Go Back   Steve's Digicams Forums > Digicam Help > General Discussion

Reply
 
Thread Tools Search this Thread
Old Jul 1, 2004, 10:28 PM   #1
Senior Member
 
VictorEM83's Avatar
 
Join Date: Jun 2004
Posts: 156
Default

I have been learning alot thanks to you guys but I am having some trouble, with the basics. I have tried searching for someof the thing but with less than desired results.

I hope you guys can fill me in on these things.

1: The F stop and the numbers used with it, what is it what does it mean and what does it do for me? Also is the F stop and the F rating on lenses?

2: ISO speeds I know the higher the speed the "brighter" the image is but what other effects does this have other than noise at higher ISO for digital cameras.

3: I understand the the lower mm number on the lens is for closer shots and the higher for further shots but what is the ranges of a 50mm, 100mm, 200mm, and 300mm lenses? this is repeat of the first question but what does the F# mean after them mm range of the lens? I know that lower numbers require less light but why/how?

Update: I do know that the DRebel had a 1.6X effect on the lenses so you dont have to explian that to me

Update 2: I found a great site explaining the F stop and the details to it @ http://www.uscoles.com/fstop.htm it has answered most my questions.

I know this is a lot to ask if you count point me to threads/web pages these are explained to save yourself time that would be great! normal responces are also very welcome.
VictorEM83 is offline   Reply With Quote
Sponsored Links
Old Jul 1, 2004, 11:55 PM   #2
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Victor:

A lens is rated by it's maximum aperture. For a prime (non zoom) lens, you'll see only one number. For a zoom lens, you'll usually see two numbers (maximum aperture at wide angle, maximum aperture at full zoom). Although, for some of the higher quality lenses, you'll only see one number (because the aperture is constant throughout the zoom range).

This rating systemlets you know how bright the lens is at both wide angle and zoom. A typical zoom lens will look something like F/2.8-4.9

Most zoom lenses don't gather as much light at the "longer end", so you can see how bright the lens is at both wide angle and full zoom with this rating system.

The lower the F-Stop, the larger the aperture (with more light reaching the sensor through the lens). The higher the F-Stop, the smaller the aperture (with less light reaching the sensor through the lens).

F/1.4 is twice as bright as F/2.0

F/2.0 is twice as bright as F/2.8

F/2.8 is twice as bright as F/4.0

etc.

The aperture scale (in one stop increments)goes F/1.4, F/2.0, F/2.8, F/4.0, F/5.6, F/8.0, F/11, F/16, F/22... Witheach one stop move to a smaller aperture, you will need shutter speeds twice as long for proper exposure.

ISO Speed is linear (ISO 100 is twice as sensitive to light as ISO 50; ISO 200 is twice as sensitive as ISO 100, etc.)

An understanding of these parameters is very important, especially if you need to shoot in "existing light" indoors.

Here is a handy chart that shows how Aperture impacts shutter speeds needed in various lighting conditions (see the table for Exposure Values, F-Stops and Shutter Speeds). Note that the chart is based on ISO 100. So, each time you double the ISO speed, you can also double the shutter speed used for proper exposure:

http://home.earthlink.net/~terryleed...tes/tables.htm

To prevent motion blur from camera shake, as a general rule of thumb, you want to use a shutter speed that is 1/focal length or faster. In other words, if shooting at 100mm focal length, you'll want a shutter speed that is 1/100 second or faster. If shooting at a 200mm focal length, you'll want a shutter speed that is 1/200 second or faster -- etc. You need to increase shutter speed as more zoom is used, because the increased magnification also magnifies any camera movement (resulting in blurred images if shutter speeds are not fast enough).

In order to get a fast enough shutter speed, in many lighting conditions, you'll need both a fast lens (lower F-Stop rating), as well as a camera that can shoot at higher ISO speeds.

Let's take an example:

You decide to purchase an inexpensive 80mm-200mm F/4.5-5.6 lens.

Your friend decides to purchase a 70mm-200mm F/2.8 "L" lens (F/2.8 throughout it's zoom range).

You both go to an indoor event, where the EV (Exposure Value) is around 6.

