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Crop factor and DOF


certx 14 415 1 United States
8 Apr 2007 7:44PM
Another question... I understand the crop factor where it relates to focal length vs 35mm equivalent focal length, but that spurred a question for me. Does the crop factor also have an effect on DOF? If so, is there some magic formula that applies across the range of focal lengths with a given crop factor? Is the difference in DOF between film vs digital cameras a product or crop factor? Or is it some other product of digital?
Carabosse 18 41.5k 270 England
8 Apr 2007 7:51PM
Depth of field is determined by focal length.
Krakman 14 3.6k Scotland
8 Apr 2007 7:59PM
Depth of field is determined by aperture and doesn't actually change with focal length.

What does happen with change of focal length is that you are 'zooming in' on part of the scene, so any lack of depth field becomes more noticeable. A bit like camera shake - which is the same with any focal length but becomes more visible with longer focal lengths.

The same is true of crop factors - because you are in effect 'zooming in' on part of the scene, although the depth of field is the same (it's the same lens after all!), any lack of depth of field becomes more obvious.
Carabosse 18 41.5k 270 England
8 Apr 2007 8:02PM

Quote:Depth of field is determined by aperture and doesn't actually change with focal length.


Nope! Incorrect. Sorry!

F4 on a 200mm lens has a different DoF than F4 on a 20mm lens.
Krakman 14 3.6k Scotland
8 Apr 2007 8:05PM
It only looks like it. Try taking the same image standing at the same viewpoint with both lenses, then crop the 20mm image to the same field of view as the 200mm...
theorderingone 17 2.4k
8 Apr 2007 8:07PM
You're all slightly wrong... sorry. :-P

It's determined by any of the following variables:

Focal length
Aperture
Distance to your subject

Getting closer will reduce DOF for any constant aperture and focal length.

Choosing a different aperture will affect DOF for any constant focal length and distance to subject.

Selecting a different focal length will affect DOF for any constant aperture and subject distance.

Try it yourselves and you'll see.
Krakman 14 3.6k Scotland
8 Apr 2007 8:09PM
p.s. it's another popular misconception along the lines that perspective changes with different focal lengths. Of course, perspective is determined by where you're standing, and not by focal length!
theorderingone 17 2.4k
8 Apr 2007 8:12PM
Carabosse 18 41.5k 270 England
8 Apr 2007 8:13PM
I was trying to keep it simple! Of course there are several variables. Size of the image capture area comes into it too in that it will determine apparent magnification.
Krakman 14 3.6k Scotland
8 Apr 2007 8:17PM
The link you've given is just about what appears acceptable to the eye - because you don't actually crop the wider angle lens to the same field of view as the 200mm lens, you can get away with less depth of field in a wide angle image without it being noticeable.

Here are a series of tests which prove the point. Depth of field does not depend on focal length.
theorderingone 17 2.4k
8 Apr 2007 8:18PM
It does, but only because you'd use a different focal length to acheive the same angle of view.

A 50mm lens will exhibit the same DOF characteristics on LF as it will on a DSLR if distance-to-subject and aperture are kept the same. Only the angle of view will change.

EDIT- and what use is that theory in the real world?

EDIT EDIT - He's changed the distance to subject, not cropped them. Of course the DOF will be very similar, he must've been about 6inches from the thing with the 17mm
User_Removed 17 17.9k 8 Norway
8 Apr 2007 8:20PM
Where that tin 'at?

♪♫ whistlin' off... ♫♪
Carabosse 18 41.5k 270 England
8 Apr 2007 8:22PM
Who will be the first to mention "Circles of Confusion"? Grin







.............oops, it's me! ;o(
strawman 17 22.2k 16 United Kingdom
8 Apr 2007 8:23PM
When I was 10 at school we used to see who could pee up the wall the highest. Things do not change much.

I read threads saying how hostile and unhelpful camera clubs are. you walk in here yup just the same.
theorderingone 17 2.4k
8 Apr 2007 8:24PM
Sorry, must take a step back...

The cash machine ate my card, and I've been stung by a wasp today!

Just waiting for the third thing to get itself out of the way so I can relax in peace for the rest of the day! :-P

P.S. How on earth did you get those musical notes on here?

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