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f numbers
Can someone help me understand f numbers..

I know that the higher the f number the smaller the apature will be but what I don't understand is if a lens is described as a f3.6 to f6.3 what does that mean?.

My camera can still dial in the full range of f numbers so are these still effective with a lens like this...

Comments (11)

Your camera body itself should be able to dial in the full range of F-Numbers, as you say. Any camera that has control over f-stops these days should do this though as they use infinite dials that simply tell the software to stop up or down. The lens itself determines what actual f-stops will be available for use though..

If the lens you're looking at is rated f3.6 to f6.3, then it will only be able to dial in the range of f-stops between those two numbers, despite the camera body itself being able to handle a wider range of f-stops. It will still work fine with your camera though, so go for it! Happy shooting!..

Comment #1

Yes, your camera is still able to use all f/numbers within the range that the mfg specs call for. The designation could mean one of two things, depending on the context you are intending:.

A. The most likely from you phrasing, is that the maximum aperture (widest opening) of the lens on your camera is f3.6 to f6.3 according to how far to the telephoto side you have zoomed the lens. (At it's widest it will achieve f/3.6 but at the most tele, it will only achieve a maximum opening of f/6.3. As you zoom, it changes.) All other apertures above those will still be useable. The change effect is only at the widest apertures at specific focal lengths. By making it variable, the manufacturer keeps the size, weight and cost of the camera down..

B. (Or) If your question is about the unusual nature of these numbers compared to those f/numbers most often mentioned: it simply means that the these specific f/numbers are in between f/2.8 and f/4 (for f/3.6)..

John.

Gleneagles wrote:.

Can someone help me understand f numbers.I know that the higher the f number the smaller the apature will bebut what I don't understand is if a lens is described as a f3.6 tof6.3 what does that mean?My camera can still dial in the full range of f numbers so are thesestill effective with a lens like this..

Van..

Comment #2

A. The most likely from you phrasing, is that the maximum aperture(widest opening) of the lens on your camera is f3.6 to f6.3 accordingto how far to the telephoto side you have zoomed the lens. (At it'swidest it will achieve f/3.6 but at the most tele, it will onlyachieve a maximum opening of f/6.3. As you zoom, it changes.) Allother apertures above those will still be useable..

Exactly. So, at it's shortest focal length, the range of apertures might be f/3.5 to f/32: at the longest end, the range is limited to f/6.3 - f/32. Since the widest aperture is what people want to know about, it is described as f/3.5-f/6.3, meaning that the widest aperture will be somewhere in this range depending on the focal length (zoom position). Everything down from the maxmum to f/32 will always be available..

Best wishesMike..

Comment #3

Gleneagles,.

Take a lens like an 18-200mm f/3.6-6.3 - this means at 18mm the lens has a maximum aperture of f/3.6 and at 200mm it has a maximum aperture of f/6.3..

As others have mentioned, as you adjust zoom from 18 towards 200mm the maximum aperture will decrease from f/3.6 to f/6.3 but you will always be able to use apertures from that maximum to such as f/32..

Regards......Caster.

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Comment #4

This is not true..

If the lens is said to have aperture of f/3.5 to f/6.3 it's the short version of the phrase "the MAXIMUM aperture of the lens is f/3.5-f/6.3". Usually the shortest focal length has f/3.5 and the longest focal length has f/6.3. The minimum aperture is also specified (something like f/22-f/36 but it's not advertized on the lens but in the technical book..

So if the maximum aperture is f/3.5-f/6.3 and minimum f/22-f/36 you can chose any f-stop between f/3.5 and f/22 at shortest focal length and between f/6.3 and f/36 at longest focal length).VictorBucuresti, Romaniahttp://s106.photobucket.com/albums/m268/victor_petcu/http://picasaweb.google.com/teodor.nitica/..

Comment #5

You said exactly what I meant, I just wasn't eloquent enough to get the point across. Obviously you can't only dial in f-numbers between the f3.6 and f6.3 range with that lens. I just meant that you could only dial in the range of f3.6 to the cameras highest f-stop at the wide zoom and the range of f6.3 to the cameras highest f-stop at the telephoto zoom...

