use all the time ...


along the river by sebastel23, on Flickr


cheers,
s.

 

[Roger Hicks]

Consider the angle at which the light rays are approaching the film....

Cheers,

R.

 

[sebastel]

hi roger,
thanks for the try, but that's not it.
i only do not have a better example online. this effect also shows when taking a portrait oriented photo of the plain blue sky. all but the long edge towards the mirror is evenly exposed, that said edge remains underexposed.

i think of taking test photos with varying exposure time, just to get more information on the issue.
regards,
sebastian

edit:
uploaded a second example here.

 

[Roger Hicks]

Yes, but the mirror is still in the same orientation relative to the film, whether the camera is landscape or portrait. And the slightly bright stripe suggests to me that some light is reflecting off the edge of the mirror, just before it's being blocked.

Have you a 200mm (or other long lens) that you could try for the same phenomenon?

Cheers,

R.

 

[zuiko85]

I think it is the mirror not retracting fast enough before the shutter travels.
Some more questions;
Is the effect the same at all apertures or is it worse at small apertures?
Is the mirror not moving fast enough at higher shutter speeds?
Is the effect the same at all shutter speeds?

I just can't think of anything else in the mirror box that would shadow the film so evenly on that one side.

Perhaps with the shutter set on 'B' and the 135 mounted you could *gently* push on the mirror back side to see if it can be pushed flatter against the top of the mirror box. May be it only looks like it is all the way up.

I think Roger could be right about the angle of the rays. The mirror may be slack but the effect won't show up with WA or normal lenses.

 

[sebastel]

nope. the 135 is the longest that i can put on the SP.
i agree, the slightly brighter stripe suggests some reflection.

so far, my assumptions go like this:

the only object in the mirror box that might cause such vignetting is the mirror itself. there simply is nothing else.
the effect does not show with the 28mm, because the rear lens is too small. light rays can not hit an obstacle close to the lens and rather far at the side.

now, the effect seems also not to occurr with the 55mm, although the 55mm has a taller rear lens. the difference here is, that the distance between rear lens and film is more than double in the 135mm, compared to the 55mm. still i'd expect some effect with the 55mm, though not as strong as with the 135mm.
so far i could not observe that at all.

now, my inspection of the open shutter (at "B" setting) had no result other than the mirror flipping up completely.

this might indicate, that the mirror might just be a tad slow, and there should be a relation between the strength of the effect and the actual exposure time (the shorter the exposure time, the stronger the effect).

i just looked into the mirror box again - the mirror really goes fully up, whether i set 1/1000sec or "B".
hm..... of course my eyes are not quick enough to see whether the mirror is up at just the right moment.

argh.
i don't want to send the camera off, it's so much fun to use it.

cheers,
sebastian

@zuiko:
those are the checks i wanted to do next. i have no indications yet about the influence of all variables (shutter speed, aperture).

looks like i first misunderstood what roger said, actually my thoughts go just the same direction.

thanks again for spending your thoughts.
s.

 

[mfogiel]

Have you used any lens shade? Also, is the rear of the lens round, or does it have a rectangular exit?

 

[zuiko85]

Just had another thought. On some SLR's the space between the mirror hinge and camera is bridged by a glued on fabric strip. I don't know how the SP is made but if it has this strip could it have become unglued and hang down in front of the film?

Or, is this the front edge of the mirror shadowing the frame. That would make more sense if the mirror misses full retraction by 2 or 3 mm at the front. If that is true the effect should have a sharper edge at smaller apertures and fuzzier edge at larger apertures.

If you could put a ground glass on the inner film rails and check if you can see the effect visually with the shutter open that might revel the cause. However normal falloff could render this hard to determine.

 

[sebastel]

hello marek,

the shade with the 135 is round, as is the rear lens - no (rectangular) baffles there that could be responsible.

the foam strip is tightly glued to the mirror box above the mirror. the inspection through the open shutter even shows, that the mirror gets fully retracted.

the best hypothesis still seems to be a "slow" mirror.

let me go through a couple of controlled test shots before we jump to conclusions. this will take some time, but i'll keep this thread updated.

thanks again for all contributions.
cheers,
sebastian

 

[wblynch]

A sluggish aperture mechanism on the lens might be enough to slow down the mirror...

It seems the slower shutter speeds would be less affected as the percentage of time the mirror is blocking would be smaller.

 

[sebastel]

good idea.
i'll test that by leaving the light meter "on" during exposure (then, the aperture pin remains pressed).

 

[Bill Clark]

Try exposing with the mirror in locked up position?

Maybe a CLA is needed?

Just a thought to help!

 

[sebastel]

bill, your comments are welcome.
the SP does not have an (official*) MLU. i know that this could be ordered, but it is a rather rare thing to find on those old spotmatics.

i guess, a CLA may help, i only wonder whether it is a reasonable thing to do (instead of finding a replacement body ... which may be in a similar state).

well. old cameras.
(head banging at the wall)

:-D
cheers,
sebastian


edit:
*
the inofficial MLU, activated by 'snicking' the release button is not reliable with my clumsy fingers.

 

[Bill Clark]

What about checking it without film, open up the back and fire on a fairly long shutter speed, perhaps 1/4 second or slower and see what can be observed. Maybe try this without a lens on, try using the "B" (bulb) and hold down the shutter button to check things out. A good tech with a good CLA might be a better investment than another body. I've tried a couple of fitness trainers for my body but they can never resist laughing! Ha!

 

[leicapixie]

My Pentax Spotmatic sometimes had a "slow" lifting mirror in cold weather. A "heavy" diaphragm, the pin, "sticky" would also cause the mirror to be on film. It always went right up and flat.
Try another 135mm lens!
The Pentax SP is a great camera. Recently bought a very good condition Black body for $10. It has no working meter, so that's the way it stays!