DOF, Crop and Bokeh (Oh My!)

[Radtech1]

It feels like I should know this by now, but I am stumped on an optics question. I was contemplating the manner in which the crop factor of a 1.6 imaging chip does or does not affect the DOF and bokeh.

I have read lots of anecdotals that indicate that a lens performs differently - in terms of DOF and bokeh - on cameras with different chip sizes. Specifically, I seem to remember reading that a lens on a smaller chipped camera (10d) will have a greater depth of focus than on a full frame chipped camera (1ds I and II) That can't be true, can it?

When I think of the lens as "projecting" an image - like a movie projector - on to a screen (the chip). It seems intuitively obvious that the DOF and Bokeh will remain unchanged - regardless of whether or not the screen (the chip) is large enough to accommodate the entire projected image (ff) or just the center (1.6)

Am I missing something here?

Rad

[Duder]

I'm no expert, but I presume that all lenses perform the same no matter what camera they are on, although using a 1.6 crop factor camera will require you to be in a different position (further back) to frame the same composition than with an full frame camera, therefore altering the DOF/bokeh.

[CyberDyneSystems]

http://photography-on-the.net/forum/showthread.php?t=45388


More specifically...

http://photography-on-the.net/forum/showthread.php?t=50556
http://photography-on-the.net/forum/showthread.php?t=66472&page=1&pp=20

I'll check back in when this hits page five... thank a lot Rad :rolleyes: :lol: :lol: :lol:

[ron chappel]

You've got one part absolutally right Rad.
If you are taking a picture from the same distance the DOF will indeed be identical no matter what format is being used.

However........If one want's to do any kind of usefull real life comparison ,one must use the same angle of view to compare formats.It's when this happens that things radically change.

So.
Using the same lens on both formats....
Because you have to move further away with 1.6 crop cameras to fit in the same scene as full frame.....this has the obvious result of moving the focus distance on the lens closer to infinity.
Now - remember how it's quite easy to get everything in focus near infinity ? But near macro distances it's the opposite-it's very difficult to get much DOF ?

So shifting the lens' focus distance closer to infinity will obviously have more of the scene in focus-more DOF

[tannoy]

I have often wondered how this works myself. I recently upgraded my 20D to a 1 DMk2 and low and behold I noticed less apparent depth of field for the same aperature than I had before, at first I thought I was nuts but at F4 on my 17-40 there really was a change in OOF areas. It did not notice a change in quality of Bokeh, just less depth of field.

If this is simply related to crop factor (or what ever you guys are calling it these days) I would love to read a good scientific explanation as to why. Good news is I need that 16-35L less now than I did with my 20D....or so it seems.

Cheers,

Darrin

[slin100]

DOF is affected by both format size and distance. Distance has a bigger effect but as Darrin (tannoy) noticed format size cannot be ignored.

This is the simplest explanation I've found of DOF.
http://www.photo.net/learn/optics/dofdigital/index.html

[rent]

the DOF & x-factor relationship has been debated heavily in the past; personally i think steven's link above establishes the answer(s) in simple and easy to understand terms.

but now... how about the x-factor and BOKEH?!

i think the shape of the bokeh would not be affected by the x-factor. the "bluriness" of the bokeh, however, should follow the same principle as DOF.

i shall be back when page count reaches double digits. :)

-alex

[tannoy]

Great article Steven, that does explain it nicely and simply.

As far as Bokeh, I for one do not see any difference in the quality on any of my lenses vs. the same on the 20D, but as Alex(Rent) pointed out, there is more OOF so the bokeh is more apparent (1.3 vs. 1.6 in my case). I would surmise that Bokeh quality becomes more important as crop factor goes down, due the apparent increase in OOF areas on the image for the same aperature on a given lens.

Cheers,

Darrin

[Curtis N]

It seems intuitively obvious that the DOF and Bokeh will remain unchanged - regardless of whether or not the screen (the chip) is large enough to accommodate the entire projected image (ff) or just the center (1.6)Correct, if you're using the same lens, zoomed to the same focal length, with the same aperture, the same distance from the subject, and the print you make from 35mm film is 1.6x the size of the comparative digital print. If you change any of these factors, it gets more complicated.

[Tom W]

This is the simplest explanation I've found of DOF.
http://www.photo.net/learn/optics/dofdigital/index.html

Good link - Bob Atkins has a great way of explaining things.