Av Mode in Canon EOS 7D

As you know (at least in Northern Hemisphere) it has been getting very dark and dim lately, so taking photos in correct exposure isn't that easy.
Yesterday I was taking some photos in a park, mainly of finches and tits. I first shot in Tv mode but then I realised the pics were quite underexposed.
Then I switched to Av. The exposure was much better, however the camera kept changing the shutter speed and usually kept it in unacceptably slow values (like 1/80).
I seriously don't understand how shutter speed can change when your ISO and aperture is constant? Is there a fourth elements involved, perhaps metering or exposure?
Anyways, do you have ANY idea how I can get a well lit image with adequate sharpness? Flash, perhaps?

Thanks for your help!

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Originally Posted by Kuzey Cem Kulaço?lu

I seriously don't understand how shutter speed can change when your ISO and aperture is constant? Is there a fourth elements involved, perhaps metering or exposure?
Anyways, do you have ANY idea how I can get a well lit image with adequate sharpness? Flash, perhaps?

Thanks for your help!

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In AV it is normal that the exposure changes depending on how and where in the scene you are metering, here are a few reasons to think about: What metering mode are you using? Are you metering consistently on the same spot? Is the light constant or changing? Are locking the exposure? Are you using exposure compensation?

Manual is easier than it sounds, perhaps that is an option for you, especially if the light on the subject is constant.

I use spot-metering. My AF point is located in center. I have used Manual before but it is also very variable in my experience.
However, you might have just found the problem: I don't tend to use exposure compensation (that horizontal scale from -3 to +3, if that's what you mean).
Also; how do you form shortcut keys to control some of the factors?

Spot metering can be tricky if you do not meter off the same area of tonality for every image. Take a Great Tit (or chickadee in our parts of the world) for example, if you spot meter off a black head you will get a different reading than if your next image is spot metered off a white cheek...you images will have different exposure for each of those two frames (one will over-expose, and the other under-expose as the meter wants to average out the scene to a neutral gray as best it can). You have to be careful to always meter off, say, the white cheek and apply positive exposure compensation to keep that cheek white (yes, that is the scale that I mean). If the light is constant, once I get the proper exposure I just keep it in manual, and fire off with a resulting correct exposure everytime no matter where or how I meter.

Not sure what you mean by your last question though...

If you want to use one of the automatic exposure modes, try a wider mode than spot. Evaluative is very "intelligent" and will often give you a good average exposure for the scene. Then do a test shot and check the histogram and blinkies and do exposure compensation as needed. (It almost always is.) Then you can set that as a manual exposure if the light is constant.

With Spot metering you'll need to meter the brightest and darkest areas and decide on a good average.

Quote:

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Originally Posted by Kuzey Cem Kulaço?lu

As you know (at least in Northern Hemisphere) it has been getting very dark and dim lately, so taking photos in correct exposure isn't that easy.
Yesterday I was taking some photos in a park, mainly of finches and tits. I first shot in Tv mode but then I realised the pics were quite underexposed.
Then I switched to Av. The exposure was much better, however the camera kept changing the shutter speed and usually kept it in unacceptably slow values (like 1/80).
I seriously don't understand how shutter speed can change when your ISO and aperture is constant? Is there a fourth elements involved, perhaps metering or exposure?
Anyways, do you have ANY idea how I can get a well lit image with adequate sharpness? Flash, perhaps?

Thanks for your help!

---

Using Av, Tv or M, you have to raise the ISO once the aperture is wide open. In Tv mode, if the aperture is wide open and there's not enough light, it'll blink at you in the viewfinder. In Av mode, when you hit the upper limit (1/8000) it'll blink, but it'll assume that you're using a tripod when SS get really low.

No matter which mode your using, you need to understand Exposure Value and how the three variables (Aperture, shutter speed and ISO) are intertwined. The light will dictate the exposure value that you need to get to. You might try Av in the "Auto" mode, so you select both an aperture and a minimum shutter speed and the camera adjusts ISO (called Auto-ISO).

You've got several helpful posts here with your query, so you should use this as an opportunity to understand with clarity how SS, Aperture and ISO interact to achieve a correct Exposure Value ("EV").

Yesterday Artie did a blog post about the different exposure modes:

http://www.birdsasart-blog.com/2014/...shooting-mode/

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however the camera kept changing the shutter speed and usually kept it in unacceptably slow values (like 1/80).
I seriously don't understand how shutter speed can change when your ISO and aperture is constant?

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Thats what AV is designed to do change the shutter speed to match the meter reading while keeping the aperture constant. Whats happening is since you are in spot metering mode, as your subject moves in the frame the camera is metering different parts of the subject or BG since they are of different tonalities the cameras is changing the shutter speed to compensate for that. As you meter different parts of the scene or subject you are telling the camera you need a different exposure so the camera is changing the shutter speed for the new exposure values. Here is a quick spot metering tutorial when in spot metering mode whatever you have that spot on the camera is reading the light that is reflecting off that area and averaging it to 18% grey, so if you are pointing it at a white subject and leave the exposure scale at 0 when you make an image the white subject will look underexposed since the meter does not know its looking at a white area. Thats where exposure compensation comes in if you change the scale to sit on +1 or +2 you are telling the camera that the area you are pointing at is brighter then 18% grey and the opposite if you are metering a dark subject. Now here is the part some people have a hard time understanding metering and exposure are two different things metering is the reading of the intensity of the light on the subject and exposure is the settings used to create an image based off of the amount of light on the subject when you grasp that it all starts to fall into place. Now a quick evaluative metering tutorial when in Evaluative metering mode the cameras is metering different areas of the frame and comparing it to a data base of images to determine the best overall exposure for the scene thats why in evaluative mode you will often get a properly exposed image with the exposure scale at 0 and why it is the best metering mode to use when you are using Av or Tv mode. This is really a quick and over simplification of metering and exposure but should set you on the right path. One last thing you know you have figured it out when you understand that there is no one correct exposure settings for any given scene, that if two people are standing next to each other and photographing the exact same scene at ISO 400 and one has an aperture value of f/16 and a shutter speed of 400 and the other is shooting at f8 and a shutter speed of 800 they both will get the same exposure.