You both set your ISO speed to 800. You take lots of photos and compare them later.

You find that your friend has ahigher number of "keepers", whilevirtually ALL of yourphotos are very blurry. Why? Your friend's lens is 4 times as bright as yours at full zoom. As a result, it can shoot at shutter speeds that are 4 times as fast (for the same lighting conditions and ISO speed). In this example, your friends camera would be using a shutter speed of around 1/64 second (still way too slow for this focal length, unless you have very steady hands), while your camera will be using a shutter speed of around 1/15 second (LOTS of blur from camera shake, trying to shoot at a 200mm focal length).

There is a LOT more to it. For example, Aperture is also used to control Depth of Field.




JimC is offline   Reply With Quote
Old Jul 2, 2004, 12:31 AM   #3
Senior Member
 
Join Date: Dec 2002
Posts: 5,803
Default

JimC gave you some good info. I wanted to add some stuff to your question #3.

First off, on a standard film camera the 50mm lens is equivalent to the human eye. It sees what you see. Since you know about the 1.6x crop factor, you know that on some digital cameras that isn't exactly true any more... but if you a 31.25mm lens (if one existed) on your DRebel, the digitial crop would turn it into 31.25 x1.6 = 50mm. So it would produce a picture that look like what you saw. Ignoring the 1.6x crop(because it makes my life easier and its 12:23 am for me!) the linese you list are:

50mm = to the human eye or 1x

100mm = kinda weak zoom, acting like 2x binoculars

200mm = not a bad zoom, acting like 4x binoculars

300mm = good zoom, acting like 6x binoculars

(obviously, on the DRebel, all those lengths act like they are 1.6x longer, and therefor the "x" power is larger too.)

There are two types of lenses. Zoom lense and "fixed focal length" lenses (called "Prime" lenses.) As zoom lens is the most common sold because it allows you to "zoom" in and out, giving you more flexability when shooting. A fixed focal length lens is like most pairs of binoculars... you get 1 view and if you want to change what you see, you have to physically move ("zoom with your feet" some call it) to change what you see. So like in JimC's example, a 80-200 zoom lens can change the focal length to all the values between 80 and 200mm. A prime can't zoom, its fixed. You get what ever it provides. You might ask why anyone would make such a lens, and there is one and only one reason, really.

Optical quality.

It's easier to make a prime lens, and they can make them with better optics. Much better optics. Zoom lenses have gotten better in the last 5 or so years, and have started to gain more acceptance (and with in the last year or so some really good zooms have been made.)

But I beleive that prime lenses are still optically better to the best zoom lenses.

I believe that JimC answered the other part of #3 for you, so I won't go into that.

And Jim is right on for #2. ISO is really a way to get a better shutter speed or larger Depth Of Field. If you can increase ISO without hurthing the picture quality (too much noise) then instead of lightening the image you would just increase the shutter speed or increase your aperture.

Eric
eric s is offline   Reply With Quote
Old Jul 2, 2004, 1:28 AM   #4
Senior Member
 
VictorEM83's Avatar
 
Join Date: Jun 2004
Posts: 156
Default

Now with taking all this in lets see if I got it right. In area thats poorly lit, for close, semi close posed for shots shots on the DRebel on a tripod, 50mm(80mm effective) F1.4 lens with an ISO of 200, with a shutter speed of 1/30 sec. it would make up for some of the low light and underexposure of the DRebel or did I miss something.

Update: new question, that I may know the answer to but just incase the camera has an F stop so I am assuming the cameras F stop is related to the lens F stop. Does this mean with a 50mm F1.4 lens I can downgrade my F stop to a 1.8 or 2.0, but not the other way around right?
VictorEM83 is offline   Reply With Quote
Old Jul 2, 2004, 4:10 AM   #5
Senior Member
 
