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the question : is DOF effected by length of the lens?
Old 11-04-2004, 02:44 PM   #1 (permalink)
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in any way shape or form ....please answer so i can pass the info to someone...
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Re: the question : is DOF effected by length of the lens?
Old 11-04-2004, 03:49 PM   #2 (permalink)
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From things I've read, the answer is YES. DOF is decreased (ie, background blur is increased) as focal length of lens is increased. This fact is making me lean toward a 85mm, versus 50mm, as my standard portrait lens, to increase the bokeh effect.
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Re: the question : is DOF effected by length of the lens?
Old 11-04-2004, 04:08 PM   #3 (permalink)
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Yes it is. DOF is determined by four main things.

Aperture
Distance to subject
Focal length
Sensor size
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Re: the question : is DOF effected by length of the lens?
Old 11-04-2004, 09:05 PM   #4 (permalink)
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Yes DOF is reduced as the lens gets longer. However, the same lense on a digital with have a different DOF longer DOF than the same lense on a film camera.

You might hear or read someone as saying the background is compressed.
 
 
Re: yes
Old 11-04-2004, 09:57 PM   #5 (permalink)
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The longer the lens, the more shallow the DOF at any given f-stop. Opening up the lens (smaller f-stop number, larger opening) and moving closer to the subject also also gives more shallow DOF. Using a camera with a larger film or sensor area, also gives more shallow DOF. That is why 4x5 view camera guys are always trying to get way into the f16 and higher range, while those using 35mm can get away with f2.8 on a 300mm, which is very popular with fashion shooters.

More than simply thinking of DOF when thinking about a longer lens, is the perspective of that lens. Sure, you could get very little DOF with a 50mm (in 35mm format), but you'd have to move in so close to the subject (for a headshot) that you would get a larger nose and features. For this reason, most 35mm shooters use at least a 105mm for headshots (sometimes even longer). In 1.5x digital terms, that would be about a 35mm focal length. If you are using that kind of digital, your 50mm is not great (its only the equivelent of a 75mm). An 85mm would give you approx 125mm effect, which would be perfect. When people talk about compression, they're talking about the flattering effect of the "compressed" image on a face especially. It makes the features smaller and just looks nice. Here's one with a 300mm, probably at f4.



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Andy Pearlman
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Thats what I though Thanks guys....
Old 11-04-2004, 10:13 PM   #6 (permalink)
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thats what i told the person....but it seems i am just an uneducated fool...because thay went to cambridge... [img]/ubbthreads/images/graemlins/crazy.gif[/img]

now i can give her a link to a few other opinions however she is so stuborn she most likely will not beleave it...
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Re: the question : is DOF effected by length of the lens?
Old 11-05-2004, 09:10 AM   #7 (permalink)
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longer lenses at wider apertures are certainly going to give you shallower DOF. Another thing to consider: zoom lenses vs. primes. as i understand it, zoom lenses focus using both the front and rear elements. primes focus with the back element only, while the front remains fixed. a prime, say 85mm or longer at a wide aperture, is going to more easily produce shallow DOF results--i.e., without being on top of the subject--and it's often why portrait photographers prefer using them.
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Re: the question : is DOF effected by length of the lens?
Old 11-05-2004, 02:12 PM   #8 (permalink)
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I agree that DOF is affected by focal length, distance to subject and aperature.

But I don't think it is effected by the sensor size.

Whether the focused image is being projected on a half frame sensor or a full frame sensor is of no consequence.

What happens is because of the "multiplication factor" of a given lense (say an 85 mm on a digital) is that you back up to get the same subject in the frame as when you are shooting it on a film. Thus as distance from the subject effects the apparent DOF, people say the sensor size effects the effect.

I beleive if you shoot the same subject from the same distance, the DOF is the same.

Let me give you this as an example, if you shoot a photo on 35 mm film and print it at 4x6.....does the DOF change because you crop the photo to 3x5...answer....no.

Format (like MF vs 35mm) the film size effects the DOF in comparing the two formats, but as a sensor is inserted into what is otherwise a 35mm body, there is no diffference between 35mm and digital.

As for the new purpose built digital cameras or the point and shoot digital cameras, I would agree that sensor size does effect the DOF, but that has to do with the distance between the lens, focal plane, and sensor.

I hope that makes sense.

Mark
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Common misconception
Old 11-05-2004, 02:51 PM   #9 (permalink)
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It is a common misconception that DOF is not affected by sensor size.... while DOF mathematical calculations to not account for sensor size, the sensor (or film) size does have a great deal to do with DOF for PRACTICAL purposes because your film/sensor size forces you to make certain lens/distance considerations.

If you try to shoot a head and shoulders portrait with a 135mm lens, on a film body, you will be at some distance "X" from the subject. To shoot the same picture (head and shoulders) with a DX sensor, you have to move farther from the subject that "X" (which give you deeper DOF) or change to a shorter lens (also gives you deeper DOF). Go to a larger film/sensor (like MF) and you can get even closer to the subject and get even less DOF, but still shooting the same picture of the same head and shoulders portrait.

In your example of trimming a photo, yes in that scenario, DOF is unchanged..(technical it does change because a smaller print will be viewed at a closer distance so you CofC calculations will change but we’ll ignore that for now) but in that scenario, you are standing at the same distance "X" and getting a "different" picture... no longer a head and shoulders portrait, but instead a tighter headshot. So by changing film/sensor size, you can hold DOF constant, and get a different picture, or try to capture the same relative picture and get a different DOF.

Even if you adjusted the apeture to try to get the same DOF between a smaller and larger film/sensor system, you will not have the same background blurring effect because the slope of the CofC to distance curve is a function of the square of the focal length so the background get blurrier “faster� with the longer lens (i.e the larger sensor/film cameras).

Bottom line is that digicams with smaller sensors (like Olympus E10) make it practically impossible to blur the backgrounds or otherwise get a shallow DOF.

Also, when you put a sensor that is smaller than the film in a 35mm body, you are doing the same thing that you get if you magically put a sensor that was the size of MF film in the same body (ignoring image circle problems).... you would get the samd DOF if you stood still, but would get a different picture. You get a very different DOF however, if you move closer/farther fromt eh subject to try to take the same "picture" (i.e. head and shoulder portrait) on all 3 formats. For comparing differnet lens/camera/film systems for technical issues like DOF, most of us will compare systems considering we will be taking the same pictures with each (i.e that head and shoulders shot).

 
 
Re: the question : is DOF effected by length of the lens?
Old 11-05-2004, 04:08 PM   #10 (permalink)
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The answer is yes. Three factors that affect DOF: Lens focal length, focal distance (which people often forget about), and aperture.

There is also an additional control over "DOF" and that is lens or film plane tilts. The caveat is that you need either a perspective control lens or a view camera for one, and for two, you're really not affecting depth of field per se, but rather the PLANE of focus (the distance of "acceptable focus" remains the same but on a different plane, allowing you to keep an entire fence trailing away in focus, or keeping eyes in focus but throwing out the body and so forth...).

If you look at DOF scales on a variety of different focal length lenses you can see this clearly. All things being equal, a 35mm lens will have greater DOF than an 85mm which will have greater DOF than a 210mm.
 
 
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