[trishytee]

I keep noticing references to 'fast glass' or fast lenses, and while I am unsure what this means I am guessing a better picture is obtained. Are my manual lenses from my old Pentax going to be 'faster' (does it mean better?) than my automatic lenses?

[sterretje]

It does not mean 'better'. 'fast' means bigger apertures (lower f-numbers). A 50mm/2.0 is faster as a 50mm/2.8

[thePiRaTE!!]

A lens of wider aperture (smaller f number) lets in more light. Because the lens is letting more light in, the shutter does not need to remain open as long and is thus faster.

[Class A]

As thePiRaTE!! explained well, "fast glass" allows you to shoot with faster shutter speeds.

AFAIC, though there is a limit to this idea. The wider the aperture (lower the f-ratios or f-number) the thinner the depth of field (DOF, the area of the image which is in acceptable focus).

When the DOF gets very thin, then

• successful focus becomes hit and miss, if you are not shooting completely static objects and have a lot of time
• the area that is captured with good focus can be extremely thin, as in not covering both the eyes and the nose

Therefore, I'd say below a certain threshold faster glass doesn't really buy you a practical faster shooting advantage but only gives you more options with really thin DOF.

In any event, fast glass is nice because every lens gets better when stopped down a bit so even if you never use the very low f-ratios a fast lens will typically yield better IQ with a normal working aperture then one that has to achieve this working aperture wide open.

In film days, fast glass was pretty much the only way to get low light shooting ability since the high ISO films brought a lot of grain with them. In the digital age, the golden route to low light shooting is good high ISO performance by the sensor/camera. Cameras such as the K20D achieve very impressive high ISO performance even for say ISO 1600 which was unthinkable in film days. Hence there is less need to buy "fast glass" to gain shutter speed (as the shutter speed can be gained by increasing the sensitivity / ISO).

[NaClH2O]

Hi Trish
Class A above gave a pretty complete explanation, but the easiest way I think of it is "fast glass" generally means better low light performance. It might be a better "all around" lens, but it will definitely be a better low light lens. Since making a "fast" lens is usually more difficult than making a "slow" lens, lens makers tend to take more care in all aspects of the lens so it is usually better than an equivalent lens with a smaller maximum aperture. In other words, the premium lenses tend to be faster than the non premium ones. Some of the old manual focus lenses are pretty fast and are really good lenses, but as you know they aren't automatic. But automatic isn't everything.

NaCl(hope that helps)H2O

[TaoMaas]

Something to keep in mind is how you plan to use a particular lens. If you're buying a wide angle lens with the intentions of shooting mostly landscapes, then having a fast lens won't be that crucial since you'll most likely be stopping it down to get good depth of field. But if you're needing a wide angle lens because you're going to shoot crowd pics at rock concerts, then you might want a faster lens because the lighting will be dim.

[Marc Sabatella]

Whether it's manual of automatic has nothing to do with it. It's all about the maximum aperture. You can find that either either by looking at the markings on the lens or by mounting the lens to the camera and seeing how large of an aperture you can set in Av or M mode. Realize - if this is new to you - that "large" aperture means small f-numbers. f/1.4 is larger than f/2, which is larger than f/2.8, which is larger than f/4, which is larger than f/5.6, etc.

If your lens has an aperture ring, you can see the maximum aperture just by looking for the smallest number available on the ring. If not, look at the front of the lens - it usually lists the maximum aperture right on the front of the lens (eg, "SMC Pentax-DA 1:2.8 40mm Limited" has a maximum aperture of f/2.8. If it's a zoom lens, it will give a range of maximum apertures, like 1:3.5 - 5.6. This indicates the maximum aperture is /3.5 at the shorter end of the zoom and f/5.6 at the longer end of the zoom.

[Lowell Goudge]

most of the things have been covered, but just to make a little summary of what "fast" glass lets you do

1) brighter viewfinder especially in low light applications
2) shoot with faster shutter speed wide open
3) control depth of field better for isolation of foreground and background
4) easier and more precise focusing since the focusing is done wide open where DOF is minimal (note ultra fast lenses like the 50mm F1.2 are soft at maximum apature negating this benefit a little)
5) quicker AF to a point, since all AF systems work faster with brighter images.

and lastly, burn a hole in your pocket quickly because speed is $$$$$

[junyo]

and lastly, burn a hole in your pocket quickly because speed is $$$$$

...most of the time. Lot's of nice MF 135/2.8's on the market for a song. Useful and plenty fast.

[asdf]

I keep noticing references to 'fast glass' or fast lenses, and while I am unsure what this means I am guessing a better picture is obtained. Are my manual lenses from my old Pentax going to be 'faster' (does it mean better?) than my automatic lenses?

Something that was probably not mentioned yet: in astrophotography, where depth of field is not a concern, a fast lens will allow you to take panoramic shots of the night sky with negligible star trails.

[Lowell Goudge]

...most of the time. Lot's of nice MF 135/2.8's on the market for a song. Useful and plenty fast.

maybe, but my 135 F2.5 is faster, and I got it for a really good price (considering its a true pentax)

A sears 135 F2.8 goes for about $10 so the 1/2 stop does cost just a little

[Marc Sabatella]

f/2.8 is usually considered the threshold for what's considered fast, but of course the faster the better. There are a fair number of f/2.8-and-better lenses available for under $100 if you're OK with manual focus, including 28mm, 50mm, 100mm, and 135mm choices.

[trishytee]

Ah! Thank you, and there I thought the glass was better! My zoom lens for my1stD only goes to 4.5. My old manual lens for my k1000 goes to F1.4.Sadly the latter is no good for my needs, as I am trying to capture Osprey, I work on 1/1000 for capturing there speed (well trying) and at 4.5 I often lose light. My camera's lowest ISO is 200 which I have been staying on. Have wondered about cranking that up to say 800, but worried about losing sharpness, as I often seem to have a halo effect as it is, and wonder if using 800 or more for light will make this worse?

[NaClH2O]

Ah! Thank you, and there I thought the glass was better! My zoom lens for my1stD only goes to 4.5. My old manual lens for my k1000 goes to F1.4.Sadly the latter is no good for my needs, as I am trying to capture Osprey, I work on 1/1000 for capturing there speed (well trying) and at 4.5 I often lose light. My camera's lowest ISO is 200 which I have been staying on. Have wondered about cranking that up to say 800, but worried about losing sharpness, as I often seem to have a halo effect as it is, and wonder if using 800 or more for light will make this worse?

Hi Trish, I don't know about the istD, I've never owned it. But I have owned the ist DS and I've found 800 ISO doesn't lose too much detail or have too much noise. Different story on the K10D where 800 ISO is borderline. In any event I'd give it a try. Make a few shots at 800, and at 400. They may work out, and if not, it doesn't cost anything. If you are looking for a reasonably good telephoto lens I can recommend the Pentax K 300mm F4.0. It's not terribly fast and it's all manual, but it isn't deadly slow either, and it's small enough to hand hold. (good thing because it doesn't have a tripod collar) They can be had second hand for less than 300 USD here in the States. I don't know what the market for them is down in Oz but I'd guess they might be easy to find. Unfortunately fast telephotos are VERY expensive.

NaCl(it's a good long lens and relatively inexpensive)H2O