Fast Glass?

[Crosby]

I am seeing a lot of threads mentioning "fast glass." What does this mean? I know we are talking about lenses.
1) Is it in relation to shutterspeed, aperture size, auto focus, or what?
2) When buying a lens, what specs should I look at to know it is 'fast glass'?

Thanks.

 

[Village Idiot]

A wider aperture is considered fast.

f/1.8 faster than f/2.8 fast that f/5.6

 

[Mav]

It means a fast aperture, or f/number. An f/2.8 zoom is much faster than the typical f/3.5-5.6 consumer zooms. Click the DSLR & Lenses 101 link in my sig and go to where it explains lens aperture.

 

[Crosby]

Thanks, I got it. I also found MAV's link to lenses and got my answer there also. I remember reading about the general rule, Mav, hope you don't mind me cutting and pasteing from your link.

"There is a general rule in photography that says you should always be shooting with a shutter speed faster than 1 over the focal length. So, if you're shooting a 200mm focal length, your shutter speed should be at least 1/200th of a second or faster. That rule was accepted decades ago, back when all 35mm cameras shot film."

Thanks for supporting the ignorant!

 

[Socrates]

Thanks, I got it. I also found MAV's link to lenses and got my answer there also. I remember reading about the general rule, Mav, hope you don't mind me cutting and pasteing from your link.

"There is a general rule in photography that says you should always be shooting with a shutter speed faster than 1 over the focal length. So, if you're shooting a 200mm focal length, your shutter speed should be at least 1/200th of a second or faster. That rule was accepted decades ago, back when all 35mm cameras shot film."

Thanks for supporting the ignorant!

Some caution in interpreting Mav's words. First, the guideline is based on the ability of a normal human being to hold the camera steady for a shot with that focal length. That's its only purpose.

Second, although quite valid for 35mm film, the FoV with amateur DSLR cameras is smaller so you need to multiply the focal length by 1.5 before you apply the rule. The sensors on amateur cameras are called "DX" by Nikon and "APS" by Canon. Yeah, technically, with Canon, you should multiply by 1.6 but that's getting too geeky.

 

[Crosby]

Thanks, good input.

 

[Helen B]

That rule of thumb is not a guarantee of critical sharpness, either. If you are using high resolution 35 mm film with good lenses you may need a significantly higher shutter speed to prevent camera shake from being the thing that limits sharpness.

Conversely, many people can achieve acceptable sharpness at much longer shutter speeds, especially when using grainy film (the film grain gives an appearance of sharpness even though the image may not be sharp).

Best,
Helen

 

[Crosby]

Conversely, many people can achieve acceptable sharpness at much longer shutter speeds, especially when using grainy film (the film grain gives an appearance of sharpness even though the image may not be sharp).

Best,
Helen

Helen, you bring up another newbe question... Are you saying all film is grainy or is there certain films that are just grainier than others?

Currently I have a 35mm film camera.

 

[Helen B]

Certain films are grainier than others, though really grainy films are getting difficult to find these days, particularly in colour. The tendency is towards fine mushy graininess rather than large, sharp graininess.

Pushed B&W film can be made to be grainy quite easily, as can some slide film. Scotch/3M 1000 and 640T were very grainy slide films even at their rated speeds, but they are long gone.

Until recently I used a lot of Ektachrome 320T (EPJ) pushed two or three stops. With a three stop push it shows a lot of sharp graininess, and a 1/4, 1/8 or 1/15 s exposure at f/1 could look sharp simply because the graininess was sharp and the film was incapable of high resolution.

Best,
Helen

 

[The_Traveler]

Some caution in interpreting Mav's words. First, the guideline is based on the ability of a normal human being to hold the camera steady for a shot with that focal length. That's its only purpose.

the operative issue here is hand-hold. If the lens/camera is on a tripod or otherwise supported and steadied, you can get by with much slower shutter speeds. Additionally if the lens has Vibration Reduction then a slower shutter speed is usable.

