The Focal Length Multiplier Confusion
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Ckiang, I'm afraid I'll have to agree with Darren and Kho King that your concept is erroneous...If you base your conclusions from the Luminous Landscape article, there is this statement in there:
"One other subtle benefit is the greater depth of field. To frame a head and shoulders portrait for example, a DSLR use will have to stand further away than a 35mm user with the exact same focal length lens. Subject distance determines DOF, for any given focal length and Circle of Confusion so it follows that the DOF will be greater by roughly 50%."
This statement says that with the same lens, same format, a DSLR will have a greater DOF than a film SLR only when subject distance is different. It says to frame a certain subject in the same proportion to the entire frame, a DSLR user will have to stand further away, thus altering subject distance and DOF.
Which means to say, with the same lens, same format, same subject distance, DOF is the same for DSLR and film SLR. Like Kho King said, it's just a crop (of course there are other ways of looking at it but this way is legitimate enough).
When you compared the lenses used by medium format, 35mm film SLR and DSLR, one of the constants is violated: lens system. A medium format image taken with a certain focal length will have different DOF from a DSLR image taken with the same focal length, because the lenses are built differently! And I believe physical aperture size etc will be different as well.
Hope my explanation isn't too flawed.
Sigh. If I wasn't confused before, this thread has made me so.
The best way to sort this out if to get a DSLR, a film camera, and possibly a medium format camera, and to sort that out. I have done.
There is a *lot* of inaccuracy floating about the Internet, including on websites such as Rob Galbraith (not Rob, but the forums anyway), and this topic was the one that ultimately made up my mind that a lot of the posh westerners really don't have much more clue than normal mortals (let's not start another western superiority thread please).
I will explain this to anyway privately, who have a legitimate reason to want to know. I.e. have a digital camera, film camera and medium format camera or at least are seriously intending to purchase one. Otherwise, it's a moot point really.
The best way to sort this out if to get a DSLR, a film camera, and possibly a medium format camera, and to sort that out. I have done.
There is a *lot* of inaccuracy floating about the Internet, including on websites such as Rob Galbraith (not Rob, but the forums anyway), and this topic was the one that ultimately made up my mind that a lot of the posh westerners really don't have much more clue than normal mortals (let's not start another western superiority thread please).
I will explain this to anyway privately, who have a legitimate reason to want to know. I.e. have a digital camera, film camera and medium format camera or at least are seriously intending to purchase one. Otherwise, it's a moot point really.
Originally posted by Tweek
Ckiang, I'm afraid I'll have to agree with Darren and Kho King that your concept is erroneous...If you base your conclusions from the Luminous Landscape article, there is this statement in there:
"One other subtle benefit is the greater depth of field. To frame a head and shoulders portrait for example, a DSLR use will have to stand further away than a 35mm user with the exact same focal length lens. Subject distance determines DOF, for any given focal length and Circle of Confusion so it follows that the DOF will be greater by roughly 50%."
This statement says that with the same lens, same format, a DSLR will have a greater DOF than a film SLR only when subject distance is different. It says to frame a certain subject in the same proportion to the entire frame, a DSLR user will have to stand further away, thus altering subject distance and DOF.
Which means to say, with the same lens, same format, same subject distance, DOF is the same for DSLR and film SLR. Like Kho King said, it's just a crop (of course there are other ways of looking at it but this way is legitimate enough).
When you compared the lenses used by medium format, 35mm film SLR and DSLR, one of the constants is violated: lens system. A medium format image taken with a certain focal length will have different DOF from a DSLR image taken with the same focal length, because the lenses are built differently! And I believe physical aperture size etc will be different as well.
Hope my explanation isn't too flawed.
What I am trying to say is, a DSLR with a 1.5x FLM, a FF DSLR or 35mm SLR, and MF are all different systems!
The LL article is right as well. But subject distance changed. If you use a 50mm on a DSLR and 50mm on a 35mm SLR, and you frame the subject the same way, the subject distance has been changed and that renders the DoF comparison invalid.
If you use a 50mm lens on a DSLR, the same 50mm lens on a 35mm SLR, shoot the same subject at the same subject distance, the 50mm lens on 1.5x DSLR will give the same effect as a 75mm lens on the 35mm camera, including the DoF.
Regards
CK
I believe it should be "... the same effect as a 75mm lens on the 35mm camera, excluding the DoF."
I think it's just as if you take a picture using the 50mm lens on a 35mm SLR, then use scissors and cut away the sides until you get the zoom equivalent to the 75mm. The DOF should remain the same as before.
But if you do that, it's different. The 1.5x FLM DSLR has inherent 1.5x multiplication.
If you just use scissors to cut, you end up with a smaller print.
When I tried a 28-70 on a D100, and a 28-70 on a F100, the 70 end on both doesn't quite look the same in terms of background blur (and DoF).
Regards
CK
The same 28-70mm lens at 70mm should produce the same image circle regardless of what camera it is mounted on, agree?
Now, fitting in the 35mm film and the CCD sensor into this image circle, we get different crops of the image circle, which ultimately ends up as our image, as shown here (image not to scale, I anyhow whip up one):
Therefore, DOF shouldn't be different.
