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Skylar McMahon
Optics Jedi Knight Capt. BlowHard Joined: April/05/2011 Location: TEXAS Status: Offline Points: 6082 |
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Posted: July/16/2014 at 16:34 |
Ted and I discussed this and I wanted to share it, because it supplies a breakdown that is very understandable.
LOWER f-number = LARGER
aperture. Larger aperture = shallower depth of field and faster shutter
speed…because more light is hitting the sensor. Shallower depth of field means
there’s less in focus fore and aft of the actual surface you’re focusing on,
resulting in a soft background. The reason lower f-numbers mean
larger aperture diameter and larger f-numbers mean smaller aperture diameter is
because the “f” is the numerator (top number) and the number is the denominator
(bottom number) in a fraction that equals the actual diameter of the aperture.
That’s the reason there’s always a “/” (divided by) after the “f”, as in “f/4.”
The “f/4” is a fraction, where (f= focal length) / (number divided into the
focal length to equal the actual aperture diameter). For example, say you have
100mm focal length and you’re shooting with an aperture size of f/2. The
fraction would then be 100/2 = 50mm aperture diameter. In the same example, if
you were shooting at f/4, your aperture diameter is 25mm, because 100/4 = 25.
f/10 would then be a 10mm aperture diameter. The reason aperture diameter is
expressed as “f-stops” that way is because even though f/4 at 100mm focal
length (25mm aperture diameter) is a different actual aperture diameter than
f/4 at 85mm focal length (21.25mm aperture diameter), the effect on field of
view and light reaching the sensor is exactly the same, because focal length
has an effect on light transmission and depth of field just as the aperture.
So, in that example, you have to use a larger actual aperture diameter at 100mm
to gain back the same amount of light than you’d need at 85mm, even though both
are set at f/4. Remember, the same rules apply for riflescope magnification
(equivalent to the focal length), objective size (equivalent to the aperture)
and amount of light (exit pupil diameter) reaching your eye (the sensor). As
you crank up magnification, you need a larger objective to get the same amount
of light to your eye. And, as you increase magnification in any optic, the
depth of field always gets shallower. Make sense? The other things that will soften the background in a photo (creating “bokeh”) besides using the largest possible aperture (again, smaller f-stop number) are using a long focal length (higher magnification) and putting more distance between your subject and the background and/or less distance between the lens and your subject. |
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Dale Clifford
Optics Jedi Knight Joined: July/04/2004 Location: United States Status: Offline Points: 5087 |
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another thing to keep in mind is the perspective given to the final picture because of the magnification being used. Camera lenses are standardized to use 50 to 55 mm as a normal viewing distance mostly duplicating the human eye's and interpretation by the brain of the spacial relationship between the objects in the photo. wider angle ie. lenses with mags. less than 50 mm change this relationship while including them in the same picture. These lenses give greater depth of field and are usually "slower" in f stops without giving up utility in ambient light requirements. Higher lenses above 50 will "stack" the perspective. If you look closely at a picture, and lots of practice, one can pretty close to guessing what magnification of lenses was used in the shot by the stacking. The resultant effect is the need for more light (lower f stops). Portrait lenses will usually fuzz the lowest f stop to "soften the features" of the person and are usually pretty fast (lower f stops), around 85mm and expensive. macro lenses need depth of field, so usually are not very fast. (higher f stops). and the focal length is based on how close you can get to the subject. and are expensive. great pictures are taken by anybody having a camera at the time, and many occur daily because of camera phones, however they are not technically great shots. being able to choose the equipment by seeing the final picture in you mind before makes the difference |
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I love little league baseball-- it keeps the kids out of the house
Yogi Bera |
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Dale Clifford
Optics Jedi Knight Joined: July/04/2004 Location: United States Status: Offline Points: 5087 |
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another easy rule of thumb is if 50 mm is the "normal perspective" then 100 mm is 2x (2x power handgun scope) and a 200 mm is a 4x etc. digiscoping adds new dimension because the mags can get from 10x-40x -- lots of planning for these shots. |
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I love little league baseball-- it keeps the kids out of the house
Yogi Bera |
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Skylar McMahon
Optics Jedi Knight Capt. BlowHard Joined: April/05/2011 Location: TEXAS Status: Offline Points: 6082 |
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That couldn't be more true. There is a social site that I'm on frequently because there are some photographers I follow as well as Nation Geographic, but many others that show up in my feed are camera phone artist.
