Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
09-17-2007, 03:35 PM   #1
Veteran Member
pcarfan's Avatar

Join Date: Sep 2007
Location: Dayton, Ohio
Photos: Gallery | Albums
Posts: 2,958
Zoom in to blur the bg...WRONG???

This luminous landscape article caught my attention as I was reading about the influnce of focal length to DOF. I need a fast lens with a decent DOF to take pictures of fighter aircraft in a low lit museum setting.

This is the article that claims the DOF to be not affected by the focal length ?

DOF2

It also has some interesting consequences to motion capture as it seems the AF-C may do a better job with a longer telephoto lens.

I thought I'll throw it out here for opinions.......

09-17-2007, 04:31 PM   #2
mph
Junior Member
mph's Avatar

Join Date: Jul 2007
Location: Johnstown, PA, USA
Posts: 44
The LL article is correct. But your title for this thread is wrong. "What is the depth of field (the region of sharp focus)?" and "How blurred is the distant background?" are two different questions.

The article's assertion, that the DOF is the same for any focal length, assuming the subject magnification and f-stop are the same, is correct. Imagine you're photographing your subject with a 100mm lens at f/8. You get a certain depth of field; in this case, say the subject is sharp over a region of 1 foot, front to back. You switch to a 50mm lens. Your experience tells you that the shorter lens has more DOF, and that's true in a sense. But to photograph your subject at the same size, you have to move in to half the distance, compared to when you were using the 100mm lens. Shortening the camera-to-subject distance reduces the DOF, and exactly offsets the DOF you gained by switching lenses. Once you've switched lenses and moved closer, there's still a 1-foot-deep region of the subject that's sharp at f/8.

But the distant background is outside the DOF either way. And the 50mm lens "takes in" more background than the 100mm, so the background is more cluttered than with the 100mm. This leads to the subject being less isolated from the background, but that's not the same as saying the DOF is larger. You can see this in the article's shots. The longer focal lengths clearly have less clutter in the background than the shorter focal lengths.
09-17-2007, 04:53 PM   #3
Veteran Member
jfdavis58's Avatar

Join Date: Nov 2006
Location: 13 S 0357397-3884316
Photos: Gallery
Posts: 876
Opinions? Pretty hard to argue with mother nature; i.e. physics. What kind of opinions are you seeking?

QuoteOriginally posted by pcarfan Quote
This luminous landscape article caught my attention as I was reading about the influnce of focal length to DOF. I need a fast lens with a decent DOF to take pictures of fighter aircraft in a low lit museum setting.

This is the article that claims the DOF to be not affected by the focal length ?

DOF2

It also has some interesting consequences to motion capture as it seems the AF-C may do a better job with a longer telephoto lens.

I thought I'll throw it out here for opinions.......
09-17-2007, 05:42 PM   #4
Senior Member
hinckc's Avatar

Join Date: Apr 2007
Location: New Jersey, USA
Photos: Gallery
Posts: 298
Here's a site that has helped me to understand how DOF relates to focal length.

Using the online calculator found there, it's clear there are really 4 pieces of info you need to calculate DOF, sensor size, focal length, aperture, and lastly subject distance.

I think that the LL article kind of takes distance to subject as an assumption, because it assumes that regardless of focal length, you always want to keep your subject at the same size. This may or may not be true. Plus, you can't always backup far enough (especially indoors) to get everything you want in frame, at a given focal length.

To relate that to your plans to shoot the indoor museum, your "faster" lens (bigger aperture) will have a more narrow DOF than a "slower" one (smaller aperture). To maximize your DOF, you'd be better off using a smaller aperture, combined with a tripod. But many museums don't allow tripods, so if the one you're going to doesn't, you'll have to compromise.

Try punching the focal lengths of the lenses you have, along with some guesses about how far away from the plane you're realistically likely to get in the museum, into the calculator. If you try some shots in your house, you should be able to get a rough idea of what aperture you need to shoot indoors at the museum. Those 3 pieces of info, plus your camera, should tell you what kind of DOF to expect.

