PDA

View Full Version : DOF, aperture, zoom.



bTaryag
08-27-2007, 07:25 AM
To get a shallow depth of field, I should use the largest aperture (i.e. the lowest number) I can.

Does zoom affect DOF as well?

If my subject is ten feet from the background, will it make a difference if I am five feet from my subject (fifteen feet from background) and don't zoom in, or thirty feet from my subject (forty feet from background) and zoomed in to my subject?

Turn
08-27-2007, 07:51 AM
well from my experience the further away the background is from the foreground the more blurred it will be

macro mode or zooming in and getting your subject isolated works best

how much zoom zoom you got?

griptape
08-27-2007, 08:51 AM
Zoom doesn't make a huge difference in point and shoots (just because there's not that much of it), but you'll get more background blur by stepping back and zooming in than you will by stepping in and zooming out.

erichlund
08-27-2007, 10:41 AM
There are lots of depth of field calculators out there, and some decent sources for the formulas used. Search on Depth of Field. Here is a Wikipedia (http://en.wikipedia.org/wiki/Depth_of_field)link that does include the formulas for calculating DOF, near limit, far limit, and hyperfocal distance. For instance, DOF is calculated as follows:

DOF = (2 N c f^2 s^2) / (f^4 - N^2 c^2 s^2) which can be simplified to

DOF = (2 H s^2) / (H^2 - s^2) where H = (f^2) / (N c)

N = lens f number
f = focal length in mm
c = circle of confusion
s = focus distance
H = Hyperfocal Distance

Circle of confusion is a constant for each format type. APS - C c = 0.018mm
- The value is approximate, since there are several ways to calculate, and different APS-C sensors have different actual sizes.
- The formala used in this case was format diagonal / 1500 (The most commonly used form)
- The original Zeiss Formula is diagonal / 1730
- Kodak recommends an angular format but that ends up equating to very close to the Zeiss Formula.

So, what's it all mean?
Well we can see that for a given f-stop and sensor format, Hyperfocal distance increases geometrically with focal length. As long as you focus inside the hyperfocal distance, your depth of field will increase the farther you focus from the camera. If you focus at or longer than the hyperfocal distance, your depth of field will be from the near limit to infinity. Also, we're all pretty familiar with the concept that smaller f-stops produce shallower depth of field.

So, to best isolate your background, the conclusion would be: You want to be as close to your subject as you can and properly frame your subject, and you want your background outside of the depth of field of the resulting shot. You want to use an f-stop that gives you sufficient depth of field to focus your subject without also bringing into focus the background you wish to isolate your subject from.

You also need to take into account the proper lens for the shot. If this is a portrait, you want a 35mm equivalent of 80-120. In APS-C (Nikon 1.5) this would be 53mm - 80mm. This being a rule of thumb, you're probably OK with 50 - 90mm. Some photographers go well outside these limits on the telephoto end, but it's rare to go wider, because wide lenses tend to distort the subject, accentuating depth.

bTaryag
08-27-2007, 02:24 PM
Getting a bit confused here!

griptape is saying that being further away from the subject and zooming in will give less DoF (i.e. more background blur).

erichlund seems to be saying the opposite.

Which one is it? I plan on photographing a bride against a backdrop of trees, and I want the trees to be blurred. Assuming the bride is ten feef from the trees, should I stand five three feet from her and not zoom, or stand twenty feet from her and zoom in on her?

BTW, I am using a Canon S5 IS.

Thanks for all the help.

erichlund
08-27-2007, 02:44 PM
Set your zoom so you can stand about 6 - 10 feet from her while properly framed. If you have aperture priority, set your aperture wide open (smallest number). This will give you your best chance. However, the small sensor on your camera gives you much greater depth of field, and you likely won't get the sort of isolation on your subject that you would get with a 35mm film camera or a dSLR.

bTaryag
08-27-2007, 08:28 PM
There is a calculator in this link:
http://www.dofmaster.com/dofjs.html

I am using a S5 IS. The calculator says to use the actual focal length, which is 6mm - 72mm. Can I assume that if I zoom in half way it is at 39mm, or does it not work so simply?

Rex914
08-27-2007, 09:50 PM
I can't speak for the S5 IS, but if you're after the blurred background, on my A640, I get the best results when I go to full telephoto (140mm), open up the aperture all the way (f/4.5 for me) and then do what's said above by isolating the bg from the fg and being as close as possible to the subject.

Even on an A640, this generates a reasonably blurred out background, partially thanks to its 1/1.8" type CCD as opposed to the smaller 1/2.5" types.

- Jon

Attached is a shot achieved using the said technique. It's not a terribly large amount of blur, but it's better than nothing.

AlexMonro
08-28-2007, 04:42 AM
Simple summary:

To get the minimum depth of field (maximum blur of background while keeping the subject sharp) (not all these suggestions may be practicable for you):

1) Get the subject as far from the background as possible (not always feasible).

2) Use the longest focal length (maximum zoom) you can. You may have to step back quite a way.
(edit) If there isn't much room, zoom to at least 50mm equivalent and put the subject 1/3 the way between you and the backgroubd.

3) Use the largest aperture (smallest f number) you can.

4) You might be able to focus slightly in front of the subject, by using manual focus or AF lock. Don't take this too far though, or everything will be blury! :)

5) If you have a choice of cameras, use the one with the largest sensor size. A full frame DSLR will be better than a 1.5 crop DSLR, which will be better than a 2.0 crop DSLR (Olympus etc.), which will be better than a large compact (Fuji S9000, S6000, F30 etc), which will be better than a medium compact (1/1.8" sensor) which will be better than an ultra compact (1/2.5" sensor). The sensor sizes for compacts are a bit arbitrary, you might have to check the specifications.

There's a good tutorial, with an online calculator here (http://www.cambridgeincolour.com/tutorials/depth-of-field.htm). Try plugging some numbers in to get a feel for how things work in situations that might be typical for you. Remember to use the actual lens focal length, usually around 6 - 18mm for compacts, not the "35mm equivalent". The actual focal length is often printed on the front of the lens.

bTaryag
08-28-2007, 06:44 AM
Thanks everyone for all the advice and tips.

On a side note, why does the S5 have such a small sensor? (1/2.5 inch).

AlexMonro
08-28-2007, 03:30 PM
On a side note, why does the S5 have such a small sensor? (1/2.5 inch).

Because it's cheaper.

bTaryag
08-28-2007, 03:36 PM
Ok, but we are dealing with a $500 camera!

AlexMonro
08-29-2007, 04:21 AM
If Canon save a dollar on the production cost of each camera, and they sell 100,000 S5s worldwide, that's $100,000 more profit. They know that most of the purchasers of ultrazooms don't know how important sensor size is to image quality, and are likely to be more impressed by features such as swivelling LCDs and My Colors. For customers who are interested in image quality, they make some great DSLRs.

Most other manufacturers do the same thing, though Fuji did buck the trend a bit with the S6000. Sadly they seem to be rejoining the megapixel race with their latest offerings.

TheObiJuan
08-29-2007, 04:27 AM
small sensor = smaller lens elements = BIG ZOOM camera for cheap.

thunderchase
08-29-2007, 08:01 AM
You should have checked the full specifications list before purchasing. Brand means nothing, every manufacturer has one jewel, Fuji has a few more in ultrazoom class. Canon has... none :D ! But they make fantastic DSLRs though...