Front vs. Rear Focussing

Hi guys and gals,

Sorry for the stupid question, but...

What's the effective difference between cameras that focus with the front standard, and cameras that focus by moving the rear standard? Do some do both?

TIA!

Steve

Yep, my Sinar focuses with either standard. The main difference is I believe that by focusing with the front standard, you are changing lens-subject distance and therefore perspective and size.

The main difference I have found is that when I use rear focus the camera back hits me in the head. I don't like trying to focus on a moving groundglass so I use front focus whenever I can, but not all my cameras have it, and sometimes my arms aren't long enough.

Richard Wasserman

Both of them simply adjust the distance between the lens and the film to bring a subject at a given distance into focus.
Leaving the back in place while moving the lens towards the subject has the effect of moving the camera slightly closer to the subject the subject. Moving the back towards you and leaving the lens in place has the opposite effect.

For tabletop work rear focus is preferable, because the lens to subject distance is more important when the subject is fairly close. For macro work it can be nearly impossible to focus with front focusing, and if the camera doesn't have rear focusing, then you need a macro rail that moves the whole camera, as you would use with a small or medium format camera.

For landscapes it doesn't matter as much, though it's nice to be able to slide the rear standard forward when you're using a wide lens. Still there are ways to manage on cameras that don't have this feature.

In general, I prefer rear focusing, but I use both, depending on which camera I'm using.

Oh, dear. There's a bit of confusion in this thread between focusing by changing magnification and focusing by moving the lens-camera assembly as a unit.

For general out-and-about photography with relatively distant subjects, I think we all focus by changing extension = changing magnfication. Its a matter of taste whether the front or rear standard is moved to change extension. With some cameras, i.e., most press and technical cameras, only the front standard is moveable. Focusing by moving the camera when working with distant subjects in in principle possible, but can't always be done.

For closeup photography, most people set magnification and move the camera-lens assembly as a unit to focus. In this situation, cameras with fixed rear standards are at a small disadvantage. One can pick a lens and magnification for which the subject has to be closer to the lens than the front of the focusing rail. Not possible, not fun.

Jay, if you change extension you change magnification. Try it, you'll see.

Moving the camera-lens assembly as a unit moves the plane of best focus without changing magnification. Try it, you'll see.

And when shooting closeup, most people set magnification and then move the camera to focus. If lens to subject distance is set first and extension is then changed by moving the rear standard to focus, magnification is changed. This plays hob with exposure compensation calculated for the initial magnification and it changes composition. This is one of the reasons why AF is not particularly helpful when shooting closeup; AF cameras change magnification to find focus, not what's usually wanted or done.

To get a little huffy for an instant, I've been shooting closeup with various flash rigs and Nikons since the early '70s. I now do the same with Graphics. Please don't tell me that my practice isn't what I say it is, and don't you dare tell me that what I do doesn't work.

jdef said:

is technically inaccurate. To select the magnification, the lens-subject distance must be determined, and remain constant during focussing.

Jay

Click to expand...

Dan Fromm said:

To get a little huffy for an instant, I've been shooting closeup with various flash rigs and Nikons since the early '70s. I now do the same with Graphics. Please don't tell me that my practice isn't what I say it is, and don't you dare tell me that what I do doesn't work.

Click to expand...

Watch out Boys and Girls......

INCOMMING!!!!!!

LOL

But since the focal length is a constant, fixing the film - lens distance and moving the object, or fixing the lens - object distance and moving the film plane, are exactly equivalent.

Here's a quick drawing to illiustrate what I said in my previous post.
Of course, the bellows extension would be slightly shorter in the back focused situation

Jay, I think we're somehow equivocating, which is a diagnosis, not an insult.

One can do the calculations starting from m and f to find extension and front node to subject distance. I think we agree on that. We can't disagree. I even have a spreadsheet that contains the calculations to help me understand what I can and can't do with my macro lenses.

