Confusion between AV Depth of field and Manual Focus ring in Canon EOS Rebel Ti

[surya_prakash1]

Hi All,

 

Iam new to SLR photography. This is my problem. It may be

silly. But since Iam new to SLR photography I thought I will clear

my head with this question.

 

1) I got a new Canon EOS Rebel Ti SLR camera

2) It has a 28-90mm EOS lens

3) I was learning the controls (AV,TV,M - User defined modes) I had

a problem.

4) The problem was when I switched my camera to AV mode and then

tried to reduce the fstop (say to a lower value where I wanted the

background to be blurred), the camera reduced the fstop, but still

my background and foreground were sharp. I did not get the depth of

field I wanted (that is foreground clear and background blurred)

5) My camera lens was switched to MF mode from AF mode.

 

Iam not sure if Iam doing somethingw wrong.

 

Because as per the Instruction manual, when the camera mode is in AV

mode and Lens is in MF mode, then while the fstops are reducing I

mean ( 1/xxx , xxx- bigger number) and make my aperture open to the

maximum possible, I should get a blurred background and a sharp

foreground ....where Iam focusing.....

 

The above does not happen. Until I rotate and change my Manual Focus

ring on front of the lens....nothing changes...Only when I adjust

the Manual focus ring I am getting the foreground sharp and

background blurr combination...

 

I know Iam definitely doing something wrong...

 

Please help me ....

 

Thanks

Surya

[surya_prakash1]

Iam sorry...

I actually meant an fstop (1/xxx - xxx- a very small number)

 

 

Thanks

Surya

[jbq]

You need to focus the cames on the part which you want to be sharp. This can be achieved by using manual focus (as you've seen), and can also be done with the autofocus of your camera by putting the subject under a focusing point (one of the brackets in the viewfinder), focusing (press the shutter button half-way), recomposing, and taking the picture (press the shutter button all the way).

[adzy]

Surya, dont confuse the two - 1. focus and 2. depth of field. WHn u use the manula focus you are basicaly changing the focal length, or basically u are bringing the point of focus nearer or further away. The depth of field on the other hand, depends on a number of factors like 1. aperture 2. focal length and 3. subject to distance.

 

U say that u are already using a wide aperture ( 3.8 i think is the widest on your lens), so thats ok. next what focal length are u using ? at 28mm more things are going to be in focus than at 105mm. Also, the closer u move to the focus object, the less DOF.

 

Also, how are u checking for blur? is the default viewfinder view ? is it the DOP preview mode by stepping down the lens, or is the actual photo ? If its the first, then u are doing it wrong, if its teh second- it can be a bit tricky, making things out in the dim viewfinder. The third ofcourse is the best test.

 

If you are doing all the things above, and still not getting the blur then something is wrong. ANd BTW, no reason to keep the lens on Manula Focus, switch to Auto- u should be ok.

[fxdonny]

> 4) The problem was when I switched my camera to AV mode and then<br>

> tried to reduce the fstop (say to a lower value where I wanted the<br>

> background to be blurred), the camera reduced the fstop, but still<br>

> background and foreground were sharp. I did not get the depth of<br>

> field I wanted (that is foreground clear and background blurred)<br>

<br><br>

That's because the Rebel Ti camera (if I'm not mistaken), does not have the Depth of Field Preview button. Changing the AV will NOT make the background blur or sharp. AV (f stop number) will take into effect ONLY when you press the shutter button. You will NOT see any background blur, unless you have a DOF preview button on your camera, which I don't think that this is the case with your camera.

<br><br>

Now, what get me confused is when you said that when you MF the camera, you can see that effect. That MUST be wrong. Auto Focus/Manual Focus, has nothing to do with Depth of Field. So, something must be wrong here.

<br><br>

Bottom line is, if you want to achieve a background blur, set the AV to the smallest number (largest aperture), press the shutter button, finish the film, process it, and see the result. That way you can tell if the background is blur or not :)

<br><br>

Hope this helps.

[mormegil]

I noticed you noted the Aperture in 1/xxx

 

Your aperture numbers should be between 3.5 or 5.6 when open all the way depending on your zoom. For example, zoomed out to 18mm (wide angle), your Aperture wide open will be 3.5. Zoomed in fully to 55mm, your Aperture will be 5.6 (I think that's right, I'm mostly using my 50mm).

 

To me, that looks like shutter speed. Shutter speed has no effect on Depth of Field (background blur).

 

Are you sure you're in Av mode (and not M or Tv mode, where the command dial changes Shutter speed)?

 

Or not hitting the "shift" button as you roll the command dial, which would also change your Shutter speed?

 

Plus, in order to get background blur, you need to be focused on something in the foreground. If you have the aperture wide open, and focus at infinity, the background will still be sharp, because it's in focus.

[mormegil]

Sorry, I was thinking about the Rebel Digital with aps 3.5-5.6. I don't know what they are on the Rebel Ti.

 

But all the same things apply from my post. Specifically, that 1/xxx is shutter speed. That does adjust the fstop, just not the Aperture.

 

Fstop is adjustable either by letting in more or less light (aperture), or increasing or decreasing the time of exposure (shutter speed). Shutter speed is usually expressed as 1/xxx (until you get to long exposure times, then it's just in seconds).

[surya_prakash1]

Hi Azi,

Thanks for your reply and others who also responded.

