Once and for all! Explain ISO?

[ronaldhanko]

Originally Posted by Tindomul

Ok, thats cool. So change the F-stop to what. How do you know what to change it to, and I forgot what f-stop means. Sorry, thanks

Tindo,

F-stop is the size of your aperture, i.e., the size the shutter closes down to when the picture is taken. The higher the f-stop number the smaller the aperture, and a smaller aperture is desirable to get greater depth of field (more of the subject in focus).

I remember how confusing I found all this at first, especially because the higher the f-stop the smaller the aperture, but if you keep working at it, it becomes second nature.

[Tindomul]

Thanks!

[Call_Me_Bob]

it took me SSOOOOO long to remember all this

[Catfur]

Exposure numbers, the quick and dirty guide:

Part 1, Aperture
We'll start here because it has terminology I want.

The aperture is technically the hole in the middle of your camera lens, it is formed by the blades of the iris (or diaphragm) (the hole is the aperture, not the blades). It is located at the point where the theoretical light rays passing through the lens are at their most constricted. The image of the aperture on the front element of the lens is called the entrance pupil, it's this image (not the aperture itself) which is measured by the f-stop (or f-number), it can be smaller or larger than the aperture itself (the image, that is). The ratio of the size of the entrance pupil to the focal length of the lens is called the f-number. It's a dimensionless number, the reason it gets "bigger" as the number gets smaller is because it's really a ratio (it's 1/f-stop). F-stops are called stops, because the iris is a spring loaded device which is normally held closed, the aperture stops are mechanical stops that prevent it from closing completely at a certain point. F-stops are a geometrical progression going by the square root of 2 (rounded to two digits, so 1, 1.4, 2, 2.8 etc...) each stop represents a halving of the amount of light allowed through (it goes by powers of sqrt 2 because as you double or halve the diameter of a circle the area is quadrupled or quartered). The concept of a "stop" is used in other areas to mean the doubling or halving of the amount of light captured in the final image. On most SLR and similar cameras, the aperture is adjustable in 1/3 stop increments (3 steps between each full "stop").

Part 2, Shutter Speed

The shutter is kind of obvious, it opens, it lets light through, it closes, it stops light. A doubling or halving of the shutter speed only double or halves the light allowed, though, unlike the aperture. Shutter speed is usually doled out in "stops" like the aperture (1s, 1/2s, 1/4s, 1/8s, 1/16s, 1/30s, 1/60s, 1/125s, 1/250s) with some rounding involved, each adjustment of shutter speed by a stop has the same effect on exposure as adjusting the aperture by a stop. Shutter speed on SLRs and such is also often adjustable by thirds of a stop.

Part 3, ISO

ISO measures the film's or sensor's sensitivity to light, the higher the ISO the higher the sensitivity. ISO is linear like shutter speed, so doubling the ISO will double the exposure, and halving the ISO will halve the exposure. You may find older literature referring to ASA or DIN, ASA is EXACTLY the same as ISO, DIN is completely different, unlike ISO/ASA DIN is a logarathmic scale, each 3 increments of DIN represents a doubling of sensitivity (and, not-very-coincidentally, each 1 increment of DIN represents our old friend, the third of a stop).

In the bad old days of film, silver halide was used to capture the image, as it is a light sensitive chemical. If enough photons are captured by a grain of silver halide, it will be turned black in the development process (it's the same in color film, just with complicated dye couplings and whatnot involved). The "enough photons" to expose a grain of silver halide does not vary linearly with the size of a grain, so smaller grains are less sensitive to light, and larger grains are more sensitive, hence more sensitive film had larger grains of silver halide, which were more noticeable in the final photograph as "grain." So more sensitive film was said to be grainier. ISO 50, 64, 100 or similar film was used for highly detailed images, ISO 1600 (typically only available in B&W) was used to gain a faster shutter speed in applications like sports photography or low light. Some early films had very low sensitivities, sensitivities that in modern terms would be ISOs in the single digits or even lower. The ISO of a modern DSLR (usually only the pro models) is also adjustable in thirds of a stop increment.

When digital sensors became available in consumer applications, they were scaled so that their sensitivity labels matched those from film, a digital sensor at ISO 100 would produce an image with the same exposure, when the same settings were used (aperture, shutter speed, lighting), as one taken with ISO 100 film. Digital sensors use tiny little photodiodes instead of grains of silver halide to capture light, however, so "grain" doesn't occur in digital photography. There is a similar issue though: noise. To allow a varied range of ISO sensitivities the output of each photodiode is amplified, this amplification creates noise (in addition to the noise already present in the original image), the more the amplification, the more the noise. The noise shows up in the image as specks of differing color or brightness, and can sometimes look similar to film grain.

A photodiode's sensitivity to light is determined by its size, so bigger photodiodes will allow images with less noise (because less amplification is needed, and because there is less of a quantum effect called "shot noise"). The size of each photodiode on a given camera is fixed, but it can vary between different cameras, a "full frame" camera like the Canon 1Ds or 5D series has a sensor that can be around 25 times bigger than the sensor in a typical compact camera (it's between 2.37 and 2.6 times bigger than the sensor in an APS-C DSLR), so each photodiode can be much larger than the ones in a compact camera (and still reach much higher Megapixel densities), this is why DSLRs have much lower noise (and reach much higher sensitivities) than compact cameras.

[Tindomul]

Very kool, greatly appreciated.

[Call_Me_Bob]

good luck!

i just got out with a F-1.8 50mm lense. i love using a low f-stop to get a selected range of focus. ill post the pics later.

[Connie Star]

Great explanation by Catfur.
I would add one thing. Like shutter speed, ISO/ASA doubles or halves the amount of light required. ie, at 400 ISO you need half the light of 200. I have a Canon EOS Xti and I find I can get reasonable images with the ISO set at 400. Higher than that and noise gets to be a problem. Less expensive models may not do well above ISO 200, and more expensive cameras can go higher. I'm rather particular about grain/noise however.
So, IMO you need to experiment to get the effect that pleases you. Set up a still life with a chid, preferably with a tripod and change each variable one at a time and see what you like. And, most important, have fun! Photography and orchids go great together.

[Izzie]

Thank you everyone for such enlightening replies! Life has suddenly become very complicated (and I'm leaving for a vacation in DC until the 11th tomorrow) so I will be sure to reply and comment when I get time. Again- thank you everyone!

[Call_Me_Bob]

have fun!!

setting up an orchid shoot is always fun!! and a good time for experimenting with your camera. if you get a black velvet back drop, and some lights it can be fun, but i dont have 'real' lights, and i dont have a black velvet, so i use what i can find. i found a peice of dark maroon velvet, and since its velvet, it doesnt bounce the light as much, so you have a neutral background. and, i just used a few lamps and stuff, but it was still fun!!

[Triffid]

Lot of interesting info already, just adding a couple of links folks may be interested in.

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