Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
07-05-2016, 06:12 PM   #1
New Member




Join Date: Jun 2015
Posts: 9
DOF on k1 crop mode?

Hi,

I read many news and threads about the crop mode on the full frame k1. I am so excited for this new function. But how is the depth of field on the crop mode?

For example, I have an DA 50 f1.8 lens and shot at f1.8 on the k1 using crop mode, Do I actually get the result of f1.8 or the result of f1.8 * 1.5 = f 2.7 ?

Thanks!

07-05-2016, 06:30 PM   #2
Pentaxian




Join Date: Jan 2011
Location: New York
Posts: 3,217
You get the center part of the 50mm f1.8 image in crop mode. It's just like using the lens on APS-C.

The outer parts of the image are cropped away in crop mode. Those edges are often blurred when you focused on a central subject, and therefore the cropped image might show less out of focus content than a full frame image at the same focal length and aperture.

Last edited by DeadJohn; 07-06-2016 at 05:05 AM.
07-05-2016, 06:51 PM   #3
Site Supporter
BigDave's Avatar

Join Date: Sep 2010
Location: Hudson Valley, NY
Photos: Gallery | Albums
Posts: 1,596
jhl, depth of field does not change with format change, only with f stop and focal length changes*. f 1.8 (or 2 or 4 or 16) is the same on a given lens for APS-C, FF, 645 etc.
*Note: this is for a given camera to subject distance.

Last edited by BigDave; 07-05-2016 at 07:35 PM.
07-05-2016, 07:13 PM - 1 Like   #4
Site Supporter
enoeske's Avatar

Join Date: Jul 2009
Location: Surprise, Az
Photos: Gallery
Posts: 1,916
QuoteOriginally posted by jhlxxx Quote
Hi,

I read many news and threads about the crop mode on the full frame k1. I am so excited for this new function. But how is the depth of field on the crop mode?

For example, I have an DA 50 f1.8 lens and shot at f1.8 on the k1 using crop mode, Do I actually get the result of f1.8 or the result of f1.8 * 1.5 = f 2.7 ?

Thanks!
If you don't change the camera settings or position, but swap between FF and crop mode, then the DOF won't change. You will just have what looks like a crop of the FF image. However, if you frame your subject the same way, the you will be closer in FF mode. You've changed your perspective. This reduced the depth of field.

07-05-2016, 07:43 PM   #5
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 41,847
QuoteOriginally posted by enoeske Quote
If you don't change the camera settings or position, but swap between FF and crop mode, then the DOF won't change. You will just have what looks like a crop of the FF image. However, if you frame your subject the same way, the you will be closer in FF mode. You've changed your perspective. This reduced the depth of field.
Well stated. I think people are often confused by something that's ultimately quite simple

FF alone doesn't give you more DoF.

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com's high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

07-05-2016, 07:49 PM - 1 Like   #6
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,904
QuoteOriginally posted by jhlxxx Quote
But how is the depth of field on the crop mode?
You have opened a huge can of worms.

...and the full answer will go against quite a bit of what you have read regarding equivalence between formats. Part of the answer is that neither the full-frame viewfinder image nor that within the crop area shows the actual DOF of an 8x10 print at about 20" viewing distance (roughly the reference reproduction ratio and standard viewing distance for that ratio). Both show somewhat deeper DOF than the reference with the area within the crop boundaries being somewhat more so. That fact should be kept in mind as you evaluate focus and DOF in both the viewfinder and rear LCD.

The last two sentences above are the working rules. What continues below is for interest purposes only.

There are three factors that determine DOF in the final display image*:
  1. Absolute lens aperture (not the same as relative aperture (aka f/ratio))
  2. Reproduction ratio (or magnification if you prefer that term)
  3. Viewing distance (visual acuity is usually no better than a little less than 1 arc
    minute so distance is critical to assessing OOF blur)
The above three factors are at the core of all DOF calculators and lens scales. Notice that focal length, f/ratio, and format are not in the list. They cancel out in the calculation.

If all of this sounds a bit abstract, simply consider my avatar image (low key B&W image of a rose blossom):



Pretty deep DOF, eh?

Now compare to a larger example at the same viewing distance from the screen:



But of course! The second image is bigger! Yes...higher magnification with aperture and viewing distance kept constant produces less DOF. If you look close enough (say a full resolution crop) DOF becomes preciously shallow and for an analog image eventually approaches zero. For those who are interested, the avatar photo was taken with an 85mm lens at f/2.8 and 80cm focus distance on APS-C. Before you whip out the DOF calculator consider that the photo has been cropped from the original capture.


Steve

* DOF is a perceptual measure made calculable by our understanding of the physics of light and physiology of our eyes.

Last edited by stevebrot; 07-05-2016 at 08:07 PM.
07-05-2016, 08:19 PM   #7
Site Supporter
BigDave's Avatar

Join Date: Sep 2010
Location: Hudson Valley, NY
Photos: Gallery | Albums
Posts: 1,596
The depth of field does not change with the size of the reproduced image, although it is more noticeable. DoF is dependent on the points of "acceptable focus" on the image plane and is measured within the subject, front to back. On the images above, there is no front to back distances as the images are two dimensional.
See the following link for the DoF calculations: Depth of Field Equations
Note there is no parameter for image size, format or reproduction size listed for the equation.
Magnification of the subject on the image plane changes DoF as you are either moving in closer (changing the focus distance) or are using a different focal length. The question was does the FF sensor change the DoF, and for a given lens at a given image plane to subject distance, the DoF for a given plane of focus will be the same for FF and crop sensors at any given f stop.
There are a lot of "givens" in that statement, weren't there!

