2.8 lens for rebel xt vs Lumix Fz30 2.8 lens

[mediyoga]

Hi all. I am gonig to ask a stupid query , bear with me on that. Being a physician I can give lectures at end on the human body but I suppose I am a newbie in cameras though I read a lot.
Aparmley very well described that a big window lets more light and hence a 2.8 lens will be very big. I have the following queries

1)how have panasonic compressed that same 2.8 into a small lens in the Lumix Fx30 also giving a 420 mm zoom? I need a tech explanation? I know of course that the sensor is smaller that Rebel xt and quality wont be the same , etc etc. The sony 828 also gives a lens with F 2.8 @ 200 mm.

2) already technology has compressed the size of the 70-300 DO lens though the aperture is still 5.6- agreed that the results are not too good with odd flares

3) Is it possible that one day we will have lens of the DO size with a 2.8 f all the way?
thanks for all help

[Rex914]

1)how have panasonic compressed that same 2.8 into a small lens in the Lumix Fx30 also giving a 420 mm zoom? I need a tech explanation? I know of course that the sensor is smaller that Rebel xt and quality wont be the same , etc etc. The sony 828 also gives a lens with F 2.8 @ 200 mm.

Because the sensor (area of exposure) is a lot smaller. In terms of area, the FZ30's sensor is about 1/16th the area of a DSLR sensor. What this means is that the FZ30 doesn't need as a large a lens to gather enough light for the sensor. The reason why DSLR's have such big lenses is because the sensor size is huge.

http://www.dpreview.com/Learn/Articles/Glossary/Camera_System/images/123di_comp_3m.gifhttp://www.dpreview.com/Learn/Articles/Glossary/Camera_System/images/123di_dslr.gif

2) already technology has compressed the size of the 70-300 DO lens though the aperture is still 5.6- agreed that the results are not too good with odd flares

The difference between a f/5.6 lens and a f/4 lens let alone an f/2.8 lens is enormous. This increases by a quadratic factor, so it takes TWICE the amount of light between each step.

3) Is it possible that one day we will have lens of the DO size with a 2.8 f all the way?

Yes, it'll eventually happen, but not for a while (we're talking many years). And it will be very expensive to begin with.

[mediyoga]

Rex
Thanks a lot for the very prompt and detailed reply. Really appreciate it. It is very clear to me now. I think I will now focus on the canon 70-200 F4 which is not as heavy as the 2.8
regds
Krishna Raman

[Rhys]

I would suggest the Sigma 70-200 f2.8.

[Vich]

I may be mistaken, but wonder if a short explanation of how F-Stops are measured could help.

Rather than making a hard calculation such as mm, they use a ratio of the focal length. So f-8 for 100 mm focal length has an apeture diameter of 1/8th of 100, or 12.5 mm. However; on a tiny sensor a P&S may be more like 6 mm to achieve the same field of view as the 100mm on the normal film camera, but for that, an f-16 apeture would be less than 1/2 mm.

Amount of light goes by Area, not diameter. Well, you can do the math.

f-stops go 1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, each successively double the light.

At 50mm on a normal 35mm film camera lense, f1.4 apeture has an area of 1,002 mm, where f2.8 has an area of 250mm. That's 4 times the light!

Point being; not everything we normally think about focal lengths translates directly to P&S.

As for zooms; I'm also curious why prime lenses (non zooming) can have such dramatically larger apetures than the largest apeture on a zoom.

I apologize for boring you if you were already aware of this. Just thought it might illuminate the subject a little if you weren't.

[timmciglobal]

You sure on the F 1.4 thing?

area = PIr2

so...

X=3.14X1.5^2 = 7.065 square inches = 179.451 mm



Tim

[coldrain]

I would suggest the Sigma 70-200 f2.8.
You always suggest the Sigma f2.8, but it is never clear why you suggest that.
It is weird in this case since weight is mentioned.

[Rhys]

I always suggest optical quality as being better than ligtweight. Economise in weight but not in optical quality.

[coldrain]

I always suggest optical quality as being better than ligtweight. Economise in weight but not in optical quality.
What has optical quality to do with f2.8?

[Rhys]

What has optical quality to do with f2.8?

The faster the lens the better the glass normally.

[Balrog]

You sure on the F 1.4 thing?

area = PIr2

so...

X=3.14X1.5^2 = 7.065 square inches = 179.451 mm



Tim
His calculations were correct .. not sure where you're getting "1.5 inches" for radius.
50mm lens, f/1.4 aperture, therefore:
aperture diameter = 50/1.4 = 35.714mm
aperture radius = 35.714 / 2 = 17.857mm
aperture area = pi * (r^2) = 3.14 * 318.878 = 1001.783 sq mm

By the way, Vich:
At 50mm on a normal 35mm film camera lense, f1.4 apeture has an area of 1,002 mm, where f2.8 has an area of 250mm. That's 4 times the light!

Point being; not everything we normally think about focal lengths translates directly to P&S.
I'd rather say, at 50mm on any lens f/1.4 aperture has an area of 1002 sq. mm.; we just have to remember to use actual focal lengths instead of "film-equivalent mm" ... in fact, the only thing that doesn't translate directly is the field of view..

