Please explain the f in lens for me.....

Hi,

I read alot lately and bought me a f2.8 lens because this is a very lightstark lens according to everyone.

Now my question, and please correct me if I'm wrong.

When I shoot I mostly shoot at Aperture sizes between 8-16 to get a good deep dof.

When using a 5.6 lens vs een 2.8 lens on aperture size f8 does the 2.8 lens STILL have more light than the 5.6 lens or does that not matter than.

Maybe a stupid question, but they always tell me there are stupid questions .

Greetings,
Frank

Say we have a f2.8 and an f5.6 lens.

If we have either one of these lenses set at f5.6 on up through the scale they will be transmitting the same amount of light to the sensor and have the same depth of field.

Where the F2.8 has the advantage is in low light situations, at f2.8 the lense is allowing twice the light through as the 5.6 and is thus faster.

I hope this answers your question.

The maximum aperture eg. f2.8 of a lens is the widest aperture the lens can use. Two lenses set at the same aperture are letting in similiar amounts of light and would use the same shutter speed. Some of the advantages of using fast lenses (low f) are that most lenses are less sharp at their extreme f stops. So an f 2.8 lens should be really sharp at f4.5 or 5.6. A lens with a maximum aperture of f5.6 might not be very sharp till f8. The wider maximum aperture gives you more flexibility in terms of setting depth of field and also shooting in very low light without flash. In addition fast lenses are usually more expensive and built better. For 10 years I used lenses with f4, and f5.6. Recently I've been able to afford the faster lenses. I've noticed a big improvement in my pictures. I can now shoot sports shots in arena lighting, something I couldn't do before due to shutter speeds that were too slow.
I hope that answers your question.

Ok,

That get's me the answer I allready knew, so that's correct, thanks for that.

Now let's go on in that area.

Let's say I photograph a waterskieër at a 30-40mtr distance will the f 2.8 give me a better DOF than the same skieër at 10mtr's distance.

Again sorry for maybe an obvious question, but I read somewhere that in f2.8 setting you only have a DOF of 5-10cm in which the focus is correct.

I shoot alot of birds and sport and when I got such a small margin for focus it will be very difficult and I wonder in that case if I have not take the wrong decission in buying a f2.8 lens.

On the other hand I expect the lens to be much better in build quality and I'm thinking about adding a 1.4x and 2.0x convertor both leaving me enough space for AF.

Greetings,
Frank

Hi JAF

Depth of field (DOF) depends on several things. The most important as far as you are concerned are the Fstop and the distance from the object. If you set you aperture to F2.8 and try and photograph the nos on someone 1 foot away you will find that their eyes will just be in focus but their ears arent. If you step back 10 feet and refocus you will now find that their ears are in focus. If your aperture remains constant then as your distance from the object increases your DOF increases. I suggest that you do a search on google for Hyperfocal distance which explains the maths behind how DOF works and includes such lovely things as Circles of confusion and lens nodes.

Hope that helped

BearSummer

As I understand it....

Since depth of field - the amount of the scene before and after the focussed point which is 'acceptably' in focus - is relative to focussing distance, then the further the distance between the camera and the focussed part of the scene, the greater the depth of field.

"Please explain the f in lens for me....."

Hey, watch the language!!


"I wonder in that case if I have not take the wrong decission in buying a f2.8 lens"

Not at all. An f/2.8 or an f/5.6 lens will be exactly the same when taking a picture at f/8 because the aperture is stopped down automatically when you take your shot, but as you're composing the image the f/2.8 lens will give you a brighter image in the viewfinder. For wildlife and sports photography that'll be easier and therefore less tiring.

f/2 is focal length [f] divided [/] by two [2], this gives the actual physical aperture size. A 100mm lens with a 50mm aperture allows the same amount of light to reach the film as a 200mm lens with a 100mm aperture, or a 50mm lens with a 25mm aperture. Makes you realise why a 400mm f/2 lens has such a huge front element!

Rich.

Allright, thanks .

Since most of my shots will be of distant subjects I will be in the clear.

I can't wait to get my lens and start shooting .

Greetings,
Frank

At f2.8 the lens will let in four times as much light as at f5.6!

Also,. even if you allways shoot stopped down, an f/2.8 lens will perform better during autofocus in low light than a lens aith a smaller aperture.

The Autofocus allways occurs with the aperture wide open,. then the lens stops down just before the actual shutter release.

There's always a trade off between shutter speed and depth of field, unless you have unlimited light. As you try to increase your shutter speed to stop action you wind up opening your aperture and decreasing the depth of field and vice versa. For portraits a narrow depth of field is just the ticket. Taking a portrait with an f stop of 2.8 - 4.5 (approx) will give you a nice blurred background and a sharp subject. It's really a matter of choice. Also be aware that depth of field does not fall evenly in front and back of what you focus on. The area in focus (depth of field) is divided 1/3 in front of your subject and 2/3 behind it. Here's an experiment I did to test my focusing that's a good example of depth of field.
Put a 50mm or so lens on your 10D. Take some cans of compressed air or soda cans and line them up one behind the other a few inches apart and about an inch to the side so you wind up with a slightly diagonal line of cans. Then open your lens aperture to its widest(smallest number). From about ten feet away take pictures of each can using the center autofocus point only. Then close down the aperture to the smallest one (largest number) and focus on the middle can. Compare the results. It's a simple way to demonstrate for yourself the power of depth of field. Have fun.

I will try that one.
The DOF is indeed TOTALLY different from the HP850 I used before because there I could almost see no difference in apperture size.

Greetings,
Frank

True - the small "consumer" digicams use very small sensors with wide angle (relative to 35mm) lenses. This combined with a general lack of lens speed means you can't do very narrow DOF shots.

I have a Minox film camera (the classic "spy camera" ) that has only one aperture setting - f3.5, and it uses guestimation focusing where you turn the control to the distance you believe to be correct - but that works most of the time because the DOF is very large (9mm film, 15mm lens.)

Most people have explained it, but I would buy whatever lens is the fastest (smallest f/#)... this will allow me much more flexability.

Then again, budget restrains me from buying an f/1.0 lens (oh if anyone has an extra 50mm f1.0 or 1.4 lens laying around that they want to donate to a good cause--me--, then let me know... heh)