P&S to SLR Lens Apertures

Buszaj · Mar 29, 2008

Hey everyone, I'm finally going to be getting an SLR soon. Just wondering, which actually lets in more light? f/2.8 on a P&S (Lumix series; same type/style camera as Canon S5/S3), or the Canon 70-200 f/4?

Thanks

christopher walrath · Mar 29, 2008

The smaller the f-number, the larger the aperture diameter in relation to the lens' focal length. So a f/2.8 will admit more light than a f/4. But the SLR type camera will afford more options and add-ons than a PhD (push here dummy) camera will.

Buszaj · Mar 29, 2008

Well that I know, I was wondering because a P&S lens is obviously much smaller than the 70-200. So, because of this, which one would actually let in more light? Its just that a P&S f/2.8 isn't really a true 2.8 (so I'm told by others). The focal length on those small lenses is tiny, though it reaches pretty far somehow...Thanks for the reply though.

Helen B · Mar 29, 2008

For the same f-number, the longer the focal length of the lens the more light it lets in from an object - but that is exactly offset by the area of the image of the object, so more light is let in but it has to cover a larger area. The end result is that the brightness of the image of an object at the film or sensor is the same for any focal length of lens at the same f-number, if the slight losses in the lens are ignored.

An exception to this rule is for images of true points of light, like stars. If they are focused to what is effectively a single point then the brightness depends on the actual diameter of the aperture (the entrance pupil, to be accurate), not the relative aperture. Therefore a star picture taken with a 210 mm lens at f/5.6 is brighter than one taken with a 21 mm lens at f/5.6, for example.

Best,
Helen

Garbz · Mar 29, 2008

It is a true f/2.8 lens. And the focal range is tiny for the same reason as the most DSLR's have a shorter ones too. Crop factor.

Man Digital SLRs have APS sensors which are 1.5x smaller than a 35mm film. So you have to multiply the numbers to get the "35mm equivalent" i.e. a 30mm on an APS sensor is very close to a 50mm on a full frame sensor.

The same applies to P&S cameras except the crop factor is closer to 3-6x which is why you often see lenses quoted in single digits like 4mm-12mm f/2.8-3.5 and despite having an f/2.8 lens (true f/2.8) you still get no depth of field.

Socrates · Mar 29, 2008

Helen B said:
For the same f-number, the longer the focal length of the lens the more light it lets in from an object - but that is exactly offset by the area of the image of the object, so more light is let in but it has to cover a larger area. The end result is that the brightness of the image of an object at the film or sensor is the same for any focal length of lens at the same f-number, if the slight losses in the lens are ignored.

An exception to this rule is for images of true points of light, like stars. If they are focused to what is effectively a single point then the brightness depends on the actual diameter of the aperture (the entrance pupil, to be accurate), not the relative aperture. Therefore a star picture taken with a 210 mm lens at f/5.6 is brighter than one taken with a 21 mm lens at f/5.6, for example.

Best,
Helen

I was going to say something like that but when I saw that you posted, I figured that you covered the issue. I was right.

Buszaj · Mar 30, 2008

Thanks for all the replies. So, by my reasoning, the 70-200 would still let in more light-->P&S lens- so lets say 20mm/f2.8=7.14mm diameter
-->70-200 lens- 200mm/f4=50mm diameter

Am I correct? Once, again thanks for the answers.

Helen B · Mar 30, 2008

Yes, a 200 mm f/4 lens will let in more light from a small area of an object than a 20 mm f/2.8 will. The ratio is the area ratio of the entrance pupils: (50/7.14)^2 which is 49x (call it 50x, because f-numbers are rounded, and not that accurate anyway).

The 200 mm lens will produce an image that is 10x the linear dimension of the image the 20 mm lens will produce. Something that is 1 mm long in the image produced by the 20 mm lens will be 10 mm long in the image produced by the 200 mm lens. That 10x length ratio is a 10^2 times area ratio, ie 100x.

This means that the 200 mm f/4 lens lets in 50x as much light from an object, but it gets spread out over 100x the area. Therefore it is half as bright - ie one stop, which is what you expect going from f/2.8 to f/4.

Best,
Helen

christopher walrath · Mar 31, 2008

Yep, Helen's spot on here. ANY lens with a maximum aperture of f/2.8 will let in twice the light of ANY lens with a maximum aperture of f/4 at those settings. If the 2.8 lens is set to f/8 and the 4 lens is set to f/5.6, then the other lens will now allow twice the light at those settings. The f number is a direct ratio between a lens' focal length and the aperture's diameter. 200mm lens/50mm aperture diameter=aperture of f/4. 500mm lens/45mm aperture diameter=aperture of f/11. Applies to any lens, even pinholes if you can mike the object you make the pinhole with and the distance from the pinhole to the film, you get the working aperture for that camera.

Helen B · Mar 31, 2008

Chris,

Are we saying the same thing? I'll re-write my earlier posts another way:

Suppose that you have a 50 mm f/2 lens on a full frame camera and a 25 mm f/2 lens on a half-frame camera (or a 4/3rds digital camera) and you stand in the same place and take the same picture with both lenses set to f/2. The 50 mm lens will let in four times as much light as the 25 mm lens, but the intensity at the film/sensor plane will be the same.

Best,
Helen

Buszaj · Mar 31, 2008

Okay! I think the last explanation helps the most. Thanks.

christopher walrath · Apr 1, 2008

I gotta disagree, Helen. f/2 at 1/125th of a second is EV9. Regardless of the focal length of the lens. The 50 does not let in more light because it is longer, it allows the same because, though the light has to travel farther, it travels through a porportionately larger aperture. f/2 is f/2. The same amount of light will travel through an aperture setting of f/2 no matter what.

Oh, and yeah we were saying the same thing. I just like emphasizing a good point when it is made. Have a great night, all.

Helen B · Apr 1, 2008

Chris,

Please re-read exactly what I wrote, and the context it was written in:

"The 50 mm lens will let in four times as much light as the 25 mm lens, but the intensity at the film/sensor plane will be the same."

It's the intensity that results in f/2 having the same effect regardless of focal length, not the 'amount of light.' The 'amount of light' travelling through an aperture from an object depends on the projected area of the aperture (usually the entrance pupil), not the f-number.

Best,
Helen

Garbz · Apr 2, 2008

Could this be explained that at a 50mm f/1.8 has a 52mm diameter but the 200mm f/2.8 has a 77mm front diameter? One of these days I'll actually open the optics textbook on my shelf behind me :er: