Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
04-04-2009, 10:59 PM   #1
New Member




Join Date: Mar 2009
Location: Bangalore
Photos: Gallery
Posts: 5
Why does brightness of the picture for the same situation vary. why ?

When I shoot indoors where lighting is limited, I see that the brightness of the picture varies significantly with the f-stop. And I do not understand why this is the case.

Let me explain how I understand things and I will appreciate it if someone can tell me where I am going wrong.

We can control aperture, speed, ISO. If I increase the aperture size, shutter speed goes up and vice versa. So the total amount of light coming in remains the same (assuming ISO has not changed). Then why would brightness of the picture taken change if aperture is changed ?

I consistently see that for one of the settings (I think larger aperture, faster shutter speed, but it could be the reverse one), my pictures are brighter.

Thanks for your help.

04-04-2009, 11:25 PM   #2
Senior Member




Join Date: Feb 2009
Location: west coast USA
Posts: 206
QuoteOriginally posted by subhrajitb Quote
We can control aperture, speed, ISO. If I increase the aperture size, shutter speed goes up and vice versa. So the total amount of light coming in remains the same (assuming ISO has not changed).
If you increase the aperture size (lower f/number) by one stop, then the shutter speed must also be increased by one stop to maintain the same exposure, yes. Whether it increases by itself or not depends on how you are controlling the camera. What mode is the camera in?

QuoteQuote:
Then why would brightness of the picture taken change if aperture is changed ?

I consistently see that for one of the settings (I think larger aperture, faster shutter speed, but it could be the reverse one), my pictures are brighter.
Something did not change as you expected. Can you post a couple of examples? Is flash involved?
04-04-2009, 11:33 PM   #3
Damn Brit
Guest




Unless you are focusing on exactly the same spot, the camera will meter differently.
Also in auto mode, the camera has more latitude and can set an aperture that you would not be able to set manually.
04-05-2009, 12:28 AM   #4
Inactive Account




Join Date: Jun 2007
Posts: 75
Flash?

I assume that you use flash indoors. Then possible causes for the phenomenon that you see are:

-with a very small aperture (like e.g. f/22), your flash may not be powerful enough to light the scene.

-shutter speed has [to a certain degree] no influence on the amount of light coming from the flash. If you use a long shutter speed (like 1/10 s), you will pick up more ambient light, but just as much flash light as with a shutter speed of e.g. 1/180s. So if your picture depends on light coming from the flash and you play with various aperture/shutter speed combinations that should give the same result, you will see that it is not because of the flash.

QuoteOriginally posted by subhrajitb Quote
When I shoot indoors where lighting is limited, I see that the brightness of the picture varies significantly with the f-stop. And I do not understand why this is the case.

.


04-05-2009, 08:05 AM   #5
Pentaxian
Wheatfield's Avatar

Join Date: Apr 2008
Location: The wheatfields of Canada
Posts: 10,158
Exposure is a combination of shutter speed and aperture. If you change one, but not the other, the total exposure changes.
04-05-2009, 12:43 PM   #6
DAZ
Veteran Member
DAZ's Avatar

Join Date: Sep 2006
Location: Everett, WA USA
Photos: Gallery
Posts: 710
One thing to keep in mind. If you are using a modern lens it start with the aperture wide open and closes it just before the shutter opens. This can produce some error not only between photos with no other changes but also between aperture settings. This can be more evident with some lenses. Some times you can see that highlights that should be round are not round. This is because some of the aperture blades were slower. As you step down more this can get to be more so.

Remember this is a camera not a scientific instrument. It is full of mechanical and electrical parts that all add errors. The camera systems we have now are so good we tend to think they are perfect. The errors are just so small that they mostly donít mater not that they canít be seen.

DAZ
04-06-2009, 05:44 AM   #7
New Member




Join Date: Mar 2009
Location: Bangalore
Photos: Gallery
Posts: 5
Original Poster
I think I know what caused it - but still not sure why my pentax did it this way.
I was shooting in aperture priority mode without flash.

Shot1
------
Aperture - 4.5 (I set this)
Speed - 1/125 s (Auto)
ISO - 480 (Auto)

Shot 2
--------
Aperture - 9 (I Set it)
Speed - 1/125 s (Remained same, I had expected it to change)
ISO - 600+ (Auto. I had not expected this to change)

I guess the change in ISO was not enough to compensate for the reduction in aperture, so shot 2 came out darker.