-Chris

09-17-2007, 05:44 PM   #5
Veteran Member
pcarfan's Avatar

Join Date: Sep 2007
Location: Dayton, Ohio
Photos: Gallery | Albums
Posts: 2,958
Original Poster
QuoteOriginally posted by mph Quote
The LL article is correct. But your title for this thread is wrong. "What is the depth of field (the region of sharp focus)?" and "How blurred is the distant background?" are two different questions.

The article's assertion, that the DOF is the same for any focal length, assuming the subject magnification and f-stop are the same, is correct. Imagine you're photographing your subject with a 100mm lens at f/8. You get a certain depth of field; in this case, say the subject is sharp over a region of 1 foot, front to back. You switch to a 50mm lens. Your experience tells you that the shorter lens has more DOF, and that's true in a sense. But to photograph your subject at the same size, you have to move in to half the distance, compared to when you were using the 100mm lens. Shortening the camera-to-subject distance reduces the DOF, and exactly offsets the DOF you gained by switching lenses. Once you've switched lenses and moved closer, there's still a 1-foot-deep region of the subject that's sharp at f/8.

But the distant background is outside the DOF either way. And the 50mm lens "takes in" more background than the 100mm, so the background is more cluttered than with the 100mm. This leads to the subject being less isolated from the background, but that's not the same as saying the DOF is larger. You can see this in the article's shots. The longer focal lengths clearly have less clutter in the background than the shorter focal lengths.
Well thanks for the explanation. I thought zooming in increses the focal length and therefore changes the DOF irrelevent of the area in view. So, for instance if I wanted to take a picture of a flower and want the flower to fill half of the view, I thought I can either shoot it at a given distance with a wide angle lens OR move back and then zoom-in to get the same field of view and the BG be more blurred.

So, what I meant in my zooming in is, actually moving back and zooming in to get the bg blurred but keeping the field of view constant.

So, why do portrait proffesionals use telephoto lens for their work. i thought a telephot zoom zoomed into a given field of view has a much narrower DOF as compared to a wideangle lens with the same field of view. Obviously I was wrong, right ?
09-17-2007, 05:56 PM   #6
Veteran Member
pcarfan's Avatar

Join Date: Sep 2007
Location: Dayton, Ohio
Photos: Gallery | Albums
Posts: 2,958
Original Poster
QuoteOriginally posted by hinckc Quote
Here's a site that has helped me to understand how DOF relates to focal length.

Using the online calculator found there, it's clear there are really 4 pieces of info you need to calculate DOF, sensor size, focal length, aperture, and lastly subject distance.

I think that the LL article kind of takes distance to subject as an assumption, because it assumes that regardless of focal length, you always want to keep your subject at the same size. This may or may not be true. Plus, you can't always backup far enough (especially indoors) to get everything you want in frame, at a given focal length.

To relate that to your plans to shoot the indoor museum, your "faster" lens (bigger aperture) will have a more narrow DOF than a "slower" one (smaller aperture). To maximize your DOF, you'd be better off using a smaller aperture, combined with a tripod. But many museums don't allow tripods, so if the one you're going to doesn't, you'll have to compromise.

Try punching the focal lengths of the lenses you have, along with some guesses about how far away from the plane you're realistically likely to get in the museum, into the calculator. If you try some shots in your house, you should be able to get a rough idea of what aperture you need to shoot indoors at the museum. Those 3 pieces of info, plus your camera, should tell you what kind of DOF to expect.

-Chris
I used the calculator to find what the DOF is for these two examples of different focal length but the same field of view.

1) Focal length of 50 mm with the subject distance of 10 feet



2) Now, I kept the camera and the aperture the same, changed the zoom focal length to 100mm but moved it back to 20 feet so the field of view is the same

(the links did not work, but the DOF was different)


Why is the DOF different ?
09-17-2007, 05:58 PM   #7
mph
Junior Member
mph's Avatar

Join Date: Jul 2007
Location: Johnstown, PA, USA
Posts: 44
QuoteOriginally posted by pcarfan Quote
So, why do portrait proffesionals use telephoto lens for their work. i thought a telephot zoom zoomed into a given field of view has a much narrower DOF as compared to a wideangle lens with the same field of view. Obviously I was wrong, right ?
You're wrong, in that they will have the same DOF. But you're right in that the telephoto will give a more blurred background. I think you're continuing to equate DOF with the degree of background blur. As I tried to make clear, these are not the same thing.

