I started using a hasselblad Arcbody with a 35mm APO-Grandagon lens and while it is slightly wider than the 38mm Biogon that I’m used to the depth of field is far greater. How is that possible ? Is it because the lens is made to cover 6x12 and at that format much wider than 35? And what format are those focal lengths based on - 6x6 ?
Focal Length is what it says, the lenght between a certain point within the lens assembly and the plain of sharp image of a subject at infinity.
It is a constant for a lens and independant from image size.
DOF is a length that depends on the degree of un-sharpness one would accept. And that again is a matter of scale of the final image respectively the format you use for taking and the respective successive enlargement.
You should read a textbook on this matter.
However your issue about a large difference in DOF between those two lenses remains.
Hmm. I have a 35/4.5 Apo Grandy and a 38/4.5 Biogon, both in Copal #0. I use both on a Century Graphic, typically at f/8 to f/11. I've never noticed much of a difference in DoF.
fatso, why do you believe that your two lenses have very different depth of field at the same aperture and focused distance?
Ok, focal length explanation makes sense.
But for an equivalent focal length the angle of views are very different; the 35 covers 120 degrees horizontally while the 38 only covers 72 degrees horizontally, making the 35 a much wider angle lens. Does this have any correlation with dof ?
Are you saying dof should be equivalent given the same focusing distance and aperture used ?
Thanks for the reply. I just asked DOFMaster what it thinks. 0.03 mm CoC, f/16, focused at 10000 feet. 35 mm, near limit of DoF 8.37 ft. 38 mm, near limit 9.86 feet. Far limits infinity. That's a tiny, as in practically speaking insignificant, difference.was doing a comparison of both lenses and I focused both at infinity, both f16. The 35mm (and also the 45mm apo grandagon) are much sharper in the foreground than the Biogon.
Everything was CLAd a few weeks ago so I don’t think there’s anything wrong with the apertures.
I used a 15x and 20x loops to evaluate my negatives as I drum scan and print 43” x 43”
The size of the Image Circle created at the focal plane is larger for a lens intended to be use with a larger format (e.g. 4x5" sheetfilm) at 120 degrees vs. one created for medium format at 72 degrees. But if both a large format 38mm lens with 120mm image circle, and the medium format 38mm with 72 degree image circle could be mounted on a medium format camera to fill the same 56mm x 56mm frame area, at the same aperture both lenses would have identical DOF (we are ignoring the fact that the large format lens might deliver fewer lines of detail resolution per millimeter since its (approx) 90mm image in 4x5" is magnified about 0.5x less than the 56mm medium format image.
I focused both lenses at infinity. And bothThanks for the reply. I just asked DOFMaster what it thinks. 0.03 mm CoC, f/16, focused at 10000 feet. 35 mm, near limit of DoF 8.37 ft. 38 mm, near limit 9.86 feet. Far limits infinity. That's a tiny, as in practically speaking insignificant, difference.
How did you focus the two lenses?
Most likely you are seeing focus shift as the lens is stopped down, and field curvature differences between the lenses. Different optical designs behave differently. If you focused stopped down the DOF would likely be very close, but focus curvature will likely be different.
sorry to ask but can you post images that express the vast differences in the DOF ?
im trying to wrap my head around how 2 views at infinity at f16 couldn't have
acceptable sharpness or DOF if they were on a tripod &c .. i never put my nose to a print
or use a loupe to inspect i look at images from a certain viewing distance maybe that is the difference ?
i would have imagined the DOF would be pretty much identical
Thanks for the reply.I focused both lenses at infinity. And both
lenses at f16. Didn’t move the tripod.
I can offer this pdf and hope it may shed some light onto the issue:BY THE WAY, a circle of confusion is a bunch of photographers sitting around discussing depth of fieldOk, focal length explanation makes sense.
But for an equivalent focal length the angle of views are very different; the 35 covers 120 degrees horizontally while the 38 only covers 72 degrees horizontally, making the 35 a much wider angle lens. Does this have any correlation with dof ?
Are you saying dof should be equivalent given the same focusing distance and aperture used ?
Thanks for the reply.
That's where, not how. I focus my lenses on the ground glass, through the lens. Did you focus yours through the lens or by scale?
Notice that at f/11 the center sharpness falls a bit but there's a huge gain half way to the corner. That is almost certainly focus shift. Try focusing away from the center wide open. When you stop down you should get a more balanced focus. Or you can figure out the degree of shift experimentally and correct focus based on that.I think that’s what I may be seeing. Here’s the mtf for the 35mm lens which shows an increase of sharpness moving away from center. Would this explain it ?
Stupid request of the day: fatso, tell us more about y'r 38/4.5 Biogon and how you mounted it on y'r Arcbody. I ask because casual searching found only three original equipment lenses for the Arcbody, and the 38 Biogon isn't one of them.
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