# F-stop and pixel burn

Discussion in 'Photography' started by Deke, May 8, 2006.

1. ### DekeGuest

2 questions.

1. Can someone please give me an easy explanation of fstop.
Everywhere I look, i get some complex answer to this and leaves me
clueless.

2. When I take a night shot, I get pixel burn on the left side of my
image...little red dots taht are obnoxious. I read somewhere that I
can take another shot of equal time with the lens cap on and can do
some magic in photoshop to correct, but I cannot find that article
again. Any assistance would be great.

Thanks.

Deke
http://photos.dietrichduke.com
http://dietrichmd.smugmug.com

Deke, May 8, 2006

It's just a description of the size of the aperture in the lens. The
aperture can be varied in size...the smaller you make it, the less light
comes through.

Now, a 1" hole is a 1" hole but the longer the focal length of a lens is the
less effectrive that hole is; consequently, f-stops are a ratio describing
the size of the hole...that way, the f-stop on all lenses effectively
describes how much light will come through at a given f-stop regardless of
the focal length of the lens.

Simple enough? The formula is f-stop = Focal length / Aperture

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3. ### DekeGuest

but how do i determine the focal length on my lense. For example.
Using my 75-300mm lens, how would i determine my fstop? (btw, im using
a canon 300d (digital rebel))

Deke, May 8, 2006
4. ### KernixGuest

I'm not a digital shooter, but for film/slide cameras there are 3
things that determine the amount of light required to make a proper
exposure - film speed, shutter speed, and f-stop or aperature. Once the
film is loaded, that variable is removed. So it's just a matter of
shutter and f-stop. I'm sure you understand shutter speed - either fast
to stop action, or slow to blur it or when there's low light.

Best analogy I ever read - f-stop is like the valve of a faucet and
shutter speed is like the speed that the water comes out of the faucet.
If you have a bucket to fill up (slide/negative) and open the faucet
only a little bit then the water will drip out and it will t ake a long
time to fill the bucket. If you open the faucet all the way, the bucket
will fill up real fast.

So, f-stop is just a hole - a hole that lets light thru similar to how
the faucet handle let water through. If you have a large f-stop opening
(there's an inverse relationship) then the shutter speed will need to
be short or you will overexpose. If the openinf is very small you need
a long shutter time or you will underexpose.

large opening is like of f-stop of f1.4 - lots of light comes thru so
use a short shutter speed
small opening is like an f-stop of f32 - little light comes thru - long
shutter - inverse relationship.

Got it? Small openings (large f-stop #) renders a larger degree of area
in focus in front of and behind the subject, than does a large f-stop.

Take a look at a magazine that has a closeup of a person - is the
background blurred to bring the attention on the person? That's done
using mostly a large f-stop. On the contrary, landscape shooters use
small f-stops.

Here's how I see it - you have 5 choices:

1. a shot where you want a large aperture (small f-stop #) - ex -
portraits
2. a shot where you want a small aperture (large f-stop #) - ex -
lansdscapes
3. a shot where you want a fast shutter - ex - sports photography to
stop the action
4. a shot where you want a slow shutter - ex - to blur a water fall or
windy field of flowers
5. when you don't need any of those 4 extremes and you want something
in between

pixel burn - what is that? What is that the result of? underexposure?
Forget that putting the lens cap thing back on. It seems to me, an
approach such as that is fighting fires rather than finding the source.

Get back with a defintition of pixel burn.
Jim

Kernix, May 8, 2006
5. ### KernixGuest

Kernix wrote:

Oops - sorry using a large aperature not a large f-stop
oops x's 2 - small aperature not small f-stop #.

I think in terrms of what effect I want then I choose the best
f-stop/shutter speed combo that the current scene allows.

Kernix, May 8, 2006
6. ### Mark W. OotsGuest

The 300D will tell you the shutter speed and f/stop at the bottom of the
viewfinder when you push the shutter release half way (focus lock position).

After a pic has been shot, you can also use the <Info> button while viewing
the shot to get the same information as well as a histogram so you can see
if you have enough detail at both ends of the curve to give decent results
or if you need to shoot it again.

Mark

Mark W. Oots, May 8, 2006
7. ### PatGuest

All of the technical discriptions are pretty much right, but I'm sure
you are sitting there saying "...but I don't see any difference when I
change the f-stop" and you are absolutely right. That's why it is
something difficult to understand.

When you look through the camera, you are seeing the lens wide open.
When you hit the shutter, little irises inside of the lens close down
to whatever size you have it set to and the picture is taken. It all
happens unbelievable fast. The good news is that you can see it for
yourself and it'll help clarify it.

Set your lens to "wide open", i.e. the smallest number (which is the
largest opening). It'll probably be something like 3.5 or 4. Could be
as low as 2 or as high as 5.6. But set it as low as it'll go. Look
around. There's not much light loss.

Now set the aperature to the highest number. It is f-22 or higher.
Hold the camera normal, with the your left hand on the bottom of the
lense, palm up, with your thumb to the left. This is a normal shooting
position. Now look at your left thumb. There's a little black button
near your thumb (on the camera), near where the lens connects to the
camera. The button pushes in towards the lens, not back towards the
body of the camera. Now look through the camera in a normal fashion
and push that button. It'll get darker. You've just previewed the
aperature setting by having the aperature close to the f-stop you have
the camera set to.

