DOF Preview on 50D, Please explain

dnme

Hi friends

I have searched the forum and other places for this but came up a blank so I hope you dont mind me asking.The 50D manual is useless. The bit about DOF preview is literally 2 sentences with no explanation whatsoever.


Can anyome please explain to me the Depth of Field Preview function on the Canon 50D.

1. It seems to do nothing in Manual mode, why not?

2. When in Av mode, if I press the DOF preview button, the image in the viewfinder seems to darken from the edges inwards. It doesn't seem to actually give me a DOF preview (I would be expecting to see say a foreground in focus with a blurred background when I have my lens at f4 etc but all the DOF preview button seems to do is darken the image). So whats going on?

Any help greatly appreciated

melekalikimaka

it darkens it cos it shows through the view finder, through the lens, normally the ap is set wide open til you click, with dof it shows with the diaphram closed to ap selected, causing less light to reach your eye, darkening the image



afaik

charybdis

Depressing the depth-of-field preview button sets the aperture diaphragm to the specified aperture, something that would otherwise only happen when taking a photograph. A worthwhile experiment would be to set your aperture to a value lower than your lens' maximum (e.g.: f/16) and look down the lens as you depress the button.

Activating the aperture diaphragm with the button has two effects: the first is that the viewfinder gets darker as there is less light entering the camera; the second is that you get to see a more realistic representation of what depth-of-field your specified aperture will provide.

It should also be noted that the depth-of-field preview button is only useful for seeing large depth-of-field as a viewfinder's ability to see shallow depth-of-field is mostly limited by the focusing screen in the camera. On most cameras, the standard screen is usually only capable of accurately representing the depth-of-field generated by apertures greater than f/2.8-f/4.

dnme

Many thanks folks, also I have just realised that it does work in manual mode but when I was trying it, I had my lens ap set to f4 (its widest) so how could it do anything.

I still dont really get it though. Whats the point of it. All it does is vaguely darken the image from the edges in. It does not give us any clue as to DOF in the viewfinder.

For example, I set my lens to f4, zoom in on something and get nice and close. I know in real life that when I take the image and look at it later, I will see my subject sharp in focus and the background all nicely blurred. But if I setup the same shot, and press the DOF preview, all that happens is that the image darkens in the viewfinder. All Im getting is a darker image preview, not a DOF preview.

So where am I missing here?

jpfahy

I could never see the point of this either, pity they didn't leave that feature out and put in something useful like WiFi, GPS or Flash Triggers

melekalikimaka

film camera legacy feature. pros bouncing from film would have made use of it, and expected it as a pro feature

DaireQuinlan

melekalikimaka wrote: »

film camera legacy feature. pros bouncing from film would have made use of it, and expected it as a pro feature

I use it quite frequently. Admittedly, using digitals I'd imagine it's probably just as easy to take the shot and chimp it. Plus you need a big bright viewfinder to begin with I think to really take advantage of it, something which is lacking in most non-pro digital bodies.

In response to the OP, it's not going to be particularly noticeable until you've stopped down a bit, maybe f/5.6 or f/8 or smaller. With the lens wide open you're not going to see any effect at all, because ordinarily the lens is kept wide open all the time for viewing until you take a shot and the aperture closes down.

DaireQuinlan

jpfahy wrote: »

I could never see the point of this either, pity they didn't leave that feature out and put in something useful like WiFi, GPS or Flash Triggers

Or maybe a mobile phone, eh ? An MP3 player perhaps ?

kumate_champ07

you can also hold the button while changing thru the aperture range, you can see the shape of the diaphram as it gets smaller and smaller(manual lenses allow you to change aperture without the need of the camera body but not canon ef lenses). most dlsr focus screens are optimised for slow zoom lenses as these became popular from the 80's onwards as autofocus and auto everything took over. when manual slr's were most popular you had standard kit lenses of f1.4 and f2, now a standard lens is an f3.5- 5.6 zoom

petercox

The DoF preview does actually show you the increase in DoF as you step down the aperture - the problem is that it gets increasingly harder to see as you go down the range due to the darkening of the viewfinder image.

It really is a holdover from film - you can't see clearly enough (read: no-one's eyes are good enough) to see if something is acceptably sharp through the viewfinder. It might look OK in a small print, but blow it up and you'll see it's not quite there.

The simplest thing to do is take the image and look at it on the LCD - zoom into the maximum magnification (or sometimes the second-highest step - some cameras will allow you to zoom past 100% magnification) and check if it's sharp that way.

If you have live view, you can use the DoF preview button and the LCD image will get brighter to compensate for the smaller aperture, but the same problem arises and you'll need to magnify the live view image to really see if things are sharp enough.

You may find my article on the subject of depth-of-field interesting.

Cheers,
Peter