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Old Dec 2, 2004, 12:49 PM   #1
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i had this as part of another discussion, however, i wanted to seperate it to avoid any confusion...here was my post...

i also have a newbie question!i'm very new to slr cameras.

in regards to the aperature range. does a digital slr camera read the aperature range from the lens and limit the settings to the range? meaning if i put in a lens with f4.5-6.7 and go to adjust the camera aperature settings, i'll only see the 4.5-6.7 range? ordo i adjust the aperature setting on the lens? if so, if i choose a aperature priority setting, how does the cameraknwo whichshutter speed would be appropriate?!

i was also reading through some of the informationfrom atutorial on lenses. part of it stated "That's why the aperture might change from f/2.6 when zoomed out to f/4 when zoomed all the way in on a subject." ... in full manual mode, if i zoomedinall the way and left my aperature at f/2.6 (smaller aperature), what would happen to the image? i'm trying to understand the direct relation with zoom and aperature.i actually found this statement:

"With zoom lenses the maximum aperture changes as you zoom the lens. It will be larger when zoomed out to a wide angle, and smaller when zoomed in to enlarge a subject."

style="BACKGROUND-COLOR: #000000"shouldn't it be the other way around though? if a smaller aperature offers greater depth of field, wouldn't i set a smaller aperature when i zoom out, in order to capture all the details at a wide angle? and when i zoom in, choose a great aperature to capture the details in front.

not sure if my questions make sense. i'm new to this, therefore, thinking of different scenarios in order to picture it..


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Old Dec 2, 2004, 2:43 PM   #2
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LOL -- that's a lot of questions. What kind of camera did you get, and what lens do you have?

I'll try to give a brief overview.

Aperture as expressed as an f/stop is is a ratio, and is determined by dividing the focal length of the lens by the size of the iris opening.

So, if you had a 50mm lens with an 18mmaperture (iris)opening, then it's aperture would be f/2.8 (50/18~= 2.8 ). Yet, if the physical size of theaperture opening (18mm in this example) did not change, and the lens was zoomedin to 100mm, then your aperture would become approximately f/5.6 (100/18=5.6).

As a general rule, metered aperture (amount of light reaching the film or sensor) and physical aperture ratio are roughly the same. That is, less light reaches the sensor through the lens when longer focal lengths are used (for a given physical size of theaperture opening).

The aperture scale (in one stop increments) 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, etc.... With each one stop move to a smaller aperture (represented by larger f/stop numbers), you will need shutter speeds twice as long for proper exposure.

Here is achart you can use to get anidea of the shutter speeds required for any EV (Exposure Value, which is how light is measured)and Aperture (but make sure to use your camera's metering, as lighting can vary -- this is only to give you an idea of how it works). It's based on ISO 100. So, each time double the ISO speed, you can use shutter speeds twice as fast:

http://home.earthlink.net/~terryleedawson/dcnotes/tables.htm

Aperture also impacts Depth of Field. Thelarger the aperture (represented by smallerf/stop numbers), and the closer you are to your subject (focus distance), and the longer your focal length (amount of zoom used), the less depth of field you will have (less of the scene in focus, as you get further away from your focus point).

A lens with a larger available aperture (represented by smaller f/stop numbers) is preferred for some subjects (so that you can blur backgrounds, helping your subject stand out). A higher quality lens with larger available apertures is also preferred for lower light (so that you can get faster shutter speeds to help prevent motion blur).

Here is an online depth of field calculator. Plug ina camera model, then change focal length, aperture and focus distance to see what impact aperture has.

Note that most Digital Cameras have a crop factor (the sensor is smaller than 35mm film, so the entire image circle is not used). For example, you must multiply the actual focal length of the lens by 1.5 or 1.6 on most Digital SLR models to get the 35mm equivalent focal length (same angle of view), since their sensors are smaller (but you must use the actual versus 35mm eauivalent focal length for depth of field computation purposes).

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

To answer your questions on how lenses communicate with a camera in aperture priority mode, etc., this is done in modern digital cameras via electrical contacts in the lens mount.If your lens has anaperture ring,you simply set the aperture to it's smallest value (represented by the largest f/stop number), and the camera then controls the aperture of the lens.

Yes, the camera is aware of the aperture range of the lens (the camera communicates with it).

Exposure is determined by several methods. If you go with manual exposure (M)mode, you set both the aperture and the shutter speed. But, modern cameras have built in metering so that they know if your settings will result in overexposure or underexposure of an image. So, this allows you to make the correct settings in manual exposure mode.

But, manual exposure can be cumbersome to use unless you have a constant light level and are shooting the same subject. This is because if you point the camera at a subject that is not as well illuminated as when you made the initial settings, you'll have an underexposed image; or if you point it at a better illuminated subject (or one with different reflective characteristics), you can get an overexposed image. So, you would need to change the settings everytime your subject or lighting changed.

Another method is autoexposure.In this mode, the camera selects both the aperture and the shutter speed for proper exposure of the image (using it's metering to determine the amount of light in the image).

Most models have a "programmed autoexposure" (P) mode that is similar. This way you have a little more control over the aperture/shutter speed combination the camera uses. For example, if the camera decided to use an aperture of f/5.6 with a shutter speed of 1/250 second, and you wanted to use a larger aperture with a faster shutter speed instead, you couldselect from more aperture/shutter speed combinations for the way the camera metered the scene.

For any given light level, there are multiple aperture and shutter speed combinations that will give you identical exposure. So, in the example above, if the camera selected f/5.6 at 1/250 second, and you moved to anotherchoice, you may see f/4.0 at 1/500 second. This is because f/4.0 is twice as bright as f/5.6, so you use shutter speeds twice as fast for proper exposure.

