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04-03-2008, 12:01 PM   #1
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DOF and sensor size

If I recall correctly although a smaller sensor makes a lens look longer than it is but keeps the DOF the same. so do I understand correctly that my old fuji will have more DOF than my K10D as the sensor crop factor is 3 times. a 28mm 135 equiv. on my dridge camera (crop factor 4.5) was in reality 6.2mm where as on the K10D 28mm 135 equiv. is 18mm, the 18mm lens will have less DOF because the DOF is always that of a 18mm and so for the fuji at 6.2mm

This all assumes of course the same aperture is used in the comparison.

does that make sense ?

04-03-2008, 01:21 PM   #2
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DOF is a property of the lens,and has nothing to do with sensor size. All lenses of the same focal length ( at the same f/ratio) with have the same DOF irrespective of the dimensions of the recording medium.
04-03-2008, 01:26 PM   #3
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Smaller sensor cameras have more DOF for the same aperture and field of view. (1.5x more for the pentax DSLRs. vs FF.) IE a PS with a 75mm equivalent lens and a k10d with a 50mm (75mm equivalent) would have different DOF at the same aperture. The PS would have greater DOF.
04-03-2008, 02:05 PM   #4
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yea thats what I am saying I have a camera that will take a certain picture at 6.2mm (28 in 35mm equi.) and another camera that from the same point will take the same picture with a 18mm lens (again 28mm in 35mm equi.) but the camera using the 6.2 mm lens will have greater DOF than the 18mm with the same aperture

04-03-2008, 02:10 PM   #5
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DOF

QuoteOriginally posted by simons-photography Quote
does that make sense ?
Not really. Assuming that with 135mm equ. you mean equivalent field of view as in 35mm.

Just look at the actual focal length of each lens. The lower the number, the greater the DOF.
Sensor size has nothing to do with that.
04-03-2008, 02:26 PM   #6
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erm yes like i said I have one camera that will be at 6.2 mm and another at 18 mm to take the same picture and the only way you will take the same picture with such different size lenses is by using a different size sensor one is 5 mm long and the other 15 mm

if the DOF is the same no matter what the crop factor then smaller lenses on smaller sensor giving the same agle of view as a bigger lens on a bigger sensor will have more DOF
04-03-2008, 02:30 PM   #7
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QuoteOriginally posted by baw Quote
Just look at the actual focal length of each lens. The lower the number, the greater the DOF.
Sensor size has nothing to do with that.
Not true. Just check a DOF calculator.

Online Depth of Field Calculator

For example
k10d @ 60mm(90mm eq) f/2.8 and 15 ft from the subject has a DOF of 2.14ft
5d @ 90mm f/2.8 and 15ft from the subject has a DOF of 1.41ft

Not coincidently 2.14/1.41~ 1.5 the same as the crop factor.

So clearly if you are taking an identical picture in terms of distance and field of view, sensor size does play a part in DOF, if only in the fact that you must use a different focal length for the same photo.
04-03-2008, 02:58 PM   #8
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Don't speculate go here
DOF
DOF2
pay attention to here:
Understanding Depth of Field in Photography great place for tutorials.

Go here for DoF calculators - one that you can customize for your own use and create DoF Scales for focal lengths you have.
Hyperfocal Distance and Depth of Field Calculator - DOFMaster

Go here to see the calculations for DoF. The calculations are effected by the CoC (Circle of Confusion) which has to do with reproduction size, lens resolution capabilities and sensor resolution. Depth of field - Wikipedia, the free encyclopedia

The issue with selective focus via DoF and the ability to control what is out of focus and what part of the subject is in focus, has more to do with the camera than anything else. P&S cameras have very small lenses (f stop wise) and very small sensors. The DoF on these cameras is not controllable as in a DSLR - that control is one reason to buy a DSLR so you can have that control.

On a sensor that is smaller than a 135 format (so called full frame) the depth of field for a given focal length is slightly different. At 50mm on my K10D at f/8.0 the DoF when focused at infinity - anything beyond 12.6 meters is in focus. On 35mm film it is 10.5 meters to infinity, on a 4x5 it is 4.47 meters. The general rule is that the bigger the sensor the greater the depth of field for a given focal length and f/stop. However, on smaller sensors a 50mm lens has a smaller FoV and therefore is more of a telephoto. On my old (now dead) P&S its max focal length was 5mm with a 2.2 MP sensor. I had no control over DoF and for that device the DoF was huge. On the first camera I used (a Brownie Box using 620 roll film with an image of 2.5 x 3.5 inches) it had one shutter speed (1/30) and two f stops - wide open at f/8 and closed down at f/11. No focus per say as the lens was focused at infinity, the closest focus was about 5 feet.

Experiment and calculate so you can learn how to exploit DoF, speculation will lead to frustration. Don't base your actions on others "opinions" - including mine - do the research and figure it out.

