AV mode and shutter speed

<p>Hi there!<br>

I have a Canon EOS Rebel T4i and was wondering, when in AV mode if there is a way to control shutter speed? Or is this a stupid question and I should just use manual?<br>

Also do you find auto focus or manual focus better? Does manual give you more control? I find my auto "seeks" the focus quite a bit.<br>

Thanks for your time,<br>

Laura</p>

<p>Hi Laura,</p>

<p>It's not a stupid question and it deals with the fundamentals of exposure. Exposure is built on three parameters:</p>

<p>ISO is the sensitivity of your camera's sensor to light. ISO 100 needs a lot of light; higher numbers like 1600 need much less.</p>

<p>Shutter speed is obviously how long the shutter is open. The longer it's open, the more light enters the camera.</p>

<p>Aperture or f-stop (the number that you control in Av Mode) represents the diameter of the opening in the lens which can be large (lots of light passes through) or small (most light is held back).</p>

<p>An exposure can be expressed as these three parameters:</p>

<p>1/60th of a second @ f/8 @ ISO 200</p>

<p>In Av mode, you control the middle parameter (aperture). You can, for example, change f/8 to f/4. This is a larger aperture that lets in much more light.</p>

<p>The camera will compensate for the greater quantity of light by making the shutter speed faster. If you change the "8" to a "4" in Av Mode, the camera will give you this exposure.</p>

<p>1/250th of a second @ f/4 @ ISO 200</p>

<p>The aperture is larger (f/4) and the shutter speed is faster (1/250s) than your original exposure. Assume the ISO didn't change because you kept it set to 200.</p>

<p>This is what Av mode is designed to do. You change the Aperture Value (AV), and the camera compensates by changing the Time Value (TV) of the shutter, more widely known as shutter speed.</p>

<p>There is a way that you can change the shutter speed in Av Mode to some degree. It's called exposure compensation. If you think that the photo is too dark at 1/250s @ f/4, dial a +1.0 value into exposure compensation. The camera will respond by doubling the amount of light that enters the camera. It will do this by changing the shutter speed, because that's what it controls in Av Mode. YOU control the Aperture Value (f/4) and ISO (200).</p>

<p>With +1.0 of exposure compensation, your exposure will now be:</p>

<p>1/125th of a second @ f/4 @ ISO 200</p>

<p>Don't forget to set Exposure Compensation back to 0.0, or you will expose your next photo.</p>

<p>Good luck!</p>

<p>Autofocus relies on contrast in the subject. Point the active AF sensor toward the edge of a shadow, for instance and it will work more reliably than pointing it toward an area with no changes in contrast.</p>

<p>Manual focus will be tricky in the Rebel's relatively small viewfinder unless your eyesight is quite good.</p>

<p>(I see Dan got to this as I was posting mine - he has an excellent answer here)<br>

There are three settings that interact - aperture, shutter speed and ISO. If you set your camera to AV mode, you are adjusting the aperture with the adjustment wheel (essentially changing the depth of field) and the camera is automatically changing the shutter speed and ISO to keep the photo lit in the way the camera judges to be correct. If you have an aperture you want (say f/11) for depth of field purposes, and want a higher shutter speed then the way to change the shutter speed is to adjust the ISO. The aperture will remain the same, so since you are adding or deleting exposure by adjusting the ISO the camera will adjust the shutter speed to keep everything even. So, staying at f/11, and raising the shutter speed from, say 400 to 800 should increase shutter speed by (I think) one stop. If you have the camera on TV then ISO will adjust the aperture to keep the photo "even".</p>

<p>If you switch to manual you can adjust all three of the factors (shutter speed, aperture and ISO), but you don't end up with more flexibility in that a correct combination is required to get the correct exposure, depth of field and action-freezing that you want to have, and you have to get all three right. If you are a beginner, I'd suggest using the AV or TV modes and experimenting with ISO until you are familiar with the impact on shutter speed and aperture. Your manual, in section 4, has a pretty good explanation of each of these modes.</p>

<p>As far as focus goes, on modern Canon cameras I almost never fiddle with manual focus. You just can't be better than AF in most cases. If the AF is seeking a lot it is likely in a low-contrast scenario (all dark, or all washed out bright), but hopefully its not too frequent.</p>

<p>Most focusing screens for DSLRs are not optimized for manual focus, and therefore it's extremely difficult to achieve accurate focus with them. But if you focus using liveview magnification rather than the viewfinder (when shooting macro, for example), manual focusing can be very precise.</p>

<p>If your question means:<br>

<em>“When in Av mode - AND I have the Aperture I want to use AND the exposure using the TTL meter is correct - is there is a way to control shutter speed?”</em><br>

Yes. You adjust the ISO.</p>

<p>As a practical example, you might be shooting outdoor sport like field hockey or football and you want to use the fastest lens aperture, such as F/2.8, because you want minimum Depth of Field for all your shots and let’s say for the sake of this example, you are set at ISO 400<br>

But you note that the Shutter Speed for your shots is moving between 1/400s and 1/640s and you know these Shutter Speeds are too slow to arrest the SUBJECT MOTION of the players – indeed you know that you want about 1/1000s as the slowest shutter speed.<br>

So, to control your shutter speed, you bump the ISO, to ISO1000~1600 which (provided the light stays about the same), will give you around 1/1000 ~ 1/1600s as your minimum slowest shutter speed.</p>

<p>+++</p>

<p>I don't use manual focus through the viewfinder, very often.<br>

If your AF is hunting: try aiming it at a CONTRAST EDGE, on the subject, lock focus and then recompose.<br>

For example a Portrait in low light - focus on the edge of the white shirt where it meets the black jacket - or the edge of the dress against the skin of the upper breast area... etc.</p>

<p>WW</p>

<p>Thank you all so much! I have been mainly shooting in AV mode and just now starting to mess with ISO changes. <br>

This really clears things up : )</p>

<p>Laura - everything comes at a price, and once you learn how to use ISO to increase the shutter speed, and to use the suggestion for over/under compensation you will find there are other "issues" that you will need to be aware of. Increasing the ISO will ultimately increase the noise in your photos - in my mind that's a welcome trade-off if the choice is not to get the shot at all.</p>

<p>In addition to all the great information provided already, check your AF mode to make sure it's not set to AI Servo. This mode continuously refocuses with subject movement which is handy when shooting fast moving subjects but invites focus hunting when your subject is sitting still.</p>

<p>@ David: Yes, I was worried about that. I'll just do my best to seek balance.<br>

@Jeff: Thank you, I did experiment with AI Servo but found exactly what you said about hunting when still. I am back to mostly using one shot.</p>

In summary, there are several ways to control the shutter speed in Aperture Priority (Av) Mode.

 

1. Change the f-stop, which represents the diameter of the aperture. The camera will adjust the shutter speed accordingly.

 

2. Change the ISO value. Again, the camera will adjust the shutter speed accordingly.

 

3. Use exposure compensation. Plus values will slow down the shutter speed to let in more light. Minus values will make it faster to let in

less light

 

4. Control the amount or light on the subject. More light will yield a faster shutter speed.