Is f2.8 the same as f2.8?

[LJ2b2c]

OK, this may seem like an silly question to some, but here goes. On the many of the point and shoot cameras the specs on the lens are listed such as "5.0-100.0mm f/2.8-5.7 (35mm film equivalent: 28-560mm)" for the Canon SX1 IS. While I'm pretty sure I understand the conversion to 35mm equivalent, I haven't heard much about the f range. f2.8 at 28mm sounds pretty fast for such a small lens, and f5.7 at 560mm sounds great, but is that true. To do that on a DSLR with a full frame or 1.6 sensor would take a monster lens. While I'm sure the picture quality is very different, is the ability of the P&S lens to gather light, really the same as that on a "real" camera? I keep think there should be something like a "35mm equivalent" for this information as well?

[ToddH]

I'm sure I'll be corrected if I'm wrong, but it is the same ratio of focal length divided by the size of the aperture, but since you are dealing with a much smaller focal length, it isn't letting as much light through, and its hitting a much smaller sensor that certainly doesn't gather light as well as the sensor in a SLR camera. So yes, it is the same, but it certainly doesn't do the same thing.





-Todd

[Tim]

I might also be corrected if wrong, but I believe the focal length has nothing to do with the aperture size. The sensor size and the crop factor are more relevant. For example, f2.8 on a full frame lens, like the EF 16-35, has a larger absolute opening than f2.8 on the EF-S 17-55. So the same goes for point and shoot cameras, where the sensor is so much smaller than 35mm, the aperture's technically a lot smaller but is effectively the same relative to the sensor size.

[Jon Ruyle]

The sensor size and the crop factor are more relevant. For example, f2.8 on a full frame lens, like the EF 16-35, has a larger absolute opening than f2.8 on the EF-S 17-55.

In theory this is false. f number is defined as focal length divided by aperture, period. Sensor size does not matter. So the EF-S 17-55 must have an aperture of 55 / 2.8 mm, while the EF 16-35 has a smaller aperture of 35 / 2.8.


However, it is true that for wide lenses, the front element is sometimes larger than the theoretical aperture because the lens has to do something weird to illuminate the large sensor. In this case, an EF-S lens may indeed have a smaller front element than
an EF lens of the same focal length and f number. I would still say the 16-35mm f/2.8 has an aperture of 35 / 2.8 mm (because that is what those words mean to me) even if the front element is larger, but maybe I *should* say "effective aperture is 35 / 2.8 mm" or something.

[neuroanatomist]

ybe I *should* say "effective aperture is 35 / 2.8 mm" or something

Nope, no 'should' about it. Aperture is the diameter of the iris diaphragm. F-stop is the ratio of focal length to aperture, i.e. = focal length / aperture. With algebraic rearrangement, aperture = focal length / f-stop.


A 70-200mm f/4 has a max aperture of 50mm, while a 70-200 f/2.8 has a max aperture of 71.4mm (43% greater diameter). That's one reason why an f/2.8 lens is so much larger than an f/4 lens of the same focal length - the optical elements need to be able to 'fill' the aperture with light, and more glass means more weight (and cost!).


Front element size is not necessarily the same as or directly proportional to aperture. In camera lenses, it's always the same size or larger, as far as I know. But in some optical systems (certain microscopes, for example), the front element of the objective lens is smaller than the iris diaphragm.

[Jon Ruyle]

ybe I *should* say "effective aperture is 35 / 2.8 mm" or something

Nope, no 'should' about it.

Okay, if "aperture" is diaphram size and is not the same as "front element size" then I don't need to say "effective aperture". Just "aperture". The world makes sense. I can breathe easy.

[msmiley]

I keep think there should be something like a "35mm equivalent" for this information as well?

There would be, but it's not as meaningful to most of the people point and shoots are primarily targeted toward, so it's not explicitly stated. To find the "35mm equivalent" aperture, you simply multiply by the same crop factor used to get the focal lengthequivalency, in this case: 5.62


So the SX1 would have a "35mm equivalent" aperture of f/15.7-32.0

[LJ2b2c]

So would an f2.8 lens on a 1.6 crop really be like a f4.4 on a full frame? I knew I could see a big difference in pics when taken bya full frame, but thought it was just a better/bigger sensor.

[msmiley]

So would an f2.8 lens on a 1.6 crop really be like a f4.4 on a full frame? I knew I could see a big difference in pics when taken bya full frame, but thought it was just a better/bigger sensor.

Correct. Sucks doesn't it?


Makes one think twice before paying for fast glass.

[ToddH]

My understanding is that f-stop and aperture aren't the same thing, but the are related. The f-stop being a ratio between the focal length and the actual size of the aperture. The actual physical size of the aperture on a F2.8 24mm lens is actually smaller than a F2.8 200mm lens, even though the F-stop value is the same, and the way I understand it, more or less the same amount of actual light reaches the sensor in both lenses at the same F-stop even though the physical aperture on the 200mm is much larger. So with a small point and shoot camera, the same amount of light hits the sensor per square inch of sensor, but the point and shoot sensor has a much smaller size, and therefor a much less light per pixel.... I think...