Focus question/confusion

[gt1]

Ok, I have an RB67 and 90mm f/3.8 lens. I've shot a few rolls with it and the in and out of focus areas in my

images have been pretty much what I expected according to my aperture. Until my last two rolls.

 

I was doing some color still life stuff on my front lawn, camera on tripod about 24" off the ground, with the

subjects on flat ground 6 feet away which I focused on. When focusing in order to get the split-image in the

focusing screen on the center of my subjects I tilted the tripod forward slightly to focus on them, locked focus,

tilted the tripod back level, and checked the focus with the matte area which at least to my eye still looked good.

 

I was shooting between f/5.6 and f/8.0 so looking at some depth of field charts I see in the in focus area should

be 5'6" to 6'7" and 5'4" to 6'11".

 

However, when I received the film back I'd say everything from 6'6" on to 15 ft is in focus. Could my slight

tilt of the tripod have thrown my focus off this much? Or maybe when I locked the focus maybe the focus knob

turned a bit? Or maybe gremlins moved my tripod when I went in side to grab a drink?

 

In any case, besides the fact that only the back half of my subjects are in focus, they were arranged within and

approx 18" diameter, I was surprised to see how sharp objects in the background were. Particularly at f/5.6 I

was expecting things 15 feet from the camera to be pretty blurry.

 

Maybe I just need to repeat the exercise to see where I went wrong.

[randall ellis]

Depending on how much you tilted the tripod to focus you may have altered the results greatly. When I am focusing on something that is not in the center of the ground glass, I usually rely on the flip-up magnifier on the waist finder to make sure that I'm in good shape. With the eye-level prism it's a bit more difficult to tell, at least for me.

 

Regardless of which finder you are using, you can use the small lever on the side of the lens to get a depth of field preview if you are unsure about how much will be in focus. Simply set the aperture and slide the square, black level down (it's on the side of the lens). This will close the aperture and allow you to see the image that will be projected onto the film plane. When doing this the rangerfinder circle will become black in part, so don't worry so much about it and pay more attention to the entire ground glass. You should be able to see the image well in good light, enough to judge focus, down to around f/11 or f/16. In lower levels of light it does get more difficult to see though...

 

It takes a little getting used to at first, but once you try it a few times you'll get the hang of it. Generally speaking, it is best to focus the camera in the position that it will be used in to ensure that problems like this do not occur. Changing position once the camera is focused will result in a change in what is in focus as you have seen here.

 

- Randy

[gt1]

Thanks Randy. I need to get used to the full ground glass of the focusing screen and the depth of field preview. I only have the waist level finder as I bought this camera for still lifes and possibly landscapes. Practice, practice, practice.

 

I'm chalking up the poor focus to my inexperience but I still don't get why I had so much DOF. With camera to subject distance of 6 ft, at f/5.6 and the 90mm lens I should have had only a foot or so in focus, right? But I had around 10 ft in focus. The exposure is pretty close so it's not like the aperture didn't open far enough, right?

[randall ellis]

I use the waist level finder almost exclusively myself, but your tastes may vary of course. The eye level finder is not so good for those of us who wear glasses, and I don't have the magnifier for my prism, but even so, I like being able to see the entire area easily.

 

As for the depth of field, it sounds odd, almost like the focal point was behind the subject after you moved the camera, which made much of the area behind the subject be in focus. The farther away the focal point is from the aperture iris, the more area will be in focus, so if the focal point moved behind the subject, more of the background would be in focus. It is possible that the aperture didn't open all the way as it should, but that's easy to check. Just set the shutter to T, adjust the aperture, and trip the shutter. The iris will close down and stay there until you rotate the shutter speed dial, so you'll be able to see if it closes down differently for the different settings. As you stated though, if this were the problem you would have exposure problems, unless you were using print film and the lab adjusted to compensate. The way to check that would be to look at the negatives.

 

It is, of course, possible that the focusing screen is not seated properly, which could throw your focus off. You might want to remove it and then put it back, making sure that it is seated all the way down in the body. Just remove the waist level finder and pull the screen out. To replace it simply set it in place and gently press it down along the sides so that you don't get finger prints in the glass.

 

I'd try to make the same picture again and try to focus with the camera in the same position it will be in when you make the exposure to see if the results are any different. Check the depth of field preview as well to get a feel for how it works - it's a very valuable tool. If none of the above help, and you still encounter this on other rolls of film, you might want to try a different lens. I suppose that it is possible that at some point someone worked on the lens and the cells are not properly aligned, although I would think that the problems would be more evident than a simple focus shift.

 

- Randy

[gt1]

Thanks again.

 

"I'd try to make the same picture again and try to focus with the camera in the same position it will be in when you make the exposure to see if the results are any different. Check the depth of field preview as well to get a feel for how it works - it's a very valuable tool."

 

I did just that yesterday, hopefully will have the film back tonight.

 

Keep in mind that I had put 2 rolls through this camera previously and everything was fine, so most likely it was me moving the camera around more than I thought.

 

One thing, though, I don't see much change in the image when I push down the depth of field preview. When I have a small aperture it does get dark so I know the lens is stopping down. So with it wide open do I just need to look closer at objects in the frame to determine what's in and out of focus?

 

Thanks for suggesting reseating the focusing screen. I hadn't thought of that. But hopefully the roll I'm shot yesterday will be ok.

[gt1]

I ran through everything above, we'll see when I get my film back but I'm pretty sure I must have flubbed something. I had some brighter subjects, stopped down to f/32, checked DOF preview, went back to f/5.6, and am starting to see the differences in the finder. I just wasn't paying enough attention to detail before.

 

And it gave me the opportunity to clean substantial dust off the focusing screen.

[gt1]

Yeah, everything's fine. I got my last roll back this morning. Although the 90mm on my RB67 at f/3.8 seems to have a lot more depth of field than when I did a very similar shot with the 80mm at f/4 on my 645:<p>

 

<img src="http://homepage.mac.com/getosx/all_stars1.jpg">

 

I'm probably comparing apples to oranges though, I think I had the 645 a lot closer to the subject than I did with my RB67.

[randall ellis]

The focal length affects the depth differently depending on the image size at the film plane. A 90mm on a 6x7 negative will be fairly different than an 80mm on a 6x4.5 negative. I'm not a math whiz, but <A href="http://www.dofmaster.com/custom.html">this tool</A> will show you visually how the depth of field changes between these situations. Plug in both your lens lengths, and then select 6x4.5 and 6x7 alternately. The width of the marks shows you how the different setups change the depth of field.

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It has to do with distance from the subject and image size at the film plane, but I'm not good enough with the concepts to get into a lot of detail. Basically, you would have the same depth of field with the longer lens if you positioned the camera in such a way that the image was an equal size on the film plane as you have with the shorter lens on the smaller piece of film (unless I've gotten off the mark that is)...

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- Randy