Mamiya RB 150mm SF lens and DOF

tkamiya · Dec 7, 2014

I am wondering....

DOF (Depth of Field/Depth of Focus) is a function of aperture. (Yes, it has more variables, but with everything ELSE being equal, it can be reduced to it being a function of aperture)

Aperture is proportional to transmission of light. (more properly expressed in reverse, however...)

Then, what happens to DOF if you keep the aperture constant but place a neutral density filter in front of the lens?
AND... what happens to DOF if you place those disks that come with Mamiya RB 150mm lens inside?

I am thinking in the first case, DOF will not change as function of ND filters but I am not so sure about the second one. The disks the said RB lens use is a series of holes. In effect, it does change the aperture opening albeit in a lot of holes, not just a single one.

Does anyone know??

I'd appreciate very much if anyone wants to post a reply, if you are guessing or making an educated guess, indicate so, and if you do know the answer either by experience or in terms of mathematical reasoning, indicate so. Mine is more of a guess with some reasoning.

Gunfleet · Dec 7, 2014

So it's your theory that changing the shutter speed changes the dof too? After all changing the shutterspeed reduces the light landing on the light sensitive material too.

cramej · Dec 7, 2014

tkamiya said:
Then, what happens to DOF if you keep the aperture constant but place a neutral density filter in front of the lens?
.

Absolutely nothing.

tkamiya said:
AND... what happens to DOF if you place those disks that come with Mamiya RB 150mm lens inside?
.

It changes. The disks are your aperture and they are meant to be used with the lens aperture wide open.

nwilkins · Dec 7, 2014

i googled this quickly and found a Popular Photography article from July 1994 claiming that you do indeed get a slightly larger depth of field with a soft focus lens, though the depth of field is only increased in front of the point of focus. No idea if it's true - I don't plan on ever shooting a soft focus lens

Kirks518 · Dec 7, 2014

It's my understanding that DOF is not related to the amount of light being transmitted, so an ND would have no effect on DOF.

DOF has to do with the "path" the light has to take to get to the film plane. With a smaller aperture, (high f/stop number), the light has to bend less, so they line up together better (producing an apparent 'in-focus' area, or greater DOF). The reverse is true with a larger aperture (smaller f/stop number). The rays of light passing through the lens are bending more, and therefore do not line up as well, except on the focal plane.

With a soft focus lens, you now have the main aperture opening, and a bunch of other smaller apertures that the light is going through. Essentially, each of those holes makes another aperture, and getting all of those apertures to line up on the film plane isn't going to happen, hence, the soft focus effect.

Kirks518 · Dec 7, 2014

Oh, and my reasoning is based mostly on research and experience (although not with SF lenses).

tkamiya · Dec 7, 2014

Kirks518 said:
DOF has to do with the "path" the light has to take to get to the film plane. With a smaller aperture, (high f/stop number), the light has to bend less, so they line up together better (producing an apparent 'in-focus' area, or greater DOF). The reverse is true with a larger aperture (smaller f/stop number). The rays of light passing through the lens are bending more, and therefore do not line up as well, except on the focal plane.

THIS is where my confusion actually started. Indeed most of standard photography books present manifestation of DOF that way. I wasn't sure if this is indeed what is happening, or just a representation of a phenomenon.

Attached is an image of the disks that 150mmSF comes with. As you can see, there is a big hole in the middle and surrounding area has smaller holes. The total diameter of the light passing area does not actually change, whether one of these disks are in place or not. Just that outer perimeters pass light less because of obstructions around the small holes. As such, the graphical representation on books tend to get fuzzy on applicability.

More over, as I understand it, the way this lens achieve soft focus lens is that center part is ground just like regular lens; however, outer area is slightly "out of curve" so that light do not focus at the same point as the center part - thus soft focus effect. Taking that with shape of the defucus inserts, it appears it wasn't intended to change DOF but amount of defocus verses clearly focused area.

I am going to experiment with this, but before doing so, I would very much like to know what I am looking to prove or disprove.

Thanks everyone.

Kirks518 · Dec 7, 2014

Here are a couple of ideas for you to experiment with (and I don't know the answers or outcomes);

Focus and recompose with the soft focus. Focus on something in the center of the screen, then recompose so that in-focus plane is close to the outer edge. Will it stay in focus, or will it get softer?

Set the aperture to f/5, f/5.6, or f/6.3, and use something where you can easily see the DOF (newspaper at an angle, or one of the many set-ups for focus testing). Take a shot. Then take the same shot with the SF disk of the same f/number, and see if the DOF changes.