[Vich]

You sure on the F 1.4 thing?

area = PIr2

so...

X=3.14X1.5^2 = 7.065 square inches = 179.451 mm



Tim
Here's a link:
http://www.uscoles.com/fstop.htm



By the way, Vich:

I'd rather say, at 50mm on any lens f/1.4 aperture has an area of 1002 sq. mm.; we just have to remember to use actual focal lengths instead of "film-equivalent mm" ... in fact, the only thing that doesn't translate directly is the field of view..

Excellent point. Of course, 50mm is only "normal" on a 35mm format, but that's irrelevant to the point being made.

"f" in "f-stop" I presume means "fraction". However; that's just an assumption.

Also; I should say that I did not do the math. I didn't have that table bookmarked when I did the initial post or would have referenced it then.

[Balrog]

Excellent point. Of course, 50mm is only "normal" on a 35mm format, but that's irrelevant to the point being made.

"f" in "f-stop" I presume means "fraction". However; that's just an assumption.

Also; I should say that I did not do the math. I didn't have that table bookmarked when I did the initial post or would have referenced it then.

Don't worry, I just did the math up there. :) Also, once you understand the math, it's easy to see precisely why the f-stops follow their seemingly arcane progression of "1, 1.4, 2, 2.8, 4, 5.6, ...". Since area varies by the square of the diameter, 2*area will be achieved by sqrt(2)*diameter .. i.e. each number is just the previous one times the square root of 2 (=1.4142).

Also, as far I know, the "f" in f-stop is just "focal length" .. i.e. f/1.4 means precisely "focal length divided by 1.4". "Stop" is just a generic photographic term for a doubling or halving of an exposure variable, I think. Dunno the etymology of that term.

[timmciglobal]

I don't understand why the numbers don't work in inches yet do in millimeters.

Here:

The lens is 3.9 inches wide or so, 1.5 is the maximum raidus of lens. 1.5X1.5 = 2.25, 2.25 X 3.14=7.065= 179 mm.

Something isn't right. I don't get it...

Tim

[timmciglobal]

1004 square millimeters cna't be right, think about it guys, 1004 square MM = 39 inches. If you made a lens a square instead it would have to be 7.5 inches X 7.5 inches to get that surface area.

Something is screwed up.

Tim

[Rex914]

How did you get 39 square inches from 1004 mm? Moreover, how did you get 7.5 by 7.5 from 39? I have a feeling that you are doing the conversion in a linear fashion and forgetting to account for the square.

[timmciglobal]

Your right :/

Gotta convert from square inches to square mm :/

Tim

[Balrog]

also, two things - one is that the focal length is listed in mm; focal length and the lens' physical length don't always match up exactly (optical tricks, don't ask me) ... second is that according to the site linked by Vich, the area calculated using this direct method would be exact if the aperture was in front of the first lens element; when it's somewhere inside the lens as is usually the case, the area can actually be much smaller than that (again, optical tricks I guess.)

Still, the principle holds; increase the diameter by sqrt(2) and the area (and thus light transmitted) increases by a factor of 2.

[ReValveiT]

Interesting...Yet,














Mind numbing...

[aparmley]

I'm not sure if anyone mentioned this but, isn't the lens on the FZ30 2.8-3.7 on the long end so . . . yea at full tele its a 3.7 .

isn't a point and shoots aperture rating equivelant to a higher value on DSLR - I can't find the conversions at this hour but I am almost positive.

So taking a guess the 2.8-3.7 lens on that P&S is something like F4 - F11 on a DSLR - not really fast at all . . . thats a guess though - hopefully one of you math junkies up there fixes my guess-timations. :p

[coldrain]

I'm not sure if anyone mentioned this but, isn't the lens on the FZ30 2.8-3.7 on the long end so . . . yea at full tele its a 3.7 .

isn't a point and shoots aperture rating equivelant to a higher value on DSLR - I can't find the conversions at this hour but I am almost positive.

So taking a guess the 2.8-3.7 lens on that P&S is something like F4 - F11 on a DSLR - not really fast at all . . . thats a guess though - hopefully one of you math junkies up there fixes my guess-timations. :p
I think the DOF is different (more) but the 2.8 is still 2.8. So, it is just as fast, just a bigger depth of field due to the smaller sensor. So if you use the aperture rating as a measure of DOF, you are right.

[mediyoga]

The lens on the Fz30 is a constant 2.8 even at the tele end. I have one but am going to sell it
Krishna Raman

[coldrain]

The lens on the Fz30 is a constant 2.8 even at the tele end. I have one but am going to sell it
Krishna RamanNo it does not. Just read the little numbers that are printed on the lens.

[aparmley]

Aperture range F2.8 - F3.7 / F11

Taken from: http://www.dpreview.com/reviews/specs/Panasonic/panasonic_dmcfz30.asp

[JTL]

The lens on the Fz30 is a constant 2.8 even at the tele end. I have one but am going to sell it
Krishna RamanThis is true of the FZ20...not the FZ30...a rather big difference, don't you think? You own it and don't know this? Regardless, facts shouldn't be difficult to get right! :rolleyes:

[mediyoga]

Thanks JTL. Big bloomer on my part!
Krishna Raman