Weird problem only appears on film

[RLangham]

So I have an SRT 101 that needs a good remedial CLA. Mainly it has mirror problems.
At slow speeds sometimes the mirror doesn't come down after exposure, which is really not that much of a problem since recocking the shutter brings it down.

There's a weird problem, though. On a lot of frames, I see the mirror coming down early. But that's not the weird part.

Picture of negative strip attached below. As you can see, this is a classic mirror fault. The weird part is, I cannot see the problem on any speed when looking through the back of the shutter. I see the entire film gate clearly.

The fault only seems to happen when there's film in the camera, and it seems to happen in the last half of the roll and happen consistently after a certain point.

What in God's name is going on with this camera?

 

[Donald Qualls]

More likely it's the mirror not going up fast enough. Check the direction of shutter travel vs. the orientation of the film in the camera at exposure time. I think you'll find the great obstruction is at the begining of the shutter path, not at the end.

 

[RLangham]

More likely it's the mirror not going up fast enough. Check the direction of shutter travel vs. the orientation of the film in the camera at exposure time. I think you'll find the great obstruction is at the begining of the shutter path, not at the end.

Oh, you know, I think you're right... the reason that appears to be a non-quadratic curve is that there are two phases in the mirror's upward travel: it moves vertically up and then swings up on its hinge. That's one of the famous features of the srT series. The concept had existed in large format SLR's but I think the sr series was the first to have a double hinged mirror in 35mm.

the thing that frustrates me is... why can't I see it doing this when I fire the shutter a hundred times in succession at various speeds with the back open and the lens off? I tried applying pressure to the aperture coupling to see if some lenses were blocking the mechanism.

I don't know, maybe it was one of the sticky little problems the camera had from sitting too long, that I've since worked out with exercise. I think the two rolls this appeared on may have been the first two I shot, but I'm not sure. It also capped at 1/1000s at first, but a few dozen cycles and it doesn't seem to do that anymore.

The seller was quite apologetic when I asked him whether it had these problems for him, but he said he had let it sit for a few years. I told him old cameras are like that and gave him good feedback anyways. Nice guy.

 

[Sirius Glass]

I had a problem in 1966 with a SR-7, sometimes there was a shutter problem that caused that. Eventually I sold it and bought a SRT-101.

 

[RLangham]

I had a problem in 1966 with a SR-7, sometimes there was a shutter problem that caused that. Eventually I sold it and bought a SRT-101.

Was it visible through the film gate when it happened?

 

[BradS]

...but it seems like it would be much too fast for the human eye to discern ???

EDIT: Especially considering that the mirror is still moving and, I assume eventually goes all the way up...that is, the blockage is only partial in space and time.

 

[RLangham]

...but it seems like it would be much too fast for the human eye to discern ???

EDIT: Especially considering that the mirror is still moving and, I assume eventually goes all the way up...that is, the blockage is only partial in space and time.

Not at all. If you can see through the entire film gate you should be able to see anything that's blocking part of the slit for any significant part of its travel. Did you look close at the picture?

 

[BradS]

.....Did you look close at the picture?

Yes. I did and based upon what I see there, I would not expect to be able to see the mirror with the naked eye.
Think about how fast the frame rate would have to be if you were to attempt to make a slo-mo video of this...hmmm...that would be something to try..eh?

 

[RLangham]

Yes. I did and based upon what I see there, I would not expect to be able to see the mirror with the naked eye.
Think about how fast the frame rate would have to be if you were to attempt to make a slo-mo video of this...hmmm...that would be something to try..eh?

I don't know what to tell you. If it's making such a completely blacked-out shadow on the film (and let me say that that part of the film is completely clear) it would be visible to the eye. I can see clear through every part of the film gate at 1/1000.

 

[BradS]

Film is far more sensitive than our eyes.

 

[Donald Qualls]

Moreover the human visual system is terribly slow. It only takes about a dozen frames per second to blend together into motion (and those frames only have to be different about four times per second -- cheap cartoons in the 1980s and 1990s were done with "four-frame" animation at 24 frames per second, giving about six changes per second).

One way to get around this is to (with no lens mounted) look through the shutter while you fire with a flash mounted (at sync speed, of course). You'll see the condition the shutter and mirror are in at the moment the first curtain passes the gate, while the second curtain hasn't yet started, and you'll most likely see the mirror blocking part of the frame at that point.

 

[RLangham]

Film is far more sensitive than our eyes.

First of all, no, our eyes detect a lot more light than most daylight film. Go outside at night. Once your eyes are adjusted you can make put detail that would take hours to expose onto even fast film.

That's a very weird thing to say and from context I think you might mean the exact opposite.

Second, you don't understand what I'm telling you. Look at the film. Clearly, there was an obstruction that was about halfway down the film when the shutter slit began traveling, which gradually raised up until it was outside the frame. This isn't some phenomenon that was only there for 1/1000th of a second. The curtains each individually take a good bit more than 1/1000th of a second (I would guess a little less than 1/125th) to travel across the frame. There's basically no delay between one curtain opening fully and the second beginning to close at 1/60th, which is the sync speed for this camera, and exposure = opening time + delay + closing time, so I would guess it takes about 1/120th of a second for each curtain to travel. That's a long time. And at speeds above X, the amount of time the shutter is traveling doesn't vary by that much, since only the width of the slit is varied.

