Lens and Aperture Discussion

Christian · February 18, 2005

I've finaly steped up to my first DSLR with a 20D and as my only lens an EF-S 17-85MM f4-5.6 IS USM.
Now I under stand the 17-85MM as it relates to the 1.6 crop factor for the 20D ( or at least I think I do)
However, what is the practical effect of the f4-5.6. I assume this is related to apature values? larger values mean less light/larger DOF?
As I eventually start finding need for other lenses I'm finding I'm not educated enough to know what any specific lense might do for me in comparison with this one.
Also I under stand the IS is the Image Stabilizer does the USM stand for anything significant?

fish · February 18, 2005

Christian,

Congrats on the 20D. The f-numbers do indeed define max aperture. Typically the lower number at the short end of the zoom and the higher number at the long end of the zoom.

Larger value (f-number) means smaller aperture and means less light, more DOF. Smaller value means larger aperture and more light, less DOF.

USM = UltraSonic Motor (silent ring-type focusing motor). As opposed to MM = Micro Motor (gear-drive, louder and slower).

wxwax · February 18, 2005

What Fishmonger said.

If you're thinking about more lenses, your needs would break into two categories: faster, and different focal lengths.

Your lens is f4 at 17mm and f5.6 at 85mm. That's OK, but not the fastest. What do they mean by fastest? And why does it matter?

As Fishfood said, the smaller the f-stop number, the bigger is your aperature. It lets in more light. More light = faster shutter speed. So.... a lens with smaller f-stop numbers is able to give you good shutter speeds even in low light. That's huge.

Good Canon L-glass zooms have a max aperature of f2.8. Not bad. But it comes at a price - add hundreds of dollars for every f-stop you gain in speed. :cry The fastest I think I've seen is f1.

As you note, small f-stop/big aperature also means narrow depth of field. So you don't want to be too close to your subject at f1. Only the front nose hair will be in focus.

DOF is definitely something to consider when you have a faster lens - ignore it at your peril. DAMHIK :uhoh

The other thing for you to consider when buying another lens is focal length. You have a nice range covered with 17-85. But lots of consumer cameras have built-in lenses that give you more zoom. So your next purchase might be something like a 70-200mm. Or a 100-400.

Careful. Buying lenses can be an addiction.

fish · February 18, 2005

wxwax wrote:

So you don't want to be too close to your subject at f1. Only the front nose hair will be in focus. DOF is definitely something to consider when you have a faster lens - ignore it at your peril. DAMHIK :uhoh

To build on what waximoto said, the big advantage of that f1 (you gotta be kidding me!) lens, is that you get a huge gain in DOF when you stop it down a couple of clicks, yet you've still got a fast shutter speed. Best of both worlds.

More realistically, the f2.8 lens is one stop faster than the f4, so your chances of getting 1/focal length is better with the faster lens. I might be building too fast, but 1/fl is the rule of thumb for hand holding and still getting a good image. I.e., on a 100mm lens, you want a shutter speed of at least 1/100s. Slow lenses (even those with IS) hamper that rule. So, if you gots the bucks, buy the fastest lenses your budget will allow. Otherwise, be prepared to use a tripod or flash a lot.

"fishfood"?

Christian · February 22, 2005

Wow, I knew I would learn something if a simply asked, but I didn't expect this amount of such rich information! Thanks fish, and wxwax for your replies. And I had never heard of the 1/fl rule, I'm going to have to tattoo that on the inside of my eyelids where I keep all my important info...like my name.

I'm sorry for the late reply, it's been a long weekend but I do appreciate the response! Now to start playing the lottery to finance those new lenses!

cmr164 · February 22, 2005

fish wrote:

To build on what waximoto said, the big advantage of that f1 (you gotta be kidding me!) lens, is that you get a huge gain in DOF when you stop it down a couple of clicks, yet you've still got a fast shutter speed. Best of both worlds.

More realistically, when you get the f2.8 lens, you can stop down to f8 or f11 and still get a 1/focal length for most outdoor shots. I might be building too fast, but 1/fl is the rule of thumb for hand holding and still getting a good image. I.e., on a 100mm lens, you want a shutter speed of at least 1/100s. Slow lenses (even those with IS) hamper that rule. So, if you gots the bucks, buy the fastest lenses your budget will allow. Otherwise, be prepared to use a tripod or flash a lot.

"fishfood"?

I know fish knows this but just to clarify his statement. It does not matter if you start with an f2.8 lens or an f4.0 lens, when you stop down to f8.0 they will use the same shutter speed.

fish · February 23, 2005

cmr164 wrote:

I know fish knows this but just to clarify his statement. It does not matter if you start with an f2.8 lens or an f4.0 lens, when you stop down to f8.0 they will use the same shutter speed.

