Understanding focal length and dof

[Paul Ozzello]

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 ?

 

[Mainecoonmaniac]

Take a look at this.
https://www.cambridgeincolour.com/tutorials/dof-calculator.htm

 

[AgX]

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:

Here you got an online DOF calculator that includes the macro range, to play with:
http://www.erik-krause.de/schaerfe.htm

You not only can calculate how at same object distance a variation of focal length results in different DOF, but also see how the ratio between near- and far-point changes with the image scale.

 

[Dan Daniel]

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 ?

Is this difference based on actual images, or on the scales on the lenses?

'Depth of field' involves an acceptable circle of confusion- a range between which an image will *appear* to be in focus. And this is heavily affected by final useage and print size. For example, the Rolleiflex manual explains that their on-camera DOF scale is based on a print up to 8x10 inches or so and for larger than that you should look at the next wider f-stop for practical depth of field. Maybe the two lenses have had scales based on different circle of confusion size.

 

[Paul Ozzello]

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 ?

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.

 

[Dan Fromm]

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?

 

[AgX]

As I said, as long as you use the same criteria for unsharpness then it does not matter what format you put behind the lens, the DOF will remain.

That the same lens acts as wide-angle lens for one format and as normal lens for another does not change that.

The idea of wide-angle lenses yielding more DOF than normal lenses is based on using lenses of different focal lenght at the same format.
Your misconception seems to originate from overlooking "the same".

 

[Paul Ozzello]

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”

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?

 

[pentaxuser]

I have just looked at the online DoF calculator and at 10ft with an aperture of f8 the DoF with a 38 is about 34ft and with a 35 is about 82 ft so yes depending on what aperture and focusing distance you choose the DoF difference is quite large. The near limit varies hardly at all but it is in the far limit that it increases substantially.

If someone had asked me to guess what difference to DoF a lens of 35 would make compared to one of 38, I don't think I'd have guessed anything so large

For information the site is www.dofmaster.com A useful site to have in my opinion

I hope this helps

pentaxuser

 

[wiltw]

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 ?

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.

 

[Dan Fromm]

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”

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.

How did you focus the two lenses?

 

[L Gebhardt]

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.

 

[Paul Ozzello]

Thanks for that explanation - that’s what I expected initially but not what I’m seeing.

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.

 

[Paul Ozzello]

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.

How did you focus the two lenses?

I focused both lenses at infinity. And both
lenses at f16. Didn’t move the tripod.

 

[Paul Ozzello]

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 ?

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.

 

[removed account4]

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

 

[Paul Ozzello]

I’ll try and scan and post some images tomorrow.

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

 

[Dan Fromm]

I focused both lenses at infinity. And both
lenses at f16. Didn’t move the tripod.

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?

 

[wiltw]

I just used the Cambridge Color DOF on-line calculation (my usual, since it allows you to specify the more exacting 20/20 visual acuity the optometrist strives to achieve, not the poor 'manufacturer standard' visual acuity assumption!) and judging 8x10 print at 25cm viewing distance.

• 35mm f/16 on 6x6 format, focus at 1200 In (100') has 20/20 vision DOF of 129 in. to Infinity
• 38mm f/16 on 6x6 format, focus at 1200 in. (100') has 20/20 vision DOF of 149 in. to Infinity

...a mere 20" difference in the DOF zone size for 3mm of FL

 

[RalphLambrecht]

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 ?

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 field

 

[Paul Ozzello]

Lol oops Yes I usually focus through the ground glass on both cameras but infinity is properly calibrated on both so I just set it there for my comparison.

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?

 

[Marco Gilardetti]

In my opinion infinity is not properly calibrated as you suppose. Redo the comparison by focusing infinity on the groundglass for both lenses.

 

[L Gebhardt]

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 ?

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.

 

[Dan Fromm]

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.

 

[Paul Ozzello]

I didn't. They're separate cameras. Arcbody and SWC/M

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.