# Question's about a 'fast lens' and auto-focus



## DScience (Apr 22, 2009)

Hello there friends!

I am trying to become familiar with the things people talk about on the forums, and I am also confused about certain things.

First question: What does it mean when people talk about a 'Fast Lens'. For instance, people will say, "I know, I need a faster lens". What is this referring to exactly? Is it the speed at which it can auto-focus?

Second question: Since I have the D60, I know that I need to get a lens that is a AF-S in order to have auto-focus enabled. So the question is, if you don't have auto-focus, how do you focus? I know this may sound stupid, but to manual focus do you just twist the zoom lens until you have the picture in focus through the view finder?


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## docphotog (Apr 22, 2009)

Hey Dscience, I'm a noob too but my take is that a "fast lens" has a lower f stop number ie. f/1.8 vs. f/4.5 or something, where the latter would be a slow lens.

"Fast" I believe is in reference to the amount of time that the camera needs to let in X amount of light. At the same time it goes hand in hand with the aperature of the lens. The greater the size of the aperature (aka smaller f stop value- yeah it's backwards and messed up!) means the more light that that can get in during a period of time and which will allow you to use a faster shutter speed. 

I have no clue what the deal is with the autofocus is all about so I hope someone else can answer that!

The advantage of using a faster lens, ie. a lens with a lower f stop #, and therefore a larger aperature is for low light scenarios. 

A zoom lens that maintains a certain f stop value the whole way, lets say Canon's 70-200 f/2.8 is going to cost you bankkkkkk over lets say Canon's 55-250 mm f/4-5.6 because at 55mm the 2nd lens will have f/4 and at 250 it will have a f of 5.6... and I'm assuming it will increment as you change the mm setting. Plus the 70-200 is much faster to begin with... that's a 1100 vs. 250 dollar price tag difference, yikes!!!

I hope I know what I'm talking about, I read all of this online somewhere, and if I can find the link I'll post it here later!


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## KmH (Apr 22, 2009)

DScience said:


> Hello there friends!
> 
> I am trying to become familiar with the things people talk about on the forums, and I am also confused about certain things.
> 
> ...


If you have to manually focus most lenses have a focus ring you turn by hand. Your D60 has a mode called Rangefinder (pg 116) that can help with MF. If you mount a Nikon AF lens on your D60 (50mm f/1.8D AF) you will have to manually focus but the focus indicator in the viewfinder will still light when focus has been acheived. 

A fast lens is one that can be opened to an aperture that is a large fraction of the lens focal length (f/2.8, f/1.8, f/1.4). That lets in a lot of light allowing a faster shutter speed.


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## Dwig (Apr 22, 2009)

DScience said:


> First question: What does it mean when people talk about a 'Fast Lens'. ...



"Fast" ,in reference to a lens, and "lens speed" are antique terms that are still in use. They date from the days when films were the equivalant of ISO 0.1 to 0.01. Lenses were generally used at their maximum aperture and exposures were controled by the exposure time (1 to 100 seconds in full sun). Hence a "fast" lens was one that allowed quicker (shorter) exposures. Such lenses had wider apertures. How wide is "fast" is a relative thing. Even today, f/4.5 is extremely fast for a 4x5 view camera lens. In smaller format cameras, like today's DSLRs, "fast" usually means f/2.8 or or wider. The term "fast" is also used to refer to films with higher ISO since they result in quicker exposures, everything else being the same.

Remember, f/stops are like fractions. They specifiy the size of the aperture as a fraction of the focal length ( focal length / aperture diameter). Just as 1/4 is larger than 1/8, f/4 is larger (wider, faster) than f/8.



> Second question: ...I know this may sound stupid, but to manual focus do you just twist the zoom lens until you have the picture in focus through the view finder?



No, that would just change the focal length causing the image to change size. You turn the other ring, the focus ring. 

But yes, you just turn the proper ring until the image in the viewfinder is sharp. You want to make sure that you've adjusted the eyepiece diopter adjustment (focus) so that you see the viewfinder screen crisply. This doesn't affect the picture but it does impact whether you can see the screen well enough to judge focus properly. This type of manual focus is difficult in low light, particularily with slow (opposite of fast) lenses. 

You can use the AF system in the body as an "electronic rangefinder" if the lens is f/5.6 or faster. The body will indicate with two pointers (check you manual for where the indicators are) in the viewfinder which way to turn the focuing ring. When both appear, it thinks you are in focus (its usually right).

If you are considering getting a manual focus lens or an AF lens without an internal motor I suggest that you practice manually focusing with your current lenses first. Simply flip the switch on the lens from AF to MF to disable the autofocus. If will give you an idea what it is like.


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## DScience (Apr 22, 2009)

Dwig said:


> No, that would just change the focal length causing the image to change size. You turn the other ring, the focus ring.



LOL I didn't even know that 'ring' existed !

Dwig, THANK YOU! That was such a perfect explanation, and giving me the background completely set it in stone. 

I was not considering getting a non AF-S lens, as I can hardly get my pics with the lens' that I have to come out perfectly in focus..I was just curious. I do however want to get a faster lens!!! LOL 

I am looking at the [FONT=Helvetica, Arial, sans-serif]*Nikon 35mm f/1.8G AF-S DX. *I'll report back to this thread whenever I get it (nexst paycheck).

[/FONT]


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## KmH (Apr 24, 2009)

DScience said:


> I am looking at the [FONT=Helvetica, Arial, sans-serif]*Nikon 35mm f/1.8G AF-S DX. *I'll report back to this thread whenever I get it (nexst paycheck).[/FONT]


Get in line. They're back ordered right now. It's a new lens. Oh, and you're welcome on the D60 info.


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