Notes on Photography

Here are a few random notes that seem interesting. Some of these words of wisdom are from my own experience, some I have picked up from others.

Exposure

• A white card is 5 stops brighter than 18% gray.
• However, another source suggests that a white card is only 3 stops brighter than a grey card. The 5 stops is for most-brilliant-white on the zone scale, which is different from a white card.
• A rule of thumb ... If metering at night from a white card when there isn't enough light for metering off of gray, only adjust 2.5 stops brighter, or so. If you go all 5 stops, the shadows will move too far up the exposure scale.
• Human caucasian skin is 1 stop brighter (Zone VI) than a grey card.
• Illuminated buildings @ night -- f/2.8 @ 1/30 with ASA 400 film

Lens Testing and MTFs

• MTF is measured along a diagonal line from the center of the film out to one corner.
• Sagital resolution is measured with lines are aligned parallel the above diagonal. In other words a radial line.
• Tangential resolution is measured with lines that are aligned perpendicular to the above diagonal. In other words, at a tangent to the radial which is the MTF line.

Macro Photography and Lens Issues

Otherwise known as Bellows Factor???

• Minimum lens extension occurs at infinity focus. Focussing closer requires extending the lens.
• For 1:1 magnification, you need a extension equal to twice the focal length of a lens. Supposing a 50mm lens, one needs 50mm of extension for 1:1. A 1/2 focal length extension of 25mm is needed for 1:2. A 1/4 focal length extension of 12.5mm is required for 1:4.
• There is a corresponding loss of light due to lens extension. At 1:1 magnification a 2 stop (4x) loss. At 1:2 magnification a 1 stop (2x) loss. At 1:4 magnification a 1/2 stop (1.4x) loss.
• Both of these effects occur even with the slight amount of extension required for focusing. There is a small increase in magnification, and a small loss of light.
• Stopping down a lens increases the size of the covering area. This is why vignetting occurs wide-open and goes away as you stop down. For view cameras and shift lenses it means that you want to stop down more to allow for larger (or any) movements without vignetting.

Lens Mounts

• M39 lens mount is Leica standard screwmount ... which is also a common lens mount for enlargers.
• M42 lens mount is Pentax standard Screwmount.
• Registration distance is the distance from the face of the mount to the film plane.
• Pentax 35mm k-mount registration is 45.46mm.
• Pentax 67 registration distance is 84.95mm.
• XXX find more of these register distances.

Stops

For aperture stops are based on aperture diameter. +1 stop == 2x light. -1 stop == 1/2 light.
Full Stops are: 1, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45
Half Stops are: 1.2, 1.7, 2.4, 3.4, 4.8, 6.7, 9.5, 13.5, 19, 27, 38

For Film stops are based on light sensitivity. 2x rating == 1/2 exposure., 1/2x rating == 2x exposure
Full Stops are: 12, 25, 50, 100, 200, 400, 800, 1600, 3200, 6400
Third Stops are: 16, 20, 32, 40, 64, 80, 125, 160, 250, 320, 500, 640, 1000, 1250, 2000, 2500

For Shutter speeds, typically half stops are available, and those are pretty obvious. I'll just list full stops.
Full Stops are: 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000

For Flash, stops are based on distance, and on flash power:

Darkroom

• JOBO Plastic tanks are IR-proof; not all plastic tanks are IR-proof. This matters if you are processing IR film!
• Beware -- plastic black-out cloth may not block IR.