New to LF: understanding what I see on the ground glass

[Keith Braithwaite]

Hi,
Completely new to large format, apologies if this is a dumb-as-rocks newbie question.

I just got a no-name 4x5 field camera with a simple glass screen (no Fresnel) and a Schneider Zenar 135 4.7.

Put them together and pointed the camera out of my office window. Now, on the glass I see the image of the scene outside, which is good, but also a very distinct sharp-edged shadow of the aperture. I notice that this shadow moves around with my gaze. What is this? I know that what my eye sees is not what the film picks up but this seems very odd. I don't want a shadow of the aperture on my negs.

Never seen anything remotely like this on any of my 135 nor 120 cameras.

Any advice on how to understad this very welcome. Is this the "hot spot" that folks mention?

Best Regards,
Keith

[Vaughn]

Odd -- the aperture is round -- so is a round shadow?

Check inside the camera -- our students sometimes have a sheet of film pop out of the holder into the camera.

Perhaps this is what you are seeing...

Light spreads out from the lens and radiates out to the GG. The image will be brightest on the GG where one's eyes are looking at the GG in the direction of the aperture. But this is the opposite of a shadow (a bright spot) and is not sharped edge.

If you have a digital camera, photograph the GG and this shadow and post it here. That will help us!

Vaughn

[Keith Braithwaite]

Odd -- the aperture is round -- so is a round shadow?

Wide open, yes, a round shadow. Stopped down the shadow is the shape of the aperture blades.



Check inside the camera -- our students sometimes have a sheet of film pop out of the holder into the camera.

No foreign bodies in the camera.



Light spreads out from the lens and radiates out to the GG. The image will be brightest on the GG where one's eyes are looking at the GG in the direction of the aperture. But this is the opposite of a shadow (a bright spot) and is not sharped edge.


Yes, I was expecting a soft vignette but this is hard edged. Note, by "shadow" I mean a hard-edged light/dark transition. The bright part is in the middle.

See the attched photo. The effect doesn't photograph well, but is in the top left quarant of the GG. Tot he nakes eye it looks like a brighter pentagon overlaid on the scene.

Thanks,
Keith

[Mark Stahlke]

You have your camera pointed at the sun or some other very bright point source of illumination. What you're seeing is lens flare. (http://www.cambridgeincolour.com/tutorials/lens-flare.htm)

[ROL]

I know that what my eye sees is not what the film picks up but this seems very odd.

Actually, one of the major advantages of LF is that you see exactly on the GG what will be recorded on the film.

Looks like lens flare. Try pointing the camera away from the sun's direction.

[Keith Braithwaite]

You have your camera pointed at the sun or some other very bright point source of illumination. What you're seeing is lens flare. (http://www.cambridgeincolour.com/tutorials/lens-flare.htm)

I did wonder if it was flare, so that's astonishingly bad. There is neither a very bright source (of any size) nor a point source (of any brightness) in the scene. Attached is what the iPhone sees from the same viewpoint.

According to this (http://licm.org.uk/livingImage/SchneiderNos.html) my lens was made in the early 50's so no surprise that it has some level of internal reflections...oh well, it was only meant to be my starter lens.

So, I need a hood the size of a bucket.

Thanks,
Keith

[Richard K.]

So, I need a hood the size of a bucket.
Thanks,
Keith

Not necessarily...have you tried just holding a darkslide to shield the brightest source of light falling onto the bellows (sun, bright sky). You might be surprised to see that bright pentagon vanish (but make sure you are not intruding into the image formation path with the slide!). Nice digi-photo BTW....:)

[Keith Braithwaite]

I haven't tried much of anyhting yet: camera and lens arrived in the post this morning. Experimentation will have wait until my return from a business trip.

Thanks all for your comments and advice.

Keith

PS: Richard, thanks. There's lots of intersting jumbled uban scenes around this part of London (which is partly what I want the LF for). Only problem is the persistantly blank overcast skies...

[Ole Tjugen]

It's not flare, it's not a strong light source somewhere inside or outside the frame.

And it won't be recorded on film either.

It's simply that your ground glass is slightly transparent, so you are looking AT the aperture THROUGH the ground glass.

Don't worry. The rest of us have learned to ignore it, that's why you got so many wrong answers!

[Keith Braithwaite]

It's simply that your ground glass is slightly transparent, so you are looking AT the aperture THROUGH the ground glass.

A-ha! That makes perfect sense. I was a bit puzzled, as with flare I'd epxect to see a bunch of pentagons.

Well, well.

Thanks,
Keith

[GPS]

With flare you wouldn't see any bunch of pentagons, just strange shine. Later change to human ground glass to be happy.

[aduncanson]

I am not sure what you mean by "simple glass" any more than what GPS means by "human ground glass." Perhaps you are saying that your focusing screen is not ground at all? It should be.

I tried to duplicate your setup with a short, fast, tessar design lens and a ground glass with no fresnel. I don't see what you see. the bright spot on the screen is very diffuse and not at all hard edged.

I think that Ole understood half of the situation, but also that your ground glass has been replaced with un-ground.

[GPS]

I am not sure what you mean by "simple glass" any more than what GPS means by "human ground glass."
...
Ground glass that doesn't make one crazy...:)

[Nathan Potter]

I think Ole is correct. The GG in question is poorly ground so is in effect translucent, providing both a hazy image of the scene as well as a hazy direct view of the aperture. Possibly a poorly, home made screen. Problem will be solved by replacement with a fully ground screen. If the OP can see enough of the scene to focus using the current screen he'll be OK to use the camera.

Nate Potter, Austin TX.

[Ole Tjugen]

could also be it's just old and greasy - try warm water and dishwasher liquid before you go to the expense of replacing it. That worked wonders on my old AVUS GG, which was almost transparent. 90 years of smudge...

[Jack Dahlgren]

Yep, the picture tells the story well. Clean it. If that doesn't work, look up some of the threads on grinding your own ground glass.

[Keith Braithwaite]

Possibly a poorly, home made screen.

I just pulled the screen and had a look at it and I think this is right, it's a rough home-brew job. Still the whole camera only cost a couple of hundred bucks and is otherwise sound.

Thanks again, all.

Keith

[engl]

It is a poor GG, it is not diffusing enough. When sufficiently ground, the GG will scatter the light in all directions, what you are seeing is a lot of light going straight through the glass as if it were plain window glass. My previous camera from the 50s had a GG like that, you should see a big improvement with another GG (such as one from Steve Hopf) or by grinding your own with a good guide.