Now, fitting in the 35mm film and the CCD sensor into this image circle, we get different crops of the image circle, which ultimately ends up as our image, as shown here (image not to scale, I anyhow whip up one):
Therefore, DOF shouldn't be different.
Originally posted by darkness
Therefore, DOF shouldn't be different.
nice diagram. look how shockingly bigger a full frame sensor is compared to the D1 and the D30 / D60 sensor size, and how much more can be done with a bigger sensor size...... :devil:
this debate about the FLM never seems to end, everywhere on the net. That's why we should all move to full frame sensors. That will end this endless debate. Focal length multipliers are stop gap measures while technology is evolving and we all know that........
The f number, focal length and distance is the same for a 1.5x FLM DSLR and a 35mm SLR.
But.... the circle of confusion is NOT the same for the DSLR and the SLR because of the difference in film image size. This is because you have to magnify the DSLR image more to get to same printed size compared to the SLR. So the DOF for the DSLR is actually less than that of the SLR.
Use this DOF calculator if you still don't believe.
http://www.silverlight.co.uk/resources/dof_calc.html
I guess the term FLM is just to let 35mm users familiar with the "equivelent" focal lengths used when using a digital camera. Focal length is the distance between the centre of the lens and the film plane when the image at infinity is focused.
Because the field of view is smaller (smaller sensor) in most digital cameras, the image will look more magnified.
FML is just like digital zoom. When you use digital zoom, though the "focal length" increases, however, in actual fact, it's just a "crop" of the original image. The perspective and the DOF still remains the same.
"The largest circle that appears as a point to the eye is referred to as the acceptable circle of confusion"
-Leslie Stroebel, View Camera Technique
Below is sorta a better explanation taken from http://tangentsoft.net/fcalc/help/CoC.htm
"Imagine a perfect white point in an empty black room. The point has no height, and no width. If you focus an optically perfect lens on that point, it forms a perfect point on the film as well. If, however, you focus slightly in front of or behind the point, the point will image on the film as a small blurry circle. If that circle is small enough, it will still look like a point when enlarged for printing. The "circle of confusion" is typically calculated as the largest on-film circle that you see as a point when you make an 8 × 12 print and view it from a "normal" viewing distance, typically 2-3 feet. Anything larger is seen as a small circle, and is therefore perceived as out of focus."
"f/Calc calculates the CoC using the "Zeiss formula": d/1730, where d is the diagonal measure of the film, in millimeters. This formula yields acceptable values for most uses."
"The film size is important because you don't have to enlarge large negatives as much to get a particular sized print. So, a 6 × 4.5 cm frame, being roughly twice the size of a 35mm frame, will have a CoC that's roughly twice the size of that for a 35mm frame. In other words, if a fuzzy disc 0.025 mm wide looks like a point when printed from 35mm film, you can have an 0.043 mm disc on 6 × 4.5 cm film and still have the same apparent degree of sharpness if you enlarge it to the same size print as you made from the 35mm frame. "
You're right, but CK was right too.
You're refering to a cropped image without further enlargement, so the DOF would be the same as the original image.
But CK was refering to a cropped image enlarged to the same size as its uncropped original. Therefore the resultant DOF is different.
so I guess the last quote is your basis for DOF?
BUT that's only if the CCD is mounted such that it has better in focus than the film plane, which is not likely. The canon glossary makes it sound more like a optical limitation of the lens in providing a true single point. i.e. cheap lenses will have larger CoCs, therefore the CoC varies from the centre of the lens to the rim. but to use the size of grain/resolution/enlargement is not valid IMO becos if I use your argument, I blow up a D60 print to 22m x 33m then would the CoC become totally humongous and the DOF is then super large/small?
the CoC corresponds with DOF becos if u draw the diagram, the coc and dof are twins on each side of the lens, so only the distance (dunno if THIS is the focal length) from lens to plane AND the finite resolution of the glass matters.
try not to talk about enlargements but to keep it in SCALE proportional to the original image circle provided/created by the lens.
otherwise nothing stays constant and the basis for comparison (light projected resolution, CoC), by right static but now varying, leads to no conclusion.
otherwise nothing stays constant and the basis for comparison (light projected resolution, CoC), by right static but now varying, leads to no conclusion.
;( :angry: you guys made me late for work!That's exactly the point! The DOF would be very small in that instance if you viewed the 22m x 33m print from the same distance as you would a 8R print.
When you talk about DOF, it is always in relation to printing a photo a certain size (say 8x10) and viewing from a certain distance (say 2-3 feet). Given this size and viewing distance, you are then judging whether a certain given point in the photo is in focus or not. Objects within the DOF range are considered to be in acceptable focus. Objects outside are not considered to be in acceptable focus and will therefore not appear sharp in the photograph.
Kho King
Senior Member
I think u misunderstood what CK wrote, read again...
If you use a 50mm lens on a DSLR, the same 50mm lens on a 35mm SLR, shoot the same subject at the same subject distance, the 50mm lens on 1.5x DSLR will give the same effect as a 75mm lens on the 35mm camera, including the DoF.
Note the "same subject distance..."
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