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 Post subject: Depth of Field
PostPosted: Sat Dec 01, 2012 10:16 pm 
I've been shooting 35mm film with a Pentax K1000 slr for years and am now confronted with the fact that film processing is becoming harder to find and takes much longer. So....I've been studying the facts about digital photography and have been looking at camera brands and features.

With film it's been fairly easy to shorten depth of field (DOF) so I can highlight my subject by throwing the foreground and background out of focus, a practice I frequently like to use. From the information I've gathered so far about digital photography, however, I won't be able to obtain the same short DOFs with a digital slr unless I spend thousands (which I don't want to do).....something about the astronomically-high cost of manufacturing the silicon wafers used for the digital sensors. The bottom line seems to be that sensor size is inversely proportional to DOF. So if I go for a less-than-full frame digital slr I'll have to settle for longer DOFs. I understand that aperture size can be selected when using a digital camera in its "manual" mode and I know that aperture size affects DOF. I've noted, however, that the smallest apertures on the lenses supplied with digital slrs are not as small as the ones my film camera lenses have (f2.8 for my 28mm, f2 for my 50mm and f5.6mm for my 75mm-300mm). There may be smaller apertures available in separately-puchased digital lenses, but that would drive the cost of my change to digital up considerably.

Can someone tell me in rough percentage terms, in adjectival terms (e.g. little, some, moderate, a lot), or in "gut" terms how much longer I should expect DOF to be with, for example, a Canon Rebel T3i digital slr w/"kit" f3.5 18mm-55mm and f4.0 55mm-250mm lenses compared to my Pentax K1000 film slr with lenses as described above?


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 Post subject: Re: Depth of Field
PostPosted: Sat Dec 01, 2012 11:11 pm 
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Tricky subject and you'll probably hear a ton of slightly different answers BUT here's the thing: the DoF won't be any different in terms of crop vs full-frame, it's just that to get identical framing, you have to step further back. This has the effect of deepening the DoF on the crop, because as you know, camera-to-subject distance also plays a large role in determining DoF.

Have a play around with this: http://www.dofmaster.com/dofjs.html

A 50/1.8 lens for Canon can be had pretty cheaply and it still delivers extremely good image quality, and narrow depth of field even on a crop frame camera like the Rebels.

In all honesty you will notice a difference, but it'll be far less jarring than say, if you moved to a micro4/3rds camera or a sensor even smaller.

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 Post subject: Re: Depth of Field
PostPosted: Sun Dec 02, 2012 12:02 am 
Plymer wrote:
Tricky subject and you'll probably hear a ton of slightly different answers BUT here's the thing: the DoF won't be any different in terms of crop vs full-frame, it's just that to get identical framing, you have to step further back. This has the effect of deepening the DoF on the crop, because as you know, camera-to-subject distance also plays a large role in determining DoF.

Have a play around with this: http://www.dofmaster.com/dofjs.html

A 50/1.8 lens for Canon can be had pretty cheaply and it still delivers extremely good image quality, and narrow depth of field even on a crop frame camera like the Rebels.

In all honesty you will notice a difference, but it'll be far less jarring than say, if you moved to a micro4/3rds camera or a sensor even smaller.


Plymer-

Thanks for reply and DOF website and lens recommendations.....

With regard to your point about framing........wouldn't I get the same framing with a given lens size because of the crop factor? For example, an 18mm digital lens (like the 18mm position of the Canon f3.5 18mm-55mm telephoto lens) would have about the same viewing angle as my 28mm film camera lens: 18mm x 1.6 crop factor (Canon's) = 28.8mm.

-Chuck/GW90


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 Post subject: Re: Depth of Field
PostPosted: Sun Dec 02, 2012 6:43 am 
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Joined: Wed Apr 08, 2009 2:29 am
Posts: 716
GW90 wrote:
With regard to your point about framing........wouldn't I get the same framing with a given lens size because of the crop factor? For example, an 18mm digital lens (like the 18mm position of the Canon f3.5 18mm-55mm telephoto lens) would have about the same viewing angle as my 28mm film camera lens: 18mm x 1.6 crop factor (Canon's) = 28.8mm.

Crop factor applies to both focal length and aperture equally. So just do as you did with focal length and substitute aperture. i.e. f3.5 x 1.6 crop factor = f5.6

So an 18mm lens at f3.5 mounted on a T3i will give you the same Field of View and Depth of Field as a 28.8mm lens at f5.6 on a full frame camera.

So basically, stop your current 28mm f2.8 lens down two stops and that's pretty much what you'll get - Mark

P.S. If you want the details: remember that f number is defined as focal length divided by pupil diameter. So increasing focal length results in a corresponding increase in f number. e.g. a 100mm lens at f2.0 has a pupil diameter of 50mm. (100 / 50 = 2) If you, say, double the (equivalent) focal length (like a camera with a 4/3 sensor would) to 200mm -- 200 / 50 = 4 (note: obviously the pupil diameter doesn't change.) So a 100mm lens at f2.0 mounted on a camera with a 4/3 sensor (with a 2X crop factor) will give the same FoV and DoF as a 200mm lens at f4.0 on a full frame camera.

Note: this also works "in reverse". So if you want to duplicate the framing and DoF that your current full frame camera has when using your 50mm lens at f2.0 on a T3i (with a 1.6X crop), you would need a 31mm (50 / 1.6 = 31.25) lens at f1.2 (2.0 / 1.6 = 1.25). Canon's EF 35mm f1.4L would get you close for ~$1,500.


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