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#1
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What's the Difference Between Aperture Setting, Exposure Compensation, and ISO?
I'm not a left-brained (or is it right-brained?) technical type, and it
seems all aspects of digital and traditional photography basically all deal with how much light is allowed into, and then captured successfully or unsuccessfully, by a camera. I've searched the archives on this group for exposure compensation, because I don't understand how it works or why it's even necessary, if a photographer can manipulate aperture setting, shutter speed, and ISO rating. |
#2
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What's the Difference Between Aperture Setting, Exposure Compensation, and ISO?
You are absolutely right - in terms of the lightness or darkness of the
final image there are only two factors: how sensitive the recording media is and how much light is let into the camera. The sensitivity of the media (digital sensor or film) is expressed by the ISO rating, which can be set by the photographer in the case of digital sensors. How much light is let in is the product of the area of the lens opening (aperture) and the length of time the aperture remains open (shutter speed). Now, if the photographer is controlling both of these factors manually (aperture and shutter speed), you are again right that there would be no need at all for "exposure compensation". But if you allow the camera to automatically set these values (as many of us do) for the sake of convenience, there may be cases where you want the camera to apply an offset to its normally calculated values in order to compensate for some condition in the scene (an overall bright scene in which the object of interest is much darker, a conscious decision to render the scene darker or lighter than normal to create a special mood, etc.). For this you have the setting for "exposure compensation" which is a user selectable amount of offset to the standard exposure calculation. You are continuing to allow the camera to calculate the exposure (so that it will quickly adjust to changing lighting conditions without input from you), but asking it to modify its normal exposure to either lighten or darken the final result to suit your creative vision. For any given value of exposure compensation the degree of offset from "normal" stays the same even though the actual exposure values (aperture and shutter speed) for each frame may vary if the lighting conditions vary. |
#3
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What's the Difference Between Aperture Setting, Exposure Compensation, and ISO?
Exposure compensation, is really only necessary if you rely heavily on
a camera's exposure meter to determine correct exposure, and you encounter an extremely bright scene (lots of snow, light sand, many people wearing white / pale clothing) or dark scene (night-time, large expanses of dark colours) , which may cause the light meter to under or overexpose that scene. Background: Light meters are usually programmed and calibrated by the manufacturer to interpret the overall light from a scene as a medium shade of grey. So when a light meter encounters a predominance of bright shades or dark shades, it tries to under or over expose them , so they become rendered as that medium shade of grey. If your scene is predominantly a mix of 'medium' colours such as tree foliage, blue sky, grey fences and telegraph poles, grey bitumen , brown rocks, people with a healthy tan complexion, which are all approximately the same brightness as that medium grey, the exposure compensation shouldn't be necessary. When you use exposure compensation, you are sort of ignoring your camera's suggested exposure, and telling it to momentarily forget what it has been programmed to do. You tell it to deliberately overexpose those bright scenes so the whites turn out truly white instead of grey, and underexpose those dark scenes, so the blacks turn out truly black instead of grey. The exact amount of excess brightness / darkness will determine the amount of compensation required. So you manipulate the camera's recommended exposure by using more exposure than recommended for very bright scenes, and less exposure than recommended for very dark scenes. I don't know exactly how your camera indicates 'correct', 'under' and 'over' exposure - maybe by flashing lights of zero, plus and minus ? Or by aligning little pointer bars so they either meet up, or a moving pointer is above/below a steady pointer. If the former, 'zero' light means what the camera thinks is 'correct', plus means the camera thinks the picture is over exposed, minus means the camera thinks the picture will be under exposed. In the case of pointer bars, when the pointers are aligned together, the camera thinks exposure is 'correct', when the movng pointer is above or to the right of the steady pointer, the camera thinks exposure is 'over', when the movng pointer is below or to the left of the steady pointer, the camera thinks exposure is 'under', Now, what's the difference between aperture , compensation, and ISO ? The aperture inside the camera lens is one of the tools you use to control the exposure. By increasing the size of the aperture (turning the aperture settings towards the numerically smaller numbers : f1.4, f2, f2.8, ) or decreasing the aperture size (turning the aperture settings towards the numerically larger numbers : f11, f16, f22), you increase the exposure or decrease the exposure respectively. Compensation (explained above) is achieved by (among other things) changing the aperture setting to increase the exposure to more than recommended, or decrease the exposure to less than recommended. In other words, compensation is the end effect or result, and the aperture setting is a method of achieving that result. Compensation can also be achieved by manipulating the camera's ISO setting, to 'fool' the light meter into believing that you have a higher or lower ISO than what's really the case. So, again, compensation is the effect or result, and maniuplating the ISO is another method of achieving that result. The effect of lowering the ISO is the same as increasing the aperture (increasing exposure), and the effect of increasing the ISO is the same as decreasing the aperture (decreasing exposure). Lower ISOs need more exposure, higher ISOs need less exposure. Don't know if I've clarified things or further confused you, but that's my 5c worth of explanation. regards daba wrote: I'm not a left-brained (or is it right-brained?) technical type, and it seems all aspects of digital and traditional photography basically all deal with how much light is allowed into, and then captured successfully or unsuccessfully, by a camera. I've searched the archives on this group for exposure compensation, because I don't understand how it works or why it's even necessary, if a photographer can manipulate aperture setting, shutter speed, and ISO rating. |
#4
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What's the Difference Between Aperture Setting, Exposure Compensation, and ISO?
