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01-30-2009, 10:41 AM   #31
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back to the OP question

as others have explained 1:1 macro is just the simple size of the image recorded on what ever recorrding medium is used.

in relation to focal length, 1:1 is easier to achieve at shorter focal lengths, because to achieve 1:1 requires the lens (assume here simple lens) to be equal distance from both subject and focal plane. As a result a 35mm macro with true 1:1 would be 70mm from the subject and also 70mm from the focal plane.

Since infinity has a lens at it's focal length, a 35mm lens normally is 35mm away from the focal plane (yes I know in complex lenses it is not that simple) and therefore to extend an additional 35mm to get 1:1 is relitively simple. by comparison, a 100mm focla length macro needs to extend an additional 100mm, that is much tougher, and usually needs extension tubes.

the benifit of longer focal length macro lenses even when working at 1:1 is that you are further away from the subject. It is easier to light, while avoidubg shadows cast by the camera and lens, and usually easier to work with things like live insects when further away.

short focal length macro lenses are however, easier to use on copy stands and for photographic reproduction of larg objects, due to the smaller working distance.

01-30-2009, 11:03 AM   #32
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QuoteOriginally posted by Blue Quote
You are changing the distance. This is straight up algebra. For a 35mm lens to have the same DOF as a 105mm lens at the same aperture, the distance has to change.
Of course - that was the point of my comment in support of Miserere. To get the SAME SUBJECT SIZE with different focal length lenses, you have to use DIFFERENT DISTANCES. As Miserere said, you have to shoot at twice the distance in order to take a macro picture of a given subject with a 100mm lens vs. a 50mm lens. The change in DOF due to the longer distance cancels out the change due to the longer focal length. The only variable left is aperture.

To show an example from the DOF calculator you linked to:

Pentax K20D, aperture f/8:

100mm lens at 10 inches: DOF = 0.05 inches
50mm at 5 inches: DOF = 0.05 inches

The reason macro photography works this way is because you're typically shooting a particular subject and you position the camera so as to get the subject to fill the frame. This is unlike, say, landscape photography where you shoot from the same position regardless of focal length.
01-30-2009, 02:17 PM   #33
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QuoteOriginally posted by Sean Nelson Quote
Of course - that was the point of my comment in support of Miserere. To get the SAME SUBJECT SIZE with different focal length lenses, you have to use DIFFERENT DISTANCES. As Miserere said, you have to shoot at twice the distance in order to take a macro picture of a given subject with a 100mm lens vs. a 50mm lens. The change in DOF due to the longer distance cancels out the change due to the longer focal length. The only variable left is aperture.

To show an example from the DOF calculator you linked to:

Pentax K20D, aperture f/8:

100mm lens at 10 inches: DOF = 0.05 inches
50mm at 5 inches: DOF = 0.05 inches

Sean, the DOF is the same because the working distance has changed to achieve 1:1. That is a result of the focal length changing. That was the point I was trying to make to Miserere's original post.

The reason macro photography works this way is because you're typically shooting a particular subject and you position the camera so as to get the subject to fill the frame. This is unlike, say, landscape photography where you shoot from the same position regardless of focal length.
Actually Sean, my original point was that they have different working distances to obtain the same depth of field with different focal lengths, you have to change the working distance to get 1:1. Miserere said focal length doesn't matter that only aperture mattered. For controlling the dof with a given lens, aperture is the most important. However, focal length is important when choosing WHICH LENS to use.

Last edited by Blue; 01-30-2009 at 02:22 PM.
01-30-2009, 02:24 PM   #34
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QuoteOriginally posted by pentaxman Quote
I guess my point is that if you are taking a picture of a 24mm bug with a FF 10Mp camera and you are taking a picture of the same bug with a 10Mp APSC camera, you will have more pixels and therefore greater resolution with the APSC camera. You will be able to enlarge the bug more with less loss in resolution with the APSC camera. Of course I am only talking about sensor resolution, not lens resolution - the lens resolution would remain the same.
That is true but has no bearing on the lens 1:1 or not. That has to do with the software and printer.

01-30-2009, 03:47 PM   #35
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DOF, focal length, etc.

QuoteOriginally posted by Sean Nelson Quote
Of course...The change in DOF due to the longer distance cancels out the change due to the longer focal length. The only variable left is aperture....
Everybody is agreeing with each other.

The DOF equation pertaining to close-ups & macros can be written without direct reference to focal length.

DOF/WOF=Constant*F-stop*WOF*(1+1/m)

Where m is magnification and WOF is width of field. For the same scene in the viewfinder (m is constant & WOF is constant) the only variable left is f-stop.

Of course one could substitute a definition for magnification like
m=focal_length/(working_distance-focal_length) to get a relationship with more variables, but both expressions mean the same thing.

DOF/WOF=Constant*F-stop*WOF*working_distance/focal_length

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01-30-2009, 05:54 PM   #36
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QuoteOriginally posted by Blue Quote
That is true but has no bearing on the lens 1:1 or not. That has to do with the software and printer.
That's not necessarily true. I agree that the lens is still 1:1 but I am looking at this as a system consisting of the camera, lens, sensor and software. The full frame camera needs 1.5X more pixels to get the same resolution. In the case of the K20D you would need a 21 megapixel full frame camera to have the same resolution.

My point is that if you are going to do an extreme closeup, you can enlarge an APSC picture 1.5X more and still have the same number of pixels as on a full frame camera. It gives you an effective 1.5 times more magnification.
01-30-2009, 07:17 PM   #37
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QuoteOriginally posted by pentaxman Quote
That's not necessarily true. I agree that the lens is still 1:1 but I am looking at this as a system consisting of the camera, lens, sensor and software. The full frame camera needs 1.5X more pixels to get the same resolution. In the case of the K20D you would need a 21 megapixel full frame camera to have the same resolution.

My point is that if you are going to do an extreme closeup, you can enlarge an APSC picture 1.5X more and still have the same number of pixels as on a full frame camera. It gives you an effective 1.5 times more magnification.
You are talking about enlarging an image after it is out of the camera and therefore has nothing to do with the 1:2, 1:1, or 2:1 macro settings on the lens.

Why are people about to pee their pants waiting on Pentax to release a FF dSLR?
01-31-2009, 08:02 AM   #38
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QuoteOriginally posted by Blue Quote
Actually Sean, my original point was that they have different working distances to obtain the same depth of field with different focal lengths, you have to change the working distance to get 1:1. Miserere said focal length doesn't matter that only aperture mattered. For controlling the dof with a given lens, aperture is the most important. However, focal length is important when choosing WHICH LENS to use.
What I said was that focal length doesn't matter if the subject size on the frame is constant. If we're talking of taking photos at 1:1 magnification with the same camera, then subject size on frame will always be the same.

Read newarts post 3 posts up; he's kindly put into a formula what it was that I was saying.


Last edited by Miserere; 01-31-2009 at 09:25 AM.
01-31-2009, 11:31 AM   #39
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QuoteOriginally posted by Miserere Quote
What I said was that focal length doesn't matter if the subject size on the frame is constant.
I think we three monkeys are describing the same elephant from different points of view...
01-31-2009, 12:40 PM   #40
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QuoteOriginally posted by Sean Nelson Quote
I think we three monkeys are describing the same elephant from different points of view...
That seems to be the gist of it.
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