The Elitist - formerly known as PDL

04-03-2008, 03:00 PM   #9
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DOF

QuoteOriginally posted by Geekybiker Quote
Not true. Just check a DOF calculator.
In your example the 60mm lens has a DOF of 2.14ft anf the 90mm lens 1.41ft.
Seems to support my statement
04-03-2008, 03:15 PM   #10
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LOL, baw and pixelsaurus were talking about same focal length, while the OP and Geekybiker were talking about the same equivalent FOV on different sensor size (thus different focal lengths).
04-03-2008, 03:16 PM   #11
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Might I suggest that the concept of equivalent focal length be abandoned. Focal length is focal length length reqardless of sensor size. Do not confuse the issue. FoV of a given focal length will change as the sensor size changes - but the focal length will NOT change.

The focal length of my FA 50mm is just that 50mm - whether it is mounted on my SF-1 or my K10D. If I photograph an object that is 10mm square on the film of the SF-1, that same object will be 10mm qare son the sensor of my K10D from the same distance. 135mm format equivalents have no place in DoF discussions. Use focal lengths - you know the ones engraved on the lens?

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04-03-2008, 04:13 PM   #12
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The thing is the DOF subject is really pretty complex, and what affects DOF depends heavily on what we are choosing to stay constant.

35mm equivalent is the best place to start IMHO. It will produce identical framing, perspective and subject distance. Perhaps FOV is a better term than focal length for comparisons between systems.

Main subject framing is iffy. You'll have extremely different pictures from a FF using a 6mm lens to a 1/1.8" using that same lens.

Really there are 4 things that play a part depending on what you chose as a constant
Aperture
Subject distance
Sensor size
focal length



Anyhow, I suggest you read through PDL's links.
04-03-2008, 04:21 PM   #13
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QuoteOriginally posted by PDL Quote
Might I suggest that the concept of equivalent focal length be abandoned. Focal length is focal length length reqardless of sensor size. Do not confuse the issue. FoV of a given focal length will change as the sensor size changes - but the focal length will NOT change.
Hear,hear.
The nitwit(s) that dreamed up the"crop factor/equivalent focal length" , a marketing ploy I guess, should be evaporated. It has done nothing but sow the seeds of confusion. When I was a 35 mm user ( I started photography in 1969), there was no confusion relating to lens focal lengths re 35mm, MF and LF cameras. I'm certain the concept of equivalent focal lengths was of no consequence to most photogs. My concern these days is that many newbie photogs (digital) really do have a lack of understanding of basic photographic and optical principles.
04-03-2008, 04:30 PM   #14
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Although people seem to have danced all around the issue, they have not hit the issue directly.

Depth of field is dependant upon two things, the first is the lens and its apature, (and this is camera and sensor independant) and the second is the magnification ratio of the print.

If I am not mistaken, DOF for a 35mm film camera was based upon an 8 x 10 print and a consideration of what "Acceptable Focus" meant.

For the sake of argument here, since the aspect ratio of the print is not equal to theaspect ratio of the film, I will assume that the 24mm width of the film, is scalled up to the 8 inch dimension of the print.

I will also here admit to being lazy, and hate rediculous and accurate calculations, so let's say that the print was made by making an 8 times (linear dimension) enlargement since 24mm is very close to an inch.

Let's now look at an 8 x 10 print made from a DSLR with ASP-C sensor like pentax. in this case we are scaling 2/3 of an inch (same lazyness as prefviously but at least I maintain the correct ratio of film to ASP-C sensor) up to 8 inches, and as a result we need a magnification of 12.

Since we are making a higher magnification enlargement, the limits of "acceptable focus" have changed, in fact reduced.

As a result the depth of field of the lens "appears" to be dependant on the sensor only because the enlargement ratio of the final print is greater.
04-03-2008, 04:30 PM   #15
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QuoteOriginally posted by pixelsaurus Quote
Hear,hear.
The nitwit(s) that dreamed up the"crop factor/equivalent focal length" , a marketing ploy I guess, should be evaporated. It has done nothing but sow the seeds of confusion. When I was a 35 mm user ( I started photography in 1969), there was no confusion relating to lens focal lengths re 35mm, MF and LF cameras. I'm certain the concept of equivalent focal lengths was of no consequence to most photogs. My concern these days is that many newbie photogs (digital) really do have a lack of understanding of basic photographic and optical principles.
But now we have a crop of cameras using at least 5 formats in DSLR's alone. (ff, 1.3, 1.5, 1.6, 2) Not to mention the whole host of PS cameras that use assorted sensor sizes. Its not like film where there were few formats to compare. There has to be some sort of useful standard to compare the performance of lenses across models. Granted 35mm equivalent isn't ideal. Maybe listing the magnification from normal perspective? Of course that breaks down with interchangeable lenses where the only constant is focal length. Then using some sort of conversion factor is the only way to compare performance.

How would you list lenses such that they are easy to compare across a different platforms?
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