Author Topic: Depth Of Field on telephoto  (Read 1405 times)

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wcfn100

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Re: Depth Of Field on telephoto
« Reply #30 on: November 14, 2018, 11:35:16 AM »
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It is confusing me that some posters keep writing that use of a telephoto increases depth-of-field.  The way I use telephotos, they decrease depth-of-field, at least for a given image size of a particular subject.

But you're talking about subjects that are many feet from the camera and a depth-of-field going into infinity.  In our case you need to think more about how a telephoto lens allows us to move farther back from an object which will effectively increase DOF for our purposes.

Jason
« Last Edit: November 14, 2018, 11:37:04 AM by wcfn100 »

Maletrain

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Re: Depth Of Field on telephoto
« Reply #31 on: November 14, 2018, 01:42:24 PM »
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Jason,

I am not talking about cases where the depth-of-field goes to infinity.  The effect is in the range of the depth of the field that is in acceptable focus. Sure, if you get farther away, the actual number of feet of depth that are in acceptable focus will be greater for lenses of any focal length.  But, the images will also be smaller. Look at the sketch DKS posted.  If you crop the image taken with the longer shorter focal length to get the same size image of the subject, you get more in the background.  That means that the background items look smaller, and thus their out-of-focusedness  :facepalm: is less apparent in the image.  So, I am still thinking that the wider angle lens gives more depth of field for the same size image of the main subject.

Rick,

The field of depth can be controlled to some extent by closing the aperture, but there are limitations.  Besides the image getting dimmer, there is also the diffraction effect that will eventually cause blurring.  So, most lenses are made to have minimum apertures that do not get very much into the diffraction range. Trying to stop-down a telephoto lens to get the same depth of field as a wide angle lens tends to make for a very dim photo and more easily detected diffraction effect.

Other solutions are becoming available:

1. "Photo stacking" software that combines several images taken from the same vantage point so that what is out-of-focus in one image is replaced by what is in-focus in the same area from another image.

2. Those tiny cell phone cameras, which are more like scale cameras when they are put into the scene to take the photograph.  They have approximately "normal" focal length for their tiny sensor size, and the newest sensors have amazing numbers of pixels, rivaling the good 35 mm SLR cameras.  So, taking a picture with one of those cell phones can result in what looks like the same perspective that a 1:1 camera would take of the prototype from the same position, with similar depth of field.  The only problem is that the cell phone that has that camera in it is huge in N scale, so actually getting it into the "right" spot on a layout can be tricky.  I hope some day those tiny cameras are made available separately, perhaps with a cord that attaches them to a cell phone for use in composing the pictures.  The technology is already here, and has already been used in satellites and terrestrial spyware.  We just need it to get into the hobby environment at a reasonable price.  There are already cheap endoscope type cameras that can be used with cell phones to look into holes, but those are designed to view objects extremely close, without much thought to perspective.
« Last Edit: November 15, 2018, 03:18:22 PM by Maletrain »

Mark W

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Re: Depth Of Field on telephoto
« Reply #32 on: November 14, 2018, 01:59:21 PM »
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Here you go:

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peteski

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Re: Depth Of Field on telephoto
« Reply #33 on: November 14, 2018, 03:39:57 PM »
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DKS, as usual you provided a great explanation of the distorted perspective. Like Tom, it took me a bit to figure out the drawing, but once I understood it, the explanation made perfect sense.
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tlaswell

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Re: Depth Of Field on telephoto
« Reply #34 on: November 14, 2018, 04:39:42 PM »
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Thank you.  I think I might have to reread all this a time or two before it sinks in.  It looks like I should stick with a regular lens or even a Cell Phone and just get as close as the focus will let me, shrink down the aperture, use a tripod and let the camera adjust the shutter for the supplied light. 

I noticed in one of the other forum areas, weekend update 11/11/18 ( https://www.therailwire.net/forum/index.php?topic=45773.msg594000#msg594000 ) it doesn't hurt to crop out the fuzzy parts of the picture if the focused section is what you need.

Lemosteam

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Re: Depth Of Field on telephoto
« Reply #35 on: November 15, 2018, 10:08:59 AM »
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This is why those photos looking down railroad tracks look like roller coasters. Depth and perspective are reduced, making gradual grade changes look drastic.