Where we disagree is on what people actually do in the field.

My practice is to decide what magnification I want. Format, subject, and intended composition give me m. f and m give me extension. I set extension, then move the camera-lens assembly to put the plane of best focus where I want it.

At lowish magnifications (<= 1:0.88) with 35 mm, I set the lens to the magnification I want, then teeter back and forth to focus.

With my Graphics, at relatively highish magnifications for them (~1:3 to 5:1), I set the camera up, put it on focusing rail on tripod in nearly the right place, and focus by moving it forwards/backwards on the focusing rail. Below 1:4, I put the camera in more or less the right place and then fine tune with extension; this because my focusing rail doesn't have enough travel.

With 35 mm above 1:0.88, I set magnification, sometimes with a tape measure, and the proceed as with the Graphics. 1:0.88 is the highest magnification attainable with a 105/2.8 MicroNikkor AI/AIS on PN-11 tube. It is at or perhaps a little beyond the sensible limit for shooting handheld.

If I were to start by putting the camera in a convenient place and then adjust extension to put the plane of best focus where I want it, I'm not guaranteed the magnification I want. This is why I disapprove of using AF close up, especially with mobile subjects.

If I haven't been clear, my Micro Nikkors have magnification scales. No calculations needed, just dial and go. With the Graphics, to get the magnification I want I have to calculate extension given m and f. I use a little tape measure to set it. And then I'm ready to try to set up to shoot.

What operations do you go through to set up for shooting closeup?

Cheers,

Dan

Jay, you wrote:

"If you want to find the extension, or lens-image distance for a given lens-subject distance/magnification, you can do it mathematically by the following formula:

v=F x (1+m)

v= lens-image distance [=, you say, extension]

F= lens focal length

m= magnification"

My comment added. So far, so good, and no disagreement.

Your equation "v=F x (1+m)" can be rewritten as m = (f/v) - 1

How can you assert that changing extension doesn't change magnification? Your own symbolic arithmetic says it does.

Cheers,

Dan

As a purely practical matter, I find rear focus useful with really wide honkin' lenses where its possible to get the bed in the shot if the front standard is moved back from the most foreward part of the bed.

Jay,

I give up. But the derivative of v with respect to m is still non-zero.

Thanks, everyone.

The only camera I own (an old Kodak 2-D) focusses using only the front standard. I've been looking on eBay at some cameras that only focus with the rear standard, and I wanted to be sure I wasn't going to buy something that I couldn't or wouldn't use.

So, to summarize, there's very little practical difference if the lens-subject distance is great enough that you're not having to compensate for the bellows extension factor, correct?

Steve

seadrive said:

Thanks, everyone.

The only camera I own (an old Kodak 2-D) focusses using only the front standard. I've been looking on eBay at some cameras that only focus with the rear standard, and I wanted to be sure I wasn't going to buy something that I couldn't or wouldn't use.

So, to summarize, there's very little practical difference if the lens-subject distance is great enough that you're not having to compensate for the bellows extension factor, correct?

Steve

Click to expand...

The only practical difference comes with short lenses. That's why some press and technical cameras have drop beds. Otherwise, extension is extension. Moving the camera fore and aft is moving the camera fore and aft.

Cheers,

Dan

Apart from close-ups, when I sometimes move the camera, sometimes move the object and sometimes move either standard (hopefully that satisfys everyone), I use front focusing when shooting 5x4, and rear when shooting 10x8. The reason? I don't have the arms of an oran utang.

Pin-hole=No focus, Problem solved.

i've always done rear focus for the "big focus" and fine tuned with the front.
never thought or cared much about what the difference was between the two ... that is unless i was using a graflex slr or a speed graphic or as john kasaian said

John Kasaian said:

As a purely practical matter, I find rear focus useful with really wide honkin' lenses where its possible to get the bed in the shot if the front standard is moved back from the most foreward part of the bed.

Click to expand...

but even then i fine tune with the front ...