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Focal length vs. the digital 1.5 multiplier
- Thread starter jcshoots
- Start date
DavidMillier
Forum Pro
Karl,
Thank you for the excellent explanation of FOV, DOF and Magnification. I think I follow your argument and I fully agree with you. What I need to understand more is the concept of DOF.
For images taken with the same lens at the same distance to the object:
1. The FOV is different between film and 1.5X digital. This is the source of the "equivalent focal length" effect.
2. The DOF is the same because the size of the OPTICAL image is the same and the true optical focal length is the same.
Please note that I am not a professional photographer. I am not trying to debate, I just want to learn more. I must say that I have gained a lot from your enlightening explanation so far.
Thanks,
Shii
Thank you for the excellent explanation of FOV, DOF and Magnification. I think I follow your argument and I fully agree with you. What I need to understand more is the concept of DOF.
Agreed
Here is what I do not understand. (I might have misinterpreted your reasoning). I thought DOF is an optical phenomenon and should have nothing to do with the size of the print (post-processing). For the same lens with the images taken at the same distance from the object, the DOF as measured on the OPTICAL image should be the same regardless of the multiplication factor. (ie. Whether film or the 1.5 X digital we are talking about.) Yes, the 35 mm image is bigger but the size of the object is the same. The 35 mm picture just has more of the surroundings. I have read the excellent article you recommended at http://www.luminous-landscape.com/dof2.htm . The author is moving the camera to get the same size image of object on the film and hence the conclusion that DOF is independent of focal length FOR THE SAME SIZE OPTICAL IMAGE OF THE OBJECT (Not blowing up the image AFTER the picture is taken). In our discussion, this is not the case. The lens and distance to the object are not changed. We are only changing the multiplication factor. (film vs 1.5X digital). This factor only affects the FOV, not the size of the optical image of the object. Therefore, is it correct for me to summarize the following?
For images taken with the same lens at the same distance to the object:
1. The FOV is different between film and 1.5X digital. This is the source of the "equivalent focal length" effect.
2. The DOF is the same because the size of the OPTICAL image is the same and the true optical focal length is the same.
Please note that I am not a professional photographer. I am not trying to debate, I just want to learn more. I must say that I have gained a lot from your enlightening explanation so far.
Thanks,
Shii
Karl Guttag
Senior Member
This is a COMMON misconception. The DoF tables most 35mm SLR users see are based on the assumption that the whole FoV from a 35mm frame will be blown up to about 8x12 and viewed closely by a critical sharp eyed person (they run studies to “create” this mythical person).
First DoF is NOT the range of being “in focus” as there is only exactly one distance (with a simple lens) that is in focus. DoF is really the RANGE of ACCEPTABLE focus.
IF you go back to the DoF equation (and not a simplified table), you will see a parameter called the Circle of Confusion or CoC. The other factors are the Focal Length, F-number, and Focus (often called Subject) distance which are all optical FACTS and are totally objective. But the CoC is a SUBJECTIVE number based on a lot of factors. Simply put, the CoC is based on how much the image will get magnified and how a viewer will perceive the sharpness. The CoC sort of “summarizes” several physical and human perception effects into a single number. If you change the CoC, the DoF will change but nothing changed about the Facts of the lens or the physical image generated by the lens.
The following site has a brief explanation of DoF and the equation.
http://www.dof.pcraft.com/dof-frames.cgi
Better yet the, site below as a DoF calculator that you can download for free, and has a help file with more detailed explainations.
http://www.tangentsoft.net/
The equation starts with something called hyperfocal distance “h.” The hyper focal distance is the nearest point you set the focus at were, everything beyond that will be in-focus (and everything half way to the camera will also be in focus).
h=f* 2 / a*c
were f 2 is the focal length square, a is the F-number, and c is the CoC
near acceptable focus = Dn = h*s / (h + (s-f))
far acceptable focus = Df = h*s / (h – (s-f))
DoF = Df - Dn
where s = focus distance
Now here is where “lying” about a smaller imager “changing” the focal length of a lens catches up somebody. You have 4 numbers to feed into the equation, Focal Length (f), F-number (a), Focus distance (s), and CoC (c). Which number do you change from using a 35mm film camera? A clue, it is NOT the focal length. The answer is ONLY the CoC. The CoC is the only thing that changes when you change the size of the original image.
Actually the CoC should be changed if the OUTPUT size is not 8x12 and viewed critically as well. A 4x6 print does not have to be as sharp on the image plane to be viewed as sharp as an 8x12 output. This is commonly ignored to keep things simple.
Now in the end you don’t want to be pulling out a calculator every time you take a photograph. But unless you have great eyesight, and bright subject and a magnifier on your viewfinder, you can’t really tell the DoF even with a DoF preview button. Thus it is good to develop a basic understanding if you want to keep everything you want in focus.
