more dumb questions.

[Tuffythepug]

the bigger the opening the more light, the more detail

actually, "the more detail" is not true. Normally a lens is sharpest a couple of "stops" below the maximum. If you wonder how is the correct sequence of f/ numbers: F-number - Wikipedia, the free encyclopedia

Don't automatically assume more light = more detail. Actually too little light as well as too much light can eliminate detail. Too little light and the details will be lost in the dark areas and too much light and the highlights will wash out. There's a middle ground that is the correct exposure in almost every shot

 

[TCampbell]

Matthew Gore (Light & Matter) posted a video here on TPF that makes I think really helps beginners visualize the relationship between aperture, shutter speed, and ISO.

You can find the thread (and video) here: http://www.thephotoforum.com/forum/...um/282344-video-three-basics-atari-style.html

 

[TCampbell]

I like to point out a few things about f-stops (aperture) and even shutter speed.

1) Camera makers round off ... everything. Even your lenses aren't exactly the focal length they state (e.g. a 50mm lens might really be a 49mm.) If each shutter speed is supposed to be half as fast as the previous one, then why do is the shutter speed that follows 1/8th called "1/15th"? Shouldn't it be 1/16th? That's all part of the "round off". It just makes everything easier for the photographer to do in their head. We do 1, 1/2, 1/4, 1/8... but then we simplify the next one by going to 1/15th... because it lets us go to 1/30th and 1/60th (instead of 1/32nd and 1/64th). We do it again when we jump to 1/125th (instead of 1/120th or 1/128th) because that simplification lets us then go to 1/250th, 1/500th, and 1/1000th.

We do the same thing with f-stops... instead of f/1.41 we just say f/1.4. Instead of f/11.31 we just say f/11. It's always truncated to just 2 digits.

2) While they seem completely arbitrary (Why, for example is f/2 letting in half as much light as f/1.4? Why not make it simply and make them ordinary counting numbers (1,2,3,..)?) there is a reason for these values.

It turns out that (a) the "f" in f-stop stands for "focal ratio". It's the ratio of the diameter of clear aperture divided into the effective focal length of the lens. If your lens is 50mm long and the diameter of clear aperture is 25mm wide then the ratio is 2 (the length is TWICE as long as the diameter).

(Sidebar: a "stop" is a term from the days of mechanical machines where any sort of lever or dial that could control a variable adjustment had pegs or notches to lock the adjustment at that particular position. Those positions (where you could lock the adjustment in that position) were called "stops" -- because they "stopped" the lever from moving. Cameras are old devices and their controls were named during the industrial revolution when the term "stop" was common ... users of the day would have recognized the term. Now that we've moved into an electronic world and we don't need many mechanical levers with "stops" on them to lock the adjustments in position, the term is a bit lost on us and we wonder who came up with that name and what it means. Well... now you know.)

And (b) the arbitrary values are based on the powers of the square root of 2. Because of the relationship between area and length, when the diameter (or radius -- take your pick, the math still works) increases or decreases based on the square root of 2 then the effective amount of light will double or halve (depending on if you're increasing the area or decreasing the area.)

Anything raised to the 0 power is 1. So the square root of 2 raised to the 0 power is 1.0. That's the first "full" f-stop: f/1.0

Anything raised to the 1 power is always itself. So the square root of 2 (which is a long number starting with 1.41... but we simply round it off to 1.4 because we don't ever express an f-stop with more than 2 digits) raised to 1st power is simply itself. That's the next "full" f-stop: f/1.4

The next stop is the square root of 2 raised to the 2nd power (or squared). In other words... we want the number itself and NOT it's square root. The square root of 2 squared is simply 2. That's the next "full" f-stop: f/2.0

And we can keep going raising the square root of 2 to the 3rd, 4th, 5th, 6th, etc. and we get: f/2.8, f/4.0, f/5.6, f/8, f/11, f/16, f/22, and some lenses make it up to f/32 (actually some lenses go even higher... but usually only on large format view cameras (those cameras where you throw a black drape over your head while looking through the back of it to focus.)

I'll color the text in this sequence in the hope that you'll detect a pattern:

f/1
f/1.4
f/2
f/2.8
f/4
f/5.6
f/8
f/11
f/16
f/22
f/32

Notice that every other value is red and the values in between are blue? Notice that every RED value is DOUBLE the previous red value? Notice that every BLUE value is DOULBE the previous blue value?

The order of f-stops actually does make sense when you see the patterns emerge. The tricky one is that double 5.6 is 11... but in reality it should be 11.2. Remember that earlier I said we never used more than the first 2 digits of an f-stop. So it's simply rounded to "11" and we keep going. The error we introduce by rounding the math is too insignificant to actually have an impact on the exposure.

I mentioned that there are some reasons why even though it's based on the powers of the square root of 2, we stick to those values and we don't simplify by normalizing the numbers back to counting numbers. That has to do with the fact that it isn't just the increasing or decreasing of the aperture that can double and halve the amount of light. If you use flash, increasing or decreasing the distance of the flash (to the subject) has the SAME effect.

Suppose I were place a fixed light exactly 10' away from my subject and this is the ONLY light illuminating the subject and I adjust the camera to take a perfect exposure of the subject using that light as my source. Now suppose I move the light farther... based on the square root of 2 (1.4). So instead of 10' away... it's now 14' (10' X 1.4) away. Believe it or not, I have just HALVED the amount of light on my camera by moving the light stand 4' back. I should now compensate by adjusting either the shutter speed, aperture, or ISO by one "full" stop to compensate and I'll get another perfect exposure.

