A head-scraching question.

[henry_ting2]

If the exposure calls for f8 @ 1/250, will there be a difference in

DOF if the exposure were made at f8 @ 1/125 with a +1 ND filter ?

So if the DOF is solely controlled by the lens iris, not by the

amount of light for the exposure, then the design of the iris (in

positioning inside the lens as well as the physical shape) will have

a relation with the intended DOF. But then how come in shooting

pictures wide-open there is a difference of DOF between a fast and a

slower lens.

I know, I have nothing better to do with my time.

[markci]

ND filters have no effect on DOF. I don't understand your last question. Faster lenses have less depth of field wide open because they have wider maximum apertures. What's not to understand?

[albert knapp md]

I agree. F/8 with a 24mm or a 240mm is proportionately the

same. The ND has no effect on DOF. As to your last question, I

think (if I understand it) the answer is simple: the faster the lens,

the smaller the DOF. This is optics 101.Go back to shooting !

(:>)))

[Rob F.]

Henry: It's the diameter of the iris that determines DOF, not the amount of light coming through. You can prove this easily: a 50mm lens at f/8 has the same DOF whether in bright sunlight, or in a dark restaurant, don't you agree? So the quantity of light is not part of it.

 

Now, let me quote from the Leica Manual, Fourth Edition, 1961, p. 60: "That is, if the object distance is held constant, all lenses of the same diameter have the same depth of field.

 

"Take, for example, a lens 1 inch in diameter. If this lens has a focal length of 8 inches (as for a 5x7 view camera), it will have a speed of f/8. If it has a focal length of 2 inches (as for the Leica), it will have a speed of f/2. But since both lenses have the same diameter, both will have the same depth of field, in spite of the fact that the f/2 lens is 16 times as fast as the f/8."

 

I know this is going to start a controversy. It should be fun, though--something we can really sink our teeth into.

 

Tomorrow.

[cmphoto1]

DOF is dependant on magnification and iris size. If you take a full frame picture of an apple at f8 with a 35mm lens and at f8 with a 350mm lens (keeping the image size the same) you will have the same DOF.

 

Let the games begin!

 

chad

[henry_ting2]

Bob, I agree this subject on the surface seems obvious, but the inquisite mind goes beyond that.

 

Is it really as simple as that and what else not to understand as some claim ?

[grant_.]

it is that simple

[albert knapp md]

Let me elaborate on my rather terse initial answer. When I said

"proportional" in a 24 vs 240mm lens (see my thread) I meant

keeping the photographed object at a similar size or distance. As

to the last part of my statement, the faster the lens or the more

opened up you are, the shallower the DOF.

[Rob F.]

Henry: You are right, it's not really that simple. Notice I didn't even claim the Leica manual was correct. The argument is directed toward comparing the DOF of various formats. Does it hold true for various focal lengths within the same format? Let's take a look.

 

Let's try comparing the DOF, at 1 meter, of 3 lenses, all of which have approximately a 25mm diameter. Sarting with a 35mm f/1.4, the table in Eastland shows that the zone of sharp focus extends from .97 to 1.04M. A 50mm f/2 has the same diameter, but the DOF goes from .98 to 1.02M. A 75mm at f/2.8 (diameter 26.7mm, close enough) allows only .987 to 1.014M.

 

We have held the object distance constant at 1M, and the diameter constant, as Morgan & Morgan stipulate, but the prediction isn't working out. We are losing DOF as focal length increases, even though the diameter is being held constant.

 

I *think* the problem with this is that M&M elected to specify the size of the circle of confusion as a fraction of focal length, rather than as a fixed diameter like 1/30mm, like everyone else does. This would be appropriate for comparing different formats, since a larger negative requires less magnification for a given size print. But it's not appropriate for comparing different lenses of the same format, because it doesn't hold a 135mm, say, to the same standard as a 35mm. The truth is that DOF fades fast as focal length increases.

 

Arthur Cox gives the applicable formulae in his book, "Photographic Optics." The formulae use focal length; distance from lens to object; the diameter of the circle of confusion (the blur circle); and f-number. If we hold all these constant (as they would be in your example) then DOF doesn't change. The formula doesn't include the transmission of the lens, which is what you are effectively changing by adding an ND filter. So, changing transmission doesn't change DOF.

 

(The last paragraph above is really the one that addresses the question. The parts before it are just where my exploration led me until I got on the track.)

[henry_ting2]

Rob, you pretty much covered the important aspect of this whole thing. However, it still does not explain the difference of DOF comparing a 35 Summilux and a 35 Summicron, both shooting wide open (taking the aperture equation out of the picture), and the DOF of these 2 lens are different. So the argument of those that the aperture have a direct bearing on the DOF is not absolute. I think there are many variables that plays a role in the DOF as far as optical lens is concerned. I'm going to do more research in this area before I make a fool of myself. Perhaps later postings will form more solid grounds.

[Rob F.]

Henry, I wonder if you are referring to the fact that some lenses exhibit a gradual falloff of sharpness outside the plane of focus, while others are more abrupt. This seems to happen with very highly corrected lenses. Lenses that are very sharp in the plane of focus make the OOF areas more obviously OOF. Is that what you're thinking of, in whole or in part?

[markci]

<i>

However, it still does not explain the difference of DOF comparing a 35 Summilux and a 35 Summicron, both shooting wide open (taking the aperture equation out of the picture), and the DOF of these 2 lens are different.

</i>

<p>

For chrissake, a Summilux is f/1.4 wide open and a Summicron is f/2. F/1/4 has less depth of field. Period. It really is that simple.

<p>

Sheesh.

[wolfgang_gressmann]

"For chrissake, a Summilux is f/1.4 wide open and a Summicron is f/2. F/1/4 has less depth of field. Period. It really is that simple. "

 

Question: Has a Summilux stopped down to f/2 the same DOF as a Summicron wide open?

[grant_.]

how many friggin ways can the same damn thing be stated...are you people dense?

[markci]

In answer to Wolfgang, yes. If focal length and subject distance are held constant, depth of field is a function of the actual taking aperture, and nothing else.