Iris at max opening = now I look very closely the iris opens up to maybe 30mm - it's in the centre of the lens half way down so hard to gauge with the opticsOriginally Posted by faberryman
There is a mask near the rear element that is 25mm dia.
You can just see a glint of its edge above, its close to the rear element, but is NOT the iris.
Rear element is 50mm dia
Sheesh, plenty of crud in that lense!
34.6mm of shutter aperture. Plenty of holes for light to tumble in!
Well if the rear lens element is 50mm, and the shutter opening is 34.6mm, then I would think that you are suffering light loss, though hard to say if it is eight stops.
Last edited by faberryman; 3-Sep-2017 at 12:55.
I think, as someone else mentioned, that you are loading the film backwards. Are you sure you have the emulsion side facing the lens on your homemade holder? Looking through the lens at the film in the holder the notch should be in the upper right corner.
Exposure on LF is exactly the same as MF and 35mm. Exposure doesn't change with format.
As the picture above shows the light actually has to get through a 25mm hole [I]after[I]the lens aperture, which is part of the lense design, maybe that's an issue?
All I can say about the emulsion is: grooves in top right corner held in the right hand makes the emulsion face towards you, left hand holds the carrier facing upwards and the right hand holds the film with emulsion also facing upwards. Slot the two together and voila!
I'm still not clear how an amount of light going through a hole of say 25mm and hitting say 36 square centimetres (6x6) has the same intensity per unit area as the same amount of light going through a 25mm hole and dispersing itself over 234 square centimetres (13x18cm or 5x8")?
I hold with Dan on this one, I have to say. Books will teach you to ask correctly, to look correctly for questions. You start to be cocky on people who give their time to help. I want to be helped as I say, not as you want to help me is selfish and childish.
You cannot learn to cook just asking your nose how it smells.
I'm really really not trying to be cocky; I've been shooting b&w on and off for thirty years 35mm then MF. I have books and I use them. I'm trying to improve my craft. Because LF facilitates craft it was my next move. I've never come across this phenomenon where the kit seems to be doing something weird. I thought it might be something peculiar to LF so I came here.
But in four pages of dialogue there doesn't seem to be an answer. I have responded as thoughtfully and thoroughly as I know. I can't go to a book and find the answer to this, I've tried. I thought that was the purpose of a forum and a community, to ask respectfully of ones peers?
It wasn't a 'random' question.
Maybe I just go and ask elsewhere.
Newbies may seem childish because of their ignorance, we all have to start somewhere.
You could always just meter, add eight stops, and expose. If that consistently works for you, then you have solved your problem.
Thanks Faberryman. Looks like I'm gonna have to.
Why dance around the problem?
If the shutter has an iris in it always leave that iris fully open. That iris is not used in the operation of the lens and the aperture numbers engraved on it are irrelevant.
Measure the entrance pupil of the lens with the lens iris fully open. The entrance pupil size is the diameter of the aperture as seen through the front glass of the lens. Measure this appearance. This can be tricky, avoid parallax error.
Measure the actual focal length of the lens. It's not the distance between the front and rear standards of your view camera if the lens is a telephoto. It's not the focal length engraved on the lens if the lens has some missing or extra glass and you don't know about it. There are a number of ways of measuring focal length. Study. Pick the one most convenient.
Divide the focal length you measured by the entrance pupil you measured and the number you get is the maximum relative aperture, or f-stop, of the lens. That's optical law and it hasn't been repealed. Compare the number you got with the number engraved on the lens. If there is a difference then the f-stop markings of the lens needs recalibration.
Check the shutter speeds for obvious errors.
Emulsion side of the film? Hold the film in portrait orientation, short edges top and bottom, long edges left and right. If the film is notched in the top edge and the notches are toward the right corner the the emulsion is facing you.
The rest is just the usual subject brightness, film speed, f-stop, shutter speed equation.
Photography:first utterance. Sir John Herschel, 14 March 1839 at the Royal Society. "...Photography or the application of the Chemical rays of light to the purpose of pictorial representation,..".
If the 8 stop underexposure would be due to an iris issue, then the iris would have to be quite tiny. 8 stops is a difference in light intensity of roughly 1:500 (1:512 in fact), so an iris that is even only half the diameter it should be, would not amount to this effect (that would only make for an underexposure of 2 stops, not 8).
Exposing through the wrong side of the film generally makes for an underexposure of 3 to 5 stops, in my experience.
If you were to add a very significantly smaller iris to film loaded the wrong way round, one would get in the neighborhood of the observed 8 stop underexposure. Either of these effects separately would in my opinion not result in this huge issue.
Aperture and focal length in principle have nothing to do with film format; think of he image as a cone of light casting a circular image, part of which you capture and the size of that part depends on the film format chosen. The light cone doesn't change in brightness depending if you capture only a small or a large part of it. Hence, it doesn't make a difference if you measure for 8x10" or 35mm; if a measurement says e.g. 1/100 @ f/5.6, then that isn't dependent on film format.
Something odd certainly seems to be going on and I can't really come up with a good hypothesis on what's going on, apart from that it looks like a pretty significant error somewhere in the process.
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