Depth of field preview?

clwyd

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Can someone please tell me how to interpret the depth of field preview on my lens? When the button is press the lens stops down to the set aperture but what does this tell me???

Thanks Chris.
 

David A. Goldfarb

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Look at what's in focus with the lens wide open, and then what's in focus with the lens stopped down.

If you're focused on a distant scene, you might not notice too much, but try focusing on something fairly close to the near focus limit of the lens. As you stop down, more will be in focus. You might even focus on a ruler parallel to the lens axis and measure how much is in focus. Once you see what's going on, you'll notice more at greater distances.
 

jd callow

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It tells you what parts of the image will be in focus and what parts will be out of focus. It is most easily appreciated when there is a good deal of distance between the point that is in focus and the other areas of the image. Such as a face in focus and the other objects in the frame being very near and or very far away.
 
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clwyd

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Thank you both, because the screen was darkening i hadn't noticed the background coming into focus as i stopped the lens down. Regards Chris
 

Anscojohn

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Chris,
That's a great little device for taking people pictures, especially outside. Compose that luverly picture; then press the DOF preview, and, lo !! a potted plant, or a telephone pole, or some such object seems magically to grow out of the person's head that you did not notice at wide open aperture.
 
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clwyd

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Hi John, I never thought about that , but yes I can see the advantages.
Thanks Chris
 

CBG

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Especially valuable when using long fast lenses. Or when focusing extremely close up. The image in each of those changes the most when stopping down. You will see far less change when stopping down a slow short lens.

C
 

Poisson Du Jour

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In a nutshell, your image should be composed such that a third of the image in front is in focus when the depth of field preview is operated, with focus extending two-thirds to the rear from the point of focus. You will see the difference by stopping your lens down (DoF preview) at f5.6, compared to, for example, f11 to f16. In small formats like 35mm, anything above f16 can result in softening of the image as light is being bent around the aperture blades in the lens (defraction). Like CBG says above, DofF needs to be critically evaluated with long/telephoto lenses, but it's less of a concern with ultra-wide angle lenses (17 to 24mm) as those lenses have inherently great depth.
 

Martin Aislabie

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The second advantage is it also has the advantage that it raises apparent contrast – useful if taking photos of people outside

Suddenly deep shadows around the eyes and/or under the chin magically pop out of nowhere.

A touch of fill-in flash will cure the deep shadows and add a key-light in their eyes.

I use depth of field preview all the time on 35mm

Have fun

Martin
 

nemo999

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clwyd said:
Can someone please tell me how to interpret the depth of field preview on my lens? When the button is press the lens stops down to the set aperture but what does this tell me???

Thanks Chris.
Click to expand...

Theoretically it shows you the precise depth of field you will get at a given taking aperture. In practice, it shows you that the focusing screen gets darker as you stop down and, since the screen has been optimized for a large aperture, you can hardly see what's sharp and what isn't. DOF preview is a very dubious benefit - to maximize its usefulness, it's good to put a plain (non-fresnel, non-microprism) screen in your camera if this has interchangeable screens. The only really accurate way to know what will be sharp is to use the DOF markings on your lens mount (if present) or else use DOF tables.
 

pentaxuser

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clwyd said:
Thank you both, because the screen was darkening i hadn't noticed the background coming into focus as i stopped the lens down. Regards Chris
Click to expand...

I am using Chris' quote because as I understand it, this is the drawback with DoF preview. Are there any "tricks" to be used which gets you the real picture of DoF when things darken or is it simply a question of being aware that things are happening which aren't obvious unless you concentrate on looking for the info.

Does a Beattie Intenscreen or equivalent work here, albeit at a price?

Thanks

pentaxuser
 

eddym

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Well, you could use the view camera trick of a dark cloth. By shutting out all the ambient light, your eyes will adjust to the darkened view in the finder and you will be able to see details better.
 

Poisson Du Jour

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The Beattie IntenScreen is "probably" available (I can't be sure); it provides a 1-1.5 stop brighter than that fitted to many SLRs as standard. The drawback however was the introduction of metering errors which were often overlooked, thus resulting in confusion and guesswork with metering, especially those with matrix/evaluative systems. As I understand it, Canon advised users against employing a Beattie screen, both for the EOS focusing point matrice and difficulty in fitting. These screens can be extraordinarily expensive (more than US$190 or AUD$200)... e.g. http://www.camerascreens.com/35mm_Intenscreens/35mm_Format.htm . Normally, as irritating as it can be, you really do have to concentrate when the viewfinder brightness drops, but this will also bring detail into sharp relief and tweaking the focus while activating DoF preview will allow you to see just where the start and finish of the range of acceptable focus (Depth of Field's correct term) is. Rather than make it a half-second affair, allow your viewing eye to adjust to the darker condition over a longer few moments.
 

pentaxuser

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Thanks. I had already assumed that the user and certainly it's true for me, has an eyecup which shuts out all ambient light. There is no DoF button on my MZ7 so I have no experience of using one but maybe with a proper eyecup on a 35mm camera the darkening problem isn't a problem

pentaxuser
 
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