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DPChallenge Forums >> General Discussion >> Is there a DPC Photography Glossary?
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 10/08/2007 08:41:09 AM · #1
Since I'm generally not averse to asking a stupid question......I'd rather express my ignorance and learn, is there anything like a glossary of terms here at DPC?

I was reading the daily lens thread and I was wondering what USM meant as it pertains to lenses.

I know that means Unsharp Mask in my PhotoShop, but it never occurred to me what it meant in a lens.

Imagine my surprise when I Googled it and found that it means the same thing.

So I was curious if we have a Wikipedia-like thing here, or could have, where the people who know this stuff could answer questions for people like me who don't.
 10/08/2007 08:49:30 AM · #2
UltraSonicMotor
just a fast focus rather than a screw focus
 10/08/2007 09:34:00 AM · #3
That's not a bad idea actually. I learnt all my 'DPCspeak' from just hanging around in the forums- as you did.

We could make a wiki one where people can update it as they go along maybe? could be an idea for the SC (site councel) to arrange.
 10/08/2007 10:03:05 AM · #4
Excellent idea. It took me a while to figure out what everyone was talking about on the forums too.

It was fun though, making names for all the abbreviations.
Photoshop -- U S M User Supplied Magic
Lenses--- Canon L refers to price Ludicrus
Nikon-- ED Extra Dollars S Sexy
Lenses -- U S M Unbelievably Slow Movement 2 Made for US Marines use, extra tough, so pricey
The list goes on.

edit spelling

Message edited by author 2007-10-09 14:26:19.
 10/08/2007 11:34:14 AM · #5
Originally posted by MelonMusketeer:
Excellent idea. It took me a while to figure out what everyone was talking about on the forums too.

It was fun thought, making names for all the abbreviations.
Photoshop -- U S M User Supplied Magic
Lenses--- Canon L refers to price Luicrus
Nikon-- ED Extra Dollars S Sexy
Lenses -- U S M Unbelievably Slow Movement 2 Made for US Marines use, extra tough, so pricey
The list goes on.


Hilarious!!!! Thanks a lot for shedding some light on this obscure terminology...
 10/09/2007 07:09:56 AM · #6
I'm bumping this to see if anyone would be interested in pitching in and doing an on-forum kind of glossary.

I wasn't even sure what a prime lens was the other day in a lens threrad.

I'm guessing from context that it's a fixed focal length lens.

Is there any interest?

I'll gladly eidt my OP and add these definitions as they come up.

*IS* fixed focal length what the definition of a prime lens is?
 10/09/2007 07:13:54 AM · #7
Fixed focal length is correct.
 10/09/2007 07:28:32 AM · #8
Originally posted by Delta_6:
Fixed focal length is correct.

Okay......next stupid question: People keep talking about "fast" lenses....what are they?

And.....I've noticed that the lenses to have seem to be the ones with the low numbers, i.e.: 1:2.8.

Is that a reference to how wide the aperture is at its largest opening?

What is this ratio in reference to?????
 10/09/2007 07:43:34 AM · #9
Fast lenses, I THINK, refers either to focusing speed or aperture. The 1:2.8 is referring to the widest aperture available on the lens. As for what the ratio is in reference to I don't know but I'll try to find out.
 10/09/2007 07:46:01 AM · #10
Update: Fast lenses refers to lenses with wide apertures that reduce exposure time.
 10/09/2007 12:26:08 PM · #11
The "f" numbers have to do with the rate light is being transfered from the environment to the sensor.
It is a log function, so that is why the numbers seem so arbritrary.
If you think of the analogy of getting a correct exposure as filling a cup with water , and the sensor being the cup, then each time the f number changes a full step, say from f-4 down to f-2.8, then the cup will be filled in 1/2 the time, other conditions being the same.
The smaller the f number, the faster a correct exposure can be made, giving the ability to expose correctly with higher shutter speeds. This is where the term "fast lens" comes from, meaning lenses having a small "f" number wide open.
The standard f numbers correspond with shutter speeds, so one click of aperture open and half the shutter speed gives the same exposure value.
An example; If correct exposure is 1/500 @ f8, then 1/1,000 @ f 5.6 will be exposed to the same level.
Another facet of this is the shallower "depth of field" (DOF) with low f number apertures, which is another subject.
10/09/2007 12:48:28 PM · #12
Melon Musketeer is correct with his analogy for f/stops. Here's the technical skinny:

f/stop is a ratio between the physical diameter of the aperture and the focal length of the lens: a 25mm diameter aperture on a 50mm lens is f/2.0. The same 25mm diameter aperture on a 100mm lens is f/4.0, on a 200mm lens it is f/8.0.

This is why you have more DOF at f/8 on a 20mm lens than you do at f/8 on a 400mm lens: DOF is a function of the physical size of the aperture, regardless of the focal length of the lens being used.

R.
 10/09/2007 02:37:35 PM · #13
Bear Music has explained the math behind aperture number very well.
The area of a round opening changes with the square of the radius. That is why the f numbers are a log scale, so that the open area is doubled for each click toward f1.

Maybe after this thread runs a while, the good definitions could be simplified, edited, and put in a thread in the tutorial section in searchable or alphebetical order. Links from there to more detailed explanations could be added over a period of time, so that it would not be such a big project all at once.
10/09/2007 02:39:28 PM · #14
Originally posted by MelonMusketeer:
Bear Music has explained the math behind aperture number very well.
The area of a round opening changes with the square of the radius. That is why the f numbers are a log scale, so that the open area is doubled for each click toward f1.

Maybe after this thread runs a while, the good definitions could be simplified, edited, and put in a thread in the tutorial section in searchable or alphebetical order. Links from there to more detailed explanations could be added over a period of time, so that it would not be such a big project all at once.


I'm all for this project. I'd be happy to spearhead it or contribute to it.

R.
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