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Old Jan 16, 2010, 10:08 AM   #1
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Default Lookin for help with the "Reciprocity Rule"

Composition doesn't seem to be a huge issue with me. Not perfect by any means, but it's not where I need to improve most. As an amateur photographer, I'm trying to pin down mastering exposure control. It's obvious that you can't arbitrarily change dials and controls and expect a great photo. Can anyone recommend any good web resources (be it on Steve's or elsewhere) that can help with nailing down the relationships between ISO, Aperture, and Shuter speed? I'm looking for something that will help my brain process the "rules" faster in the field.

I'm sure you understand there's nothing more frustrating than missing a great shot because you need to do math problems in your head.

Are there any shorcuts I can try to remember as starting points? For example, if I'm shooting at ISO 1600, my shutter speed should be around 1 sec, and aperture around f/12? If I'm completely off base here, please let me know. I'm looking to learn.
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Old Jan 16, 2010, 11:04 AM   #2
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The amount of light you have is missing from that.

Yes, there are some "rules of thumb". But, your camera has a meter to measure light levels. That's what it's for. ;-)

You can't just use any combination of settings you want to. For example, if you try to use a shutter speed that's too fast for the lighting, iso speed and aperture, you'll get an underexposed (too dark) image. If you try to use a shutter speed that's too slow for the lighting, aperture and ISO speed, you'll get an overexposed (too bright) image.

I answered a similar question earlier this morning. So, I'll copy the contents of that post into this one. I probably need to create a sticky with some answers to some of those types of questions.

You have 4 main variables to take into consideration for exposure (and I use the term "main" since there are a lot of nuances to how you measure the light, for example, your metering mode), as well as different film characteristics if shooting film, and camera settings if shooting digital for the desired tone/contrast curve within an image and more). Once you have a better idea of how these 4 variables work together to give you a properly exposed image, the other fancy features will make more sense. These variables are light, aperture, ISO speed and shutter speed.

Light is typically measured as EV for Exposure Value in Photography.

Aperture (which works similar to the pupils in your eyes, where you can open up the aperture iris wider to let in more light, or close it down to let in less light). If you let in more light with a wider aperture, you can expose the film or sensor faster. If you let in less light with a smaller opening, it takes longer to expose the film or sensor. Note that aperture is normally expressed as f/stop, which is a ratio between the focal length of the lens and the diameter of the aperture iris. So, smaller values represent a larger iris diameter.

When you vary the aperture, you're controlling the iris in the lens (which like a pupil in your eye, can be opened up to let in more light or closed down to let less light in). So, this impacts the shutter speeds you'll need for proper exposure (since more or less light is getting through to the sensor). Aperture also impacts Depth of Field.

The aperture scale in one stop increments (with larger than f/1 apertures possible but very rare in lenses) goes f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22... With each one stop move to a smaller aperture (represented by higher f/stop numbers), you will need shutter speeds twice as long for proper exposure for the same lighting and ISO speed (only half the light gets through compared to a one stop larger aperture).

ISO speed. This is how sensitive the film or sensor is to light and is the same thing as the older ASA rating for film. The higher the ISO speed, the faster you can expose it (each time you double the ISO speed, you can use shutter speeds twice as fast for the same lighting and aperture.

Shutter Speed is how long the camera's shutter stays open to expose the film or sensor).

IOW, it all boils down to how sensitive the film or sensor is to light (which you control via the ISO or ASA speed of the film you use with film, or the ISO speed settings you use with digital), and how much light you need to let it see to "expose" the image (which you control via the aperture opening size and shutter speed).

So, when you see someone mentioning a concept like a one stop darker exposure, they can mean using a shutter speed twice as fast for the same lighting, aperture and ISO speed. Or, they can mean using a one stop smaller aperture (higher f/stop number, as in going from f/4 to f/5.6) for the same lighting, ISO speed and shutter speed. Or, they can mean using a one stop lower ISO speed setting (for example, going from ISO 400 to ISO 200, decreasing the sensitivity by half) for the same lighting, aperture and shutter speed, etc. These variables all work together to insure a properly exposed image.