I can now understand why my 2nd shot came out darker but is the camera behavior expected ? Essentially, how do I predict my camera behavior ?

When I tried to attach my pics, all I got was some text with a URL as below. Sorry.
[IMG]file:///C:/pics/expts/aperturePriority/100_0405/__IGP6265.jpg[/IMG]

Last edited by subhrajitb; 04-06-2009 at 05:49 AM. Reason: Could not paste pics.
04-06-2009, 06:20 AM   #8
Pentaxian
Wheatfield's Avatar

Join Date: Apr 2008
Location: The wheatfields of Canada
Posts: 10,158
QuoteOriginally posted by subhrajitb Quote
I think I know what caused it - but still not sure why my pentax did it this way.
I was shooting in aperture priority mode without flash.

Shot1
------
Aperture - 4.5 (I set this)
Speed - 1/125 s (Auto)
ISO - 480 (Auto)

Shot 2
--------
Aperture - 9 (I Set it)
Speed - 1/125 s (Remained same, I had expected it to change)
ISO - 600+ (Auto. I had not expected this to change)

I guess the change in ISO was not enough to compensate for the reduction in aperture, so shot 2 came out darker.

I can now understand why my 2nd shot came out darker but is the camera behavior expected ? Essentially, how do I predict my camera behavior ?
Read the manual.
Buy a book on basic camera theory.
Read it.

04-06-2009, 07:56 AM   #9
Senior Member




Join Date: Feb 2009
Location: west coast USA
Posts: 206
QuoteOriginally posted by subhrajitb Quote
When I tried to attach my pics, all I got was some text with a URL as below. Sorry.
The images need to be resized in an editor so they are at most 850x567 pixels, then you can click the "Manage Attachments" button when you post/edit, and upload them there. Drag & drop won't work.

QuoteOriginally posted by Wheatfield Quote
Read the manual.
Buy a book on basic camera theory.
Read it.
If I were confronted with this behavior, I'd be confused too. How is a book on basic camera theory going to explain how a particular camera implements Av mode with auto ISO? Not even the Pentax manual explains the camera's internal priorities. There are posts in forums everywhere containing various tests people do to uncover them.

If you're the type of person who puts seemingly unrelated facts together, you might guess that the shutter speed didn't decrease due to the focal length guideline for avoiding camera shake, but it takes a bit of a logical leap to come up with that. Especially since this is probably a camera with SR, and SR is supposed to work better than that old rule -- or so the literature implies.

And it doesn't explain the ISO choice.

Care to share the particular manuals and books you have in mind? They must be amazingly well-written.


subhrajitb, Wheatfield is right that your best chance of understanding what's going on in general is to get as much information and theory as possible, even if he was a bit short about it. You're doing well so far.

Last edited by Quension; 04-06-2009 at 08:14 AM. Reason: modify tone a bit
04-06-2009, 08:08 AM   #10
Pentaxian
Wheatfield's Avatar

Join Date: Apr 2008
Location: The wheatfields of Canada
Posts: 10,158
QuoteOriginally posted by Quension Quote
The images need to be resized in an editor so they are at most 850x567 pixels, then you can click the "Manage Attachments" button when you post/edit, and upload them there. Drag & drop won't work.



If I were confronted with this behavior, I'd be confused too. How is a book on basic camera theory going to explain how a particular camera implements Av mode with auto ISO? Not even the Pentax manual explains the camera's internal priorities. There are posts in forums everywhere containing various tests people do to uncover them.

If you're the type of person who puts seemingly unrelated facts together, you might guess that the shutter speed didn't decrease due to the focal length guideline for avoiding camera shake, but it takes a bit of a logical leap to come up with that. Especially since this is probably a camera with SR, and SR is supposed to work better than that old rule -- or so the literature implies.

And it doesn't explain the ISO choice.

Care to share the particular manuals and books you have in mind? They must be amazingly well-written.
This stuff isn't rocket science, and the fundamentals of camera theory, photographic theory and exposure theory haven't changed in something like 150 years, and are brand independant.
The Camera by Adams would be a good place to start.
Anything by John Hedgecoe would be a good read.
Photographic theory isn't dependant on shake reduction, or digital sensors or crop factors or any of the other things that get tossed around as complications.
These things are distractions, nothing else.
04-06-2009, 08:13 AM   #11
Veteran Member
octavmandru's Avatar

Join Date: Jan 2009
Location: somewhere around
Posts: 615
many factors could be implied... Wheatfield means you are probably the only good judge in your own situation.
-weather you you use or not Metering locked with AF
-spot metering or not
-Iso WILL change if is set to Auto, within set limits.
And a whole lot of other factors.