imagejunkie wrote: You are absolutely right - in terms of the lightness or darkness of the final image there are only two factors: how sensitive the recording media is and how much light is let into the camera. The sensitivity of the media (digital sensor or film) is expressed by the ISO rating, which can be set by the photographer in the case of digital sensors. How much light is let in is the product of the area of the lens opening (aperture) and the length of time the aperture remains open (shutter speed). Now, if the photographer is controlling both of these factors manually (aperture and shutter speed), you are again right that there would be no need at all for "exposure compensation". But if you allow the camera to automatically set these values (as many of us do) for the sake of convenience, there may be cases where you want the camera to apply an offset to its normally calculated values in order to compensate for some condition in the scene (an overall bright scene in which the object of interest is much darker, a conscious decision to render the scene darker or lighter than normal to create a special mood, etc.). For this you have the setting for "exposure compensation" which is a user selectable amount of offset to the standard exposure calculation. You are continuing to allow the camera to calculate the exposure (so that it will quickly adjust to changing lighting conditions without input from you), but asking it to modify its normal exposure to either lighten or darken the final result to suit your creative vision. For any given value of exposure compensation the degree of offset from "normal" stays the same even though the actual exposure values (aperture and shutter speed) for each frame may vary if the lighting conditions vary. Excellent explanation, simple and to the point! |
#5
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What's the Difference Between Aperture Setting, Exposure Compensation, and ISO?
wrote in message ups.com... I'm not a left-brained (or is it right-brained?) technical type, and it seems all aspects of digital and traditional photography basically all deal with how much light is allowed into, and then captured successfully or unsuccessfully, by a camera. I've searched the archives on this group for exposure compensation, because I don't understand how it works or why it's even necessary, if a photographer can manipulate aperture setting, shutter speed, and ISO rating. You are right. It is most useful in auto-exposure modes to compensate without switching to manual exposure for under/over-exposures caused by subject reflectances that are not average (snow, coal-bins are some extremes...;-) In thorough form, exposure is a balance between the amount of available subject light, sensor/film sensitivity (ISO/ASA), lens aperture (f-stop), and shutter speed. Lower subject light requires a higher sensor/film sensitivity, and/or a wider aperture, and/or a slower shutter speed. Sensor sensitivities come in numbers like 50, 100, 200, etc., with each higher permitting shooting in 1/2 the light level (1/3rd in-between values also exist). Apertures come in whole values of f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32, etc., and these (with larger numbers) represent 1/2 the light exposure as you move up in value (in-between values exist, and many lenses with separate aperture rings permit infinitely variable settings for precise exposure settings). Shutter speeds come in whole values (in seconds) of 4, 2, 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/125, 1/250, etc. (each exposes 1/2 the value of the one before, and many cameras permit in-between values, especially in some auto modes). The choices of what to use depend on conditions. Low light requires high sensor/film sensitivity (400 or so, risking greater noise/grain), and/or a wide aperture (f2.8 or so, risking little depth of field and so-so lens performance [the latter is also a problem at stops of f16 and smaller due to diffraction]), and/or a slow shutter speed (like 1/15, 1/30, etc., risking camera shake - most evident at a speed about 1/lens-focal-length=shutterspeed [like for 125mm, figure the shutter speed should be at or higher than 1/125mm = 1/125th second] or slower). As a result of the above, many combinations give the same exposure, like ISO 200, f5.6, and 1/60th second equals ISO 200, f8, and 1/30th second, which equals ISO 400, f8, and 1/60th a second (for a given available light level...). Either the shutter speed in Shutter priority mode (watching for appropriate aperture selection by the camera), or Aperture in aperture priority mode (watching for appropriate shutter speed selection by the camera), or Program mode (watching to see that appropriate values have been selected by the camera - and many cameras allow you to "slide" the equivalent values to some more to your liking) can be used under any lighting condition . In addition, many cameras permit you to bias the exposure toward darker or lighter if you do not like the average value of the brightness of the pictures you get from your camera (judged on a reasonably well calibrated monitor). Personally, I prefer to use "A" mode and occasionally add a bit of exposure composition - but others prefer other ways of working. And, the flash on most cameras can be turned off when not needed... So there...! ;-) -- David Ruether http://www.ferrario.com/ruether |
#6
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What's the Difference Between Aperture Setting, Exposure Compensation, and ISO?
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#7
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What's the Difference Between Aperture Setting, Exposure Compensation,and ISO?
I'm new to digital cameras so I'll ask this question:
On a film camera exposure compensation and changing the ISO you set the camera to are exactly the same thing, since the film is fixed. However, on a digital camera, while the sensor is of course fixed, how the processor processes the analog out signal is not necessarily fixed. It can and surely does change how analog levels are converted to bits, even in the "raw" files. Because of this. it is not clear to me that exposure compensation and changing the ISO setting are identical. I suspect they are not. I have been using exposure compensation for getting rid of white clipping (Canon 30D) and making the image viewed on the camera LCD screen "correct". (I use raw files, however.) Is this the correct way? Explanation requested. Doug McDonald |
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