On a sidenote, do you find much use for the SF lens? I'm a gear junkie, and have an RB67, but I don't think I'd actually ever use an SF lens.

tkamiya · Dec 7, 2014

Hi!

I have this lens, 150mm SF. I played with it when I got it, then promptly forgot about it.... My girlfriend laughed at me saying, "You spend all this money and now you are buying a special lens to make it fuzzy??" She has a point there!

I was going play with this again because we need to take our self portrait. Then I started to wonder, how would these disks work in conjunction with controlling DOF?? (yes, I do know these are for reducing exposure with lens itself wide open....) I'm also wondering, would the degree of de-focus-ness stay constant?

I figure I should know my baring before spend a roll of film. Hence this post.

I think your plan is good. I was thinking something similar. The disks are supposed to make wide open (f/4) equivalent to f/5, f/5.6, and f/6.3. So half-click is involved. I'm actually not sure how much effects I'll actually see but I guess that's the whole point of experiment.

I might make use of a long wooden fence and take an oblique shot. That should show the results..... (or measuring tape with surveyor flags)

Xmas · Dec 8, 2014

The discs are for the age of the person

no disc 50, then 45, 40 and 35 your gfriend(s) should understand...
The depth of field alters with the disc.
but the soft blur and depth of field interact... fence posts wont help you... and you need to test with each 1/3 stop with each filter (and no filter).
But it is a studio lens you need to bracket 1/3 stops with transparency film.

RalphLambrecht · Dec 8, 2014

a ND filter has no affecton DOF.anyway, I set my f/syop always around f/8-11 to hit the lens' sweet spot and get a decent DOF.what are you trying to achieve?:confused:

cramej · Dec 8, 2014

tkamiya said:
The total diameter of the light passing area does not actually change, whether one of these disks are in place or not. Just that outer perimeters pass light less because of obstructions around the small holes.

It does change because the total area that allows light to pass changes with more holes or less holes. This is calculated to give you the 'effective aperture' that is printed on the disc for exposure.

tkamiya said:
More over, as I understand it, the way this lens achieve soft focus lens is that center part is ground just like regular lens; however, outer area is slightly "out of curve" so that light do not focus at the same point as the center part - thus soft focus effect. Taking that with shape of the defcus inserts,

The soft effect of the lens is caused partly by aberrations in the lens itself which are apparent above f/8 and partly by the discs with diffraction.

tkamiya said:
it appears it wasn't intended to change DOF but amount of defocus verses clearly focused area.

Yes, but you do not have a 'defocused area' and a 'clearly focused area' like you would on a Nikon DC lens that remains sharp at the in-focus areas and allows you to change the amount of defocus. You have an overall diffusion at different strengths when you use (or don't use) the discs. Basically the effect is strongest wide open with no disc, then the largest aperture disc, and so on. You can also change the effect by stopping the lens down while using the discs.

tkamiya · Dec 8, 2014

RalphLambrecht said:
a ND filter has no affecton DOF.anyway, I set my f/syop always around f/8-11 to hit the lens' sweet spot and get a decent DOF.what are you trying to achieve?:confused:

I am talking about RB 150mm SF which is SOFT FOCUS lens. So "sweet spot" takes different meaning. By F8, the soft focus effect is GONE. According to instructions, it is designed to be used wide open or with the inserts I've been talking about.

Xmas · Dec 8, 2014

tkamiya said:
I am talking about RB 150mm SF which is SOFT FOCUS lens. So "sweet spot" takes different meaning. By F8, the soft focus effect is GONE. According to instructions, it is designed to be used wide open or with the inserts I've been talking about.

It behaves like a normal lens by f/11...

John Koehrer · Dec 8, 2014

Gunfleet said:
So it's your theory that changing the shutter speed changes the dof too? After all changing the shutterspeed reduces the light landing on the light sensitive material too.

Nope OP's question was limited to ND filter and effect if any, on DOF.
Shutter speed has no effect on DOF

Gunfleet · Dec 8, 2014

That was my point, but thanks.

flavio81 · Dec 9, 2014

Xmas said:
The discs are for the age of the person.

LOL!!! Bravo !!

tkamiya · Dec 9, 2014

Xmas said:
The discs are for the age of the person

no disc 50, then 45, 40 and 35 your gfriend(s) should understand...
The depth of field alters with the disc.
but the soft blur and depth of field interact... fence posts wont help you... and you need to test with each 1/3 stop with each filter (and no filter).
But it is a studio lens you need to bracket 1/3 stops with transparency film.

