question about depth of field

Hi all if someone wanted a extremely shallow depth of field say f2.8 to blur the background on portraits for example. To obtain an apeture of f2.8 would this be down to the lens you buy or cameras settings ??


Pete

Both.

The lens would need to be at least f2.8 and the camera you use would need settings to allow you to specify f2.8. The best choice would be to use 'Aperture Priority', where you select the aperture, and the camera selects the speed. If you need to over-ride this, select 'Manual' and set both the speed and the aperture to your requirements.

I'd suggest a read of http://en.wikipedia.org/wiki/F-number

From that, it should be clear that for a given lens, the minimum F-number (which corresponds to the widest aperture) is dependent on the focal length, and the maximum size of the hole that the light gets in through.

For instance, I have a Sigma 400mm lens, designated as F5.6. This means that its minimum F-number is 5.6.

400 divided by 5.6 is 71, so it should come as no surprise that the size of the filters it takes is 72mm.

To get to F2.8, it would need to be twice the diameter - that's a big chunk of glass, so not surprisingly, long telephoto lenses with small minimum F-numbers are much more expensive, since they're much more difficult to make.

So the maximum aperture (minimum F-number) is very much dependent on the lens - no matter what you set the camera to (if the camera controls the lens aperture), you cannot get an F-number smaller than the lens's minimum.

All that said, it remains true for a given lens, that to get the shallowest depth of field, you need to set it as wide open as possible.

Be careful though. For portrait work, it's possible to get DoF so shallow, that the eyes can be sharp, but the nose and ears aren't. This may not be what you want.

Sorry, but the f(No.) and the focal length have got bugger all to to do with the filter size ?

If that was the case, why do my f1:4 -100mm and my f1:1.4 - 50mm have the same filter size ?

Galoot is right by saying it has nothing to do with the filter size. However it is the diameter of the front lens that has something to do with it but for the obvious reasons the filter size can never be less than the diameter of the front lens.

Take a look at the 100mm and the 50mm lens, assuming that you are correct in saying that they are both f1.4, then the 50mm front lens should be half the size of the 100mm. Either that or the 50mm lens does not use all of the front lens

No, they are not both f1.4, look again at the post.

The 100mm is f1:4 and the 50mm is f1:1.4. but they both have the 49mm filter size.

Did I not say 'assuming that you are correct in saying that they are both f1.4'. This was not a statement that lenses are not both f1.4

Like I said, it is the front lens and not the filter size. However, a 100mm f1.4 lens with a 49mm filter size, hmm!

Please re-read the post.

This time note, f1:4 for the 100mm lens, NOT - f1.4, as you seem to imply and YES it is a 49mm filter size.

Sorry, you lost me here,

"Did I not say 'assuming that you are correct in saying that they are both f1.4'. This was not a statement that lenses are not both f1.4"

Duh ????

Well, Ok, fair cop.

I had a look at my 400mm F5.6 lens - 400/5.6 = 71.

I also had a look at my 200mm F4 lens - 200/4 = 50

Each of these is within a millimetre or two of the filter size - which is obviously the size of the front of the lens - and in those cases also the front element of the lens.

I should indeed have looked a bit more closely at my 50mm F 1.7, and 135mm F 3.5 lenses, each of which has a front element where the diameter of the disc through which the light passes is smaller than the diameter of the front. In the 50mm case it's 29mm - and 50/29 is 1.7. For the 135 it's about 38mm - and 135/38 is about 3.5.

So Ok, I was a bit loose in my description - in the interests of strict accuracy I should have noted that not all lenses have a front element the same size as the filter. My apologies.

I do apologies, it was the way your describe the two lenses f1:1.4 and f1:4, guess I need to recheck my glasses. However, you need to re-check your brain cell. For the last time, it has nothing to do with the filter size but the front element lens. Please read ChrisA’s description very careful.

For a simple one element lens the f number is obtained by dividing the focal length (say 50mm) by the diameter of the lens (say 25mm) which in this case would give us f2.

If the focal length had been 100mm then if the glass diameter stayed the same the f number would be f4.

With a complex lens of many elements the front element may have to be much much larger and what we need to calculate the f number is the effective diameter of the optics.

It's also easy to measure the focal length of a single element lens, but in a complex lens we measure from the nodal point which lies within the lens.

When we talk about many of these things it's helpful to restrict it to the simple lens as it makes a general understanding of the principles a little easier.

Just to add to johnriley's description. The reason why lenses don't have one simple element is because that single element would act like a prism when it refracts the light, spliting up the light into red, green, etc. So it needs another element to counter act the prism effect, which produces another effect, so we need another set of elements to counter act that effect and so on until the end image is near perfect. How perfect depends on the quality of the elements.

BTW I thought we were talking about depth of field

We were, but one thing leads to another and certainly the concept of f numbers is relevant to understanding.

One of the great things about photographic theory is that it all inter-connects, so when all the pieces fall into place it's quite fascinating. Certainly there's plenty to keep us amused for a very long time.

As has been pointed out, lenses need more than one element to work effectively as photographic lenses. The most versatile lens is probably our own, everyone has one in each eye, and there's a simple one element lens that is capable of remarkable things.

Another interesting thread, though perhaps not what petekd was after!

Suffice it to say that whether you select the aperture directly from the lens or from within the camera, that aperture has to be available ie. a lens with a maximum aperture of F2 cannot be opened to F1.4.

John is quite right to point out that modern, complex lens designs means that lenses of the same focal length and maximum apertures can vary enormously in their physical dimensions. The quoted focal length is really more to do with the angle of view, rather than any actual physical measurement - 50mm lenses, irrespective of their design, have a 47 degree AOV.

For any given focal length, a faster lens (wider max. aperture) with have a larger diameter front element and a really fast one might be so large as to require a wider body and therefore a larger diameter filter.

Prime example (pardon the pun): SMC Pentax A 50mm F2, 50mm F1.7 and 50mm F1.4 have progressively larger front elements but all take a 49mm filter and only become slightly larger and heavier. The 50mm F1.2 however requires a 52mm filter and weighs almost three times as much as the F2 lens.
The F2 features 5 elements in 5 groups (seperate elements, in other words), whereas the F1.2 has 7 elements in 6 groups (two of the elements are placed or bonded together).
The respective physical dimensions (max. diameter and length) are: F2 lens is 63.0mm by 31.0mm and the F1.2 lens is 64.5mm by 47.5mm.

Faster lenses are bigger, heavier and more expensive than their slower cousins but in some cases can be relatively poor performers - the more glass you have, the more critical the tolerances need to be!

And the more glass you have the more internal reflections. Even with 0.2% reflection thanks to SMC coating, that adds up with every surface in a 17 element lens...

Without coating, reflections can be 5% per surface, so the development of modern lenses needed superb coating technology.

This is one reason why the aforementioned 50mm lenses often amaze digital ohotographers - with only 6 or 7 elements there is much less relection, therefore higher contrast.