what is relation of ISO and Apeture (F/Number)

[legolas]

Hi
Thank you for reading my post
Can some one explain what is relation of ISO and F/number?
In what situation we change ISO to keep F/number without change?
I saw many Macro photos that has big iso number, is that a rule of thumb to have big iso number and small apeture for Macro photos?

here is the link to those photos I said.
http://www.treknature.com/photos.php?cat=lens&id=473

[Bynx]

F-number is the size of the aperture letting light in. ISO is the sensitivity to that light. You always want the ISO to be as low a number as possible. As the number gets higher so the noise increases. You would only want to use a high number as a last resort to increase the shutter speed if necessary, to increase the F-number for increased depth of field if necessary, or under low light conditions to have the sensitivity to get the shot keeping in mind the picture will have a fair amount of noise.

[Rhys]

ISO - the higher the number the more noise
Aperture - the higher the number the greater the focal depth
Shutter - the higher the speed the less the risk of blurred images.

Yes - they are mutually exclusive. The art of photography is in selecting a happy medium while ensuring great photos.

[griptape]

They have no direct relation to each other. They're simply measurements of how much light enters the sensor, and how sensitive the sensor is to that light. You're likely seeing high ISO wide aperture shots for macro because flash (without a flash ring) can create hard to avoid, and often time undesirable shadows, and when your lens/flash is close to an object, it tends to leave a ring shaped shadow around the object. So the pictures you described, the settings were probably just to avoid using the flash.

If your question is "what settings are better to get more detail at very close distances", then the answer to that is... the lens. The minimum focus length of the lens can't be changed by any exposure settings.

[Rooz]

griptape indicated the main reason in this case becasue the flash was used to freeze the motion of the insects. so to try and bring out as much background as possible and minimise shadows, he used a high iso.

also remember that a correct exposure is a combination of aperture, (fstop), shutter speed and ISO. so the other reason to use a high iso is to keep the shutter speed up high while maintaining the desired depth of field. you can use any one of those variables to achieve the exposure you want.

i strongly recommend Bryan Petereson's book, "understanding exposure."

btw: since you have the sigma and are intersted in macro, you may find this link interesting. :)
http://www.flickr.com/photos/xtwizx/

[Rhys]

They have no direct relation to each other. They're simply measurements of how much light enters the sensor, and how sensitive the sensor is to that light. You're likely seeing high ISO wide aperture shots for macro because flash (without a flash ring) can create hard to avoid, and often time undesirable shadows, and when your lens/flash is close to an object, it tends to leave a ring shaped shadow around the object. So the pictures you described, the settings were probably just to avoid using the flash.

If your question is "what settings are better to get more detail at very close distances", then the answer to that is... the lens. The minimum focus length of the lens can't be changed by any exposure settings.

Eh? They do have a direct relation to each other.

I'll try to post an EV table.


a p e r t u r e v a l u e
1.0 1.4 2.0 2.8 4.0 5.6 8.0 11 16 22 32 45 64
s 1 0 1 2 3 4 5 6 7 8 9 10 11 12
h 2 1 2 3 4 5 6 7 8 9 10 11 12 13
u 4 2 3 4 5 6 7 8 9 10 11 12 13 14
t 8 3 4 5 6 7 8 9 10 11 12 13 14 15
t 15 4 5 6 7 8 9 10 11 12 13 14 15 16
e 30 5 6 7 8 9 10 11 12 13 14 15 16 17
r 60 6 7 8 9 10 11 12 13 14 15 16 17 18
125 7 8 9 10 11 12 13 14 15 16 17 18 19
s 250 8 9 10 11 12 13 14 15 16 17 18 19 20
p 500 9 10 11 12 13 14 15 16 17 18 19 20 21
e 1000 10 11 12 13 14 15 16 17 18 19 20 21 22
e 2000 11 12 13 14 15 16 17 18 19 20 21 22 23
d 4000 12 13 14 15 16 17 18 19 20 21 22 23 24

Ok. That didn't paste all that well but...

This will be fine for most light levels at 100 ISO. If you use a higher or lower ISO then the EVs have to be raised or lowered.

This should go some way toward showing the relationship between ISO, shutter and aperture.

[griptape]

For the purpose of answering the OP's question, there is little direct relation. Yes, there are formulas for how to let in the same amount of light, but add shutter speed in there and there's 100 different ways to get the exact same exposure with different ISO, Shutter Speed, and F numbers. So yes, they are related, but are also independent.

[Rhys]

For the purpose of answering the OP's question, there is little direct relation. Yes, there are formulas for how to let in the same amount of light, but add shutter speed in there and there's 100 different ways to get the exact same exposure with different ISO, Shutter Speed, and F numbers. So yes, they are related, but are also independent.

ISO, shutter speed and aperture are independently controlled but all 3 are needed in order to take a photo so they are very closely related.

