Depth of field

bateman

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Faster apertures and longer focal lengths have shallower depth of fields...
So my question is, Which has a shallower depth of field? A 35mm F/1.4 or a 50mm F/1.8.
Thanks,
Brandon
 

Ysarex

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Faster apertures and longer focal lengths have shallower depth of fields...

It's quite a bit more complicated than that.

So my question is, Which has a shallower depth of field? A 35mm F/1.4 or a 50mm F/1.8.
Thanks,
Brandon

However, at that very simplistic level an equally simply answer would be the 50mm.

Joe
 

KmH

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Faster apertures and longer focal lengths have shallower depth of fields...
Longer focal lengths don't have shallower DoF when subject scale is maintained in the field of view and the same aperture is used for each focal length..
In that case, the DoF is the same regardless the focal length, because the point of focus distance changes with the focal length used.

50mm56Feet5.jpg


200mm56Feet20.jpg
 

Ysarex

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No offense here Mike -- that's a fairly common illustration and I've seen it and/or variants of it over and over it all kinds of media. It can be misleading. So instead of griping I finally took the trouble to fix it.

Joe


$DOF x 3.jpg
 

Big Mike

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I don't really think it needed to be fixed, as it isn't 'from the view of the lens/photographer'. It's an overview of the scenario.

The way it looks to me (your version) is that someone is taking photos of three different sized people.

I know what you're saying...that the person (in the resulting photos) would be a different size on the paper/screen....but I don't think that its necessary to show that in this graphic.
Now, if you wanted to make one, that was actually showing what the resulting photo would look like, in each situation...that would be great.
 

Gavjenks

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Yall are making it way too complicated.

Larger apertures and greater MAGNIFICATION = shallower DOF.

^ sentence explains it all, short of an equation.
 
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Ysarex

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Yall are making it way too complicated.

Larger apertures and greater MAGNIFICATION = greater DOF.

^ sentence explains it all, short of an equation.

That's better than the OP's original statement, but it's still a bit more complicated than that. For example, the distribution of DOF is unequal around the plane of focus and with decreasing magnifications and increasing f/stop values that forward/back DOF ratio becomes markedly different depending on focal length: e.g. at the same magnification and f/stop for a short and long lens such that the DOF is the same the DOF distribution will be less unequal for the long lens. And there are other complications that sentence doesn't quite cover.

Joe
 

Helen B

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Yall are making it way too complicated.

Larger apertures and greater MAGNIFICATION = greater DOF.

^ sentence explains it all, short of an equation.

I think you have that the wrong way round. You might also want to define which magnification.
 

Ysarex

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Yall are making it way too complicated.

Larger apertures and greater MAGNIFICATION = greater DOF.

^ sentence explains it all, short of an equation.

I think you have that the wrong way round. You might also want to define which magnification.

Oops! That one slipped right past me -- I just read larger apertures as larger f/stop values. I'm gettin' old.

Joe
 

Ysarex

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I don't really think it needed to be fixed, as it isn't 'from the view of the lens/photographer'. It's an overview of the scenario.

The way it looks to me (your version) is that someone is taking photos of three different sized people.

I know what you're saying...that the person (in the resulting photos) would be a different size on the paper/screen....but I don't think that its necessary to show that in this graphic.
Now, if you wanted to make one, that was actually showing what the resulting photo would look like, in each situation...that would be great.

I don't think it's necessary to show the scale change either, but those figures can be misleading especially to a novice. Not including the scaling is fine but then how about marking the focus plane with and X.

Joe
 

Ysarex

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Wow fail on my part. Post above un-reversed.

I'm not sure I understand what un-reversed means, but I'm going to defend you at least a little here.

Still a bit simple but, DOF can be pretty well defined as a function of f/stop, focal length and distance to subject. Magnification is a function of focal length and distance to subject so DOF then is a function of f/stop and magnification. I think that's what you meant to say (that's what I was reading) and that's reasonably sound. There are fine-tuning factors and some caveats in there like the existence of infinity focus and hyperfocal distance but that's a fair starting point. I got what I think you were trying to say. Helen of course is always the final word.

Joe
 

Helen B

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(Feeling a little uncomfortable)


Yall are making it way too complicated.

Larger apertures and greater MAGNIFICATION = shallower DOF.

^ sentence explains it all, short of an equation.

As with other simple statements about DoF this might be best when accompanied by the condition that it doesn't really apply when comparing between formats, unless both the magnification of the object at the film/sensor and the magnification from the film/sensor to the final image remain unchanged between formats. There are other conditions when it doesn't apply even for constant format size. In fact, it only really applies when the lenses being compared are focused significantly closer than their hyperfocal distance, as Ysarex mentions.
 
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