Difference between Av and Tv

[groo]

Hello,

I am new to Photography and I just purchased a Canon t3i recently. I have been playing around with the settings a bit and I came across a situation that I did not understand, so I thought perhaps you guys could help.

I am taking a picture in a fairly dim lit room and I am comparing the setting between different camera modes:

green auto mode sets the iso at 400 shutter speed at 1/60 and aperture to 5.0 using the flash.
It try tv mode and set the shutter speed and iso manually to 1/60 and 400 using the flash, and aperture gets automatically set to 5.0, so the same as full auto mode
but when I try Av mode and set aperture to 5.0 and iso to 400 using the flash, it wants to set shutter speed to 0"6 instead of 1/60 and takes a very different picture due to the longer shutter speed.

So my question is why is Av mode setting the exposure so much differently than Tv mode? if in Tv mode with a shutter speed of 1/60 and aperture of 5.0 is correct for proper exposure why is it not correct (or perhaps I should say different) in Av mode?

I would say that Av mode looks more true to life because the lighting isn't as white from the flash as it is in Tv mode, and has a more natural yellow glow from the light in the room.

Thanks,
Keir

 

[groo]

hmm, maybe I should have turned the page in the manual ... I see that Av mode set the shutter speed longer so the flash exposed the foreground and the longer shutter speed exposes the background.

So since in this case I was take a picture of my wife lying on the couch, the Av mode would be the appropriate mode to use correct (I'm not quite at the point of using M mode yet.) I would use Tv mode only if I am taking a picture of something with movement?

Thanks

 

[Nikon_Dude]

In general, Tv (shutter priority) is more useful when you have movement, because you can set the shutter speed fast enough to "freeze" the motion and not get motion blur in your pictures.

If shooting a still subject, you can use Av(aperture priority) to get the desired Depth of Field you want, and allow the camera to set the shutter, and if it sets it slow so be it, nothing is moving that can blur.

 

[pgriz]

When controlling the amount of subject motion blur is important (either fast enough to freeze the action, or slow enough to get a nice blur), then Tv is the way to go, as Nikon_dude already said. Av controls the aperture, and thus the depth-of-field, again as Nikon_Dude said. When using flash, the camera computer now needs to figure out how much light to put out (if shooting with the built-in flash, or using ETTL with an external flash), and also takes into account the amount of background (ambient) light. Here it starts getting complicated, as the programming that the engineers put into the camera is supposed to be good for "most" situations, but may not be in fact good for your specific situation. You also have to switch gears a little, since the assumption of the camera is that in the Av+flash combination, the aperture is set at a specific value to control the flash exposure, and it has to come up with acceptable background exposure by controlling the shutter. Obviously, if the shutter speed falls below hand-holding speed limits, you need a tripod to prevent blur. Personally, when I shoot flash with ambient light, I usually get better results if I set the aperture based on DOF considerations, adjust the flash power manually to match the aperture, and set the shutter speed manually to get the right amount of exposure of the ambient light. This means that I'm shooting manual on the camera, and manual on the flash. This approach gives controllable results in static situations, but is generally too slow if you're moving around and the light/subject is constantly changing. If you use ETTL (ie, camera is controlling the flash output), and the flash head is NOT pointed straight ahead (ie, bounced light or used with modifiers), then the results become harder to predict and I get a lot of not-so-great shots because the camera's calculations don't often reflect my intent. Apparently, the higher-end Canon cameras handle ETTL better than the lower-end cameras, but with my T1i, I have to check the images all the time to make sure that I am getting the shots I want.

 

[Big Mike]

Welcome to the forum.

hmm, maybe I should have turned the page in the manual

You beat me to it...read the manual. Then read it again.

As you are seeing, the camera will act quite differently in the priority modes (Av & Tv) than it will in Auto...especially when it comes to flash. In P (I'd suggest using P as your auto mode and avoid the 'green box of death')...in P mode, the camera won't let the shutter speed fall below 1/60, at least when flash is being used. This mean that if you are in a lower light situation, your background will likely be dark because the flash will only light up the subject/foreground. But, you are unlikely to get blurry shots...which is why they designed them that way.