Regards,
07-05-2016, 08:43 PM - 1 Like   #8
Pentaxian
Not a Number's Avatar

Join Date: Mar 2012
Location: Venice, CA
Posts: 4,112
Depth of Field, Digital Photography and Crop Sensor Cameras - Bob Atkins Photography

07-05-2016, 10:25 PM   #9
New Member




Join Date: Jun 2015
Posts: 9
Original Poster
Detailed explained! Thank you everyone. You are awesome! I really learned something new.
07-06-2016, 02:49 AM - 1 Like   #10
Pentaxian




Join Date: Apr 2007
Location: Romania
Posts: 9,004
Steve is correct IMO, as DoF is based on assumptions regarding the visual acuity and viewing distances. Challenge these assumptions e.g. by printing larger and looking closer, and you'd get different results.
07-06-2016, 08:50 AM - 1 Like   #11
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,904
QuoteOriginally posted by Kunzite Quote
Steve is correct IMO, as DoF is based on assumptions regarding the visual acuity and viewing distances. Challenge these assumptions e.g. by printing larger and looking closer, and you'd get different results.
Thanks for the comment.

QuoteOriginally posted by BigDave Quote
See the following link for the DoF calculations: Depth of Field Equations
Note there is no parameter for image size, format or reproduction size listed for the equation.
Thanks for the link. The circle of confusion term in the initial equation on the linked page is derived from final reproduction ratio to a standard size, viewing distance, and visual acuity. Final reproduction ratio is affected by the capture format.

Most discussions of DOF are concerned with the description from subject to focal plane. That is most appropriate because we are, after all, discussing a range of distance in front of and behind the subject. In addition, the diagrams are easily understood and allow the reader (and author) the ability to skip page after page of equations decribing DOF for many different cases.* The part that is usually not discussed is what constitutes acceptable blur, the so-called circle of confusion (CoC). CoC is usually given as a constant for a particular format, though the actual derivation is a matter of convention.** At various times, there have been different schools of thought (e.g. Leitz vs. Zeiss vs. Kodak vs. GOST) where different values have been used. That provides one explanation why the distance scales on lenses from different countries, makers, or era don't agree. CoC as it relates to photography and the pertinent considerations are well treated in the Wikipedia article on the subject. There is also a nice discussion of the history of the concept.

https://en.wikipedia.org/wiki/Circle_of_confusion#Circle_of_confusion_diamet...in_photography

To summarize from the article, CoC is derived from: 1) visual acuity, 2) viewing distance, 3) enlargement from the original image.


Steve

* How DOF is calculated varies by subject distance, lens design, and image magnification. Fortunately we almost never need to calculate DOF! DOF Formulae and their derivations

** Embarrassingly, the convention is simply d/1500 where d = format diagonal in mm. I wonder what rule applies to panos!

Last edited by stevebrot; 07-06-2016 at 09:03 AM.
07-06-2016, 11:42 AM   #12
Pentaxian




Join Date: Dec 2010
Location: Ontario
Photos: Gallery
Posts: 2,435
Great and thorough explanation by Steve.

I just want to add that there's nothing special about the relationship from FF down to APS-C. If you want a hands on practical demonstration of what happens to DoF or anything else when you crop, you can go ahead and use your APS-C dslr and do an analogous 1.5x crop of your photos using your favorite software. Try printing cropped and uncropped at the same size, try changing distances to keep the cropped and uncropped framing the same, etc.

Of course you are better off getting the K-1 and doing your tests with its built in crop mode (because then you'd have a K-1) but you can work with what you have.
07-11-2016, 01:22 AM   #13
Forum Member




Join Date: Nov 2015
Location: British ExPat in Bucharest
Posts: 84
QuoteOriginally posted by jhlxxx Quote
For example, I have an DA 50 f1.8 lens and shot at f1.8 on the k1 using crop mode, Do I actually get the result of f1.8 or the result of f1.8 * 1.5 = f 2.7 ?
both yes and no sort of you actually still get the same depth of field as for a f1.8 lens *but* with a cropped sensor body you would need to move backwards to frame the shot to match what you see in FF mode. moving backwards would increase the depth of field.

this video from tony northrop demonstrates it
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
24x36mm, crop, dof on k1, f1.8, full-frame, k1, k1 crop, mode, pentax, result
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
FF mode - DA15 - crop factors & crop sizes acoufap Pentax K-1 6 05-09-2016 05:52 PM
K1 autofocus with aps-c lenses in crop mode slip Pentax K-1 1 05-09-2016 04:51 PM
What will the crop mode be like on the FF? mikeodial Pentax Full Frame 19 05-08-2015 08:59 PM
DOF and Crop Sensors edl Pentax SLR Lens Discussion 16 11-28-2009 01:11 PM
DOF for 6 x 7 on Crop Sensor Jewelltrail Pentax Medium Format 46 04-12-2009 01:29 PM



All times are GMT -7. The time now is 09:52 AM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top