BetterPhoto Q&A
Category: New Answers

depth of field ..popular and confusing

I am using a G9 camera.. to take pictures..

SO my qestion is I have read.. the depth of field(blur in the background) is the most shallow or more blur at the largest aperature... in my case its 2.8

But I am amazed the g9 camera when I take picutre in 2.8 the blur in the background is not much but when I zoom a little bit and take the picture... the aperature is 4.5 and the depth of field(blur in the background) is very shallow there..

So how come the blur is increased in the background with 4.5 aperature than the 2.8 aperature..

i know I am misunderstanding something here...

can some one help me...

thanks in advance...

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December 21, 2007

DoF is not just a function of aperture, but also of focal length. The longer the focal length, the shallower the DoF. And you zoomed from wide angle to tele. With considerable consequences for the DoF, as you've seen.

Have fun!

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December 21, 2007

Depth-of-field is that span of distance both fore and aft of the point focused upon that remains acceptably sharp. Depth-of-field is not split down the middle. It extends 2/3 away and 1/3 back towards the camera, as measured from the point focused upon. As an example, if you are focused upon an object 6 meters away, the depth of field is likely 4 meters ~ 15 meters.

Depth-of-field is controlled by four factors:
1. Aperture; the smaller the lens opening the greater the depth-of-field.
2. Focal length; Depth-of-field is increased when a short lens is in use. Depth-of-field decreases when longer lenses are used. In other words, short focal length lenses have expanded depth-of-field – long lenses have reduced depth-of-field.
3. Distance focused upon; Depth-of-field is reduced when you close focus. Depth-of-field is increased when the point of focus is distant.
4. The size of the display screen or print size. Tiny images appear to have greater depth-of-filed than large images.

Now to answer your question: When we zoom-in the focal length becomes longer and longer. We are magnifying the subject. Optically we pay a price for this magnification. The price is a loss of light at the chip plane (focal plane). In your case, as you zoom from short to long, the aperture changes; it becomes smaller. The change is from f/2.8 to f/4.5. That’s 1 ½ stops. Now f/4.5 is smaller in diameter than f/2.8 and logically one would expect to see an increase in depth-of-field. This is not the case, as you have zoomed to a longer focal length. Longer yields reduced depth-of-field. The amount of change in focal length is overbearing in magnitude as compared to the 1 ½ f/stop stop-down. In other words focal length rules and the depth-of-field become shallower.

f/4.5 does display more depth-of-field than f/2.8. Had you only changed apertures and not zoomed, the depth-of-field increase would have materialized.

Alan Marcus (marginal technical gobbledygook)
ammarcus@earthlink.net

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December 21, 2007

[f/4.5 does display more depth-of-field than f/2.8. Had you only changed apertures and not zoomed, the depth-of-field increase would have materialized.]

hey alan do u mean there woud be no difference if I hand not zoomed.. what did u meant by materialized..

By my experiment with the camera if I dont zoom changin the aperature would only change light intensity ..... not the DOF..

One more question you know I have g9 camera with f/2.8 to f/8, with 6x zoom what would be the best setting for most shallow depth of field...

from my experience I am getting that in f/4.5 at max zoomed position .. or are there any other better sweet spots...
I tried to experiment.. but I think some theory makes it more clear....

and there is an unknown quote ... in the lens 7.4mm to 44.4mm in the lens may be its focal length.. should I be happy with that.. or its normal.. or .. how should I react with this number

Pls forgive my english

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December 22, 2007

f/4.5 does display more depth-of-field than f/2.8. Had you only changed apertures and not zoomed, the depth-of-field increase would have materialized.