Second, although quite valid for 35mm film, the FoV with amateur DSLR cameras is smaller so you need to multiply the focal length by 1.5 before you apply the rule. The sensors on amateur cameras are called "DX" by Nikon and "APS" by Canon. Yeah, technically, with Canon, you should multiply by 1.6 but that's getting too geeky.

Altho the FOV is smaller, that shouldn't effect the allowable shutter speed limits because it is only the Field of View that is snipped, the lens isn't getting longer.

 

[Helen B]

I've always thought of it as being dependent on final magnification, so the focal length of the lens is only one of a number of factors. You can hand-hold a 15 mm lens at very slow shutter speeds with a full frame camera because the low magnification reduces the image dispacement caused by camera movement. The displacement at the image plane would be the same for a P&S camera with a small sensor, but the image has to be enlarged more, so the small degree of displacement that is imperceptible with the moderate degree of enlargement from full frame becomes noticeable with the larger degree of magnification from the small sensor.

How does that sound? (It's a bit rushed, and I apologise for not putting enough efort into explaining well enough, but at least I know what I mean!)

Best,
Helen

 

[The_Traveler]

Helen,

I absolutely agree with your explanation about the displacement of any point at the image plane. My belief that the rule of thumb needn't be recalculated is based on a somewhat tenuous chain of understandings.

Full frame capable lenses are said to be able to focus from x to y, irrespective of whether they are mounted on a full-frame or cropped frame body.

This implies that the distance of the focal plane from the focal point of the lens is the same for both full and cropped frame bodies.

(Otherwise the lens could focus closer in one body and beyond infinity in the other)

Thus a 50 mm lens focussed on an object at 25 feet would produce an image of that object the same height in pixels on both a full-frame and cropped frame sensor (assuming same pixel density on the sensors). (This can be tested on a D3 body with different crop factor and full-frame lenses.)

Thus the displacement of any object would be the same on a cropped and full-frame sensor and

thus the 1/focal length rule of thumb would be the same.

Since this experiment was all done in my mind while lying on a couch staring at the ceiling, let me know if I have strayed.

Lew

 

[Helen B]

Yes, I agree, but I think that there is one more step. The image displacement is the same, and lets say it is 5 pixels. To produce the same size print those 5 pixels might be translated into a blur size of, say, 0.2 mm when enlarged from the full-frame sensor and 0.32 mm when enlarged from the 'cropped' sensor.

Suppose that the limit of our perception is about 0.25 mm. The enlarged blur from the full frame sensor is below this number, the enlarged blur from the cropped sensor is above it.

That's just an example to try to explain what I'm thinking. It tends towards counting angels on the head of a pin, but if the crop factor was bigger, the difference would be bigger.

Best,
Helen

 

[The_Traveler]

Yes, I agree, but I think that there is one more step. The image displacement is the same, and lets say it is 5 pixels. To produce the same size print those 5 pixels might be translated into a blur size of, say, 0.2 mm when enlarged from the full-frame sensor and 0.32 mm when enlarged from the 'cropped' sensor.

Suppose that the limit of our perception is about 0.25 mm. The enlarged blur from the full frame sensor is below this number, the enlarged blur from the cropped sensor is above it.

That's just an example to try to explain what I'm thinking. It tends towards counting angels on the head of a pin, but if the crop factor was bigger, the difference would be bigger.

Best,
Helen

That makes full sense because the pixel density is higher on the cropped sensor.

So it might make sense to count the extra implied focal length.

Good and sensible discussion - a relative rarity in this kindergarden.

 

[passerby]

I am seeing a lot of threads mentioning "fast glass." What does this mean? I know we are talking about lenses.
1) Is it in relation to shutterspeed, aperture size, auto focus, or what?

Shutter Speed (SS) is what is all about. A fast lens (glass) is a lens that makes the camera able to use faster SS than the slow lens.
For an example. I point my camera with 50mm f1.8 to the window.
I set the aperture at f5.6 and the SS registers it as 1/320 of a second. When I change the aperture to f1.8 the SS registers it to 1/3200 of a second. That is 10 times faster than f5.6.

But if you take pictures in the day time you don't need fast lens at all, indoors or outdoors. Beside they are expensive you know.