Karl
--Karl
E
Eugene Powers
Guest
E
Eugene Powers
Guest
Karl,
Let's take a look at something different than just theory.
Let's take a picture of the ruler.
Let's say it is 12 inches and we sharp focus in the middle abd get DOF from 3 to 9 inches.
And this will be done with film camera.
In the film print we will see full 12 inches with above parameters.
With D1 we will see same ruler cropped at about 2 and 10 inches
but with the same DOF between 3 and 9 inches.
So, how does DOF changes again?
Eugene
Let's take a look at something different than just theory.
Let's take a picture of the ruler.
Let's say it is 12 inches and we sharp focus in the middle abd get DOF from 3 to 9 inches.
And this will be done with film camera.
In the film print we will see full 12 inches with above parameters.
With D1 we will see same ruler cropped at about 2 and 10 inches
but with the same DOF between 3 and 9 inches.
So, how does DOF changes again?
Eugene
E
Eugene Powers
Guest
E
Eugene Powers
Guest
Of course it does. What I meant is that 60mm lens will behave as 90mm (smaller FoV or cropping, pick your term here) but perspective stays the same as long as you don't step back.
If you step back to frame in full just like what you see through 60mm lens on 35mm film camera you will change perspective and "loose" (it is a bad term ) the amount of pixels covering whatever you covered before.
That is why if you have full size sensor but same amount of pixels as in D1x,
let's say, you will not get any better picture if you will take it from the same
distance as with D1x right now. Your picture will be bigger in size of course
( more surrounding area in the frame) but not better. It is all in the amount of pixels falling on your subject.
On the other hand if you take picture of the group of people and use the same 60mm lens you would not have to step back to frame all of them
if you had full size CCD. But your picture will be the same as with D1x right now since the amount of pixels is the same.
Hope this helps.
Eugene
If you step back to frame in full just like what you see through 60mm lens on 35mm film camera you will change perspective and "loose" (it is a bad term ) the amount of pixels covering whatever you covered before.
That is why if you have full size sensor but same amount of pixels as in D1x,
let's say, you will not get any better picture if you will take it from the same
distance as with D1x right now. Your picture will be bigger in size of course
( more surrounding area in the frame) but not better. It is all in the amount of pixels falling on your subject.
On the other hand if you take picture of the group of people and use the same 60mm lens you would not have to step back to frame all of them
if you had full size CCD. But your picture will be the same as with D1x right now since the amount of pixels is the same.
Hope this helps.
Eugene
DavidMillier
Forum Pro
Eugene
I know I shouldn't say this as it's irrelevant to the discussion and everyone makes typos but your continuous mis-spelling of "lose" keeps distracting me! Loose means "not tight" rather than "no longer possessing".
Apologies for this but it brings out the real pedant in me. Feel free to point out my own grammatical errors!
Regards
Dave Millier
I know I shouldn't say this as it's irrelevant to the discussion and everyone makes typos but your continuous mis-spelling of "lose" keeps distracting me! Loose means "not tight" rather than "no longer possessing".
Apologies for this but it brings out the real pedant in me. Feel free to point out my own grammatical errors!
Regards
Dave Millier
Karl Guttag
Senior Member
If you print out both images to the same size print and view them at the same distance, the D1 image will be less sharp. The DoF will be 1.5X less on the D1 image because everything captured by the D1 will be (assuming you don't apply software sharpening) will be less sharp by 1.5X
Karl
Karl
--Karl
DavidMillier
Forum Pro
It is an optical phenomenon but the image also has to be created in your brain and because of the way we perceive images the overall size of the image has an effect on DOF. More about this at the end of this post.
I like to avoid such complexities in this sort of discussions by making my DOF comparisons between the original optical images e.g. if I was comparing the DOF of a 35mm and a 120 neg I would make that comparison on the negs. Karl prefers to make his comparisons on prints of a given size as do published DOF tables etc.
Absolutely correct as long as the aperture is also unchanged
Still correct
Michael Reichman is correct about this
Yes. I would alternatively term this as " the magnification is different" although Karl disapproves of my terminology
Yes
You should note however, that if you choose to compare DOF on enlarged prints instead on the optical images things are different.
This is because (as Karl points out) there is only one true distance at which an image is truly sharp.
At all other differences (closer and further) the image is progressively more out of focus.
However, the human eye can only perceive detail above a certain size and as long as the bluriness is slight enough that what is supposed to be a point in an image is still rendered small enough to be below the eye's threshold, your brain will be fooled into accepting a small fuzzy circle as a point and it will still seem sharp. Technically the small fuzzy circle is called the Circle of Confusion (and isn't it confusing!).
