Understanding of DoF in Av mode

[Rex_Ham]

Hi all

Would I be right in saying that when using Av mode, that unless you are close up to your subject and want to get good bokeh, it is adjusted mainly to give the right shutter speed for the available light to prevent image blurring?

Now this might sound like an obvious statement to you, but as I'm just still getting to grips with the control features of DSLR's, I often wondered when viewing peoples exif data, why they had chosen, say, f9 on a view of a valley for instance....! Now that I've been out and about with my camera more as the weather's taken a turn for the better, I've been experimenting with Av mode and found unless there's nothing in the foreground that's sneaked into the viewfinder by mistake, such as a bit of grass or a bush etc....the image at f4 looks the same as the one shot at say, f13 - it's just that the shutter speeds are vastly different (when using the same iso setting)

It's taken a while - but the penny's finally dropping on understanding the differing aspects of photography :bonk:

 

[pxl8]

Kind of...

The focal length also plays a factor, well magnification to be precise. If your subject is far away then the magnification is low and so you get more DoF for your aperture setting. The closer the subject gets the more you'll notice the fore/background going OOF.

Shooting landscapes at f/4 at very wide angles will get most of the scene in focus but stopping down to f/9 will get the points furthest away from the focal distance even sharper and for landscapes it can make a big difference.

 

[Rex_Ham]

Kind of...

The focal length also plays a factor, well magnification to be precise. If your subject is far away then the magnification is low and so you get more DoF for your aperture setting. The closer the subject gets the more you'll notice the fore/background going OOF.

Shooting landscapes at f/4 at very wide angles will get most of the scene in focus but stopping down to f/9 will get the points furthest away from the focal distance even sharper and for landscapes it can make a big difference.

Thanks for that.

On about focal lengths - is there any way to see what focal length you're at - apart from looking at the reading on the focus ring?

 

[pxl8]

No, you could check the EXIF data later to see what you used for a shot but the guide on the lens is just that, a guide. The actual focal length will change depending on focus distance - a 50mm lens is only 50mm when focused at infinity, as you focus closer the focal length gets shorter and how much depends on the design of the lens.

And finally... the quoted focal lengths of a lens are normally rounded slightly, so a 300mm lens might be 295mm give or take.

It's bit like forecasting the weather with seaweed

 

[will69]

Hi all

Would I be right in saying that when using Av mode, that unless you are close up to your subject and want to get good bokeh, it is adjusted mainly to give the right shutter speed for the available light to prevent image blurring?

You would choose Av when you are primarily interested in Depth of field, and motion effects are less of a priority (e.g. freezing motion; or showing movement).

Once you have chosen the aperture you would check shutter speed to ensure that you won't get any unwanted camera shake and if necessary adjust aperture and / or ISO accordingly.

 

[Rex_Ham]

I'm aware of using Av for dof in scenes where there is some sort of feature in the foreground of the shot, but for general scenic views, where everything is at a reasonable distance and dof isn't an issue, almost any f/ stop can be selected - just so long as the shutter speed is OK to prevent camera shake - is that right?

Thanks

 

[Lensflare]

All Av is doing is setting the shutter automatically, according to the light meter reading in the camera to balance the aperture that YOU have chosen, for whatever result you are looking for.

It doesn't matter whether you are in Av, Sv, P or M the aperture selected will still give you the same depth of field for the situation pertaining.

If you want shallow depth of field, go closer to your subject and increase the focal length of the lens - both of these things will reduce your depth of field for whatever aperture you set - BUT f2.8 will have less than f22. Even so, the same rules apply - closer and longer reduce the D of F regardless of the actual aperture (so even f2.8 will have more depth of field on a wider lens, or when the subject is further away.) At f22, you will still have less deptjh of filed on a longer lens than a shorter one, and less deptjh of field as you get closer to your subject. Combine the two (close subject distance AND wide open aperture) and your depth of field disappears very quickly.

So when shooting in Av (what I use most of the time, even for sport - you want a fast shutter, you know you have to keep it open!) simply choose the aperture for the result you want, you might not LIKE the shutter speed the camera gives you - but then you have to do something about it, like put up with it, or up the ISO in an attempt to gain some light gathering capability, OR put more light on the subject.

You want shallow D odf FIeld, or faster shutter - you have to keep the aperture open.

You want greater depth of field and can put up with slower shutter - or you want a slow shutter speed (waterfall to cotton wool effect? Motion blur?) then you simply stop the lens down to suit. Dead simple.