This exposure simulator may help you to understand it better.

http://www.photonhead.com/simcam/shutteraperture.php

Note that aperture also influences depth of field. See this handy calculator for more information about it:

http://www.dofmaster.com/dofjs.html

Note that a good book on basic photography can also help you understand these concepts (and it doesn't have to be specific to digital, as the same exposure concepts apply to film).

But, your meter can do the work for you in most lighting (although with very long exposures a meter may not help a lot).

If you're in Av (Aperture Value, a.k.a., Aperture Priority) mode and vary the aperture (your f/stop setting), you're controlling the iris in the lens (which like a pupil in your eye, can be opened up to let in more light or closed down to let less light in). When you use Av mode, you select the desired aperture (usually via a control dial). Then, the camera selects the correct shutter speed for proper exposure, based on how it's metering the scene.

Note that you also have more than one metering mode (spot, center weighted, and multi-segment) with most cameras that will influence what it thinks is needed for proper exposure, based on the areas of the scene it's evaluating.

If the camera is metering the scene where you end up with a darker or brighter exposure than desired, you can use Exposure Compensation to get a brighter or darker exposure. If you use a -EV setting (needle to the left of center), you'll have a darker exposure than the camera would have used. It gives you a darker exposure by using a faster shutter speed with the selected Aperture.

If you use a +EV setting (needle to the right of center), you'll have a brighter exposure than the camera would have used. It gives you a brighter exposure by using a slower shutter speed with the selected Aperture.

If you use manual exposure (setting both the aperture and the shutter speed), that takes the place of Exposure Compensation because you're setting both. Just use the meter in your viewfinder to see how your settings are working (if the needle or display is left of center, you'll be exposing darker than the camera metered the scene). If it's to the right of center, you'll be exposing brighter than the camera measured the scene.

Also note that a variety of settings combinations will give you the same exposure. For example, if the lighting was the same and your iso speed is set the same, these combinations would all result in the same exposure:

1/800 second at f/2.8
1/400 second at f/4
1/200 second at f/5.6
1/100 second at f/8

But, you may want to use different aperture settings for depth of field purposes (or a different combination resulting in faster or slower shutter speeds for other purposes, like freezing a moving subject).
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Old Jan 16, 2010, 11:31 AM   #3
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The Reciprocity Rule you are looking for is likely the Rule of Thumb which states that for a given focal length, in order to avoid motion blur, the shutter speed should be the reciprocal of the focal length. I.E. for a 200mm lens, shutter speed should be 1/200s, and you should choose ISO setting and aperture to give you this minimum.
This is just a rule of thumb, though, and Image Stabilization or use of tripod will change the values. How you apply the rule will also depend on how much DOF you need in the shot. (more DOF means smaller aperture, which would require higher ISO)
Apologies if this isn't what you meant.

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Old Jan 16, 2010, 12:31 PM   #4
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Jim,

thanks for the great explanation. Particularly the part about the light meter. I'm still figuring my way around the viewfinder and interface. Wasn't 100% clear that when metering the shot, that actually told me how my exposure was. That'll be a huge help. Question though, where the exposure meter is in the viewfinder, when I press the shutter halfway, there is a series of bars under the exposure meter, that show up and slowly disappear while auto focusing. What are they, and what are they doing exactly? I've attached a screen shot of the viewfinder LEDs I found on the web to show you what I mean.

VT,

That is a round about way of what I was referring to. Specifically, I'm looking to try and get some baselines in my head for reference. Sort of as "a start here and adjust accordingly" type of thing. Here's a link I found that explains it a little, I'm just not too sure how accurate it is. Also, I'm using a Canon T1i, so my understanding is that I need to actually measure 1.6 x the focal length to measure shutter speed, correct?

http://www.diyphotography.net/exposure_demostrated
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Old Jan 16, 2010, 12:36 PM   #5
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That's your meter. See the bottom half of my post above, where I'm talking about the needle being to the left of center if your settings are going to give you a darker exposure than the camera is metering, or to the right of center if your settings are going to give you a brighter exposure than the camera is metering. When it's in the Center, your settings match up with how the camera is metering the scene. Only instead of a needle, you're looking for the bar that's lit up with your camera. ;-)
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Old Jan 16, 2010, 12:45 PM   #6
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Quote:
Originally Posted by fbords View Post
...Also, I'm using a Canon T1i, so my understanding is that I need to actually measure 1.6 x the focal length to measure shutter speed, correct?
The rule of thumb for a hand held camera is to make sure your shutter speeds are the reciprocal of the 35mm equivalent focal length. In other words, your shutter speed should be 1/35mm equivalent focal length or faster.

