UNDERSTANDING FOCAL LENGTH

Nici

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This may sound like a dumb question but maybe its because I fail to understand if shooting with a prime 35mm and shooting in either aperture priority of program mode, do I need to adjust the focal length on the camera settings and how? Please be kind, I'm here to learn.
 

snowbear

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^Exactly.

For a quick run down:

Aperture priority is where you set the aperture, or the lens iris size, and the camera sets the shutter speed.

Shutter priority (Tv in Canon) is where you set the shutter speed and the camera decides what aperture to use

Program mode is in between - you pick a preset aperture + shutter speed pair.
 
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480sparky

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+2.

You cannot alter the focal length of a prime lens as, by definition, it only has one.
 

wfooshee

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I think the question amounts to, "Does focal length affect exposure, and do I need to tell the camera what focal length I'm using?"

Focal length is not a camera adjustment; it's a lens adjustment which affects the field of view of the lens. That's the mm number you see with lenses. a larger number mean a narrower field of view which brings a smaller portion of the subject area into "closer" view. In your question, the 35mm prime, the 35 is the focal length. you adjust focal length by zooming a zoom lens, or changing lenses if you have primes. If you have a 35mm prime on the camera, your focal length is 35mm, period. nothing to adjust as far as focal length is concerned, and the camera itself doesn't really care what the focal length is as far as exposure is concerned.

Exposure is determined by the combination of ISO, shutter speed, and aperture, and focal length is irrelevant. Maybe the question is about aperture on different lenses. How can f:2.8 on one lens be the same aperture as f:2.8 on another lens? That's one of the happy things about how lenses work. A 50mm lens with an iris 12.5 mm across passes the same amount of light as a 400mm lens with an iris 100 mm across. If you notice, in the first case 12.5 mm is 1/4 of 50, and in the second case 100 mm is 1/4 of 400mm. In other words, in both cases, the iris size is 1/4 the focal length. If you represent focal length with the letter f, then you can write it as f:4. Does that look familiar? That's the aperture setting for both cases.

That's why focal length is irrelevant as far as exposure is concerned; the aperture setting itself is related to the focal length, and the ratio of aperture to focal length is a constant for light passage in a lens. A longer lens needs a larger iris to pass the same amount of light when compared to a wider-angle lens, but since the aperture setting is that ratio, f:4 on a longer lens is a physically larger iris opening than f:4 on a wide lens, so setting both to f:4 results in the same exposure setting.

If aperture were expressed as a physical opening diameter rather than a ratio to focal length, then things would be considerably more difficult!
 
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budget cruncher

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This may sound like a dumb question but maybe its because I fail to understand if shooting with a prime 35mm and shooting in either aperture priority of program mode, do I need to adjust the focal length on the camera settings and how? Please be kind, I'm here to learn.
Nope. As has already been explained, the focal length is simply a function of the lens, and cannot be altered in any way by the settings on the camera.

BTW: your camera has the capability to recognize the focal length and include that information in the EXIF of each shot you take with that lens. All you need to do is recognize the qualities of each lens that you use to fully realize the image that you wish to achieve.
 

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As the posts above have pointed out the camera doesn't usually need to know the focal length to get the image right. Some cameras can require the user to input focal length to operate in body image stabilization. Most of the time the camera will be able to get this info direct from the lens but this isn't always the case.

My Pentax DSLRs (which have IBIS) ask for the focal length to be entered when they are turned on with a lens they don't recognize mounted. If it's ignored the camera assumes it's unchanged from last use..
 

xenskhe

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if shooting with a prime 35mm and shooting in either aperture priority of program mode, do I need to adjust the focal length on the camera

You would adjust the focus, and also select the desired aperture (or let the program mode select the aperture for you).
 

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