The DOF tells you how much of the subject is in acceptable focus. It does not tell you about the appearance of the out-of-focus background.
09-17-2007, 06:17 PM   #8
Veteran Member
pcarfan's Avatar

Join Date: Sep 2007
Location: Dayton, Ohio
Photos: Gallery | Albums
Posts: 2,958
Original Poster
QuoteOriginally posted by mph Quote
You're wrong, in that they will have the same DOF. But you're right in that the telephoto will give a more blurred background. I think you're continuing to equate DOF with the degree of background blur. As I tried to make clear, these are not the same thing.

The DOF tells you how much of the subject is in acceptable focus. It does not tell you about the appearance of the out-of-focus background.

In your first response you did mention the clutter of the background and how it's different in both circumstances, and I noticed it when I read the article, and infact the article eludes to the fact that they are different. I understand that. At least I think I do

But, my question is,.... is the DOF (I know it's different from the degree of blurriness of the background) the same for different focal lengths if the field of view is kept the same?

I had done the calculations given in the link by the other poster even before I posted this thread. As you say in your first response should'nt the DOF (not bg clutter) be the same when I punch in 50mm at 10 feet VS 100mm at 20 feet ? Why is it different ?

09-17-2007, 06:19 PM   #9
mph
Junior Member
mph's Avatar

Join Date: Jul 2007
Location: Johnstown, PA, USA
Posts: 44
QuoteOriginally posted by pcarfan Quote
Why is the DOF different ?
The simple equations assume that the DOF is "thin" compared to the camera-to-subject distance. If you're 10 feet away, and the DOF is 8 feet, the equations break down. Try repeating your experiment with a fast aperture like f/2, and you'll see the DOF is the same, or very close, in the situations you tried.
09-17-2007, 06:29 PM   #10
Veteran Member
pcarfan's Avatar

Join Date: Sep 2007
Location: Dayton, Ohio
Photos: Gallery | Albums
Posts: 2,958
Original Poster
QuoteOriginally posted by mph Quote
The simple equations assume that the DOF is "thin" compared to the camera-to-subject distance. If you're 10 feet away, and the DOF is 8 feet, the equations break down. Try repeating your experiment with a fast aperture like f/2, and you'll see the DOF is the same, or very close, in the situations you tried.

Whew!, now it makes sense. I was wondering why the wikipedia on DOF has different calculations based on 'moderate to large distance' vs 'close-up' etc.......It changes indeed.

Thanks for your help.
09-17-2007, 06:31 PM   #11
mph
Junior Member
mph's Avatar

Join Date: Jul 2007
Location: Johnstown, PA, USA
Posts: 44
I think we've moved past this point, but I just spent a few minutes making a drawing to illustrate the difference between DOF and background clutter, and gosh darn it, I'm going to use it!

Our subject is Mr. Blue Circle. First we photograph him with a 100mm lens, shown by the black lines. Then we switch to the 50mm lens, and move to half the distance, shown by the red lines.

If we shoot with the same aperture in both cases, the DOF will be the same, which I've illustrated with the green bar alongside the subject. Everything within the bar will be in acceptable focus. (As mentioned previously, this is an approximation and isn't quite true when the DOF is "thick" compared to the subject distance. I probably should have put the cameras further away from Mr. Blue Circle.)

Note that even though the DOF is the same, the 50mm lens includes more trees and other junk in the background, making the background more cluttered.