 

1) To your question "what focal length are u using ? " - I just focused my Canon lens cap and a dollar note a little bit away. I used a long focal length. (probably the longest on my lens - 90mm). I set my mode to AV. I understand the more wider u go the more things will become sharp. But I used the longest focal length from a distance - 90mm and used the aperture was open the maximum (1/xxx, xxx- very less number) for the object I was trying to capture.

 

2) To your question "how are u checking for blur?" - I am actually NOT seeing the blur in the viewfinder and everything is sharp in the above setting (both the cap which is closer to the camera than the dollor note). I am not sure if I develop the image would I get a blurred image ? with this setting (say for ex: f/2, 90mm).

 

3) All I want to know is how can I make my viewfinder show the image blurred without adjusting the focus ring at the tip of the lens.

 

4) When should I actually use the focus ring..I mean in what situations...

 

5) In User-Defined mode (M,AV,TV) should the lens Focus mode switch be switched to MF or AF.

 

Please correct me If Iam wrong.

 

Thanks

Surya

[phule]

<< Please correct me If Iam wrong. >>

 

You seem to be very wrong about many issues with regard to your camera.

 

At this point, I'm not sure if a message board on the web is the sort of help you need. It might be better for you to take the camera to a local camera shop or to a local university and ask someone with a lot of experience to show and explain to you what you're doing wrong.

 

You are mixing up terms and I'm afraid you're only going to get more confused without someone standing next to you telling you what you're doing wrong.

 

With all that said, you are NOT going to be able to evaluate the Depth of Field until you get your film developed.

[fxdonny]

I need to make a bit correction:

 

> Changing the AV will NOT make the background blur or sharp.

 

I mean, changing the AV will NOT make the background blur/sharp ON VIEWFINDER, unless your camera has DEPTH of FIELD preview button, which in this case, your camera doesn't have it. When you are viewing the pic via viewfinder, the lens is in its open up position (to it's largest aperture, in your case, 3.8). Only when you press the shutter button will the lens aperture close to the selected aperture as you select using the AV. Once again, you will NOT know the effect of the Depth of Field on the Viewfinder, UNLESS your camera has a Depth of Field Preview button.

[phule]

<< Once again, you will NOT know the effect of the Depth of Field on the Viewfinder, UNLESS your camera has a Depth of Field Preview button. >>

 

However, in this case, DoF Preview would not show anything. As you've pointed out, the lens is already wide open which is where Surya set the aperture.

[eric bond]

F D,

 

This camera DOES have a Field Of View preview button. From the front side of the camera, it is on the right-hand side of the lens mount, right below the button to release the lens from the mount. It is a small, round, black button.

 

Please see http://www.canoneos.com/rebelti/html/instruct.pdf for a detailed manual. Page 41 addresses the use of the DOF Preview button.

 

Using this button in the Creative modes will show you how much the lens will stop down and what kind of depth of field will be on your film. Obviously, the more you stop down, the less light will show through (and make it harder to evaluate the DOF).

 

Good luck,

 

Eric

The information about the DOF preview button is on page

[eric bond]

Here is an illustration of the button. You must press and hold it for it to show you the current DOF.

[phil vaughan - yorkshire u]

To clarify;

at 90mm (the longest focal length of your lens) your maximum fstop is 5.6, (not really wide enough for a shallow dof) therefore if you put your lens cap about 1foot away and your dollar bill about 18 ins away, you should see the effect.

 

The effect you're going for is usually achieved with a telephoto lens 100mm+ and an aperture of 2.8 or smaller.

 

You can leave your lens to auto focus (even when you're in M AV or TV) 99.9% of the time and you'll learn when it won't work. Then you can use that focus ring on the end.

 

You should read through this:

http://www.photo.net/making-photographs/

 

To the other helpful posters:

You don't need depth of field preview to show the depth of field at the widest aperture!

[fxdonny]

Eric, thank you for the correction. It's good that the DOF Preview button is on Rebel Ti as well.

 

Surya, then, the answer is easy. Just play with the button that was indicated by Eric, and see how you like the effect on the viewfinder by changing the AV button.

[jay_.]

Surya, the smartest and best thing you can do is buy yourself an *old* book on basic photography, set your Ti to "M" manual and pretend like it's an old camera. All those modes are just going to confuse you. They can be really handy for someone who knows the theory but a bane to the newbie. Exactly the opposite of what the camera manufacturers want you to think. A 747 has an automatic pilot function but that doesn't mean anyone can plop down in the captain's chair and fly it.

[andys]

Some Links on photo.net about the basics and an old forum question about DoF.

 

Articles about the Basics

 

http://www.photo.net/making-photographs/exposure

 

http://www.photo.net/making-photographs/

 

 

Forum Q about DoF.

 

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=003bxW

[surya_prakash1]

Hi Eric,

 

Thanks for the reply. I used that button and saw the preview.

One more thing I learnt by playing with the camera is the lens made with the widest aperture possible by the camera to allow lots of light into the viewfinder.

 

If the preview button is pressed the camera stops down the aperture and the amount of light entering the viewfinder is cut short. This makes viewing the image a bit tricky/difficult.

 

I understand the best way to see it clearly is to wait till it gets processed correctly.

 

I would like to thank all the people who put the effort to reply this much and help so much for a new comer to photography like me.

 

I would take all the comments in a positive way. It was a good learning experience.

 

Thanks all

Surya