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Depth of Field and Lenses in the RED range

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explosive

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The general question.
Which lens in the RED range will give the shallowest DOF?

The specific question.
Suppose we take 4 Lenses at 50mm, F4, object 2 meters distant.
Which of the following lenses will give the shallowest DOF, and which will give the deepest DOF?

- 18-50 CF zoom
- 50-150 zoom
- 18-85 zoom
- 50mm prime

My guess for shallowest would be the prime or the 50-150...
Any guesses for deepest DOF in this case?

Or is the difference really not that great?

See I ask this question as I actually want a lens that is capable of reasonably shallow DOF when needed.
 
Depth of field is affected by focal length, lens aperture, distance focused, and degree of magnification / circle of confusion chosen. The differences caused by different lens designs, their MTF, where the actual exit pupil is due to the size of the zoom, etc. is less significant than the basic optical factors.

Of course, which lens has the widest lens aperture would be an important factor. If you want the shallowest focus at 50mm, I'd look for a fast lens.

However, note that for large screen projection, 50mm at f/4 on a close-up (let's say, focused at 5') in 35mm / RED full-sensor capture is fairly shallow in focus in terms of how soft the background looks generally.
 
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David Mullen ASC said:
Depth of field is affected by focal length, lens aperture, distance focused, and degree of magnification / circle of confusion chosen. The differences caused by different lens designs, their MTF, where the actual exit pupil is due to the size of the zoom, etc. is less significant than the basic optical factors.

Of course, which lens has the widest lens aperture would be an important factor. If you want the shallowest focus at 50mm, I'd look for a fast lens.

However, note that for large screen projection, 50mm at f/4 on a close-up (let's say, focused at 5') in 35mm / RED full-sensor capture is fairly shallow in focus in terms of how soft the background looks generally.
Click to expand...

Thanks for the response. That, was in fact, exactly what I wanted to know. So a fast lens would give the shallowest DOF, but this is less significant than the actual optical factors. Good, thats actually what I was hoping for.

So whilst the Prime would perform best (i am guessing it is the fastes at f1.4), if one wanted to they could achieve a similar result (insofar as DOF is concerned) with the zoom?
 
explosive said:
So whilst the Prime would perform best (i am guessing it is the fastes at f1.4), if one wanted to they could achieve a similar result (insofar as DOF is concerned) with the zoom?
Click to expand...

Well, you'd be limited by whatever the fastest aperture was on the zoom -- in other words, f/2.8 on a prime or zoom would be about the same depth of field, matching distance and focal length, but if the prime could open to an f/2 or f/1.4, then you'd get less depth of field with the prime at that wider aperture.

Most 35mm zooms aren't faster than an f/2.8 anyway, so you don't have much choice if you want to use zooms. But f/2.8 in 35mm photography is fairly shallow-focus except in wide-angle wide shots where you can't judge how soft the background is anyway.
 
>Suppose we take 4 Lenses at 50mm, F4, object 2 meters distant. <

Given that all of the lenses are at the same focal length, f-stop, and focus distance in this scenario, all of them would have the same depth of field. If you are asking which of the lenses would allow shallower depth of field in general, then it would generally be the longest and fastest one. They may have different qualities in terms of sharpness or bokeh, but if all of the mentioned factors are the same, then depth of field is the same.
 
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chuck colburn said:
Actually you need to define your terms. By DOF do you mean depth of field or depth of focus?
Click to expand...

Isn't the depth of the visable field also the depth of what's in focus?

ANyways, I meant field.
 
parisrem said:
>Suppose we take 4 Lenses at 50mm, F4, object 2 meters distant. <

Given that all of the lenses are at the same focal length, f-stop, and focus distance in this scenario, all of them would have the same depth of field. If you are asking which of the lenses would allow shallower depth of field in general, then it would generally be the longest and fastest one. They may have different qualities in terms of sharpness or bokeh, but if all of the mentioned factors are the same, then depth of field is the same.
Click to expand...

See I understand the logic in what you said, but I was also thinking that whatever internal mechanism is in place to make the 18-50mm Close Focus, well focus close, could also effect the depth of field somehow. Ie. make it generally deeper.
 
explosive,

Depth of field is what is in acceptable focus in front of the lens. This is based upon a chosen circle of confusion diameter for a given format. And other considerations, it can be a deep subject. lol
Depth of focus is plane of acceptable focus behind the lens.
Wide angle lenses have shallow depth of focus and longer lenses have deeper depth of focus. The subject of depth of field has been disscused in other parts of this site. I'm not touching that one with a ten foot pole. LOL

Chuck
 
David Mullen ASC said:
Well, you'd be limited by whatever the fastest aperture was on the zoom -- in other words, f/2.8 on a prime or zoom would be about the same depth of field, matching distance and focal length, but if the prime could open to an f/2 or f/1.4, then you'd get less depth of field with the prime at that wider aperture.

Most 35mm zooms aren't faster than an f/2.8 anyway, so you don't have much choice if you want to use zooms. But f/2.8 in 35mm photography is fairly shallow-focus except in wide-angle wide shots where you can't judge how soft the background is anyway.
Click to expand...

Gotcha :)
 
chuck colburn said:
explosive,

Here's a neat site that explains a lot about optics. If you want, open up the Optics section and scroll down untill you see the words depth of field and click on that. It's a good article on the subject.

http://www.vanwalree.com/about.html
Click to expand...

neato -- will check it out when i get home. and yes, defn meant depth of field according to what you described above.
 
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