Depth of Field

[MIKEROPHONICS]

Apologies for the question, but I get the concept of opening up to a wide aperture for reducing the depth of field and have tried this quite successfully.
I also get that long focal lengths compress the backgrounds (like in the tracking zoom shot in Jaws when Brody is on the beach).

The bit I am struggling with is why longer lenses reduce the DofF. Why do portrait people use 70-200 f2.8? I like the look of those shots, but want to understand the implementation of the technique. I get the 2.8 bit and I know that the lens compresses the backgrounds, but why do people say this reduces the DofF?
I would love a Nikon 70-200 f2.8, but I have a D7K crop sensor and suspect that it may be too long for a crop sensor?

 

[vizzair]

this is a good site to browse
http://www.cambridgeincolour.com/

sometime a bit technical but loads of info.

 

[HoppyUK]

Apologies for the question, but I get the concept of opening up to a wide aperture for reducing the depth of field and have tried this quite successfully.
I also get that long focal lengths compress the backgrounds (like in the tracking zoom shot in Jaws when Brody is on the beach).

The bit I am struggling with is why longer lenses reduce the DofF. Why do portrait people use 70-200 f2.8? I like the look of those shots, but want to understand the implementation of the technique. I get the 2.8 bit and I know that the lens compresses the backgrounds, but why do people say this reduces the DofF?
I would love a Nikon 70-200 f2.8, but I have a D7K crop sensor and suspect that it may be too long for a crop sensor?

Couple of things:

Longer focal lengths don't 'compress the backrgound'. It's the greater shooting distance that flattens perspective, eg shoot the same scene with a wide lens and a longer one, then enlarge the centre of the wide shot to match - perspective will be identical.

Longer lenses don't reduce depth of field either. If the subject is framed the same, ie same size in frame, DoF will be the same. What happens is that with the longer lens you're shooting from greater distance so the narrower field of view cuts out a lot of background. This reduces any distracting clutter and helps the make the subject stand out more. It looks a bit like more shallow DoF but actually it's not.

Edit: Good DoF calculator here http://www.dofmaster.com/dofjs.html If you input say a 50mm lens at 3m and then a 100mm lens at 6m, you'll get the same answer. Near as damn it anyway, though things do shift around a little when you compare extremes like a 20mm used close and a 200mm lens from much greater distance, but that's not really a relevant comparison.

 

[woof woof]

The bit I am struggling with is why longer lenses reduce the DofF. Why do portrait people use 70-200 f2.8? I like the look of those shots, but want to understand the implementation of the technique. I get the 2.8 bit and I know that the lens compresses the backgrounds, but why do people say this reduces the DofF?

Longer lenses don't actually give you less DoF but in use it looks very much like they do and therefore (in use) I think it's perfectly ok to think that for all practical purposes they do.

It's also worth keeping in mind that longer / shorter lenses and different camera to subject distances can create very different looking images that really can't be made to look the same by simply cropping.

Have a look at this Luminous Landscape write up and you'll see that even 28 and 50mm focal lengths produce quite different looking results and foreground and background relationships when the subject remains the same size in the image.

http://www.luminous-landscape.com/tutorials/dof2.shtml

This is something that you can easily test yourself assuming you have access to the various focal lengths.

 

[HoppyUK]

Longer lenses don't actually give you less DoF but in use it looks very much like they do and therefore (in use) I think it's perfectly ok to think that for all practical purposes they do.

It's also worth keeping in mind that longer / shorter lenses and different camera to subject distances can create very different looking images that really can't be made to look the same by simply cropping.

Have a look at this Luminous Landscape write up and you'll see that even 28 and 50mm focal lengths produce quite different looking results and foreground and background relationships when the subject remains the same size in the image.

http://www.luminous-landscape.com/tutorials/dof2.shtml

This is something that you can easily test yourself assuming you have access to the various focal lengths.

Apart from being factually wrong, I find that a truly astonishing statement for someone with a scientific background like yourself, and a notorious stickler for detail It is extremely misleading.