NOW I get it!

robert3985

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Re: Depth Of Field on telephoto
« Reply #36 on: November 20, 2018, 11:58:57 PM »
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But you're talking about subjects that are many feet from the camera and a depth-of-field going into infinity.  In our case you need to think more about how a telephoto lens allows us to move farther back from an object which will effectively increase DOF for our purposes.

Jason

I'm sorry to disagree, but in the world of pro photography, wide angle lenses INCREASE DOF and telephoto lenses DECREASE DOF.  There is no way in my train room that I can move far enough back with my 105mm f2.5 lens set at f16 to have the same DOF as my 16mm f2.8 set three inches from my subject at f16.

Additionally, the main subject would be so small in the telephoto shot that cropping down to it would reveal it to be totally unusable because of noise and pixelization, not to mention a total lack of resolution.

However, in the studio or on location, I can move far enough back with my 180 f2.8 to include my model's entire body, with much less distortion because the telephoto flattens the image, and throws much of the background out of focus to isolate the subject.  With my 11-16mm Tokina ultra-wide zoom set at f2.8, I can include her entire body from (I'm guessing) about ten feet or so...with plenty of distortion and also with the background mostly still in focus.   I use the ultra-wide to get closeups in my figure photography that emphasize a body's curves and forms, has excellent DOF even at maximum apertures and will distort the forms a LOT for artistic purposes. 

Likewise for model railroad photography, I sometimes want to use the distortion of converging perspective lines to emphasize distance or closeness often at the same time, which my wide-angle lenses provide to differing degrees.  With my ultra-wide 11-16mm f2.8 Tokina zoom, it also increases DOF to the point that everything appears in-focus from a couple of inches away from the surface of my objective lens to over 24 feet away...stopped down to f22, with no discernible diffraction effects.

Photo (1) - Here's the setup with my Nikon D7200 and my Tokina 11-16mm f2.8:


Photo (2) - Here's the Photoshopped result, the pink Echo Cliffs being about 22 feet away, still in focus with this single exposure/no focus stacking shot:


On the other end of the lens spectrum, a telephoto used with the same f stop will have a very narrow DOF focused close...or even semi-close.

Photo (3) - This photo illustrates the narrow DOF a tele gives you, which in this photo is about 18" with critical focus being on the smokebox door.  Nikkor 18-200mm zoom...200mm @ f22, which is minimum aperture for this lens:


So, no...telephoto lenses do NOT increase DOF with respect to equal apertures and distances compared to wide and ultra-wide angle lenses as these photos graphically illustrate.

Cheerio!
Bob Gilmore
« Last Edit: November 21, 2018, 12:08:05 AM by robert3985 »

wcfn100

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Re: Depth Of Field on telephoto
« Reply #37 on: November 21, 2018, 02:56:37 AM »
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So, no...telephoto lenses do NOT increase DOF with respect to equal apertures and distances compared to wide and ultra-wide angle lenses as these photos graphically illustrate.


If only that was what we were talking about.

Using zoom to keep more of a scene in focus is a tried and true tradition of model railroaders and it works for that specific goal.  There are several members, past and present, who use that technique regularly for the images they post here.  You can always tell by how flat the perspective is.


Jason

robert3985

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Re: Depth Of Field on telephoto
« Reply #38 on: November 21, 2018, 11:44:02 AM »
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If only that was what we were talking about.

Using zoom to keep more of a scene in focus is a tried and true tradition of model railroaders and it works for that specific goal.  There are several members, past and present, who use that technique regularly for the images they post here.  You can always tell by how flat the perspective is.


Jason

DOF of telephoto lenses is what the OP is talking about, since this thread is titled "Depth Of Field on telephoto".  What are YOU talking about then?

I am fully aware of what model railroad photography is, since I have been doing it for over 30 years.  Just because several members use a photographic technique regularly has no bearing on whether that technique is good, or bad...nor does it change the fact that telephoto lenses are used to isolate subjects and have a more shallow DOF than wide angle lenses.  Telephotos do not keep more of a scene in focus, they keep less of a scene in focus.

I can post more photos that graphically illustrate the facts.  However, I'm late for breakfast! :)

Cheerio!
Bob Gilmore