Depth of field and focus distribution.

This question might fall in the general photography class too, but I'm using a

GL690 right now, so any camera specific comments are welcome.

I've always had problems with DOF scales on lenses being too loose for big

enlargements. Now using a 50mm lens on the 690, it seems to behave the same way.

But aside that, one thing that really bothers me is even when using the right

aperture, focus never seems to be distributed evenly. The front half of the DOF

range always seems sharper. And shooting mainly landscapes, that comes out

painfully obvious when comparing front plane subjects with the skyline in the

background.

I experiment the same using sharp lenses on 35mm (usually a 35mm f/2 from Canon)

and now with the 50mm on the Fujica, it's far worse, the focus difference being

much more obvious in the far larger slides.

The 50 is terribly sharp, and comes close to a good 35mm prime (I wouldn't dare

to say equal, but i didn't see any difference under the loupe). And what comes

up: the sharper the lens the more obvious the difference between center DOF

areas and marginal ones. And when you want an even focused relaxed picture, it's

a bomb.

The center of my question would be: is there a problem with perception in the

distribution of focus? I mean, knowing that the front half of the DOF range

might be a couple of meters and the back half goes to infinity, may be the

marginally out-of-focus range is far longer in the back so we notice it a lot more?

Could there be an improvement if we shifted the hiperfocal distance a little

towards infinity so we change this perception? or stop down another point AND

shift it?

I'm not saying DOF range calculations are wrong, I just say subjective

perception of it, when you have at the margins of your DOF range 1 cm of barely

usable focus in the front, and 100m of it in the back, It looks as if the focus

is only good on the front planes. And when it's printed huge from a 6x9 slide

and the lens is terribly sharp as to render the center DOF range areas in

perfect focus, it ruins the even focus appearance effect. I even thought of

using f/32 only to get diffraction to even down the focus.

Or may be I'm getting it all wrong, Any opinions? Thanks in advance.

The word for the problem at hand is Hyperfocal focusing.<p>

DOF ranges are determined by first choosing a certain Circle of Confusion (CoC) - the acceptable atomic 'fuzziness' of a focus point. If you are particularly critical, or especially if you make enlargements viewed closer than the nominal 'normal' viewing distance, then the manufacturer's chosen CoC might be too generous (too large). In their literature for their earlier Super Wide camera, Hasselblad warns of exactly this and suggests that for large work the photographer not use the DOF scale on the lens, but one for a smaller CoC.<p>

Here's what you do - shortcut- choose a DOF scale from a lens of the same focal length intended for the 35mm format.<p>

About the foreground/background issue - indeed, that is exactly my experience. The area of "acceptable focus" isn't really 1/3 behind and 2/3 in front of the lens. It is less behind.<p>

TIP: Usually objects closer to the camera are larger, so will appear sharper (compared to the background) so you should bias the focus toward the background, or Infinity.<p>

If you like, I have a pretty good photo that demonstrates this. I'd have to go into work to put it up. Let me know.

Thanks Pico. I'm aware of hyperfocal distance and the CoC differences. The Fujica 50mm's scale works for a CoC around 45 microns, while for 35mm 30 microns would be more like it. It's easy to use the next smaller aperture marks to solve this (and a little more just to be sure).

What bothered me is the last point you addressed, that focus seems to be always sharper in the front half of the DOF range (not half in terms of distance, but in terms of the lens focusing scale).

Max

 

What do you mean by "next smaller aperture mark" ? ? ?

Do you mean the next smaller aperture opening or the next smaller (lower) aperture figure on

the

scale ? ? ?

Regards Jurgen

<i>What bothered me is the last point you addressed, that focus seems to be always sharper in the front half of the DOF range (not half in terms of distance, but in terms of the lens focusing scale).</i><p>

I cannot explain it, but like yourself after experiencing it often enough it's become intuitive.<p>

Have you considred a view camera? Or maybe something like the old SL66 setp with front tilt?

Jürgen , my fault, that didn't come out clear. I meant that when you know the DOF scale is designed to achieve a certain circle of confusion diameter, you can always set the focus for the DOF you want to achieve but use the next smaller aperture (if the dof scales show you could achieve it using f/11, you stop down to f/16).

Also, I was wondering if we could define some pragmatic rule that improved focus near infinity. For example, using the regular scale for the closer end, at f/11, but setting the infinity end at f/16 and see if and how that works at different apertures. That would produce a sort of hyperfocal distance biased towards infinity focus, I guess.

It's late and it's getting hard to think, but shouldn't that be "using the regular scale for the closer end, at <b>f/16</b>, but setting the infinity end at <b>f/11</b>"

Oh yes Q., Thanks. I got through it several times and seems I got mixed up. Infinity should be on the wider aperture, as you point out. What do you think about it? Shooting at F/16, setting hyperfocal distance with infinity set at the f/11 marking?