Comment #6

I just wanted that the OP not to be more confused than he is right now.I hope I didn't offend you.VictorBucuresti, Romaniahttp://s106.photobucket.com/albums/m268/victor_petcu/http://picasaweb.google.com/teodor.nitica/..

Comment #7

No no, not at all. I fully admit on most message boards I post on I'm not the best at successfully writing what I'm thinking, so half the time some simple advice I give gets all misconstrued. ..

Comment #8

Actually, it is more likely f/6.3 - 96 since the lens shifts all apertures, noit jsut the max aperture in the vast majority of cameras.Van..

Comment #9

Yes, that makes sense. I've never used f/96... I wonder what the diffraction effects would be like?!Mike..

Comment #10

Gleneagles wrote:.

Can someone help me understand f numbers.I know that the higher the f number the smaller the apature will be.

The easiest way to remember and understand this is to think about the correct way that lens apertures should be expressed - as a fraction - because it is actually a ratio. The ’ stands for focal length..

It has become the norm to specify apertures with the shorthand terms of f2.8, f5.6 etc. which can initially seem very confusing - 'what, a smaller number means *more* light?' Actually, they should be expressed as a fraction (’/2.8), to describe the ratio between the focal length and the actual diameter of the chosen aperture of the lens..

So a 50mm lens at f2.8 will have an actual physical aperture opening in the lens blades of 17.9mm in diameter. 50mm / 2.8 = 17.9mm. The same 50mm lens at an aperture of f5.6 has a diameter of 8.9mm - so you can see that the hole is getting smaller as you stop down, thereby letting less light in. You can illustrate this to yourself - stick a lens on your camera - check the focal length, set an aperture, work out the focal length divided by the aperture, look at the front of the lens and hit the DOF preview button (if you have one of course) and see if the lens aperture closes to the number of your calculation. Try different focal lengths and apertures and you'll see that it consistently always does this. At the maximum aperture, nothing happens as the lens rests fully open..

When you realise that it should actually be expressed as a fraction, it's easier to remember that ’/2 allows more light than ’/4 - a half is bigger than a quarter after all - a useful aide memoire..

Each full stop of aperture (the full stops are 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22) results in a doubling or halving of the amount of light reaching the sensor/film plain - but the numbers aren't halved or doubled at first glance, so that doesn't make sense initially either - but that's because they're expressed as a ratio with diameter, but of course it's a [roughly] circular hole - and the area of the whole circle lets in light, so where exposure is concerned, it's the area of the aperture that counts. So ’/5.6 lets in half as much light as ’/4 because the area of the aperture is half the area that it is at ’/4, thus:.

So, to take our 50mm lens example;’/4 = 12.5mm diameter aperture (50/4) = 123mm area (pi r i.e. 3.14 x 6.25)’/5.6 = 8.9mm diameter aperture (50/5.6) = 63mm area (pi r i.e. 3.14 x 4.45).

I don't expect you to memorise the maths by any means, but once you grasp the basic principle that the aperture numbers used are a ratio between focal length and aperture hole diameter and that each full stop of aperture change halves or doubles the amount of light, as the overall area of the aperture is increased or decreased by changing the diameter of the circular opening, it makes the entire subject much easier to grasp and apply in the field..

It also explains the answers you've had from others about the maximum possible aperture often being expressed as a range of apertures, which are determined by focal length..

For example, I have the inexpensive Sigma 70-300mm lens on my 20D here within reach - the lens is specified as ’/4-’/5.6 - ’/4 being possible at 70mm, the maximum aperture I can get at 300mm is ’/5.6. For the lens to be able to achieve ’/4 at 300mm, it would need to be 75mm+ in physical diameter (300/4)- it's not, it's actually narrower than that - so the maximum aperture is limited by the actual build of the lens in this case. This tends to be the case in lenses in the lower price ranges, to get a constant maximum aperture over a range of focal lengths generally costs lots of money as it involves much more complicated optics and physically bigger lenses..

So many photos, so little time.http://www.peekaboo.me.uk - general portfolio & tutorialshttp://www.boo-photos.co.uk - live music portfoliohttp://imageevent.com/boophotos/ - most recent images.

Please do not amend and re-post my images unless specifically requested or given permission to do so...

Comment #11

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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