By equivalent, I mean the same angle of view you'd have with a given focal length on a 35mm camera.

With Canon models with an APS-C size sensor like your T1i, you multiply the focal length by 1.6x to see how they compare. For example, a 50mm lens on a Canon dSLR with an APS-C size sensor would give you the same angle of view you'd have using an 80mm lens on a 35mm camera (50mm x 1.6 = 80mm).

So, in other words, you'd want to make sure your shutter speeds are 1/80 second or faster to help reduce blur from camera shake if you're not using a tripod (or stabilization via a stabilized lens). Note that this rule is only for camera shake, not blur from subject movement (which may require much faster shutter speeds).

But, that's only a rule of thumb (as some people can hold a camera steadier than others, and some people may require even faster shutter speeds to prevent blur from camera shake.

However, that is unrelated to the exposure questions you were asking That's only a rule of thumb for shutter speeds needed to reduce blur from camera shake.
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Old Jan 16, 2010, 12:57 PM   #7
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Quote:
Originally Posted by JimC View Post
That's your meter. See the bottom half of my post above, where I'm talking about the needle being to the left of center if your settings are going to give you a darker exposure than the camera is metering, or to the right of center if your settings are going to give you a brighter exposure than the camera is metering. When it's in the Center, your settings match up with how the camera is metering the scene. Only instead of a needle, you're looking for the bar that's lit up with your camera. ;-)
I understand that from your original reply. My question though, is why do the lines show up and disappear? They show up before numbers and needle do on the meter, and they disappear in a count down fashion while I hold the shutter button. Is that like a progress bar while metering the shot? Sorry, I may not be explaining it clearly.
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Old Jan 16, 2010, 1:00 PM   #8
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No, it's not a progress bar. It's an indication of how your settings are impacting exposure.

With most cameras, the meter turns on with a half press of the shutter button. After you release the shutter button, it will only stay on for a finite period of time (which varies by camera model). The meter indicates how your settings are impacting exposure based on what you're pointing it at.

With some models, exposure is locked with a half press (if you hold the shutter button down that way). With most models, it's not (a half press is only locking focus, not metering, so metering will still vary if lighting is changing). Note that if you're shooting in non-manual modes, it's going to try and keep it centered (varying shutter speed and/or aperture to insure proper exposure), unless you use Exposure Compensation to move the indicator right of center for a brighter exposure, or left of center for a darker exposure). So, it's not of any value other than to adjust Exposure Compensation.

IOW, in an Auto type mode (programmed auto, aperture priority, shutter priority, etc.), it's only purpose is to let you use Exposure Compensation to change the camera's behavior and see how you're impacting exposure (unless you are trying to use settings that are outside of the range the camera or lens is capable of supporting for the conditions you're shooting in, like using a faster shutter speed in Shutter Priority mode than the lens can support, because it can't open up it's aperture any wider, resulting in an underexposed image).
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Old Jan 16, 2010, 1:13 PM   #9
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IOW, if you're using Manual Exposure with something like these settings at ISO 100:

1/400 second, f/4

... then, you may see the needle to the left or right of center (telling you if you'll have an underexposed or overexposed image using those settings).

If it's one stop the to left of center (exposure would be too dark), just slow down your shutter speed to 1/200 second instead (exposing it for twice as long) which should bring the indicator back to center (unless you wanted a darker exposure). Or, you could simply use a wider aperture instead (for example, f/2.8 versus f/4, which lets in twice as much light) to accomplish the same thing. Or, you could use a higher ISO speed (ISO 200 versus ISO 100, which is twice as sensitive) to accomplish the same thing (expose the image one stop brighter so the the needle is back in the center).
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Old Jan 16, 2010, 1:42 PM   #10
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Jim,

Thanks again. As always, your information is very useful. I'll try playing around with the Exposure meter some more today and see what I can do. This may be the help I needed.
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