The subjective quality comes in when assessing what maximum size of circle of confusion is acceptable in the image. If the COC is too big, the image will seem to have little DOF. If it is small the image will appear to have a bigger DOF.
However, when you enlarge a negative, you make the COC bigger as well as the image so the blurriness that might be acceptable in a smaller image can become objectional in the enlargement.
Presumably what Karl is saying is that because the sensor image is smaller in area than the film image if you want to produce prints of the same size you have to enlarge the original images by different amounts so the COC on the final prints is of different sizes and therefore this changes the apparent DOF from one print to another.
This is true but it's not how I prefer to consider the question. Probably this is why Karl and I seem to agree on some points and disagree on others.
Regards
Dave Millier
ps you might like to consider WHY different focal length lenses produce different DOF. I believe it has to do with the absolute size of the aperture hole. For a 100mm lens the aperture hole at (say) F4 is larger than that of the apperture hole for a 50mm lens at F4.... but this could open up another can of worms!
DavidMillier
Forum Pro
You mean that there will less overall DOF rather than that the area of the image at the plain of focus will be less sharp...
I'm not sure that applying a sharpening algorithm can make much difference to DOF. Sharpening only works on exaggerating the contrast alongside already sharp edges - I think it just makes blurred regions look more obviously blurred...
I'm not sure that applying a sharpening algorithm can make much difference to DOF. Sharpening only works on exaggerating the contrast alongside already sharp edges - I think it just makes blurred regions look more obviously blurred...
Karl Guttag
Senior Member
DMiller,
I through in the qualifier of sharpening because we are talking about optical effects. Things get REALLY MESSY if you throw in non-linear sharpening algorithms.
Karl
I through in the qualifier of sharpening because we are talking about optical effects. Things get REALLY MESSY if you throw in non-linear sharpening algorithms
.Karl
--Karl
D
Dick-
Guest
Wow based on that ...my 4 by 5 is cropping my 8 by 10..and I am "loosing" pixels...must be the be the vibration working them loose and they fall off.
I guess the APS is cropping the 35mm even though the standars lens is different... hmmmm.
I guess the APS is cropping the 35mm even though the standars lens is different... hmmmm.
--****
Paul Rumohr
Well-known member
Oh, man, this is way too funny. We need a pro digital comedy thread!
Paul
DavidMillier
Forum Pro
****
I don't like the "cropping" terminology in this context either and you are quite right with your comments but it probably amounts to just another way of expressing the same idea.
Regards
Dave Millier
I don't like the "cropping" terminology in this context either and you are quite right with your comments but it probably amounts to just another way of expressing the same idea.
Regards
Dave Millier
ianR
Veteran Member
Hello
Perhaps some of you gentlemen might like to see this explanation of the multiplier effect.
http://www.lonestardigital.com/multipler.htm
regards
Ian
Perhaps some of you gentlemen might like to see this explanation of the multiplier effect.
http://www.lonestardigital.com/multipler.htm
regards
Ian
Karl Guttag
Senior Member
An absolutely horrible set of explainations. The diagram of the camera and lens if flat out wrong. It implies that the outer part of the lens generates the outer part of the image when it contributes to the whole image.
This is a perfect example of how a marketing department is putting out misinformation in the guise of technical content. People see that junk and come here all confused.
Karl
This is a perfect example of how a marketing department is putting out misinformation in the guise of technical content. People see that junk and come here all confused.
Karl
--Karl
jcshoots
Member
Hi all....I started this post a day ago and can't still get my answer....I'm going to ask the question again like this:
You have a beautiful subject (let's say a vestal virgin a week before her wedding) you are going to shoot with an F5....you will not use your 8mm Fisheye (big nose and all that) nor will you use your 600mm that you keep around for birds and the like.....You will probably use something around an 100mm lens....some prefer the 85mm, some the 105mm.
My question is: If you're using a D1 for this purpose and you want the effect of the 100mm lens, should one use the 105mm or the 60mm?
Thanks,
Jim--'It was the best of times, it was the worst of times'www.jimcoxphoto.com/
You have a beautiful subject (let's say a vestal virgin a week before her wedding) you are going to shoot with an F5....you will not use your 8mm Fisheye (big nose and all that) nor will you use your 600mm that you keep around for birds and the like.....You will probably use something around an 100mm lens....some prefer the 85mm, some the 105mm.
My question is: If you're using a D1 for this purpose and you want the effect of the 100mm lens, should one use the 105mm or the 60mm?
Thanks,
Jim--'It was the best of times, it was the worst of times'www.jimcoxphoto.com/
ianR
Veteran Member
That's not at all what it's implying. It's not suggesting anything the outside of the lens produces the outside of the image, it's simply showing the relative sizes of the images obtained form different cameras, and the relative fields of view. How else could you represent it?
Ian
Ian
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