Exposure question

[guest]

This may sound like a beginers question but in school the answer was a big mess with different people having different answers.If you shoot k-40 outdoors and the built in light meter is giving a f 4 as an example. If you stop down to an f2 this would give more light to the scene(underexposing)
Sound simple but a teacher says this will cause more grain build up and not a better lit scene, is this correct . Scott from "In my Image" movie.
Says to always stop down to allow more light because low light gives the home movie look. Can someone confirm that stopping down does give more light to a scene.But more grain?

[super8man]

Can't really comment on the grain but generally, I think yes, grain will become more of an issue as you OVEREXPOSE your film.

Really basic:

F4 is almost wide open

F2 is considered wide open in many lenses.

F4 will let in less light than F2, f8 lets in less light than f4, etc, etc.

Soooo, if the scene is measuring f4 and you shoot f2, there will be more light hitting the film and more overexposure.

As for stoppoing down (like from f2 to f4 or from f4 to f11 and so forth), you will actually INCREASE your depth of field - this is generally a BAD thing for a PRO look (ok, here come the flames). No, if you want the tight action scene look, focus on a single subject, like a face - focused on the eye, and shoot at f2. It will look marvellous. If you shoot someone next to a car in bright daylight, the only way you can get that short depth of field look is with faster shutter speeds and fast lenses and fast film. K40 will never allow you the ability to create some of the scenes that can be made with a 500 assa film (like vision 500T) - and I am only talking exposure here, not the look of the film itself.

In still photography this is why pros have monster lenses. Not only are they easier to look through due to being brighter, they allow you to shoot at like f2.8 and 5000th of a second with 800 ASA film and get awesome CONCENTRATED photos.

Same reasoning goes with movies too. But other factors come into play.

Sooo, no, I disagree - stopping down give you that HOME MOVIE look while shooting wide open on tight subjects concentrates the action and gives a more PRO feel.

If you are into that kind of thing.

Me, I like it all if it is done well. If I like what I am seeing, I really don't care how they got the image to do that - stopped down, wide open, in the middle, whatever. And that is the real point in all of this talk. Just make beautiful images!!!

[monobath]

The subject of apertures confuses a lot of people. It doesn't work quite as one would think.

"stop down" means to close the aperture so that it gets smaller, and admits less light. Going from an aperture of f4 to f2 is not stopping down, however. f2 is a larger aperture than f4, so you are really making a bigger hole to admit more light. Strangely enough, no one really ever says "stop up", although this is what you are doing.

The bigger the f number, the smaller the aperture, and the less light admitted for any given time that the shutter is open. The smaller the f number, the bigger the aperture, and the more light is admitted for any given time the shutter is open.

So, if your meter reads f4, and you open up your aperture to the larger size of f2, thus admitting more light for the duration of the shutter opening, you will overexpose. Going the other direction, that is, stopping down from a metered f4 to an aperture of f5.6 or f8 would admit less light for the duration of the shutter opening, resulting in underexposure.

So, basically, you get the idea that changing the aperture lets in more or less light, but you are using the terminology in the opposite sense that it is actually employed.

Assuming that the metered reading of f4 is accurate, then stopping down means to make a smaller aperture (e.g., f4 to f5.6), and to let in less light, leading to underexposure. Opening up the aperture means to make it larger (e.g., f4 to f2), thus letting in more light, resulting in overexposure.

I hope that makes sense. If you want to know why the f numbers go up as the aperture gets smaller, and go down as the aperture gets larger, it is because the f number is a ratio of the focal length of the lens to the diameter of the aperture. Thus, when a 100mm lens has an aperture with a diameter of 25mm, the f number is 4 (f = F/D, where f is the f/stop number, F is the focal length of the lens, and D is the effective diameter of the lens opening, aka aperture). If you close the aperture down (stop down) from 25mm diameter to 17.86mm, the f/stop will be f5.6, one stop smaller. If you stop down to 12.5mm diameter aperture, the f/stop will be f8, two stops smaller.

[stoney]

Highly recommended source is
Photographer's Mate 3 & 2, pages 83-91

For graininess refer to
http://www.filmshooting.com/community/a ... /grain.php

Factorial stop system
'Think' of the f stop as a fraction relating to the size
of the diaphragm/iris opening and how much light is
transmitted to the film plane. The actual relation is
f=F/d or T=F/D which is listed further below with certian
assumptions.

- 'Open up' the lens and let more light in.
- 'Stop down' the len and reduce the amount of light.
- Think fraction. 1/8 to 1/11 is stopping down one stop.
- Open up the lens a full stop and the amount of light doubles.
- Stop down the lens diaphragm a full stop and the amount of
light is cut in half.

You may calculate an equivalent exposure or compare the
different intensities of light by squaring the f stop
number and using it as 1/x.
fstop squared relative_amount_of_light
f8 8*8=64 1/64
f11 11*11=121 1/121

Beware of the first and last steps on a lens. If the next
number is not very close to 1.4 times the previous stop then
the stop is an intermediate stop. This is done to give
you the maximum/minimum diaphragm diameter. You may turn
the f/t stop in between to get an intermediate stop.

