DOF-I'm Confused?

[VinceC]

>>Ok, I'm always hearing people talking about stopping down their 1.5 Summicron to give them the same DOF as an Elmar 3.5 My question is this. Is DOF governed by the len's light transmitting design or by the fact that stopping down a fast lens gives it the pinhole effect? A small enough pinhole should theoretically have an infinite DOF. My 500mm f8 SLR lens has about 6 inches of DOF but my f1.2 stopped down to f8 doesn't need to be focused. Please unconfuse me. Or unconfuse "them". Stu<<

I'll hazard another stab at this.

The "pinhole" idea is pretty close to what really happens. It's my understanding that as the lens aperture gets smaller, an increasing portion of the transmitted light rays are parallel or closer to parallel when they strike a given point on the film -- the above-mentioned cone-shaped zones of focus are increasingly long and narrow, which stretches out the area of acceptable focus. There was a group of large-format photographers in the 1930s (or thereabouts) who named themselves the f/64 Group with the goal of eliminating selective focus and showing everything sharp.

Pinhole cameras do have very extreme f/stops -- in the realm of f/200- f/300 or more. They work by transmitting only the parallel light rays, eliminating the need for a lens to shape the flow of light. So pinhole photographs have remarkable depth of field. Another example: I'm nearsighted and need eyeglasses, but if I make a tiny pinhole between the tips of my fingers, I can see a dim sharp image without my glasses because all the light rays are parallel or nearly parallel.

Another variable, mentioned above, is the focal-length of the lens. This also impacts whether or not the optical rays are paraellel, because the f/stop is based on the focal length. A 500mm lens at f/8 has a much larger opening than a 21mm lens at f/8. This is especially easy to see on a rangefinder format because most 35mm, 28mm and 21mm lenses are non-retrofocus designs where you can see the tiny size of the aperture openings.

For the most part, lenses of the same focal length (say, 50mm) will behave more or less alike at comparable f/stops, as far as what's in focus and what isn't; that's because the size of the aperture circle would be more or less the same. With a longer lens, like a 500mm, f/8 is still a pretty large circle, so the light rays aren't very close to parallel. On a 21mm lens at f/8, the aperture opening is so tiny that the light rays hitting a given spot on the film are nearly all parallel and the depth of aparent focus is quite deep -- 3 feet to infinity.

 

[VinceC]

>>why should an inefficent lens design, a slow lens, have the same dof at a given distance at a given aperture simply because of it's poor design.<<

A faster lens of the same focal length effectively increases the maximum size of the aperture circle. That's why an F/1 lens is huge and an f/3.5 is svelte. It takes an enormous amount of research and precision for the image quality of the f/1 lens to even begin to approach the inherent quality of the f/3.5 lens -- again, because of the percentage of light rays that are at or close to parallel.

 

[johne]

In practice, one might want to look through the lens stopped down to determine the depth of focus but that would eliminate the rangefinder and return the field to the SLR. Alas!

Johne

 

[Stu W]

Vince, I think the "more or less" clinched it for me. I was thinking in absolutes. Thank you. Stu

 

[VinceC]

Photography is a wonderful blend of absolutes and variables, science and art, moods and physics.

 

[sfb_dot_com]

OK, I meant to say Summarit, not Summicron as was pointed out. My initial confusion was that why should an inefficent lens design, a slow lens, have the same dof at a given distance at a given aperture simply because of it's poor design.

I'd dispute the assertion that a slower lens is an inefficient design. Slower only refers to the length of time the shutter has to be open for sufficient photons to reach either the sensor or the film. In fact I'd say that the f3.5 Elmar may well be a more efficient lens for a number of reasons. Since Schrodinger's cat has already been yanked mewling and puking out of it's metaphysical box, I'll therefore employ that other well known paradigm of Occam's razor to support my assertion.

My apologies for being a little contradictory, but on to my main point which is that this thread has been an excellent one for stimulating the little grey cells into action. I think that all the posts have helped to shed a little light on to an issue which whilst apparently straightforward is actually pretty fundamental to our pastime of photography. It has certainly helped me with my understanding of these matters. And whilst it has required some thinking to be done on my part, I feel that the way some abstruse concepts have been explained would put most textbooks to shame.

Regards to all

 

[HansDerHase]

I thought the smaller aperture diameter should increase depth of field, thinking back to pinhole cameras. I'm convinced by the math but still confused by the concept. Stu

Well the confusion might come from that you equal f-number with aperture-diameter. This is only true at the same focal length (yes mr. lens-designer it also depends on the lens design but we are close enough so keep quiet).

That means your physical opening
at f/1.2 with a 35mm lens is approx. 35/1.2 = 29 mm
while
at f8 with a 500mm lens is approx. 500/8 = 62,5 mm

The smaller the actual/physical opening the wider the DOF given the same focus distance (and film format and resolution and so forth).

 

[bmattock]

Stu,

Lenses in which the aperture measures the amount of light passing through them are known as 'cine' lenses and have the apertures marked in 't-stops' instead of 'f-stops'.

Adding a non-clear filter does reduce the AMOUNT of light passing through a lens, but does not alter the size of the hole through which the light passes. Therefore, the DOF remains the same. This is also true for slower or faster film - no effect on DOF.

In general, use this:


* To increase DOF, stand farther away or stop down to a higher aperture number.
* To decrease DOF, stand closer or open your lens to a lower aperture number.
* Longer lenses have less DOF for any given aperture if the distance remains the same.