Flashes are rated based on something called a "guide number". It's the distance the flash can illuminate a subject adequately for an exposure. If my flash has a "guide number" of 100 feet then it means the flash can provide adequate lighting to illuminate a subject 100' away. But there's a base assumption which states that you will use ISO 100 (no problem... every camera can do ISO 100) but ALSO it assumes you'll use f/1.0. Well there's a problem... your camera lens doesn't let you set an f-stop of 1.0. Neither does mine. (Canon used to make a 50mm f/1.0 lens... they discontinued it well over a decade ago). Now we're in a panic... we don't have an f/1.0 lens. What do we do?

THIS is where those screwy numbers become REALLY helpful: All you do is DIVIDE the "guide number" for the flash, by the f-stop you plan to use. Plan to take the shot at f/4? No problem... divide 100 by 4 (100 ÷ 4 = 25) and that's your new distance. The flash will adequately illuminate a subject 25' away using ISO 100 and f/4. Set any f-stop you want. Just divide the guide number by the f-stop and you're done. Now you know where to put the light. If we didn't use these screwy numbers for f-stops, this math wouldn't work. Working with lighting would be much more complicated. Thankfully, the math is REALLY easy... once you know the secret.

It turns out manual flashes usually let you adjust the power output... full power, 1/2 power, 1/4 power... notice it's always "halving" the value of light. If I halve the number of photons that I emit, that's like moving the light back 1.4 times farther away. Cut the power level in half again and it's as if you moved the light twice as far away as the original position.

This is all based on something known as the "inverse square law" of light. ( Inverse-square law - Wikipedia, the free encyclopedia ) Each time you increase the distance by a ratio based on the square root of 2, you actually DOUBLE the physical area that the light has to cover.

Given that the light source is giving off a constant number of photons of light, if those photons are now spread over twice as much area then it means that the same size area now has only half as many photons landing within that same spot. In other words... the light is now HALF as intense.

An even easier way to think of the math is that each time you DOUBLE or HALVE the distance of the light, the impact of the light on your subject is either 1/4 or 4x (depending on whether you're going closer or farther with the light). So if I move that light from 10' from my subject up to only 5' from my subject, then I'll get 4 times the light intensity on you my subject. I now need to reduce the exposure in the camera by TWO stops to compensate. See how easy that is? I don't need to look at a light meter. I don't need a calculator.

That might be a little complicated but here's the good news... it's probably just about the MOST complicated thing you'll ever have to learn about photography, and it's not really that hard to grasp the concept. ...especially when you see it illustrated in a nice diagram.

As for Sparky's comment about the relationship between detail and aperture is spot on. While I said the math of f-stops is "the most complicated thing" you'll ever have to learn... there are some things which are more complicated (so I lied)... it's just that you don't actually need to learn the math (ok, I didn't _really_ lie). There's some math about optics. That math gets complicated and it has to do with the fact that light isn't really a straight beam... but actually a wave. So we're dealing with the "wave" nature of light (an area of physics that you do not need to learn to be a good photographer.)

Just know that as a "generalization", the middle f-stops on a lens tend to yield the most detail if you were to scrutinize an image under a magnifying loupe. 2 stops down from whatever 'wide open' is for a particular lens is often the sweet spot in terms of it's ability to yield the highest level of detail. If you take a photo at f/32 and notice that when you magnify the pixels under a magnifying glass that it is not quite as "sharp" as the same photo taken at f/8 (even though we all know the "depth of field" or area of acceptable focus is considerably greater at f/32) do not call your lens manufacturer to declare that the lens is defective... the lens is working perfectly. It's simply that the lens is not able to violate the laws of physics which dictate something known as "diffraction limits".

If you're really into science and math and want to learn about the problem, learn about the Airy disk (Airy disk - Wikipedia, the free encyclopedia) and this tutorial on diffraction limits: Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

(Incidentally... I get in trouble with my other half every time I write one of these posts here instead of blogging it. I finally started a blog. I'll probably copy this to my blog later... maybe along with some diagrams.)

 

[agnatha]

okay based on you guys' good advice i took some decent pictures at work.
now, they aren't great yet, but for me they're good. as far as an action shot goes, i'm still having issues in that i need to predict where the owls are going to fly. and that's just going to come down to experience. but i did get some nice shots. haven't touched them with photoshop and i thought that i'd share.
maybe you guys like owls. most people do.
http://agnatha3141.tumblr.com/image/29280573055
http://agnatha3141.tumblr.com/image/29280764519
http://agnatha3141.tumblr.com/image/29282747333
i personally enjoy bats. so there's one of my buddy the bat.

 

[agnatha]

oh and by the way, the thing you said about the lens speed just a little below, - that is exactly the thought that i was having earlier today. i had this weird feeling that the max wasn't necessarily the optimal.
you guys really are the best at explaining this.
i've been helped more by you then any other site and i've only posted three times. it's quite remarkable.

ok, i hadn't even seen the second page of advice before i got to work to take some photos.
the one right above about light distance focal length etc. i was really enjoying and even taking notes. then i got to the end and said "how do i apply this to my owl picture the other day? oh, crap, what's the distance to the sun again?"
so, in response to it, let me say that it's awesome advice for indoor stuff but i do a lot of outdoor stuff. is there numerical rules for that too? cause, i for one, would enjoy another explanation for that too, if you're up for it.

and also, how do i find out my guide distance? i've never seen that term before, is there another word for it?

 

[Buckster]

and also, how do i find out my guide distance?

Check in the technical specifications that came with your flash, or in most advertisements for the product, especially the more thorough ones like the manufacturer, B&H or Andorama, even usually on Amazon.