misc. lens questions - no recs wanted

[asimh]

Ok, i have couple of questions about lenses. This was after reading the

nice writeup

 

http://www.dpchallenge.com/tutorial.php?TUTORIAL_ID=19

 

Anyhow,

 

1) with regards to hyperfocal distance (i searched on this but didn't

really find what i wanted to know). Why is it that the HFD/2 is the

front focus length? i mean, why not just label HFD as the front focus

point at which everything beyond is infinity. Basically, why mess

with that additional 1/2 factor? Is there some historical/anecdotal

usage or importance of this?

 

2) When they say constant aperture, say the 24-70/f2.8, isn't that

really a misnomer? I mean, if fstop is the ratio of focal length to

aperture, whenever you zoom in, to maintain that ratio, isn't your

aperture diameter actually getting larger in magnitude? Oppositely,

say on the 28-105, f3.5-4.5, if you set the aperture wide open, and

then zoom in, your aperture magnitude is still the same, yet your

fstop ratio has increased (yet maintaining a constant aperture

opening). So isnt it more appropriate (and correct) to say constant

aperture ratio? or do people just leave out the ratio part and abbrev

it as constant aperture? just making sure i am not missing something.

[richard_cochran]

<cite>

2) When they say constant aperture, say the 24-70/f2.8, isn't that really a misnomer?</cite>

<p>

Technically, perhaps, but photographers always refer to a lens

aperture by its ratio, or f number. (As an aside, astronomers

refer to a telescope's aperture by its measurement in units of

length, and they use THAT as the primary number to describe

the optic. A lens with focal length of 800mm and aperture of

100mm would be called a 100mm f/8 by astronomers, or a 800mm

f/8 by photographers.)

<p>

When you change focal lengths, you must change the

f number or the aperture diameter, or both. Variable

aperture zooms almost always change both, so they're

"variable aperture" by anybody's definition. "Constant aperture"

zooms hold the f ratio constant, so you can zoom back and

forth without having to adjust any exposure settings. They

hold constant the aspect of aperture that's important

for exposure to photographers, that is, the f ratio.

[pdumais]

1) It's typical to have your main subject exactly in focus; the HFD/2 tells you where your "in focus" distance reaches to. But that part is not really in focus, it's calculated with an assumption on either film resolution or print size.

 

2) Yes, constant aperture in this case is a misnomer of sorts. I uppose you should say constant aperture ratio. But if you point that out to photographers, you will get dirty looks and shrugs.

[asimh]

gotcha. many thanks.

[imaginator]

1. I don't use hyperfocal methods... just figure aperture and focus distance to get results I need (I seem to have good sense of distance and do this almost by instinct) 2. Ratio... yes! Aperture ratio or focal ratio... same thing. By the way, one more thing to consider is the ratio is only valid at infinity focus... changes with close focus. I learned optics as an astronomer... we say focal ratio, and speak in terms of actual aperture diameter when describing optics... photographers speak of focal lengths (like Richard explained) It bothered me to hear the term aperture used to describe what I call focal ratio, but then I realized photographers are indeed using the term correctly by stating the aperture is f/8... it means the aperture is f (focal length) divided by 8. By the way, don't try to figure zooms... they aren't that simple and are based on moving negative lens groupings and the aperture control (adjustable diaphragm) is within the lens system... between elements, so technically you have to think interms of "effective aperture" (ratio, of course) or "entrance pupil".

[cole_petersburg]

The factor of two is an approximation. It's an extraordinarily good one at the hyperfocal distance for all focal lengths.<BR>

However, the hyperfocal serves us in other ways. It tells us depth of field for any focusing distance. The DOF markings on a manual focus lens barrel are found using the following equations.<BR>

Of course the depth of field acceptable by convention may not be acceptable by you, and the correlation may vary with focal length. E.g. I now stop-down my 28mm Yashica lens by one stop to get acceptable DOF after getting some blurry prints.

<BR>

According to http://www.outsight.com/hyperfocal.html#math,<BR>

H = (L x L) / (f x d)<BR>

NF = (H x D) / (H + (D - L))<BR>

FF = (H x D) / (H - (D - L))<BR>

f is f-stop, L is focal length, D is distance, d is the diameter of the circle of least confusion. By convention, d=0.03mm on a 35mm frame.<BR>

Here's something I'll throw in just because it was so hard to figure out because no one will explain it: Light travels from each point on your object in all directions, and travels to your circular lens in the shape of a cone. The light from each point is spread out among concentric cones. The lens shapes the cones and finally makes them converge on your film plane. The intersection of each cone and the film plane is the circle of least confusion, which you'd like to be sufficiently close to zero diameter. AND because the cones are concentric, stopping down the aperture eliminates some outer cones of light from each point, therefore stopping down does not crop your image.