If you moved to the next choice in the opposite direction, you may see f/8 at 1/125 second available (because only half as much light gets through the smaller f/8 aperture opening compared to f/5.6, shutter speeds must be twice as long for proper exposure in the same lighting). How you make your selections in this mode will depend on the camera model. Many will use a "command dial" that you spin to toggle between available choices that all still give you proper exposure of the image.

Another mode is Aperture Priority (Av) mode. This is where you select the desired aperture, and the camera selects the shutter speed for proper exposure. This is easier to use versus full manual exposure (since you only need to select one parameter versus two, letting the camera make the appropriate choice for the lighting conditions).

For example, if you are shooting in lower light, you may want to select the largest available aperture (where you'll get the fastest shutter speeds to help prevent motion blur), and let the camera select the appropriate shutter speed for any subject you point it at (keeping in mind that depth of field is shallower at larger apertures). Or, you may want to select a smaller aperture for greater depth of field, letting the camera select the appropriate shutter speed for proper exposure (keeping in mind that smaller apertures require slower shutter speeds, so you need to watch out for motion blur from camera shake or subject movement if you choose one that's too small for the conditions).

Another mode is shutter priority mode. In this mode, you select the desired shutter speed, and the camera selects the appropriate aperture to insure proper exposure of the image. But, make sure you don't select a shutter speed that's too fast for the lighting conditions, the ISO speed selected, and the largest available aperture (otherwise, you'll "run out of aperture", and have no choice but to underexpose the images unless you change the settings).

When selecting a lens, you need to decide what focal range, optical quality and maximum available aperture you need (with a larger available apertures being preferred for lower light conditions, since it will let you use faster shutter speeds). Lenses with larger available apertures are also preferred for subjects that require a shallow depth of field (where you want to make your subject stand out more from distracting background).

With a prime (non zoom) lens, you will see one aperture listed (maximum available).

With a zoom lens, you usually see two apertures listed (the maximum available aperture at wide angle, and the maximum available aperture at full zoom). Some higher quality zoom lenses can maintain a constantaperture throughout their focal range. For example, zoom lenses that can maintain a constant f/2.8 aperture throughout their focal range are popular fornight sports ina stadium (or in any other conditions where faster shutter speeds are needed to prevent motion blur from camera shake or subject movement).

As a general rule, the cost of a brighter (a.k.a., fast) lens with larger available apertures will be substantially higher than the cost of a slow lens. These will also be larger and heavier.



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Old Dec 3, 2004, 7:50 AM   #3
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jimc:

style="BACKGROUND-COLOR: #000000"wow, i really appreciate the detailed response! sorry if it consumed a lot of your time. your examples really helped put a lot of it into perspective.

style="BACKGROUND-COLOR: #000000"i'm actually using an old school nikon to play around with, then when i get the hang of it, i want to invest in a digital slr. it'll probably be easier to start with a digital slr so i can see the results right after i takea picture, then play around with the settings to get the image i want. but it is a lot of money, so i'm trying to work with what i got.

style="BACKGROUND-COLOR: #000000"my last question, i hope you dont mind and which i'm stilltrying to figure out was in regards to zoom. you stated earlier:

style="BACKGROUND-COLOR: #000000"" style="BACKGROUND-COLOR: #000000"With a zoom lens, you usually see two apertures listed (the maximum available aperture at wide angle, and the maximum available aperture at full zoom)."

style="BACKGROUND-COLOR: #000000"i'm trying to tie that into this statement

style="BACKGROUND-COLOR: #000000"" style="BACKGROUND-COLOR: #000000"With zoom lenses the maximum aperture changes as you zoom the lens. It will be larger when zoomed out to a wide angle, and smaller when zoomed in to enlarge a subject."

style="BACKGROUND-COLOR: #000000"if i want to capture a landscape scenario, i would zoom all the way out for the widest angle, however according to that comment, my aperature wouldchange to the largest aperature? if i wanted a greater depth of field, wouldn't i need a smaller aperature? or in the statement, do they mean "larger" in terms of number, which would be a smaller aperture?

i may have answered my own question, but i'm trying to make sense of it in my head.

thanks and regards,
joe
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Old Dec 3, 2004, 10:50 AM   #4
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cable wrote:
Quote:
style="BACKGROUND-COLOR: #000000"if i want to capture a landscape scenario, i would zoom all the way out for the widest angle, however according to that comment, my aperature wouldchange to the largest aperature? if i wanted a greater depth of field, wouldn't i need a smaller aperature? or in the statement, do they mean "larger" in terms of number, which would be a smaller aperture?
The largest available aperture with most (but not all) lenses is only available at the wide angle lens setting.

However, you don't have to use the largest available aperture. A full range of apertures is available. For example, a 35-70mm lens rated at f/3.5-4.5 may have apertures from f/3.5 to f/22 available at it's wide angle lens setting.

In good light, the Camera's Autoexposure algorithms won't be using the largest available aperture (they'll be using something smaller, only using larger apertures when shutter speeds become too slow for the lighting conditions).

If you are not happy with the Camera's Autoexposure Choices, you can also use Aperture Priority Mode to set the desired Aperture you want (within the range of the settings available for a given lens).

Most lenses will be a little softer at both extremes of the available aperture settings, too. So, you have to balance your depth of field (or shutter speed needs) for a given shooting condition and lens.


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Old Dec 3, 2004, 11:58 AM   #5
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thanks again jimc!.

ya, i was trying to understand the statement that i read elsewhere. of using a larger aperture when zooming out.

makes sense, just have to try it out....
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