Transmission stop system
The transmission stop system was developed in order to
standardize the calibration of lens diaphragms taking into
account light loss by reflection, absorbtion, and
manufacturing variation. It uses a perfect lens with
no light loss as the reference (vacume or air?). When you
calibrate the lens T-stop you would open up the diaphragm
wider than the reference in order to compensate for the
light loss of the lens. This was done for color films
and motion pictures to obtain consistency between different
lens since the f-stop calibration method was not adequate
even for the same model of lens.

Factorial stop system
f=F/d
where f=relative factorial stop
F=Focal length
d=diameter of lens diaphragm/iris to be calibrated
assuming
film plane one focal distance from optical center of lens
object >8 focal lengths away (infinity)

Transmission stop system
T=F/D
where D=diameter of standard diaphragm with no light loss.


NOTE: approach the T-stop from the same direction in order
to experience consistent 'backlash' from the diaphragm;
especially for smaller diameters.
Example: stop the lens down past the intended setting
one stop and open it up one stop so that you
are always approaching the T-stop from the
smaller diameter.

NOTE: Remember the change in depth of field for different
f/T stop settings.

Brightness Range exposure method
Take a reflective 'spot' reading of the darkest region where
details will be revealed and then take another reflective spot
reading of the brightest region that you want. If you have a
light meter with a light intensity range in addition to f stops
you may take the geometric mean of the two readings.

geometricMean = squareRoot(darkestRegionIntensity x brightestRegionIntensity)

The reflective spot readings of the darkest and brightest areas
that you want to be able to see will also help you to determine
if you are exceeding the latitude (exposure range) of the film
that you are using.

If you are using your motion picture camera light meter your
meter will deviate from a spot meter (4 degrees vs aproximately
25 degrees)
can use these two f stop readings and compare them against
your average brightness reading as a sanity check. The
average brightness reading gives you the proper exposure
for an average scene %18 grey which you may accurately measure
by placing a matte %18 grey card at the subject if this is
possible.

Stoney

[studiocarter]

Film grain is increased with darker pictures when using a Video camera and a WorkPrinter in my experience. That is because the video camera must open up more to allow more light in and also it increases GAIN onto the video which puts grain onto the image. Look at night images with a Hi-8mm video camera. They are grainy. There was no film used but they are grainy.
If dark film were projected with lots more light I don't know if there would be more film grain(uals) or not.
When overexposed light film was transferred, I have seen the opposite. If the thinner lighter emulsion was transferred with the same video camera then there is less grain. The video camera can stop down and lower or eliminate any gain increase it wants to apply. The lighter images are low in contrast, soft gray, and very smooth. The darker images are more contrasty, dark sepia toneish, and very grainy.

Reversal film properly exposed and processed is very smooth and has fine grain. If it is processed to negative only and left that way then grain is increased. That is because larger silver crystals react faster to light than smaller ones. Reversal film is composed of large and small crystals. During reversal bleaching, large crystals are removed and the smaller ones remaining that are as yet unexposed are then exposed completly to light and processed. This results in a fine grain reversed image.

Reversal film that is underexposed and processed properly will be darker than correctly exposed films. A higher wattage bulb may be used to project it with that (does?) result in a more saturated image (?)

Please post your experiences here.

My experience is that using underexposed film is difficult because it is hard to see it projected and in the editor viewer. I'd rather it were a bit lighter so I can see it easier. Then, I had to go out and buy a white beaded projection screen because my gray 'silver' screen was too dark. Unless the projector was very close to the screen, making a small image, the full size picture was dark. The room needs to be black. Street lighting makes that difficult as it shines through the light curtans. Underexposed film does not look good projected with my setup. My big old Bell & Howell built like a tank sound projectors have huge fast lenses and very bright bulbs. In my black celler with a white screen I can move the two far apart and get a big bright image. But that is for 16mm film. Perhaps I could see the difference there when shooting a little underexposed with K40.

Maybe it looks better in a professional Rank transfer. In that case all the above does not apply. What situation is it you are in?

[scottbobo2]

I total agree with you regular 8. I find that slightly overexposing Super 8 k-40 gives a beautiful projection on my sankyo projector and video transfer (Paclab here in NY ).In New York we get alot of dull days in the fall and winter and stopping down is usually never that much of an option.I have never seen overexposing causing any grainy image. You need so much light to light k-40 why would you underexpose it? Maybe the california people can stop down but in NY it is rarely an option in my opinion shooting here

[mattias]

I have never seen overexposing causing any grainy image.

i tend to agree if we're talking video transfers, but projected reversal is definitely less grainy when slightly underexposed.

You need so much light to light k-40 why would you underexpose it? Maybe the california people can stop down but in NY it is rarely an option in my opinion shooting here

huh? that doesn't make sense. if you decide to underexpose you need *less* light, not more.

/matt

[scottbobo2]

You are right mattias.

[Patrick]

"The only way you can get that short depth of field look is with faster shutter speeds and fast lenses and fast film"

Actually, the use of fast film usually leads to an increase in depth of field because it forces the photographer to use a smaller aperture. With cinematography, the shutter speed is usually fixed (at standard filming speed) so the aperture must be regulated (in this case made smaller.)