[sla]

In analog photography sensitivity is also called "film speed".
Film speed is used in the exposure equation to find the appropriate exposure parameters. Four variables are available to the photographer to obtain the desired effect: lighting, film speed, f-number (aperture size), and shutter speed (exposure time).
More details in wikipedia. (http://en.wikipedia.org/wiki/Film_speed)

regards

[DonSchap]

ISO is considered to be your media sensitivity to light. It is usually measured in full f-stop steps ... but recently has been divided up into 1/3 stop steps in some of the more advanced digital cameras.

If possible, the lowest available ISO setting is 50 in digital cameras, but normally is 100. So, for the sake of argument ... let's say 100 is the least sensitive ISO-setting you can have.

The next setting is 200 ... with is effectively an f-stop higher in sensitivity or TWICE as sensitive to light striking it. On it it goes ... doubling the setting, doubles the sensitivity.

100 -> 200 -> 400 -> 800 -> 1600 -> 3200 -> 6400 (only recently made available)

The problem usually encountered with the ISO set to anything higher than 800 is the introduction of "digital noise" ... because the sensor is being energized to such a level, it "creates" what looks to be distortions in shadow areas ... and solid color areas. This is not a desirable effect ... and can be minimized through use of either software ... or some tedious touch-up work with a post-processing program, such as Photoshop.

In low light situations ... where color is already pretty minimal, this can be also be quickly minimized by converting the color image to a grayscale image and printing that instead.

Aperture does not play with the sensitivity of the digital sensor, at all, but is a way of optically restricting the amount of light getting through the lens and eventually hitting the sensor.

Again, this is measured in F-stops or 1/3 portions thereof. Remember, the lower the f/ #, the wider the aperture is.

Scale
Widest
Aperture
F/1 - (usually) This setting allows the most amount of light to the sensor.
f/1.4 - light is cut by half - 1/2 original light hitting lens
f/2.0 - light is cut by half, again - 1/4 original light hitting lens
f/2.8 - light is cut by half, again - 1/8 original light hitting lens
f/4 - light is cut by half, again - 1/16 original light hitting lens (use flash indoors)
f/5.6 - light is cut by half, again - 1/32 original light hitting lens (use a brighter flash)
f/8 - light is cut by half, again - 1/64 original light hitting lens (better get some sun)
f/11 - light is cut by half, again - 1/128 original light hitting lens
f/16 - light is cut by half, again - 1/256 original light hitting lens
f/22 - light is cut by half, again - 1/512 original light hitting lens
f/32 - light is cut by half, again - 1/1024 original light hitting lens
f/44 - light is cut by half, again - 1/2048 original light hitting lens
f/64 - light is cut by half, again - 1/4096 original light hitting lens
f/89- light is cut by half, again - 1/8192 original light hitting lens
f/128- light is cut by half, again - 1/16384 original light hitting lens
f/172- light is cut by half, again - 1/32768 original light hitting lens
Narrowest in use -> Pin hole camera = f/196

Okay ... how do you compensate for such little amounts of light? The answer is: TIME!

That brings us to shutter speed ... or how long do we let the light hit the photo-sensitive media or sensor.

As with most of the other light relationships, the amount of time has an additive effect. It you double the time, you double the amount of light (only if the light source is constant, of course).

Now shutter speed and the sensor do have a serious part to play, together. The sensor can become totally saturated if exposed for too long a time ... and that effectively neutralizes any sensitivity you may want to introduce, so watch your time with bright light sources.

So ... doubling the time. The shutter setting of 1/250 second allows a certain amount of available light to the sensor in 250th of a second. If we need to brighten the image, without doing anything else than altering the time, we double the length of it ... and thereby double the intensity of the brightness. Set the shutter speed to 1/125th and it is done.

Not bright enough ... double it again, set to 1/60th. So on and so forth. The unfortunate side-effect (or fortunate ... depends on what you are trying to depict) is that things change over time. The shutter stays open longer ... if anything in the frame-area (that area you have the camera's lens pointed at) moves ... while the shutter is open ... the light coming from that object moves, too ... and you will see it as a motion blur.

Normally, sitting people stay still for about 1/60th of a second, then you get blinks, hand movement, toe-tapping, head-bobs ... etc.

If people are walking, 1/250th may get you there ... 1/500 makes it certain.

Sports ... 1/800 sec. or faster.

Shutter-speed conveys movement. You want more ... leave the shutter open longer. Freeze 'em ... speed it up.

I hope this has been informative.

Good luck ;)

[AcroRigo]

DonSchap, Great Information! Good to see people with as much knowlege as yourself willing to take the time to share it! :cool:

[TheWengler]

i strongly recommend Bryan Petereson's book, "understanding exposure."

Hey Rooz, how advanced is that book? I've been looking for a good digital photography book and have heard this one mentioned a couple times. I don't necessarily need something that starts from square one as I would consider myself on more of an intermediate level.

[TheObiJuan]

Consider that book required reading.

Don, great write up.
That should be quoted for how easy it is to read and understand.