The mirror would be in frame for about half of the time the shutter is open, per the shadow on the film, so about 1/250th of a second. Open an SLR, take the lens off and fire it at 1/250th. You will be able to see clearly through the film gate. You will not be able to see the mirror.

A half-blind person could see the mirror if it were doing it now.

 

[BradS]

....Once your eyes are adjusted.....

This exactly proves the point. The film is far more sensitive, it does not need five minutes to 'adjust' to the darkness.

 

[Donald Qualls]

On the other hand, I can routinely see my hands (faintly, with averted vision) while loading film in my darkroom -- but I can't see the source of the light, and it has not yet produced any detectable fogging on even 800 speed film. At their best, eyes are FAR more sensitive than the fastest film you can buy.

 

[BradS]

I swear I can close my eyes in the darkroom and see my hand waving in front of my face...I think the brain sometimes plays tricks.

 

[RLangham]

This exactly proves the point. The film is far more sensitive, it does not need five minutes to 'adjust' to the darkness.

Absolutely immaterial. That's the photosensitive chemicals in your eyes being replenished to full sensitivity. You can see things moving in the dark. You don't have to readjust your eyes to the dark every time what you're seeing changes in some way. It is not equivalent in any way to making a long exposure on a camera.

Your eyes have a certain sensitivity. In bright light it is diminished by the photisensisitive chemicals continuously being used as fast as they can replenish themselves.

You would be hard pressed to find film the same sensitivity as fully dark adjusted eyes. Now, because the most sensitive cells in our eyes aren't color cells, you will find film that can render color in comparative darkness with a time exposure, but our eyes don't need a time exposure.

Furthermore, I can point a camera without a lens at a bright scene and see the full area in detail through the shutter even at 1/1000th, so you're concretely wrong that a person can't see a mirror that happens to be in the way for part of the exposure. I don't know what to tell you. These are the facts.

 

[BradS]

...and yet you say that you cannot see the mirror intruding on the frame with your eyes but the film obviously did. Quite the conundrum, I'd say.

ok...another example.

Have you ever taken a photo using an electronic flash where the shutter speed was set to one or two stops faster than the synch speed? What happens? and yet, as you correctly observe, even at 1/1000 second our eyes see the shutter fully open. We do not see the slit pass by but the whole frame.

 

[RLangham]

...and yet you say that you cannot see the mirror intruding on the frame with your eyes but the film obviously did. Quite the conundrum, I'd say.

ok...another example.

Have you ever taken a photo using an electronic flash where the shutter speed was set to one or two stops faster than the synch speed? What happens? and yet, as you correctly observe, even at 1/1000 second our eyes see the shutter fully open. We do not see the slit pass by but the whole frame.

Oh? Really? Only a slit is open at speeds above X? I didn't know that!

I'm done here. You're not listening to me.

 

[Pieter12]

Oh? Really? Only a slit is open at speeds above X? I didn't know that!

I'm done here. You're not listening to me.

That is how a focal-plane shutter works at high speeds.

 

[RLangham]

That is how a focal-plane shutter works at high speeds.

I know. That was sarcastic because if he read my prior posts he would know that I have an understanding of that.

 

[BradS]

Furthermore, I can point a camera without a lens at a bright scene and see the full area in detail through the shutter even at 1/1000th, so you're concretely wrong that a person can't see a mirror that happens to be in the way for part of the exposure. I don't know what to tell you. These are the facts.

...but this experiment demonstrates exactly why you cannot see the mirror....at least it demonstrates why I would not expect to see the mirror. Maybe your visual perception is different from mine.

When I do the experiment, I see the whole frame at once - as if the shutter were completely open. I definitely do not perceive the tiny slit moving by the frame.

 

[BradS]

Moreover the human visual system is terribly slow....

I think this answer by Donald is better than the one I offered.
I used the wrong term when I said sensitivity.

 

[RLangham]

...but this experiment demonstrates exactly why you cannot see the mirror....at least it demonstrates why I would not expect to see the mirror. Maybe your visual perception is different from mine.

When I do the experiment, I see the whole frame at once - as if the shutter were completely open. I definitely do not perceive the tiny slit moving by the frame.

Ok, now we're getting somewhere. Yeah, I understand what you mean, but you should still not be able to see through the parts of the frame that are completely blocked by the mirror when the slit passes. Maybe "seeing the mirror" is not the word for that. I apologize for the confusion.
How would you see through the slit when the mirror is down? By the time the mirror is up the slit is long gone. Persistence of vision dictates you should see the curved shadow that the film sees, as the mirror raises and the segment of the slit that is blocked decreases or... that's all I can figure.

 

[BradS]

maybe so...It is difficult for me to visualize the mechanics and timing.
I like Donald's idea of using a strobe to attempt to see more clearly what is going on....but even then, I suspect that the strobe light would overwhelm the eye...so, maybe a cleverly rigged camera ? It's an interesting question.

 

[Sirius Glass]

I had a problem in 1966 with a SR-7, sometimes there was a shutter problem that caused that. Eventually I sold it and bought a SRT-101.

Was it visible through the film gate when it happened?

I only saw it on occasional slides.