The above will cause purists and photo math weenies to groan, but it is 'true enough' for most straightforward photography, presuming the same camera and film size used.

There are a lot of DOF calculators on the web. All ask the same things:

1) The size of the sensor.
2) The focal length of the lens.
3) The distance of the subject from the lens.
4) The aperture used.

Without regard to ANY lens design, the effective DOF can be calculated. I am speaking in general terms that make sense to the human eye, without regard for 'circles of confusion' and size of the resulting print or the distance of the subject, etc. Most people who talk about DOF don't mean that anyway. They're talking abou practical things, like "How do I make the background in a portrait less focused and more 'fuzzy'?"

The way to increase those nice OOF (Out-Of-Focus) effects in a portrait is to use a longer lens, or move closer to the subject, or open up the aperture. Or, a combination of any of the three.

OOF effects are a function of taste, but generally are seen by the viewing public as a sign of photographic quality, even without knowing why. I feel that's because most snap-shot type cameras use high aperture values (f8 or higher) combined with flash to create deep DOF (everything in focus from up close to far away). When a photographer has successfully used selective focus to isolate a subject and leave some foreground and background blurred out with OOF effects, it is a visible mark of advanced photographic ability. It is also one of the first things that advanced amateurs learn after taking their auto-everything SLR off of 'auto' for the first time.

The next step after getting a good handle on DOF and OOF effects is to give some thought to mastering 'hyperfocal' focusing. This allows you to place your subject anywhere in the DOF range (be it 1 inch or 100 feet) that you prefer, instead of the standard 1/3 in front and 2/3 behind your focal point.

Hope you find this helpful!

Best Regards,

Bill Mattocks

 

[jaapv]

Efficient is not the right word in lens design anyway. The most efficient lenses ever built are the Leica Telyt V 400, 560 and 800.
One element, two or (800) three kitted lenses, near Apo quality, a brilliant (in all senses ) design. These, btw., are RF lenses.Still,far less efficient tele designs caught up on them in size and speed and handling.

 

[bmattock]

Efficient is not the right word in lens design anyway. The most efficient lenses ever built are the Leica Telyt V 400, 560 and 800.
One element, two or (800) three kitted lenses, near Apo quality, a brilliant (in all senses ) design. These, btw., are RF lenses.Still,far less efficient tele designs caught up on them in size and speed and handling.

It is common for people to equate faster lenses with higher quality. After all, they tend to cost more.

By the same token, I've spoken to friends at Kodak who explained why they are changing their film designation for consumer color print films - ISO 100 and ISO 800 seem the same to many consumers - so since 800 is more expensive, they think it is 'best' and buy it without regard for how they intend to use it.

When speaking of film, higher ISO obviously does not equate to 'better'. Better for a given set of circumstances would be more appropriate.

The same can apply to camera lenses. A 'faster' lens (ie, a 50mm f1.4 versus a 50mm f1.8 from the same manufacturer for the same camera) is 'better' for a given set of circumstances:

* Less available light / slower film and no flash wanted or available.
* More extreme OOF effects desired via shallower DOF.
* Different lens design creates a different 'look' in the resulting image.
* Lens manufactured to a higher set of quality standards.

This last item can cause more confusion. One can GENERALLY say that it is easier to manufacture a high quality lens that is slower rather than one that is faster. That is, all things being equal, a slower lens of the same focal length and the same manufacturer for the same camera would be capable of better image quality wide-open than than a faster lens.

However, this is sometimes NOT the case, due to politics and some other concerns that have more to do with marketing than anything else. You see, once upon a time, a 50mm lens was considered a 'standard' lens and was sold with most SLR cameras. Manufacturers would therefore tend to make them their showpiece - some say that Canon, Nikon, and some others made them at a loss to impress people into buying MORE lenses from the same maker. Thus, more R&D and precision manufacturing was given to the venerable 50mm f1.4 than to the f1.8, and the end result COULD BE that the f1.4 would be superior in technical quality to the f1.8 at every aperture.

A word about different lens designs. Commonly-seen designs on RF and SLR cameras are typically Tessar and Gauss - many more Gauss than Tessar. A Tessar design is never (as far as I know) seen with an aperture faster than f2.8, while a Gauss design is often f2.0 or faster. The differences in lens design ensures that Tessars are considered to have sharper edge-to-edge sharpness than Gauss designs, but that comes down to personal taste - not really an issue of 'efficiency'.

Best Regards,

Bill Mattocks

 

[Nikon Bob]

Interesting thread and I am with Bill in keeping it simple. Sometimes knowing what is might be better than knowing the technicalities of the why it is which cloud the issue of using DOF in a practical way.

Bob

 

[mongo141]

AMEN! Bill

 

[jaapv]

You're right, Bill, but still "horses for courses" will apply. In the case of 50 mm lenses, i would prefer a Summicron 2.0 to a Noctilux 1.0 for summer shooting on the beach!

 

[bmattock]

You're right, Bill, but still "horses for courses" will apply. In the case of 50 mm lenses, i would prefer a Summicron 2.0 to a Noctilux 1.0 for summer shooting on the beach!

Personal preference should always be paramount when making a lens choice! I think it can also be helpful to have a basic understanding of why one lens gives a certain effect over another.

Best Regards,

Bill Mattocks