 

[woof woof]

Well Richard, for decades many people actually believed that focal length changed DoF and it was even taught that way and printed in many text books.

And to be clear, what I meant was that if you keep the subject the same size in the frame the DoF will be the same, and you said the same thing. The Luminous Landscape piece says the same thing too.

As for it being ok to believe that in use focal length changes DoF, I personally think that it is ok to think that for all practical purposes it does as it looks like it does. If you don't agree then I can live with that but we're (I hope) all adults here and can make our own minds up.

And no, I'm not that much of a stickler for detail but I am finding your continuous argumentative posts aimed at me a little irritating and bordering on trolling. Lets just co-exist and let each other live and let live with our own personal opinions.

OP and Richard...

Here's a good write up on the subject and looking at the examples you can see the different effects although DoF is the same.

http://www.completedigitalphotograp...5cdContents/Chapter9/Focal Length and DOF.pdf

 

[HoppyUK]

Well Richard, for decades many people actually believed that focal length changed DoF and it was even taught that way and printed in many text books.

And to be clear, what I meant was that if you keep the subject the same size in the frame the DoF will be the same, and you said the same thing. The Luminous Landscape piece says the same thing too.

As for it being ok to believe that in use focal length changes DoF, I personally think that it is ok to think that for all practical purposes it does as it looks like it does. If you don't agree then I can live with that but we're (I hope) all adults here and can make our own minds up.

And no, I'm not that much of a stickler for detail but I am finding your continuous argumentative posts aimed at me a little irritating and bordering on trolling. Lets just co-exist and let each other live and let live with our own personal opinions.

OP and Richard...

Here's a good write up on the subject and looking at the examples you can see the different effects although DoF is the same.

http://www.completedigitalphotograp...5cdContents/Chapter9/Focal Length and DOF.pdf

It makes no difference whether it's you or anyone else making untrue statements. I just found that one coming from you to be particularly noteworthy.

It's important to be clear on this point, because it is quite a common untruth that longer lenses have some inherant optical characteristic that delivers less depth of field, and then it doesn't happen. It only looks like less DoF when you have a busy background that can be rendered less distracting when it's cut away and simplified by the narrower field of view.

That's quite a different effect, and in fact you don't even need a low f/number and shallow DoF to get it. Couple of examples - DoF the same, background rendered very differently from greater distance and longer focal length. But take away the background and the result is the same.

70mm f/4

200mm f/4

 

[woof woof]

It makes no difference whether it's you or anyone else making untrue statements. I just found that one coming from you to be particularly noteworthy.

I see.

So if I make sure to highlight the fact that DoF is the same when the subject size is the same more clearly you'll stop telling newbies that you can't get limited DoF from CSC?

And as I said, I do wish that you'd drop the attitude with me. As far as I remember I've always tried to be civil with you and yet IMVHO you repeadedly post snide and argumentative comments aimed at me in thread after thread. So far I have resisted going to mods as I do believe in live and let live but I'm getting tired of it and I would apptreciate a more mature and less argumentative approach from you.

 

[HoppyUK]

Just stick to the facts Alan and we'll get along fine.

 

[woof woof]

Still an attitue eh? Unbelieveble. Grow up Richard.

Sorry for spoiling the thread OP, but I at least hope that the links I provided prove interesting.

 

[HoppyUK]

Still an attitue eh? Unbelieveble. Grow up Richard.

Sorry for spoiling the thread OP, but I at least hope that the links I provided prove interesting.

Seriously Alan, I don't have a problem with you personally at all. Quite enjoy many of your posts actually. I'm not picking on you, just on untrue statements. Unfortunately, you have a habit of making them on this particular subject.

And when you do, you should expect to get jumped on, and not just by me as you know only too well.

You do get less DoF with smaller format cameras, and none of your arguments and thousands of words going back years now can change that. And here, depth of field and field of view are two very different things and it's important to understand the difference. Which I've explained and illustrated.

You stop making untrue statements, stop referring to me as the internet policeman, with an attitude, or trolling, and needing to grow up. Or carry on regardless, I really don't mind!