RB67 90mm F3.8 hyperfocal distance - how?

I was just looking at my camera. The focussing is done on the body, and at the front of this lens (non C version) I can turn this diaphragm. So on the scale I can set it at say F16 or F22 and it shows infinity and 7 feet would be in focus. But if the focusing is done on the body how do I achieve this? Do I pick something in the centre (where the red dot on the scale - like 14 feet) and imagine that and manually focus onto that?


Thanks.

rayonline_nz said:

I was just looking at my camera. The focussing is done on the body, and at the front of this lens (non C version) I can turn this diaphragm. So on the scale I can set it at say F16 or F22 and it shows infinity and 7 feet would be in focus. But if the focusing is done on the body how do I achieve this? Do I pick something in the centre (where the red dot on the scale - like 14 feet) and imagine that and manually focus onto that?


Thanks.

Click to expand...

Yes, that should work.
Or you could focus through the viewfinder on a point, and then read the scale on the right side of the body to see what the distance indicated is.
Adjust the DOF scale on the lens to match, and see where the indicated hyperfocal distance indicators take you for the aperture that you have selected.
Cheers
T

There is ( or should be) a plate on the right hand side of the RB67.
You would use the DOF scales on the lens ( which are read only, having no connection to the camera or lens)
Select a position on the lens scale that covers your required dof , far and near.
The red line then indicates the hyperfocal setting
On the camera plate, the 90mm lens is coded green ( the lower green curve is the 90mm one)
Adjust the focus rail so the green curve intersects the hyperfocal distance setting.
It is only approximate !

Explanation best done with an illustration. Look at this photo (borrowed from B&H)


You will see it is currently focused at about 3.1'. The DOF scale shows that

• with f/16, the zone in-focus should be about 2.75' - 3.5'
• with /8, the zone in-focus should be about 2.9' - 3.2'

So in practice, let's assume you are intending to shoot with f/16 and have focus from whatever-near-distance out to Infinity...put the Infinity mark aligned with right DOF scale mark, and the left DOF scale mark shows the nearest distance which will be 'in focus'.

Now let's deal with REALITY! The average DOF Scale on lenses, and the average DOF calculation program assume 'manufacturer standard' vision in the viewer, which is POORER than the 20/20 vision that optometrists strived to achieve with corrective lenses. IOW, the average viewer will NOT perceive a blur circle to be 'in focus'. Using focus point at 3.1' (above), the standard DOF scale and the standard DOF calculator both say f/16 permits the in-focus zone to the 2.78' - 3.5' when focused at 3.1'
But if we assume 20/20 vision in the viewer, the f/16 DOF zone when focused at 3.1' is actually only 2.99' - 3.02' !

Most shooters of 135 format who understand this gross overstatement of DOF zone compensate by using the one-aperture-larger marks...if shooting at f/16, they look at the f/11 marks instead in the DOF Scale. Consulting the image of the Mamiya 90mm lens, I might be inclined to use the f/5.6 marks or the f/8 marks on the DOF Scale, to estimate my true DOF Zone size...the marks for +2EV aperture size in lieu of my actual shooting aperture with the Mamiya 90mm might be best to use for a more realistic determination of DOF Zone extents, but maybe the +3EV marks are even better to use.

If you use somewhat longer focus distances (maybe at 12' rather than the very close distance illustrated -- at 12' distance the DOF Zone is 10.3'-14.3' with f/16), you can get a somewhat better idea of what degree of aperture 'compensation' you need to use in the DOF Scale, to narrow down the precision a bit better than I guessed in the above paragraph.

wiltw said:

...
You will see it is currently focused at about 3.1'. The DOF scale shows that ...

Click to expand...

Forgive me if you already know this, but that lens isn't focused anywhere.

On the RB67 lenses, that scale is a guide only for the DOF calculations; actual focusing is performed by the rack on the body.

The issue the OP was describing involved how to determine the distance the lens is focused at - and that is done by the scale inscribed on the side plate.

From the interweb:

http://farm8.staticflickr.com/7268/8153300585_4782f942ec_b.jpg

............and by examining the viewing screen

Theo Sulphate said:

Forgive me if you already know this, but that lens isn't focused anywhere.

On the RB67 lenses, that scale is a guide only for the DOF calculations; actual focusing is performed by the rack on the body.

The issue the OP was describing involved how to determine the distance the lens is focused at - and that is done by the scale inscribed on the side plate.

From the interweb:

http://farm8.staticflickr.com/7268/8153300585_4782f942ec_b.jpg

Click to expand...

Oh, duh!

wiltw said:

Click to expand...

Yeah, sorry. I figured you probably knew it, but others not familiar with RB lenses might think the moving ring on the lens actually focuses.

Excellent DOF description though.

There are at least two mamiya RB lens catalogs on the Butkus website, they have depth of field tables which you can use to find the hyperfocal distance that suits your usage.

