Lenses? Fs? Differences?

padocon

Hey all,
I understand a bit about lenses but I was thinking:
If I have a nifty fifty which is F1.8
And I have a kit lens F5 ish.

And say I am shooting at an aperture of F6 on both lenses, what is the difference? Is there a major difference between the two?

The above lenses are an example. I am wondering about lenses in general.

Thanks in advance

Rb

http://en.wikipedia.org/wiki/F-number

padocon

Thanks but I still don't know what the difference will be if I shoot at F6 on both.

DaireQuinlan

padocon wrote: »

Thanks but I still don't know what the difference will be if I shoot at F6 on both.

Assuming they're the same focal length (ie the kit zoom is set to 50 and we're still talking about your 50mm aswell) then not much of one. Define 'difference'

Every lens has it's own subtle or not so subtle 'look'. The kit lens might be more flarey or less prone to flare, or softer or perhaps sharper, or might have more or less distortion than the 50mm. Some lenses have a slight warmer or cooler colour cast. Some might have a touch of spherical aberration, or chromatic aberration, or some other flaws. Some might not. It's kinda like asking what the difference is between two pieces of string, if by 'string' you mean anything from some sewing thread to some multicoloured climbing rope, to a cable hawser used to moor ships, and everything in between

pullandbang

The prime (50mm 1.8) would probably be sharper than the zoom at 50mm for the same f stop.

CabanSail

The f number is just a ratio of Focal length & the Diameter of the Aperture. It is a way to quantify the amount of light that will illuminate your sensor/film.

If you have a selection of lenses of various focal lengths, all set to f8 and all focussed on the same light source, then the level of illumination they each provide will be the same.

DaireQuinlan

CabanSail wrote: »

If you have a selection of lenses of various focal lengths, all set to f8 and all focussed on the same light source, then the level of illumination they each provide will be the same.

<pedant> This is only true if we discount the transmissibility of the lens. I just mention this because it almost ALWAYS gets dragged up by somebody. In RL situations this difference can almost always be discounted for all intents and purposes. </pedant>

notspav

I'm still a little confused about so-called fast lenses, even after reading this thread.

Is it as simple as a 'fast' lens allows you shoot at a larger aperture, and therefore is capable of taking in more light at once? And if you shot at, say, f8 the whole time then there's no point in using a fast lens?

I realise that this is an idiotic question, but the minute I try and read up about this my brain turns into a fine paste.

DaireQuinlan

notspav wrote: »

I'm still a little confused about so-called fast lenses, even after reading this thread.

Is it as simple as a 'fast' lens allows you shoot at a larger aperture, and therefore is capable of taking in more light at once? And if you shot at, say, f8 the whole time then there's no point in using a fast lens?

Pretty much, yeah. They're also generally heavier than their slower counterparts.

There are pros aswell though. Most lenses improve when they're stopped down a few stops in terms of vignetting and sharpness. Your slow cheap kit lens is at its widest at f/5.6, and your 50mm will have been stopped down a bit. In general (and this is in no way a universal thing :-)) the faster lenses perform better at (say) f/5.6 than the equivalent slower lens would at the corresponding aperture. Stop down another stop and the differences will start to disappear. This isn't an optical thing though, just down to compromises made in the design and manufacture of the slower (and generally cheaper) lenses.

notspav

DaireQuinlan wrote: »

Pretty much, yeah. They're also generally heavier than their slower counterparts.

There are pros aswell though. Most lenses improve when they're stopped down a few stops in terms of vignetting and sharpness. Your slow cheap kit lens is at its widest at f/5.6, and your 50mm will have been stopped down a bit. In general (and this is in no way a universal thing :-)) the faster lenses perform better at (say) f/5.6 than the equivalent slower lens would at the corresponding aperture. Stop down another stop and the differences will start to disappear. This isn't an optical thing though, just down to compromises made in the design and manufacture of the slower (and generally cheaper) lenses.

Like how a Ferrari would perform better at 80mph than a Fiat Panda? That kind of thing?

I get ya, thanks.

padocon

CabanSail wrote: »

The f number is just a ratio of Focal length & the Diameter of the Aperture. It is a way to quantify the amount of light that will illuminate your sensor/film.

If you have a selection of lenses of various focal lengths, all set to f8 and all focussed on the same light source, then the level of illumination they each provide will be the same.

But if there is little difference ie if one lens is slightly sharper why do pros pay big for lenses?

kensutz

Big lenses = big egos. L lenses are worth the money in both image sharpness and image quality. Tamron and Sigma can't quite match the L lenses although some give them a run for their money.

padocon

kensutz wrote: »

Big lenses = big egos. L lenses are worth the money in both image sharpness and image quality. Tamron and Sigma can't quite match the L lenses although some give them a run for their money.

So quality and sharpness. Thats all? Sure that surely is not worth the extra 700 euro or so for a "pro" lens. Is it?

kensutz

Yes it is. Build quality, weather proofing etc.

CabanSail

There are lots of factors why you would pick one lens over another.

Many of the Pro Lenses are BIG as they are Fast. You need to be able to have the size to have the bigger aperture.

The quality of the glass, the prescision in which it is ground and the R&D that went into the design are all things which will have impact on the image quality. This includes sharpness, distortion, Chromatic Abberation, Bokeh, susceptibility to flare etc.

The Build Quality of the lens is also a consideration. It's no good having a great quailty lens but you dare not take it outside. Then there is the quality of the motors & the mechanisms which will determine how fast & precisely it will focus.

There are other design factors like if the front element rotates (a PITA when using a CPL) how smooth the rings are for manual focus & zoom. If the lens creeps etc.

All these factors will combine to give the characteristics of a particular lens 7 how much it will cost. You then have to decide what is the best value for money for your application. The "Pro" glass ussually has a lot fewer compromises but this can add to size & price.

democrates

'Fast' refers to shutter speed, but it's the wide aperture that allows you to shoot fast.

Let's say you're indoors using ISO100 film, no flash just available evening light through the window. A lens that only goes down to f4 will mean your shutter speed might be 1/30th, but an f2.8 lens will get the same exposure in terms of light at 1/60th, hence the f2.8 is 'faster'.

melekalikimaka

if both a 50mm, the prime will win

cnocbui

Faster lenses also afford brighter viewfinder images, which I find nice.

Faster lenses also allow for a shallower depth of field, when you want to isolate the subject from the background. Some fast lenses have a superior bokeh.

I have a Zuiko 50mm f1.2 as the standard lens on my Olympus. At f1.2 it is really soft, but it affords a terrifically bright viewfinder image. Stopped down to f5.6 or 8, it has centre to edge sharpness and higher contrast than other Olympus Zuiko 50mm variants.


(bokeh does no t just mean out of focus, it is a term that refers to the quality of the blurry bits)