If you switch the camera to Av or Tv, the camera will no longer limit the shutter speed to 1/60. Instead, it will try to give you exposure settings that would properly expose for the ambient light. And in low light situations, that will likely require a long (slow) shutter speed and/or high ISO (your aperture is probably already maxed out at around F5).
That slower shutter speed, may or may not give you blurry images, it depends on the amount of ambient light.

Like pgriz, I usually use Manual mode on the camera when shooting with flash. I set the aperture for the DOF that I want (keeping in mind that a smaller aperture (higher F number) requires more flash power). I set the shutter speed for the amount of ambient light I want....and I set the ISO, also for the amount of ambient exposure that I want, keeping in mind that lower ISO requires more flash power and higher ISO means more digital noise.
I usually keep the flash in E-TTL mode though. And if it gives me more or less flash expose than I want, I just use FEC (flash exposure compensation) to adjust it.

So going back to the original question..."Difference between Av and Tv ".
The obvious difference is that in Av, you pick the aperture and the camera gives you the shutter speed.....and in Tv, you pick the shutter speed and the camera gives you the aperture.
But if you think about it, those two modes are really the exact same thing. I never use Tv....not because I don't care about the shutter speed...but because I know that I can control the shutter speed by controlling the aperture. For example, if I want the fastest shutter speed that I can get away with, then I'll just set my largest aperture (lowest F number). If I want a longer shutter speed, then I'll choose a smaller aperture. It doesn't matter if I'm using Tv, Av or even manual. It still holds true that if you want a faster shutter speed, you can do that with a larger aperture (or higher ISO).

by the way, I teach for a Photography School and we have a class that would be perfect for you. We currently teach in Edmonton and maybe Reddeer, but there are plans to offer classes in Calgary as well.

 

[groo]

The obvious difference is that in Av, you pick the aperture and the camera gives you the shutter speed.....and in Tv, you pick the shutter speed and the camera gives you the aperture.
But if you think about it, those two modes are really the exact same thing.

But this is what confused me, they aren't the same thing, In Av mode it was impossible for me to produce the same picture that Tv mode was taking (1/60 5.0). in Av mode I couldn't achieve a shutter speed of 1/60, because it seems programmaticly the camera want's to expose the image differently than Tv mode.

 

[Big Mike]

Well, the reason for that, is that the lens aperture is much more limited that the shutter speed. So when you use Tv mode and choose 1/60...the camera gives you F5 (and I bet the F5 was flashing) because that's the maximum aperture of your lens (at that zoom). So the photo was different because it was underexposed. If the lens would allow for it, the camera would have chosen a larger aperture, thus giving you the same exposure that you would get in Av mode.

The reason I use Av mode, rather than Tv mode...is that the shutter speed isn't as limited as the lens aperture. So in your example, you choose an aperture of F5...and the camera gives you a shutter speed of 6 tenths of a second. But that is the shutter speed that you need to get a 'proper' exposure...in the light that you are in.

So what you need to know, when shooting in Tv mode...is that when the aperture value is flashing...it means that the lens has reached it's limit and for 'proper' exposure, you will need to dial down the shutter speed. In this case, the aperture value would probably have kept flashing at F5, until you dialed the shutter speed all the way down to 0"6.
So even though you are in Tv...you really don't get to choose the shutter speed that you want...you still have to choose a shutter speed that works for the light that you are in.

So, what I would do, knowing that there isn't much light. Is to use Av mode and choose my largest aperture (F5 for your lens at that zoom level). I then know that the camera will give me the shutter speed...even though it's much too slow to get a sharp photo (if either the camera or subject are moving).
And knowing this, I would increase the ISO, which would then allow the shutter speed to get faster. It becomes a trade off between how high I want to turn the ISO, and how fast I can get the shutter speed. The trade off is that as the ISO gets higher, you get more digital noise and overall lower image quality....but that's still better than a blurry photo.

 

[Smitzel269]

Don't worry, I just got my t3i yesterday, and I'm just as confused. I'm probably going to read the manual again and then just play around with the settings lol.

 

[Big Mike]

Keep in mind that the stuff we're discussing here, isn't specific to the T3i...it isn't even specific to digital. So while I do strongly recommend reading the manual over and again...you can learn a lot of the basics from many different sources.

 

[groo]

Thanks, that makes more sense.