30 minute LF camera fail!!

[philslizzy]

Thinking about my 5x4 camera. I remembered a youtube video where two japanese guys made a large format camera using a standard 50mm lens and an enlarger lens.

So let the pics do the talking

Try out



a meritar m42 on one end and a El Rokkor 50mm on the other end



A bit of tube -an arbitary 9" length as the enlarging lens will perform the frame coverage, some scotch tape over the end as a focusing screen.

Bingo it works, my camera shake.



So to make a big one















Result: a one inch projected image.



what happened and where did I go wrong??

[Lloydy]

Well..........the arrow is pointing the right way, so it can't be that. And your dexterity with a cardboard box, kitchen knife and black gaffer tape has to be applauded.
But I'm afraid I can't help with the optical solution.

[iangreenhalgh1]

The enlarger lens is acting to focus/collimate (not sure of the right term) the image circle of the Meritar, sort of like a condenser lens in an enlarger.

You need bigger lenses with bigger image circles, sorry to say.

[Lloydy]

Here's my theory, the two lenses on the bellows will have to have their focal points meet at some point in the bellows? The front lens sends the picture to the back lens. If the back lens is a 'long' lens then there will be a very small angle of spread of the image?
Perhaps a wide angle lens at the back might work?

[philslizzy]

[David]

I think you're losing your image circle in the bellows. I suspect you'll need an MF or LF bellows with a pyramid-like shape.

Edit: Nevermind. I forgot about the lens inside the camera, so that theory is incorrect.

[philslizzy]

Thats Ok David:D

If you watch the video of the camera, the image is upside down like a normal view camera but, although you cant see it, mine is the right way up. I assume because two lenses are used in the optical path. one inverts it and the other inverts it ending with an image the right way up.

I wondered if the guys on the video had placed a small ground glass behind the Yashinon, and that the enlarger lens just projected that image. But if that were the case the image would be the right way up.

An enlarger lens is designed to pick up an image from a 35mm neg, so it would have an image circle of at least 60mm, and the front is designed to be projected to almost any size. The screen is about 6 inches from the lens so I would expect it to be small - but not that small.

However, I will try again with a longer box and see what happens.

Or, perhaps the two lenses are too far apart??

[philslizzy]

YAAAY! When I was writing the above, I realised the image circle was small due to the distance between the enlarger lens and focus screen.. I doubled the length of the box with some black paper and got an image circle about 4.5 inches across. The loss of light is incredible! I will make a better box, about 14 inches long and see how it goes.

It would be nice having a LF camera that uses regular M42 lenses.

So lets do the exposure maths: Paper rated at 3asa, one 50mm f2 lens, one 50mm f4.5 lens, 40cms distance to film plane, erm, the law of inverse squares hmm, the intensity of the light will be inversely proportional to the square of the distance. So if we take a distance of 2 and square it, we get 4, the inverse of which would be 1/4 so effective aperture of, erm....hmm, erm, I'll make a digital back and use the NEX to measure it.

It will work.

Probably

I am remembering now how, at school we would turn a microscope into a telescope using the condenser lens like I have used the Meritar and the objective as I have used the El Rokkor.

[David]

Well, let's rough out some bracketing times. For easy math, this is calculated assuming 100 ISO and I'll recalc it at the end of paper ISO.

f2 with sunny 16 is 15 EV for 1/8,000th of a second exposure. Throw in a light reduction to f4.5, and you're down to an EV of 13. If we throw in a correction for inverse squares, assuming a loss of hald the light, we're at EV 12.

So far we're at the second lens and this rough estimate assumes os far the the light loss equals the difference in light between a sunny 16 object and a half-full moon seen through a long telephoto. That sounds, to me, not too far off.

Then add the space from the second lens to the paper back and let's roughly guess that's worth 7 EVs. Just a guess. Total guess. The 7 EV reduction takes us to 5 EV, about the light of a candle in a dim room.

At 100 ISO, 5 EV at f2 is a 1/8th second exposure. Paper is five stops slower than 100 ISO film, so four seconds on a bright day may be a good starting point.

Shooting both lenses wide open on a sunny day, I would pre-flash the paper and then bracket 2, 4, and 8 seconds. A good idea may be to take a single piece of paper and cut it into three strips or four quarters and also try a 16-second exposure. That would give you a good idea of how far off I am.

[philslizzy]

WOW! I was guessing I may have no use for a shutter and guesstimated a couple of seconds. So what is flashing the paper (I presume exposing to light first) and how would you suggest I do it?

Do you teach? I did back in the 70's

[Farside]

[philslizzy]

Its my coat not a goat!!

I could put this photo on youtube entitled: A ghostly goat in my camera WOW!! MUST BE SEEN!!! REALL!! It'd fit in nicely with all the other crap.

[David]

http://www.ilfordphoto.com/aboutus/page.asp?n=125

I don't teach (I probably could). Flashing is really well described in that link, better than I can do.

[philslizzy]

Thanks David, looks complicated without any dark room facilities. I understood the principle but never applied it and your link explains how to apply it. My own darkroom experience has been just dev and print, nothing fancy.

I seem to remember a member of my class (advanced photography) telling me he pre flashed recording film prior to using it in his camera. He was always experimenting with strange emulsions.

Anyway, the principles are established with my cardboard camera, something more permanent is required before I can put a film or paper in it.

Thanks David for your help, plus Ian and Dave and everyone else for your input!

[cn]

Guys, it's simple optics: you're going to get a huge amount of light loss whatever you do with this set-up.

The enlarger lens is made to project a 35mm frame, but that's only half the story, as the light entering the lens must be converging towards the lens. If that 35mm plane is projected by a photographic lens, the light exiting that lens is, in most cases, diverging, so you'd need a way of "bending" the light towards the enlarger lens at the intermediate image.

The only lens designs I know of that don't have that strong diverging behaviour are "telecentric" ones. Some modern lenses made specifically for digital are designed to be telecentric. So I guess your best chance is a combination of two of those back-to-back.