Yes, of course you are right, Charles. Not sure what I was thinking, but I went back and corrected it. Thanks for pointing out my error.

leebase · February 25, 2005

f stops are a ration between the opening circle of your lens and the focal length.

Here's how I explain it. Imagine you had a hose filling a bucket with water. When the bucket is full...you have your correct exposure.

If you had a 1" round hose....let's say it takes 10 seconds to fill the bucket. The amount of time is equivalent to your shutter speed.

If you doubled the hose's area...you get twice as much water coming through....so you'd fill your bucket in half the time. If you cut the size of the hose's opening in half, you'd need to let the water run twice as long to fill the bucket.

Your aperature is the size of the hose...and your shutter is the amount of time you let the water flow....and the filled bucket is your optimal exposure (photo lighted just right, not to light, not too dark).

So....let's say you have a situation where your camera is at f5.6, and the shutter speed you are getting is 1/30 at that speed. 1/30 is so slow, that if your subject is moving you'll get blur.

So...you "open up" the aperature "1 stop". Each "stop" is twice the size of the fstop smaller, and half the size of the f-stop larger. Smaller numbers are larger f-stops.

In order: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32

As you can see, every other f-stop is a double of one, or a double of 1.4

Back to our story...you open up the aperature 1 f-stop. From 5.6 that would be 4. So you have light coming in at twice the rate...and you can reduce the time by half to 1/60 and have the exact same "exposure".

1/60 is still to slow to stop action. So you open up the aperature 1 more f-stop to 2.8 -- but wait...you can't with our lens, and the maximum aperature is f4. But, let's press on for the story.

So now you have f2.8 and a shutter speed of 1/125 which stops a certain amount of action...but your toddler is really moving....so you open up 1 more f-stop to f2.0 and shutter 1/250...and now you have the photo you wanted.

Of course, as you open up the aperature...you narrow the "depth of field"....the amount of the photo that's still "in focus" in front of and behind the plane of focus. For portraits you freuently want a narrow DOF...to have just your subject in focus, with a blurred background.

Now, there are two other things you could do besides opening up the aperature....either because you can't (your lens doesn't support it) or you don't want the narrow dof. You can "add light" via flash....or you can increase the sensitivity of the sensor....by setting the ISO.

The ISO values are also twice as sensitive as the stop before them.

ISO 200 is twice as sensitive as ISO100 and so on.

So in our example...lets say those values were at ISO100. When you got to f4...and still didn't get the speed you wanted. You could have popped up the flash....OR...you could bump up the ISO.

Upping the ISO one stop is equivalent to opening the aperature one stop...EXCEPT...that the ISO doesn't affect depth of field....AND...the higher the ISO, the more noise that's in the photo.

Your job as photographer is to balance these variables in the most pleasing way (in addition to the composition of the photo).

Lee

bkriete · February 25, 2005

Lee-
Thank you so much for the definition and description of f-stops and their effects. Very clear and helpful, and not at all obtuse in the manner of many technical descriptions. Great info on ISOs also. I wish I had the privilege of reading your explanation earlier in my photographic learning. Would have saved me a lot of

and :slosh

wxwax · February 25, 2005

Lee, could you please explain the theory of relativity in similar language? Thanks in advance.

cmr164 · February 25, 2005

leebase wrote:

...
So...you "open up" the aperature "1 stop". Each "stop" is twice the size of the fstop smaller, and half the size of the f-stop larger. Smaller numbers are larger f-stops.

In order: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32

As you can see, every other f-stop is a double of one, or a double of 1.4
...

Great explanation but I would like to add a clarification for those who can still handle high school math. The difference between each f-stop is the previous f-stop times 1.414... (square root of 2) (Some of you may remember from school days the formula for the area of a circle.) That is why a 1.4 extender changes the f-stop by 1 and 2x extender by 2 stops. Thus the f-stop is the ratio between the width of the opening and the focal length and the width of the opening is related to amount of the light (size of pipe) by powers of 1.414... (because the f-stop relates to diameter but the area relates to radius which is 1/2 diameter)

leebase · February 26, 2005

wxwax wrote:

Lee, could you please explain the theory of relativity in similar language? Thanks in advance.

You are driving 75mph in the left lane. Gramps in front of you is doing 56...and grandma in the right lane is doing 55.

While the motorist fixing their tire on the side of the road sees you going 55 and honking and flashing your lights....YOU see yourself going 1mph in relation to grandma and you give grandpa the bird incase the lights and horns don't communicate that he's going SLOW in the FAST LANE!

That's relativity

:

Lee

bkriete · February 26, 2005

So relativity explains why everyone going faster than you is a moron and everyone going slower is a dumbass?

leebase · February 26, 2005

bkriete wrote:

So relativity explains why everyone going faster than you is a moron and everyone going slower is a dumbass?

