DOF Advice

neonpollen

neonpollen

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Name
Andy
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Hi,

Wednesday night there is a huge bike meeting in a nearby town and there will be literally thousands of bike to photograph but I also want to get candid shots of the people enjoying themselves and try to capture some of the atmosphere of the whole event.

My problem is I can never seem to judge properly what f-stops to use to get the right DOF for people faces as I walk around. How does DOF vary with distance to the subject ? is there a rule of thumb i.e. if the subject is twice as far away, the image will have twice the DOF ? or say at 15 feet a good f-stop would be F8 for example. My guess is it's just down to experience with the lenses ?

I only have two lenses for my 350D at the moment a 28-135 1:3.5-4.5 and a 70-200 1:2.8L so I'm restricted in choice, I was only going to take the 28-135 as I'll be going on my bike and as yet I don't have a decent camera bag to put my gear in (I'm also worried about the big lens getting damaged in the crowded streets).

Any advice would be gratefully received.
 
Have a look at this DOF calculator , it lets you get an idea of the effect aperture/length has.

On a more important note , is that one of the best bike/colour schemes ever in your Avatar :D
 
for my shame, i tend to stick to around F8 and use manual mode, flick the shutter speed around to keep it slightly under exposed....im such a cowboy
 
This a huge and potentially confusing subject. If you have a look at the bumph which came with your lens, you may find that the manufacturer has included some Depth Of Field tables for your lens showing the extent of DOF at varying distances and apertures. A Google search may also turn up these tables for you. They're interesting in that they'll give you a good grasp generally speaking, but let's be honest, there's no way you're going to commit the info to memory and you certainly don't want to be peering at DOF tables before you take every shot.

There used to be very handy DOF scales on lens barrels which enabled you to work out what was going to be in focus at a given distance and aperture. Sadly we don't get them now on AF lenses, or we sometimes get an apology for one which is of little practical use whatsoever.

What we do have though is the Depth Of Field Preview Button, which briefly stops down the lens to the actual taking aperture so that you can judge what is going to be in focus and what isn't. The screen goes somewhat darker obviously because it reduces the amount of light entering the lens, but it is a very useful facility for judging DOF.

Have a read of this article for a better understanding of DOF...

CLICKY

Hope that helps somewhat. Sing out if not. :)
 
Hands up everyone who never uses the DOF Preview Button. :wave:
 
LOL that is a problem Ian, particularly in poor light. If you focus on something though at full aperture - say 5 or 6 feet away, you can clearly see that the DOF doesn't extend very far past the subject or towards you. As you gradually decrease the aperture and press the preview button you should see the dof noticeably increasing with each reduced f stop despite the darkening of the screen, although the stronger the light the better. In reasonable light the screen should still be viewable down to around f8 or f11

It's best to avoid wide angle lenses for these sorts of tests as they have such inherent great DOF anyway, but with a standard 50mm or tele lens you should see the effect quite noticeably.
 
IanC_UK said:
I have actually started using that now ! ;) but all i see is the pic getting darker ! ;) LOL :help:
Click to expand...

Same here, I can't say I have found it useful yet - unless I'm doing something drastically wrong :wacky:
 
Thanks for the help and links the DOF article is excellent, though I may need to read it a few times to get it all to sink in :)

The DOF calculator has open my eyes a little, I didn't realise that the longer the focal length the shorter the DOF :( That explains why a lot of my bike racing pictures are out of focus, some of the track I go to it's possible to get quite close to the racers and according to the calculator the DOF can be reduced to inches.

I do try and use the DOF preview button but unless it's a very bright day I can't really tell the difference, other than the viewfinder going darker. By the time I found the damn button, pressed it the shot has gone :)

mrgubby: I agree, much better than those horrible blue ones :D
 
For candid shots, you'll be better off with a wide aperture (narrow DoF). This'll give you nice fast speeds and blurred backgrounds, helping seperate the subject from what's going on around them.
Remember light levels will be lower as it's in the evening.
 
I went out yesterday afternoon to practise getting the right DOF while my daughter was fly the power kites, got a few decent pics and learnt a lot.

In the evening I carefuly packed my camera & lens into my tank bag and rode over to the event, unpacked the camera only to find I had left my CF card in my card reader :( ... what an idiot. So sadly I didn't get any images from the bike event at all.
 
That's bloody annoying. :(

Go into the menus on your 350D and disable 'Picture Taking Without Card', then as soon as you try to take a pic you'll get a 'No Card' message on the screen. It still doesn't protect you from going out with no card in the camera, (been there - done that) but at least it's one less thing which can spoil your day. It's something you should do as soon as you get home and take your new camera out of the box.

Edit... Sorry just realised it was two separate outings. The only thing you can do is try to drum it into your head as part of a check list before you go out. I've chucked a spare card into the glove box in the car and it lives there. Easily done though.
 
Yup, I've got me free 256mb card in the bag pouch for emergencies.
 
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