So I need to get 3 girlfriends and test each disks. Got it.

Xmas · Dec 10, 2014

tkamiya said:
So I need to get 3 girlfriends and test each disks. Got it.

close but the fourth option is no disc with two ND filters for f/4, 5.6 & 8 exposure at f/4 on lens. Or for f/5.6, 8 and 11 at f/5.6 on lens, etc.

Pack two ND filters x2, x4 and two film speeds for out of studio!

tkamiya · Dec 10, 2014

Thank you.

Can you give me a general trend? I know, without the disks, smaller the aperture (size) less softness. What would happen with the disks? Smaller the center hole and more numerous those little holes, more softness or the other way around? Based on your age comment, I'm guessing number of little holes are proportional to the degree of softness....

So wide open and whole bunch of holes disk, I get the maximum softness?

Xmas · Dec 10, 2014

Ok I don't think of it from that direction

My lens is normal sharp at f/8 or smaller - photo of a star a point.

At f/4 no filters you get approximately 3 times the light in a big circle spread around the core point.

The discs reduce both the amount of spill in total intensity and the taper of the spill.

There is a convolution ( a transform ) between the disc pattern and the spots surround.

You need to waste a 120 on Sirus two shots each no filter /11, /4 and each filter in turn @ their effective apertures.
Print each at your normal enlargement.

Real portraits will be a different signature.

Ladies won't mind the soft effect.

M Carter · Dec 10, 2014

This comment struck me:

Xmas said:
But it is a studio lens you need to bracket 1/3 stops with transparency film.

The SF is primarily a portrait lens - I can't imagine shooting product or landscapes with it... does anyone actually bracket a portrait session?

Does anyone shoot portraits on E6? I shot years of fashion with E6, but again, I never bracketed a fashion shoot - what if the best shot, the shot the client wanted, was an off-bracket?

People on E6 (to me, back in the day) = meter, polacolor 100 test, open 1/3-1/2 stop from 'roid exposure (for EPP anyway), snip test at the lab, adjust processing to suit, usually 1/4 push or so for some snappiness. Maybe a 10 or 20% magenta CC filter, too. But bracketing? Can't see it for E6 people.

Just recalling how good those polaroid emulsions were a couple decades ago... 4x5 polaroid, a blow dryer and flatbed scanner? That was my "digital camera" for several years.

Xmas · Dec 10, 2014

M Carter said:
This comment struck me:

The SF is primarily a portrait lens - I can't imagine shooting product or landscapes with it... does anyone actually bracket a portrait session?

Does anyone shoot portraits on E6? I shot years of fashion with E6, but again, I never bracketed a fashion shoot - what if the best shot, the shot the client wanted, was an off-bracket?

People on E6 (to me, back in the day) = meter, polacolor 100 test, open 1/3-1/2 stop from 'roid exposure (for EPP anyway), snip test at the lab, adjust processing to suit, usually 1/4 push or so for some snappiness. Maybe a 10 or 20% magenta CC filter, too. But bracketing? Can't see it for E6 people.

Just recalling how good those polaroid emulsions were a couple decades ago... 4x5 polaroid, a blow dryer and flatbed scanner? That was my "digital camera" for several years.

I always bracketed wedding formal shots 1/3 of a stop.
My 150 has been out on rainy days.
I've only got the 150 between 90 & 250.
At f/8 it is a normal lens.
I do use Fuji pack film.

flavio81 · Dec 10, 2014

Xmas said:
I always bracketed wedding formal shots 1/3 of a stop.

You are very exacting!

But were you able to do this on the RB? I ask because the lenses have half-stop aperture clicks, and the shutter can not be set in-between stops, as far as i know.

M Carter · Dec 10, 2014

Xmas said:
I always bracketed wedding formal shots 1/3 of a stop.
My 150 has been out on rainy days.
I've only got the 150 between 90 & 250.
At f/8 it is a normal lens.
I do use Fuji pack film.

Just curious - what happens when the bride wants an off exposure? Or do you only show the "on" exposures?

That would stress me out a bit, I think, cutting my deliverable frames by 2/3 (but I never did portraits or weddings, just fashion print where you wanted as much deliverable as possible). Never heard of bracketing people shots is all.

Mamiya RB 150mm SF lens and DOF

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