Marc H's Avatar
 
Join Date: Dec 2003
Posts: 130
Default

VictorEM83 schreef:
Quote:
Now with taking all this in lets see if I got it right. In area thats poorly lit, for close, semi close posed for shots shots on the DRebel on a tripod, 50mm(80mm effective) F1.4 lens with an ISO of 200, with a shutter speed of 1/30 sec. it would make up for some of the low light and underexposure of the DRebel or did I miss something.
Quote:
Lets complicate things
Quote:
In theory youre right, but theres one in important factor, Depth of field. With a 50mm F/1.4 lens, youcan get a very shallow DoF, depending on the distance of your model. If you use an average distance between camera and model, the DoF at F/1.4 will be just a few centimeters, leaving a lot of your model out of focus.
Quote:
Update: new question, that I may know the answer to but just incase the camera has an F stop so I am assuming the cameras F stop is related to the lens F stop. Does this mean with a 50mm F1.4 lens I can downgrade my F stop to a 1.8 or 2.0, but not the other way around right?
Quote:
Wrong, a Drebel has no Fstop, its all in the lens. But every lens has a minimum and a maximum F-stop, depending of the lens. The canon 50mm f/1.4 has a maximum opening of 1:1.4 to a minimum of 1:22, so you can with most lenses close the lens to a higher f/stop, but you cant open a lens more than its design spec
Marc H is offline   Reply With Quote
Old Jul 2, 2004, 7:38 AM   #6
Senior Member
 
Chako's Avatar
 
Join Date: Jun 2004
Posts: 301
Default

To answer your last question.

Faster lenses have lower f stop ratings. This means that if the lighting is fairly low, a faster lens will get you a faster shutter speed. However, you will still have control over the full range of f stops afforded to you by your lens. Remember, 2 things are important for proper exposure, shutter speed (controls the time the film/sensor plane is exposed to light), and aperture (controls how much light strikes the film/sensor plane).

With a faster lens, your able to "open" the lens more, thereby letting in more light. Slower lenses can only "open" so much, letting not as much light in.

Aperture is connected to depth of field as mentioned by other posters. Greater depth of field is a result of restricting the amount of light hitting the film/sensor. Thus, for greater depth of field, you would want higher f numbers. Fast lenses, wide open, will diminish your depth of field, but their prime advantage is in low light conditions. That is why a pin hole camera has supreme depth of field (everything it can collect on film, is in focus) due to a small amount of light being allowed through a small hole, meaning a large aperture.
Chako is offline   Reply With Quote
Old Jul 2, 2004, 11:00 AM   #7
Senior Member
 
Join Date: Dec 2002
Posts: 5,803
Default

I just wanted to add that the reason a lens with a larger aperture (low f-stop) is called "Fast" is because it allows you to use a higher shutter speed (a "faster" shutter speed.) It allows you to get pictures you wouldn't be able to get otherwise, so they are nice to have. But they are more expensive (harder to make, more materials) and heavier (more materials) so there are downsides to "faster" lenses.

Unlike focal length on a prime lens, you can always change the fstop. Lenses have a max and a min aperture (but you rarely hear about the min.) And you can change between them on the fly. So yes, what you asked (can I change the 50mm f1.4 to f1.8?) is yes. I wouldn't call it "downgrading" but you can do it, and you will want to it is a very handy thing to do.

Eric
eric s is offline   Reply With Quote
Old Jul 3, 2004, 4:21 AM   #8
Senior Member
 
VictorEM83's Avatar
 
Join Date: Jun 2004
Posts: 156
Default

Well downgrading most likey wasnt the best term but I think I got the full idea behind the F stop. Its whole point is to gather(allow/enable) or restrict light into the lens to get the effects your looking for.

Its such a simple thing yet complicated to say the least

The more I learn the more I am deing to get my camera its will be here at the end of next week due to a address verifing issue with newegg :P I really need to get testing everything out ASAP but I dont think they get that
VictorEM83 is offline   Reply With Quote
Old Jul 5, 2004, 2:32 AM   #9
Senior Member
 
Mikefellh's Avatar
 
Join Date: May 2004
Location: Toronto, ON, Canada
Posts: 1,707
Default

Suggest you hit the following site where it teaches the basics with photos:
http://209.196.177.41/

Also, head to the library and check out books on basic photography...it doesn't have to be about digital photography to learn about f-stops and ISO.
Mikefellh is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 4:22 AM.