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Skylar McMahon
Optics Jedi Knight Capt. BlowHard Joined: April/05/2011 Location: TEXAS Status: Offline Points: 6082 |
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Thanks, Stephanie who works here is very interested in Digiscoping. We may have to do some experimenting.
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tejas
Optics Journeyman Joined: March/08/2010 Location: Lone Star State Status: Offline Points: 575 |
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Just a little something to add. Most Nikon DSLRs use ccds that are smaller than a frame of 35mm film. Therefore, if you have a DX type camera, which is most of them, your focal length is actually 50% larger than the lens your using. For instance, a 50mm lens, close to the same focal length as your eyeballs, is actually 75mm when attached to a DX camera. If you want "normal" you really need something close to a 35mm lens.
Here's an example of depth of field vs distance from subject. These are from the same photo I took a few months ago of a walking stick. I used a 100mm F3.5 macro lens set at f7.1. If I had opened it up to f3.5 the depth of field would be even less. As it is, it's only sharp to for the first 1/8 inch or so of depth. I was only a few inches from his head.
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tejas
Optics Journeyman Joined: March/08/2010 Location: Lone Star State Status: Offline Points: 575 |
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The other pic wouldn't load on the same post.
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Son of Ed
Chuck Norris Joined: June/18/2011 Location: TEXAS Status: Online Points: 122202 |
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umm......I always thought my 50mm lenses made things appear a little wrong. My lawn always looked too big. The fence looked farther away than my eyes saw it. The valley looked farther away than when I was standing up on the hill.....
I always thought that 80mm was basically duplicating your eyesight.... Anybody know about this?
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RifleDude
MODERATOR EVIL OPPRESSOR Joined: October/13/2006 Location: Texas Status: Offline Points: 16337 |
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The focal length equivalence part is correct. However, the focal length of any lens is always the same no matter what camera sensor size it's being used with. The DX sensor size (APS-C) just crops the scene more, giving the appearance of greater focal length. It doesn't actually cause an increase in the focal length, the sensor is just smaller, so the image projected onto the sensor takes up more of the sensor area. Another way to look at it is the DX sensor crops the scene by a 1.5X factor vs the full frame (FX in Nikon nomenclature) 35mm equivalent sensor size. So, a given lens on a DX camera body provides an "35mm equivalent" FOV of a lens having 1.5X greater focal length, but the effect the focal length has on the subject remains the same as if the shot were taken with an FX sensor. Other sensor sizes have an even greater "crop factor." For example, a micro four thirds sensor has a 2X crop factor, so a given focal length on a MFT sensor camera has a FOV equivalent to a lens having twice the focal length on a full frame sensor camera. Some Canon APS-C sensors have a 1.3X crop factor and others have a 1.6X crop factor. The Nikon 1 cameras have a 2.7X crop factor. |
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Ted
Money can't buy happiness... but it's much more comfortable to cry in a Porsche than on a bicycle. |
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RifleDude
MODERATOR EVIL OPPRESSOR Joined: October/13/2006 Location: Texas Status: Offline Points: 16337 |
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I always thought the same thing, Ed; somewhere around 75 - 85mm looks more equivalent to normal vision to my eyes as well. However, 50mm is the generally accepted "rule of thumb" (full frame equivalence) in the camera world for normal human vision equivalence. |
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Ted
Money can't buy happiness... but it's much more comfortable to cry in a Porsche than on a bicycle. |
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Bitterroot Bulls
Optics Master Extraordinaire Joined: May/07/2009 Location: Montana Status: Offline Points: 3416 |
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80mm seems like it has a much narrower FOV than my eyesight. The accepted "normal" 50mm seems just about right to me for a "normal" FOV, FWIW.
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-Matt
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Son of Ed
Chuck Norris Joined: June/18/2011 Location: TEXAS Status: Online Points: 122202 |
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" Bitterroot wants to see it WIDEWAYS correctly.....but FRONTWARDS can be all fouled up!! He's been smelling elk droppings too long! " |
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