Hey Alan do you mean there would be no difference if I hand not zoomed? What do you mean by materialized.

f/2.8 is the largest aperture (lens opening) made available with your lens. f/2.8 has diminished depth-of-field. f/4.5 has increased depth-of-field. So I am saying there is a difference. As you set your camera to smaller and smaller apertures, depth-of-field increases. You should know that when I say smaller, I am talking about the size of aperture. The confusing part is small apertures are labeled with big numbers. f/32 is a vary small aperture with vast depth-of-field. f/22 is just a little bigger, depth-of-field is slightly reduced. f/16 is bigger yet, depth-of-field continues to reduce. f/16 is a yet bigger aperture, depth-of-field continues to lessen. The word materialized means to become apparent.
By my experiment with the camera if I don’t zoom changing the aperture would only change light intensity ..... Not the DOF.

This is false. As you change aperture the light intensity diminishes if you make the aperture smaller, depth-of-field increases. As you change to a larger aperture more light can enter the camera and depth-of-field shrinks.

One more question you know I have g9 camera with f/2.8 to f/8, with 6x zoom what would be the best setting for most shallow depth of field...

Setting the lens aperture to f/2.8 gives you the shallowest depth-of-field. As you zoom, your lens will change to f/4.5. We would like the lens to remain at f/2.8 as we zoom but this would greatly increase the cost of the lens. f/4.5 while not as shallow as f/2.8 continues to be shallow. Depth-of-field is most shallow at 6x zoom with the lens set to f/4.5.

from my experience I am getting that in f/4.5 at max zoomed position .. or are there any other better sweet spots...
I tried to experiment.. but I think some theory makes it more clear....

At max zoom set to f/4.5 depth-of-field is the shallowest setting you have.

and there is an unknown quote ... in the lens 7.4mm to 44.4mm in the lens may be its focal length.. should I be happy with that.. or its normal.. or .. how should I react with this number

The Canon Powershot G9 is supplied with a zoom lens. The zoom range is 7.4mm ~ 44.4mm. This is the focal length of the lens. When set to 7.4mm the lens is a wide angle. When set to 44.4mm the lens is a telephoto. Be happy! This camera also has an additional 4x more digital zoom, that’s a lot of zoom.

Alan Marcus

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December 22, 2007

I read urs suggestion many times...

SO this is my conclusion...

1) Depth of field means the region in a picture that's in sharp focus (or, to be more precise, acceptably sharp).

2) Depth of field is shallower(decreases) in larger opening of the aperature.. which is f/2.8, f/1.8....

3)Not only aperature but focal length of your lens is also important. The focal length is just a measure of your lens's ability to magnify a scene. Your camera might have a 28-105mm lens, for instance, in which the low number is its wide-angle mode and the higher number is telephoto. The more you magnify your subject, the shallower the depth of field becomes. When shooting with a normal or wide-angle lens, you have a lot of depth of field(Sharp foucs every where). If you zoom in to a telephoto magnification, your depth of field drops dramatically giving you the best and shallowest depth of field. For use in portrait photography and other...wher u need shallow depth of field.

http://www.it-enquirer.com/main/ite/more/expoaperture/

http://www.pcworld.com/article/id,125378/article.html

Think we should use the term shallow or deep when talking about depth of field.. because a newbie cant understand the meaning of increased or decreased depth of field unless he knows that depth of field means sharpness of the objects..
If some one understands that DOF means blurring of object it would be harder to grasp the meaning like me....hehe
Just a thinkin...
Thanks

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December 22, 2007

http://upload.wikimedia.org/wikipedia/commons/3/38/DOF-ShallowDepthofField.jpg

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December 22, 2007

I would thank u for urs kind help..

And about the newbies... that was my opnion... I got the answer.. but for others ... its a great deal... to understand...

Thanks again
Bigyan M dixit

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December 22, 2007

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December 24, 2007

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December 25, 2007

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December 26, 2007

is a hand held.. shot...

what r u tryin to explain to me samuel..

am afraid of gettin more confused....

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December 26, 2007

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December 27, 2007

There's no arguing with the truth.

Season's Greetings!

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December 27, 2007

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December 29, 2007