I think the notion that "shallower DOF = more background blur" comes from the way we learn about DOF. We think of putting the camera on a tripod, mounting a lens, and stopping down that same lens from f/2, f/2.8, f/4, all the way to f/32. The DOF increases, and at the same time the background gets less blurry. We associate these two effects in our head. But switching lenses and shooting at the same aperture isn't the same as keeping the same lens on the camera and stopping it down. When you switch lenses, you can end up with the same DOF and different blur. Or even shallower DOF and less blur, or vice-versa.
Attached Images
 

Last edited by mph; 09-17-2007 at 06:34 PM. Reason: Typo... wrote "more", meant "less".
09-17-2007, 06:49 PM   #12
Veteran Member
pcarfan's Avatar

Join Date: Sep 2007
Location: Dayton, Ohio
Photos: Gallery | Albums
Posts: 2,958
Original Poster
That is one heck of an illustration. Bravo! I hope it will help others who may wander into this discussion from a web search as it did for me.
09-17-2007, 09:17 PM   #13
Veteran Member
WMBP's Avatar

Join Date: Dec 2006
Location: Dallas, Texas
Photos: Gallery
Posts: 2,496
QuoteOriginally posted by pcarfan Quote
So, why do portrait proffesionals use telephoto lens for their work. i thought a telephot zoom zoomed into a given field of view has a much narrower DOF as compared to a wideangle lens with the same field of view. Obviously I was wrong, right ?

Yeah, but there's a lot more to portrait photography than depth of field!

The distance you are from the subject makes a huge difference to the perspectival relationships of the elements of the photo. When you stand on the front porch of a house, you can't see the driveway on the side; when you view the house from across the street, you can see the driveway on one side and the tree on the otherside, and things seem to relate to each other normally. In the same way, if you shove a camera in someone's face, well, it distorts the way the parts of the face will appear. In particular, noses get bigger. Step back a bit, and things seem to get into their proper proportion -- indeed, if someone does have big ears, a big nose, an extra big chin, then shooting from 15 ft away with a focal length of 100mm may flatter the subject or at least not accentuate the negative. Memorable example: You're a glamor photographer shooting a beautiful model and you want her legs to look extra long. What do you do? Get down on the floor and shoot up. It's not the shooting up that matters here -- it's the fact that her feet are closer to the camera. You get the same effect if the model lies on a sofa with her legs stretched out toward the camera. Alternative example: Jay Leno hires you to take a shot of him that accentuates his chin. Do you shoot from twenty feet away with a telephoto lens? No, you shove the camera in his face and shoot at 20mm.

I've enclosed two simple shots that illustrate the point. Mr Snowman is a bit sensitive about the size of his nose. He hates the shot made at 28mm, where I shoved the camera in his face. He much prefers the shot made from over twice the distance, at 75mm.

Other shots in the same series and illustrating depth of field can be seen here.

Will
Attached Images
View Picture EXIF
PENTAX *ist DS  Photo 
View Picture EXIF
PENTAX *ist DS  Photo 
09-18-2007, 04:50 AM   #14
Veteran Member




Join Date: Dec 2006
Location: Outside of Philly
Posts: 1,564
Cute snowman, Will! I'm sure he'd REALLY hate his nose at 10mm
09-18-2007, 05:32 AM   #15
Veteran Member




Join Date: Mar 2007
Location: Boston, PRofMA
Photos: Albums
Posts: 3,053
LOL. As all the male readers hurry to their bathrooms w/ a tripod and camera set to self-timer w/ a wide angle lens :-)
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!
article, camera, dof, dslr, length, lens, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Shutter Blur: K7/K5? skyoftexas Pentax DSLR Discussion 35 03-25-2011 03:07 PM
Macro Blur of Wings eaglem Post Your Photos! 2 08-24-2010 07:43 AM
Macro Wings a Blur eaglem Post Your Photos! 1 08-17-2010 01:12 AM
Macro lilies-a-blur shooz Post Your Photos! 1 05-19-2010 07:21 PM
Blur possible at 1/3000th? boom Troubleshooting and Beginner Help 9 05-05-2009 02:18 PM



All times are GMT -7. The time now is 02:37 PM. | See also: NikonForums.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