If you hold down the button and look into the end of the lens, you
should be able to see the aperature. Let go and you see it open up.

Pat, May 9, 2006
8. ### DekeGuest

Excellent description and help...however. That little button doesnt do
much of anything, and im having issues determining the aperature value
as im getting numbers like 4000 and stuff.

Deke, May 9, 2006

RTFM

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10. ### DekeGuest

Manual has been lost for quite some time, hence the post

I do have this Lark book on the camera, but it offers no useful help on
changing the aperature.

Deke, May 9, 2006
11. ### DekeGuest

Ok, I got it to work. Took some basic understanding on lenses for me
to really get it, but now its all good. That little button will come
in handy now that i know what its for!

Thanks for the help on the Fstop, now I need to work on this pixel burn
(red dots) on my night shots. Any ideas?

Deke, May 9, 2006

That's the way to learn (anything)...get a basic understanding of something
so you can use that knowledge in other situations.
______________
Uhhh, Deke...you could have pushed it all on your own. Experimentation is a
good way to learn too
_______________
NP. BTW, f-stops are generally marked in whole stops and they are a
geometric progression...1.0 1.4, 2.0, 2.8, 4.0, 5.6 etc. Each admits 100%
more/less light than the preceding/following whole stop...makes it easy to
correlate with shutter speed.
________________
Nope, my experience is film.

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....a help file of info about MP3s, recording from
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13. ### dj_nmeGuest

"pixel burn"?
Do you mean the red and green speckles that appear in the shadow areas
or that are evenly distributed acros the image when using a high ISO
setting?
That is just image noise, it is caused by trying to use too high and ISO.
For example, most small sensor digicams (basicaly anything other than
modern DSLR cameras) have sensors that require a fairly large amount of
light or the electronic noise from the sensor and prosessing electronics
will overwhelm the image data and show up as coloured speckles across
the image.
Turning the ISO setting up is akin to turning the volume up on a hifi
stereo, if the scene isn't bright enough (signal isn't strong enough)
then coloured speckles (for audio, hiss and crackles) will appear.
Long exposure times are a bit like turning the ISO setting up as far as
noise goes, there is unfortunately no "free lunch".
Most of the long exposure time photos, even from DSLR cameras have some
sort of noise reduction performed, done either in-camera or in post
prossessing on a computer.
Noise reduction can make the resultant image either "plastic" looking or
slightly fuzzy if done to extremes, but the alternative is a grainy
image or none at all.

dj_nme, May 10, 2006
14. ### KernixGuest

Deke,

If dj_nme is correct (I'm a film slr shooter) then you need to change
your approach. I suppose pushing your digital camera to as high an ISO
setting as possible which results in the pixel burn you mention, is
akin to a film/slide shooter who chooses a really high film ISO and the
results are grainy images.

I assume you are selecting the high ISO because of the darkness. Well,
there's no getting around the need for light. What shutter speeds are
you using? Night shots typical use a combo of large aperature (small
f-stop #'s) and long shutter times. And usually, the shutter is where
you make the shot. I've taken nights shots with shutters speeds
anywhere from 1 sec to 1 minute long depending on the light level of my
subject. If you do not own a tripod and cable release - then you MUST

How is your cash flow? Cable releases are cheap, but tripods vary from
cheapo \$10 models to Bogen & Gitzo models which run \$100-200. If I
recall correctley, I bought a Bogen Tripod for \$150 and a Gitzo head
for another \$50 or so. I was working part time in a photo store so I
got a decent employess discount - 20% I think.

It's a stainless stell kick-ass tripod and ball head mount.

Get back with the shutter speeds you haver been using.
Jim

Kernix, May 10, 2006
15. ### DekeGuest

I have a shutter release and a tripod, so that part is covered. What I
ahve been doing is this: Turning my camera to BULB setting, turning
ISO to 800 - 1600 (depending on light) and aiming and shooting. I
haven't, obviously, used the aperature or shutter speed changes
yet...but thats this weekends project. Ill let ya konw.

Thanks

Deke, May 10, 2006

Then set the ISO lower and leave the shutter open longer.

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____________________________

....a help file of info about MP3s, recording from
LP/cassette and tips & tricks on this and that.
Get it at http://mysite.verizon.net/xico

17. ### KernixGuest

Yeah - lower the ISO waaaaaaaay down and increase shutter speed.

Since you have a bulb setting, and most like a auto mode, do you have
an aperture priority mode, and shutter priotity mode as well? I assume
the bulb setting is in manual mode? Are you going to blow up the final
shot? If so - choose a small aperature / large f-stop #. Secondly, make
sure to set the ISO to 100 or lower. Then in aperture priority mode,
see what shutter speed is recommended.

Are you getting a flashing on your display indicating that you are
going to get an underexposure? If not, set the shutter and take the
pic bracketing as necessary.

If you are getting a warning that you will have an underexposed pic,
set the shutter to the slowest setting in aperature mode (not bulb),
increase the aperture (smaller f-stop #'s) until the blinking goes
away. Make a note of the shutter speed and how many stops away from the
smallest aperture you ended on. Then go to manual mode, set the shutter
that you had in aperture mode, set to the smallest aperture that you
started with in AM, THEN manually increase the shutter speed in the
same amount as the # of f-stops it took you to get a reading of a
properly exposed pic in aperture mode - then take the shot.

Jim

Kernix, May 11, 2006