Question about aperature and focal length

DS Williams · June 3, 2009

Hey guys

So if the f number = focal length/aperature entrance pupil size

and longer focal lengths like 100mm lenses have larger entrance pupils at f4 than shorter focal lengths have...

then how do both lenses at f4 pass the same amount of light to the film stock/sensor?

Chris Keth · June 3, 2009

Hey guys

So if the f number = focal length/aperature entrance pupil size

and longer focal lengths like 100mm lenses have larger entrance pupils at f4 than shorter focal lengths have...

then how do both lenses at f4 pass the same amount of light to the film stock/sensor?

Lenses passing light is basically like a refined form of a tube and the relative aperture is basically like the diameter of that tube. You can think of it as the iris circle is always covering the same angle of view from the point of view of the film. A circle of 50cm diameter at 1 meter distance looks the same size to a viewer as a circle of 100cm diameter does at 2 meters.

Imagine you are standing in front of an evenly lit white wall. You hold your incident lightmeter toward the wall with a tube over the sensor that is a foot long and 3 inches in diameter, or f/4. To get the same reading at the same place through a tube that is 2 feet in diameter would require the tube to be 6 inches in diameter, which is also f/4.

I have stated this as a theoretical situation, but you can really do this and verify the result if you like. I did it in a photographic technology class in college and I got it to work out to the 10th of a stop by flocking the inside of the tube so take light bouncing around the inside of the tube out of the equation. I did it outdoors on a bright day where full sun was hitting the wall. The lightmeter was mounted on a tripod and locked down so it didn't move. The only thing that changed was the tube, and the duvetyne draped over the meter and back of the tube to catch non "image forming" light.

Edited June 3, 2009 by Chris Keth

DS Williams · June 6, 2009

Lenses passing light is basically like a refined form of a tube and the relative aperture is basically like the diameter of that tube. You can think of it as the iris circle is always covering the same angle of view from the point of view of the film. A circle of 50cm diameter at 1 meter distance looks the same size to a viewer as a circle of 100cm diameter does at 2 meters.

Imagine you are standing in front of an evenly lit white wall. You hold your incident lightmeter toward the wall with a tube over the sensor that is a foot long and 3 inches in diameter, or f/4. To get the same reading at the same place through a tube that is 2 feet in diameter would require the tube to be 6 inches in diameter, which is also f/4.

I have stated this as a theoretical situation, but you can really do this and verify the result if you like. I did it in a photographic technology class in college and I got it to work out to the 10th of a stop by flocking the inside of the tube so take light bouncing around the inside of the tube out of the equation. I did it outdoors on a bright day where full sun was hitting the wall. The lightmeter was mounted on a tripod and locked down so it didn't move. The only thing that changed was the tube, and the duvetyne draped over the meter and back of the tube to catch non "image forming" light.

Thank you Chris for your informative and quick reply.

I have one more simple question, do the larger entrance pupils in longer focal length lenses become a factor in their shallower depth of field? It seems logical that that would be the case...

K Borowski · June 6, 2009

Hi, what's a meter? Especially in LA?

<_<

Chris Keth · June 6, 2009

Thank you Chris for your informative and quick reply.

I have one more simple question, do the larger entrance pupils in longer focal length lenses become a factor in their shallower depth of field? It seems logical that that would be the case...

Not such a simple question, really. I have a feeling that has something to do with it (like a larger pinhole makes a softer image than a small one, ignoring diffraction), along with the greater magnification of the longer lenses. I admit I can't explain all of the reasons long lenses have less depth of field than wide ones.

Hi, what's a meter? Especially in LA?
<_<

On digital shoots, it's that thing the DP puts in my frontbox and then forgets about. :lol:

K Borowski · June 6, 2009

On digital shoots, it's that thing the DP puts in my frontbox and then forgets about. :lol:

Good answer Chris. In this country, a meter shouldn't be anything other than that :-p

Chris Keth · June 7, 2009

Good answer Chris. In this country, a meter shouldn't be anything other than that :-p

What about for posing when the EPK crew is around?

Tom Jensen · June 7, 2009

What about for posing when the EPK crew is around?

Whether shooting film or video, one should be ready to pose when the EPK arrives.

Kirsty Stark · June 8, 2009

Hi Chris,

Just wanted to check... you've used double and half distances, and double and half-sized openings in your example.

I know you said that the measurements are only theoretical, but the double and half theory would still be correct, yes?

Chris Keth · June 8, 2009

Hi Chris,

Just wanted to check... you've used double and half distances, and double and half-sized openings in your example.

I know you said that the measurements are only theoretical, but the double and half theory would still be correct, yes?

Yes, the measurements used could be verified by testing. They do happen to be different by factors of 2 because it makes typing up the post easy and calculator-free but you could do the same thing with odd numbers.

If you wanted to, you could do the same thing with any diameter and length tube that is blacked out in the inside. For example, if you used a 10 inch 2.5 inside diameter tube (f4) and then accounted for the 3 stop difference, you could use a 10 inch long .91 inch inside diameter tube (f11) as the other part of the experiment.

Edited June 8, 2009 by Chris Keth