[subhrajitb]

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.

[Quension]

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?

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?

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.

[Andreas]

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.

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.

.

[Wheatfield]

Exposure is a combination of shutter speed and aperture. If you change one, but not the other, the total exposure changes.

[DAZ]

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 don’t mater not that they can’t be seen.

DAZ

[subhrajitb]

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.

[Wheatfield]

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.

[Quension]

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.

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.

[Wheatfield]

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.

[octavmandru]

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.

[Quension]

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.)

The Camera by Adams would be a good place to start.
Anything by John Hedgecoe would be a good read.

Thank you.

[OregonJim]

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).

[heliphoto]

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.

[subhrajitb]

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 !