Help a noob understand the exposure conundrum

[Dan Fromm]

Maris, koraks, the OP seems to have hung a 300 mm telephoto lens in front of an Ilex #3. This puts the lens' rear node a good distance in front of the lens' diaphragm and farther in front of the shutter's aperture. It might act as a stop.

[TimberwolfII]

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 That's optical law and it hasn't been repealed.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.

Thank you for the comprehensive answer it is appreciated.
I'll do the math and see whether the lens is doing what it should. I will study.
I believe I am putting the film in correctly, but obviously I will be very conscious to be aware when loading.

[TimberwolfII]

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.
.

Now I understand why the light intensity doesn't change between formats. Thank you koraks succinctly and clearly described.

[TimberwolfII]

This is the lens/adapter/shutter configuration. The lens has the mount thread just behind the iris, about a third the way along the barrel, hence the long adapter otherwise the lense would clash the shutter.

Sorry bout the dark image, it's early morning and I don't want to disturb my significant other turning on lights...

[koraks]

Maris, koraks, the OP seems to have hung a 300 mm telephoto lens in front of an Ilex #3. This puts the lens' rear node a good distance in front of the lens' diaphragm and farther in front of the shutter's aperture. It might act as a stop.

I understand, but how could that amount to an 8 stop difference? 2 or 3 stops, sure, and maybe even 4. But 8? Even quite far from the nodal point, the iris would have to be quite tiny to get even close to 8 stops!

[Tim Meisburger]

Right. The eight stop difference is huge, and the reason I though the OP was shooting through the film base (although I'm not even sure that would give you eight stops).

[Dan Fromm]

I understand, but how could that amount to an 8 stop difference? 2 or 3 stops, sure, and maybe even 4. But 8? Even quite far from the nodal point, the iris would have to be quite tiny to get even close to 8 stops!

Fair question. I too have trouble with 8 stops.

Here's a test for the OP. Hold the lens/shutter at arm's length with the shutter open and its and the lens' diaphragms both fully open. Looking through the lens from the rear, slowly close the lens' diaphragm until the blades just become visible from the rear. What's the marked aperture at that point?

[TimberwolfII]

Immediately. I can see them start to move off max 6.8. I can see them past the 'mask' near the rear element. It's like an uninterrupted tunnel, all the projections - lens iris, mask, shutter iris all look the same diameter.

I'm hoping I can get time to contact print the under-ok-over negs in next couple of days so I can post them with accompanying data. Maybe they will shed some light, if you'll excuse the pun.

Rob

[TimberwolfII]

Ok, lets go back to check the film base way round. Maybe I can put it to bed yes I've put it in right or no I've screwed up. I've looked at the negs and I know the subject was looking to MY left when I took one of the images. My brain is hurting trying to juggle which way the notches/emulsion/head direction is cos everything turns upside down and back to front.

[Mark Sawyer]

If your subject on the negative is looking the same direction as in real life with her head at the top of the image, the notches should be in the top left while held vertical.

Only if he loaded the film with the notches at the bottom of the holder, or put the holder in the camera from the bottom.