Help with lens choice for extreme close up on 8X10

[Leigh]

At the same magnification and relative aperture and desired image quality (that's what the dread circle of confusion is about) all lenses give the same depth of field. At the same magnification, a longer lens requires more extension but gives greater working distance. That's all you have to play with, Randy.

Leigh's statements

are dead wrong. Leigh, before you vanished you and I sometimes breathed fire on each other. Please check what I posted and note that I quoted you exactly before breathing fire on me.

Hi Dan,

No, the flame thrower is in the closet, disassembled. ;-)

We're talking about two different aspect of lens usage, and both statements are correct.

I'm talking about focal length. You're talking about magnification.

Short FL lenses have great depth of field.
That's why cell phone cameras don't have focusing mechanisms for their 5mm or whatever lenses.
Yet they resolve everything from the tip of your nose to the moon.

- Leigh

[Eric Woodbury]

Here's a DOF calculator. I set it for 100mm focused to 200mm and a 200mm focused to 400mm. Both 1:1 mag. Same depth of field at the same f-number. Focal length of the lens drops out of the equation. If your subjects are still, consider a pinhole. Rules are different without a lens, that is to say, the focus is the same everywhere, blurry. Or use less than 1:1 and enlarge the image from the negative. Or simulate an electron beam microscope with a slit light source and a scanner.

http://www.cambridgeincolour.com/tut...calculator.htm

If you don't believe it, try it.

[Leigh]

I set it for 100mm focused to 200mm and a 200mm focused to 400mm.

You're making the same error as Dan, calculating DoF based on magnification.
That's certainly valid if you want to move the camera.

It's NOT valid if you change lenses while maintaining the same camera-to-subject distance, as is usually the case.
You normally change FL because you want to adjust the framing from the current camera position.

I calculated DoF using your calculator for 100mm and 200mm f/5.6 lenses both focused at 10 meters.

DoF for the 100mm lens is 24.02 meters, while that of the 200mm lens is 3.39 meters.

DoF is inversely proportional to lens focal length, as I said.

-Leigh

[Dan Fromm]

Leigh, you're a little confused. DoF is determined by magnification, relative aperture and desired image quality. Always and everywhere.

You went wrong by forgetting that at the same film plane-to-subject distance a short lens gives lower magnification than a long lens.

[Leigh]

You went wrong by forgetting that at the same film plane-to-subject distance a short lens gives lower magnification than a long lens.

No, Dan.

I did not go wrong in any respect.
My statements were absolutely correct, and supported by the online calculator supplied by others.

You're talking about changing lenses and moving the camera to maintain a particular magnification.
But people don't do that in the real world.

They change lenses while maintaining the camera-to-subject distance in order to change the framing.
That also changes the magnification.

I stated that quite clearly in my previous post. End of discussion.

- Leigh

[Randy]

OK, since I am having a difficult time with this, let me propose a situation:

With the two lenses I have, 150mm and 115mm, both at a distance from the subject to give a 1:1 magnification, both at minimum aperture of f/45, will one give greater depth of field than the other - if so, which one?

Leigh, I don't understand why I wouldn't have to move the camera closer or farther away to achieve the desired magnification when changing lenses. How could we assume that just changing the lens will give me the desired magnification without some modification of the camera to subject distance?

[Leigh]

Leigh, I don't understand why I wouldn't have to move the camera closer or farther away to achieve the desired magnification when changing lenses.

Yes, if you want 1:1 magnification you must move the camera when changing lenses.

But why would you do that?
At 1:1 you get the same Dof with any lens, so why change lenses in the first place?

- Leigh

[Dan Fromm]

Randy, @ f/x, x an arbitrary number, and 1:y, y another arbitrary number, all focal lengths give the same DoF.

By the way, @ 1:1 with the aperture set to f/45 the effective aperture is f/90. At f/90 the diffraction limit is on the order of 16 lp/mm at very low contrast centrally, less the farther off-axis you go. Y'r negs won't be enlargeable, may give unsatisfactory contact prints. Don't take my word for this, try a few shots and please me by telling me that for your purposes and standards I'm mistaken.

I have to catch a train, will reply more fully about part two of your question later today.

[ic-racer]

I guess my question is pretty basic - at 1:1 will the 115mm lens offer more depth of field than the 150mm lens when both are stopped down to f/45?

You need to already know the answer to your question to sift through the wrong answers with a question like that.

[Randy]

You need to already know the answer to your question to sift through the wrong answers with a question like that.

?

Dan, I will be contact printing making 8X10'ish cyanotypes - no enlarging. I will set up the camera and do some experimenting today.
The reason I wanted to get an idea on what lens to use is because I suspect that at f/45 at 1:1, I will not be able to see the image on the ground-glass to determine if I am getting the depth of field I want.