This is why I use Manual mode. Will not make it's own choices, and I can use the same good settings how many times I want.
04-06-2009, 08:20 AM   #12
Senior Member




Join Date: Feb 2009
Location: west coast USA
Posts: 206
QuoteOriginally posted by Wheatfield Quote
This stuff isn't rocket science, and the fundamentals of camera theory, photographic theory and exposure theory haven't changed in something like 150 years, and are brand independant.
That's more of an answer to his first question of "why is the image darker?". It doesn't seem to shed light on his newer, more refined question: "why did the camera do this unexpected thing?". (Do you already know the answer to that? I don't, and am curious.)

QuoteQuote:
The Camera by Adams would be a good place to start.
Anything by John Hedgecoe would be a good read.
Thank you.
04-06-2009, 08:21 AM   #13
Veteran Member
OregonJim's Avatar

Join Date: Mar 2009
Location: Willamette Valley, Oregon
Posts: 1,329
QuoteOriginally posted by Wheatfield Quote
This stuff isn't rocket science, and the fundamentals of camera theory, photographic theory and exposure theory haven't changed in something like 150 years, and are brand independant.
The Camera by Adams would be a good place to start.
Anything by John Hedgecoe would be a good read.
Photographic theory isn't dependant on shake reduction, or digital sensors or crop factors or any of the other things that get tossed around as complications.
These things are distractions, nothing else.
Wheatfield,

While these references are appropriate for any of us to read, I don't think they'll solve the OP's problem. Besides, they're probably not easy to find in India.

The problem he's having is specific to the "quirks" of his particular camera - as Quension pointed out, the auto-ISO behavior and the program line for Av that limits shutter speed choices based on focal length. I think that this forum is the proper place to ask such a question (and to expect a friendly answer).
04-06-2009, 09:27 AM   #14
Veteran Member
heliphoto's Avatar

Join Date: May 2008
Location: Region 5
Photos: Gallery
Posts: 2,540
Wheatfield, I think this is a more complex issue than you are implying. I can think of several avenues to pursue, but they are camera specific, not general photography knowledge.

For subhrajitb, I'd recommend you set the ISO manually to a fixed setting (not auto), and set the camera on a tripod, so that you can ensure that it is metering off of the same areas of the scene. I'd also set Matrix metering or maybe center weighted, because if you are using spot metering and you move the center spot a tiny bit, the exposure value can be very different (using a tripod should allow you to use spot metering, but I would choose one of those more averaging metering modes for your testing). Once you've removed those variables, the shutter speed should correspond to the aperture in more the way you are expecting - then you can change various settings to see when they start affecting the exposure in an unexpected way.

My guess is that you're camera is set for spot metering, and is therefore being more influenced by the exact brightness under the center spot than the overall brightness of the scene.
04-06-2009, 09:58 AM   #15
New Member




Join Date: Mar 2009
Location: Bangalore
Photos: Gallery
Posts: 5
Original Poster
First, thanks to all who even bothered to reply. I guess folks on this forum really care and I am simply overwhelmed.

To me the lesson is that I should not assume my camera will behave in a certain way. I will keep tab from now on.

Quension, thanks for your theory on SR. Will keep that in mind too.

As for books, I have The Digital Photography book by Kelby, and Understanding Exposure by Peterson. The latter explains all about ISO, aperture etc. It is definitely not rocket science, but my take is it will take quite a bit of experience to understand how to get one's camera to make use of the elements.

OregonJim, You are right, I did not find a single good book on photography in my local book store. My colleague got the above books for me when he was visiting the US

Hope to be back sooner or later to the forum !
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
aperture, brightness, camera, iso, pentax help, photography, picture, shutter
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
What can I do under this situation? dragonwsun General Talk 20 10-21-2010 10:08 AM
SDM situation is much improved (i think) philbaum Pentax SLR Lens Discussion 55 06-06-2010 08:22 PM
Black & White A thorny situation coachteeter Post Your Photos! 4 04-21-2010 08:52 PM
tricky wedding situation...how would you handle this? jshurak Photographic Technique 8 05-15-2008 07:04 PM
Memory card situation mysterick Pentax Camera and Field Accessories 8 03-05-2007 09:14 PM



All times are GMT -7. The time now is 07:22 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top