Max,<br><br>I do not 'believe' in DOF as a quantifiable entity. There are too many variables, and in the end is a subjective thing. You gave a good description of DOF's nature in your 'original post' above.<br><br>So my approach is different from yours. Since there's only one plane of sharpness, and you can't rely on DOF getting things in front and beyond that plane of sharpness in 'quasi focus", i don't even try. When shifting the plane of focus in an attempt to fit a scene into a DOF-range, you give up control of what is in true focus. Instead i rather focus on that thing in the scene that is the attention-getter, the 'focal point', the main subject (it may be a personal thing, but scenes that are 'interesting' from 'here to infinity' usualy aren't 'interesting' at all. A picture needs a focal point!).<br>The aperture, DOF, is then used to control the way the sharp, main bit of the image relates to the rest, and that in the usual way: you can isolate the main subject, draw more attention to it by reducing DOF, do the opposite, let it fade into the rest by increasing DOF. You then never do, or need to, rely on DOF stretching from well defined point A to ditto point B. It never does anyway.<br><br>Al this 'hyperfocal' and 'DOF range' business is squandering your chance to use sharpness as a technical and creative tool. It passes control to two supposed boundaries of an illusionary entity, and where true sharpness comes to rest becomes a matter of chance.

Here is how I focus for maximizing even focus:

Focus on nearest object I want in focus<br>

16----11----8-----5.6-----4-----2-----4-----5,6------8------11-----16<br>

....................................................l......................................<br>

Move focus ring to place that point on the stop that is one stop less than the stop I will be using (using f16 in this example).<br>

16----11----8-----5.6-----4-----2-----4-----5,6------8------11-----16<br>

...........l........................................................................................*(inf)<br>

 

If the infinity mark is also under a stop that is less than the one I am using the entire range should be in acceptable focus.

Q.G.

 

Very well said and i fully agree . The play with DOF is a very powerful tool in photography .

However , its not accepted by many clients

Many images require a big DOF , for example furniture , machinery or motorcars (look at

VICTOR by HASSELBLAD , page 36-41) , the Bugatti . The spectator should see every detail

.

Other items , like juwellery or portraits , for example , might gain or loose with a big DOF .

Two years ago , i had to photograph a hand crafted diamond ring collection . It were very

dainty , wonderful pieces , and i took very small and beautyful flowers as decoration .

(Goldregen and Glycinien) and a very small DOF , just to show a bit more than the

diamonds . The client liked all the arrangements and the lightning with soft shadows but

he wanted all details to be seen clearly and sharp . So my ideas did not match his and i

had to take all images again . I was frustrated , because it was hard work .

So again , the play with DOF is powerful , but might also be misunderstood by many

people .

Just as an addition , for those guys , who work in LF , there is a very good DOF and

Scheimpflug calculator available from RODENSTOCK , designed by Walter E. Sch�n , and he

recommends , that , if you want highest quality , to step down up to one complete value of

aperture . (f16 instead of f11 , for example) as long as you dont go beyond f32 , because

of increased diffraction .

This calculator is a very helpful tool and also a must for LF photographers .

I think that the impression that the far distance is not as sharp is because an object in the distance is recorded on the film in a much smaller area than that same object in the foreground. It contains less information and under enlargement may appear less sharp. Imagine an index card with typing on it. Placed in the foreground and sharply focused, the typed text should be legible. At 20 feet, no mater how sharp the focus you may not be able to read it and it may appear to be out of focus.

Maybe it helps to clearly state a DOF-related distinction.<br><br>A picture with a definite focal point expresses a view about the thing pictured, It says <b>this</b> is it!<br>It is an image.<br><br>Pictures without a single 'focal point' - those infinitesimal DOF thingies - certainly can have a valid reason for being. These thingies are meant for the viewer to roam and select his or her own points of interest in.<br>They are illustrations.<br><br>It's up to all of us to decide what we like to produce: images or illustrations.

Very interesting ideas. I agree on most cases, but sometimes I feel two points of focus in different planes can give an idea of direction. I like plane interaction and perspective, and that requires some of these tricks. On the way infinity looks, I posted a shot that shows that behavior, everything looks a little softer near infinity. It's very sharp though, more than I'd need, but the difference is there. Could it be an atmospheric thing? It was shot with the lens set at infinity.

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00J1XR

> even when using the right aperture, focus never seems to be distributed evenly. The front half of the DOF range always seems sharper. And shooting mainly landscapes, that comes out painfully obvious when comparing front plane subjects with the skyline in the background. <

 

I have picked up upon and refer to this part of your question only,

 

I have only scanned the other answers, forgive if this point has been mentioned.

 

I note you mention this phenomenon in landscape photography, it could be that your lens is getting too warm and the infinty end of its focus is not where it shoud be, hence a problem if you are working from your infinity mark on the lens barrel to set your focus for a particular aperture.

 

Some lenses allow for the heating of the lens and allow focussing `beyond` infinity.

 

A practical field solution would be to set the infinity mark of the lens barrel about one half stop [or to be super safe one full stop] larger than the actual aperture being used, thus ensuring everything to infinity is in focus.

 

Granted this will create a smaller depth of field.

 

Regards WW