Thanks. Good point the scale on the side of the body. I would probably close down the aperture by 1 step further too ....

The Mamiya RB67 user manual gives an example of DOF Scale use, with the 90mm at f/11 and focused at 15' , the DOF zone is said to be 10' to 30'.
In comparison, a DOF calculator programs says

• 'manufacturer standard' vision results in a DOF zone of 11-26'
• 20/20 vision results in actual DOF zone of 13'-17'

So that gives you an idea of just how good (not) the DOF Scale on the side is...it would take use of f/32 on the 20/20 vision DOF calculator (11' - 25') to match what Mamiya says happens at f/11 !

I don't think the scale on the body is very useful for focus; it's primarily there to show exposure compensation for bellows extension.

DOF charts and scales are created for fairly not-critical use. They cannot take into consideration all of the factors necessary for good photography, such as subject matter and individual user's preferences. Consider the typical pinhole photograph: it appears unsharp, but that's what the photographer intended. Consider the degree of enlargement: Ansel Adams photographs that are magnificent when presented as he intended are dismally unsharp in some posthumous exhibits. There is much information on DOF available online. Here are two articles: http://trenholm.org/hmmerk/SHBG09.pdf and http://trenholm.org/hmmerk/SHBG10.pdf.

wiltw said:

The Mamiya RB67 user manual gives an example of DOF Scale use, with the 90mm at f/11 and focused at 15' , the DOF zone is said to be 10' to 30'.
In comparison, a DOF calculator programs says

• 'manufacturer standard' vision results in a DOF zone of 11-26'
• 20/20 vision results in actual DOF zone of 13'-17'

So that gives you an idea of just how good (not) the DOF Scale on the side is...it would take use of f/32 on the 20/20 vision DOF calculator (11' - 25') to match what Mamiya says happens at f/11 !

Click to expand...

I tried to figure what circles of confusion (COC) your DOF app. surmises :

10' to 30' DOF for 15' focusing relates to COC = 0.08mm
11' to 26' DOF for 15' focusing relates to COC = 0.06mm
13' to 17' DOF for 15' focusing relates to COC = 0.02mm

The DOF given by Mamiya is a little laxist indeed.
The DOF tables for medium format (6x9 or 56x72) are usually computed for COC = 0.06mm
Of course, if you use very fine grain films, shoot on a tripod, want to enlarge much, you may choose a smaller COC, but 0.02mm seems to me more like the limit for 35mm shooting.

What seems clear to me is that distance scale on a plate on the camera body and only informative DOF scales on the lens, is not very comfortable (nor even suitable) for precise DOF control.

However, I use this (in my opinion) very simple method to compute mentally my own DOF tables "on the spot" :

First, I have computed and remember the hyperfocal of the given lens at a given diaphragm using this simple (however almost exact) formula :

h = f² / cD

h is the hyperfocal to remember, f is the focal length of the lens, D the diaphragm and c the COC that I want. Let's do it for the lens mentionned (f = 90mm) at D = 16 and with a rather small (for 56x72) COC of 0.05mm ; you find h = 10m (and have to remember just that).

Now, here are two simple (however almost exact for any focusing distances) formula that use only the focusing distance d and the remembered hyperfocal h to compute :

d' = hd / (h+d) the nearest acceptable distance for your chosen COC
d" = hd / (h-d) the farthest acceptable distance for your chosen COC

The interesting fact with this computation is that it uses only an addition or substraction, a multiplication and a division, which can be performed (almost) mentally and on figures in the same scale (all in meters !).

For instance, take d = 2m

d' = 10x2 / (10+2) = 20 / 12 = 1.67m (down to 33cm in front of the focused distance)
d" = 10x2 / (10-2) = 20 / 8 = 2.5m (up to 50cm behind the focused distance)

Easy ?

However, you had to remember the hyperfocal h = 10m only for D = 16 !

Because (look at the first formula) you see that D is under the fraction bar. so when you open the diaphragm (smaller Fstops) h will be proportionnaly larger and vice versa. So knowing only h=10m for D=16, you easily deduce :

h=14m for D=11
h=20m for D=8
h=28m for D=5.6

etc. and :

h=7m for D=22
h=5m for D=32

and so on.

I say I do this mentally, because DOF control needs not to be utterly precise, but using these simple computations instead of the "informative" scales on the Mamiya lenses, gives you the opportunity to choose your own COC ! And apart the "one to-be remembered hyperfocale h", the focusing distance d is the only parameter that you need to figure (roughly - either reading it on the distance scale plate or by pure estimate).

Bye, Paul

I'd use the scales provided by Mamiya. They take into account the exact optical design of the lens. While a DOF calculator is more generic.

Remember, Mamiya loves us. Mamiya is salvation, peace, love and understanding.

If you want more precision (i.e. a smaller circle of confusion), just use the scale that corresponds to a wider aperture than the aperture you'll be using. (i.e. shoot at f8 using the DOF values that correspond to f4)