Lee

wxwax · February 26, 2005

bkriete wrote:

So relativity explains why everyone going faster than you is a moron and everyone going slower is a dumbass?

Great!

ashby · February 27, 2005

wxwax wrote:

What Fishmonger said.

Good Canon L-glass zooms have a max aperature of f2.8. Not bad. But it comes at a price - add hundreds of dollars for every f-stop you gain in speed. :cry The fastest I think I've seen is f1.

I think Canon had 50mm/f .98 in a 35mm format lens back in the late 60's. It was pretty big news then because f1.0 was supposed to be the theoretical max.

gus · February 27, 2005

Christian wrote:

apature values? larger values mean less light/larger DOF?

Think of it like the pupil in your eye....at night your pupil opens right to get as much light in as possible to help you see & in bright sun it shrinks right down to only let a small amount of light in.

luckyrwe · February 27, 2005

ashby wrote:

I think Canon had 50mm/f .98 in a 35mm format lens back in the late 60's. It was pretty big news then because f1.0 was supposed to be the theoretical max.

Point nine FIVE!

canon%207%202.jpg

wxwax · February 28, 2005

Humungus wrote:

Think of it like the pupil in your eye....at night your pupil opens right to get as much light in as possible to help you see & in bright sun it shrinks right down to only let a small amount of light in.

Except when you drink a lot of kava.

gus · February 28, 2005

wxwax wrote:

Except when you drink a lot of kava.

But you will not see the spirits if you dont....

.

wxwax · February 28, 2005

Humungus wrote:

But you will not see the spirits if you dont....

.

With six mangoes a day going into that intestinal tract, I'm surprised you don't hear the spirits.

fish · February 28, 2005

leebase wrote:

you give grandpa the bird incase the lights and horns don't communicate that he's going SLOW in the FAST LANE!

That's relativity :

well done!

fish · February 28, 2005

luckyrwe wrote:

Point nine FIVE!

Some additional info:

August 1961. This lens is developed as one of the standard lenses for the Canon 7 rangefinder camera. It had the largest aperture in the world for photographic lenses available in the market at the time. It attracts a great deal of attention as a "dream lens", as it is four times faster than the human eye. The lens mount is clip-on, 3-lug bayonet system like the Mirror Box II (cameras have S mount inside bayonet mount). The edge of the rearmost lens element is cut off to avoid friction with interlocking roller of range finder mechanism. It is Gauss type lens with 7 elements in 5 groups.

fish · February 28, 2005

Can you get any faster than an f0.0 lens?

luckyrwe · February 28, 2005

Four times faster than the human eye? So the human eye is about f/3.8?

Andy · February 28, 2005

the hall of wisdom belongs to wxwax as a moderator, but he's probaably too shy to move this thread there so i'm doing it.

thanks to christian, for asking a question many want to ask but dont.
thanks to fish, wax, and leebase for the really thorough explanations!

luckyrwe · February 28, 2005

And thanks to Andy for putting up with the other 1,995 of us!

Eric&Susan · March 1, 2005

I think I understand all the numbers and technical jargon but how does it relate to real life situations? What would be good for landscapes? sports? portraits? everyday practical use? Can this be answered or are the questions to general? Obviously there are going to be variables in each situation but is there a general rule of thumb?

Eric

wxwax · March 1, 2005

Eric&Susan wrote:

I think I understand all the numbers and technical jargon but how does it relate to real life situations? What would be good for landscapes? sports? portraits? everyday practical use? Can this be answered or are the questions to general? Obviously there are going to be variables in each situation but is there a general rule of thumb?

Eric

Eric, it really does depend on what you're trying to achieve. It helps to visualize what you want the final shot to look like, then figure out how to achieve it.

That being said, typically landscapes are shot with small aperatures/high f-stop numbers so that they have big depth of field. Andy's looking to buy a wide angle lens and expects to use it in the f8 to f11 range. On a wide angle, that's a huge depth of field.

For portraits, you'll notice that the background is often blurry. That helps focus attention on the subject. Portraits tend to be shot with longer lenses, often but not always somewhere between 80mm and 135mm, with a smaller aperature/smaller f-stop number - and a narrow depth of field. That's what allows the subject to be in focus and the background blurry. The danger is that the depth of field is too narrow, and that the subject's entire face isn't in focus (although some people go for that effect on purpose.)

For general walking around, you need to think about the lens you're using, and what constitutes narrow or deep depth of field for that particular lens. I've scrwed up many a time. :cry

mercphoto · March 1, 2005

fish wrote:

Can you get any faster than an f0.0 lens?

That was a joke, right?

fish · March 1, 2005

mercphoto wrote:

That was a joke, right?

Nope.

Lens and Aperture Discussion

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