Pentax LX long exposures

[JCJackson]

Does anyone have experience using a Pentax LX in automatic (Aperature Preferred) exposure mode for exposures of 2 to 10 minutes duration. The manual says that the LX should do this up to 125 seconds, but when I was exposing a dim interior (lit only by streetlight spill) the shutter stayed open for 5 minutes before closing. This may be the correct exposure (I'll know in a day or so when I develop the film), but it also seemed to be substantially outside the range represented by Pentax specs. Is this longer time still within the capabilities of the metering system, or was I witnessing some kind of malfunction?

 

[dr. ego]

the light meter goes all the way down to -6.5 ev
that makes 125s at 100asa and f1.2
the easiest way to find out i guess is to put your camera empty on iso 11 and f11 and time it in a very dark room.
i am gonna try this with my pentax lx, im curious if the pentax lx can keep the shutter open for longer then 125s on automatic mode.

 

[benjiboy]

If you are giving exposures of 125 seconds with an electronic shutter your batteries aren't going to last long, for exposures of 2 to 10 minutes you need to set the shutter on B and give it a time exposure .

 

[werra]

Those 2 to 10 minutes are metered from film surface. Batteries are cheap anyway.

 

[Philippe-Georges]

The metering and automatic exposure system of the LX is the best I ever had and have.
I made exposures in Bourgogne's whine cellars for more than 25 min. YES, about 25 minutes on automatic drive!
This was on FUJI-CHROME Sensia 100. No problems, a perfect exposed slide, I still wonder why the Schwarzschild effect did not occur.
Anyway, the LX can expose as long as it 'feels' that it is necessarily. It changes the exposure time when the light conditions are changing during exposure, that's unique.
I have three of them, very convenient when I have to shoot a lot on location, indoors in ambient light. I have two tripods and 9 lenses (all primes), I just put one on a spot where very long exposure are needed and the second one on an other spot and the third one is handhold. So, I do not have to wait for these long exposures, and can go on with the shooting.
I made a little, dim, led light thing I can connect in to the PC slot, so I can see when the shutter is still open. The LX's PC slot is closed when the shutter is open.
And the batteries are lasting for at least 2 years, under 'normal' conditions, regardless the long exposures.

Have fun with your LX, it's a good and reliable instrument. Just watch out for the sticky mirror syndrome, it is nasty and expensive to have it fixed, but a LX is worth the effort.

Philippe

 

[JCJackson]

My LX and I thank you

This is a very encouraging and enlightening answer. I am shooting Fuji Acros, which claims that only 1/2 stop of reciprocity compensation is needed at 1,000 seconds, so I am hoping to avoid the Schwarzschild effect. Maybe Fuji slide film has similar miraculous immunity? It also occurs to me that your long battery life could be due to use of the power winder - I think that when the winder is installed, the camera electronics draw power from it and not the button cells.

Thanks!

The metering and automatic exposure system of the LX is the best I ever had and have.
I made exposures in Bourgogne's whine cellars for more than 25 min. YES, about 25 minutes on automatic drive!
This was on FUJI-CHROME Sensia 100. No problems, a perfect exposed slide, I still wonder why the Schwarzschild effect did not occur.
Anyway, the LX can expose as long as it 'feels' that it is necessarily. It changes the exposure time when the light conditions are changing during exposure, that's unique.
I have three of them, very convenient when I have to shoot a lot on location, indoors in ambient light. I have two tripods and 9 lenses (all primes), I just put one on a spot where very long exposure are needed and the second one on an other spot and the third one is handhold. So, I do not have to wait for these long exposures, and can go on with the shooting.
I made a little, dim, led light thing I can connect in to the PC slot, so I can see when the shutter is still open. The LX's PC slot is closed when the shutter is open.
And the batteries are lasting for at least 2 years, under 'normal' conditions, regardless the long exposures.

Have fun with your LX, it's a good and reliable instrument. Just watch out for the sticky mirror syndrome, it is nasty and expensive to have it fixed, but a LX is worth the effort.

Philippe

 

[Philippe-Georges]

This is a very encouraging and enlightening answer. I am shooting Fuji Acros, which claims that only 1/2 stop of reciprocity compensation is needed at 1,000 seconds, so I am hoping to avoid the Schwarzschild effect. Maybe Fuji slide film has similar miraculous immunity? It also occurs to me that your long battery life could be due to use of the power winder - I think that when the winder is installed, the camera electronics draw power from it and not the button cells.

Thanks!

I was not talking about a winder ore something alike. I just have one, I mostly use it for handheld shooting because, due to its weight and bulk, it stabilises the camera. The LX is, compared to the 'others', rather light and small witch is typical Pentax.
No, the long exposures, I made, are powered by the button cells, no external power at all, and it never came up to me to look into that.
Just buy the good ones, to me, Kodak's KS76 SILVER OXIDE 1,5 V batteries are the best. The non silver oxide, the lithium ones I think, are not that long lasting, but cheaper.
About the miraculous FUJI; I do not believe in miracles, it might be just a coincidence, but a good AND repetitive one. Ever since, I shoot that film when long exposures are the issue, and it just never failed.
Perhaps this 'miracle' is due to the slightly discolouring/darkening of the emulsion when exposed? Look at the discolouring of the (FUJI) film's start strip that is hanging out of the cartridge. Times longer than a few minutes, like 25 min, are rather long and perhaps just enough for that changing.
As the LX is measuring the light reflected from the film surface and is continuously monitoring and correcting the amount and intensity of the light during exposure, this might be an explanation, who knows?
Anyway, this is working to my full satisfaction, and that is just what I want.

Philippe

 

[Philippe-Georges]

the light meter goes all the way down to -6.5 ev
that makes 125s at 100asa and f1.2
the easiest way to find out i guess is to put your camera empty on iso 11 and f11 and time it in a very dark room.
i am gonna try this with my pentax lx, im curious if the pentax lx can keep the shutter open for longer then 125s on automatic mode.

You better put some film in. The pressure plate is black and hardly reflective, film is somewhat greyish and slightly reflecting. And yes, please do the test.
Put the camera, loaded with (colour-) slide film, in your cellar just lightened by one candle with a clock next to it so you can see the (small-) hand's movement on the picture, and have a drink to my health while waiting. Do not forget to fully close the aperture.
Have the film developed, slide film never lies, I am looking forward to your comments!

Philippe