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 Author Topic: understanding exposure and infinity focus  (Read 10289 times)
eatstickyrice
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 « on: July 26, 2007, 10:13:16 AM » Reply

Hi,

Right now I am working my way through Bryan Peterson's book "Understanding Exposure". On page 39 he mentions that for storytelling apertures he sets his lens to a maximum aperture of f/22, and then he presets his focus so that the distance of two feet is aligned directly above the center mark on the lens. However, I'm a bit confused if the two foot mark is supposed to be one of the green numbers on Canon lenses? Bryan's point in his explanation seems to be that doing so would allow for everything in the picture to be in focus. Is he going for infinity focus here? I think I am confusing Bryan's approach with infinity focus. Could some of you please clarify this for me?

Rick
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richarddd
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 « Reply #1 on: July 29, 2007, 12:07:52 PM » Reply

For maximum depth of field, try focusing at a distance about 1/3 of the way from the nearest to the farthest point you want in focus.  Google "hyperfocal distance."  Experiment a bit (or more than a bit).

Usually, the two foot mark means just that for a lens with a distance scale.
 « Last Edit: July 29, 2007, 12:08:44 PM by richarddd » Logged

Mort54
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 « Reply #2 on: July 29, 2007, 04:25:55 PM » Reply

Quote
Hi,

Right now I am working my way through Bryan Peterson's book "Understanding Exposure". On page 39 he mentions that for storytelling apertures he sets his lens to a maximum aperture of f/22, and then he presets his focus so that the distance of two feet is aligned directly above the center mark on the lens.
From your description, it sounds like he's talking about prefocussing his lens at the hyperfocal distance. The hyperfocal distance varies by focal length, so each different lens focal length will have a different hyperfocal distance. 2 feet would be the hyperfocal distance for a wide lens (I don't have a hyperfocal table in front of me, so I can't tell you exactly what focal length lens he's talking about). You'd need to use a different distance if you aren't using the same focal length lens that he's talking about.

Be advised that setting a lens to f/22 will rob the image of some sharpness due to diffraction effects. Yes, you'll get greater depth of field, but the whole image will not be as sharp as if it had been shot at larger apertures. Also note that using hyperfocal focussing does not yield as sharp an image as if you had focussed exactly on the point you want to be optiimally sharp. Hyperfocal is a convenient compromise setting that yields an overall mostly in focus image, but it's not necessarily the optimal focus. If you need a specific part of the image to be tack sharp, you're better off focussing specifically on that particular part of the image.

By the way, beware of approaches that tell you to use a fixed collection of settings for all images. These approaches can be convenient under the right circumstances, but otherwise they are just poor compromises. The beauty of a modern DSLR is the degree of control it gives you over the photographic variables. Advising use of f/22 and hyperfocal focussing just turns your wonderful DSLR into an expensive point and shoot.
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GregW
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 « Reply #3 on: July 29, 2007, 06:43:23 PM » Reply

Peterson is describing Hyperfocal distance but doesn't refer to it specifically.  He writes well imo but the book could do with a 'digital update'.  For example maximum depth of field and sharpness of the flower field on p.39 would not be achieved with a digital SLR using his f/22, 20mm setup.  This is in large part due to diffraction but that's another topic.

I'll try and offer a very practical way of describing hyperfocal distance to fill in some of the gaps in Peterson's description.  As he's a Nikon shooter the calculated hyperfocal distances I quote are based on a 1.5x Nikon sensor.

Peterson describes the following scenario, focal length of 20mm, f/22.  For that scenario the hyperfocal (H) distance would be 0.90m (Calculated based on body and lens) everything from H/2 (H divided by 2) would be in or of acceptable sharpness i.e. 0.45m.

You can dial 0.90m in using the distance scale window on top of the lens.  If you don't have one or it's not very detailed you can either measure or approximate the distance, focus, lock the focus and recompose.

If you were to change the aperture to a more digital friendly f/11 the hyperfocal distance would be 1.8m and everything from H/2 would be of acceptable sharpness i.e. 0.90m

The theory is one based on producing an 8*10 print with acceptable sharpness.  As has already been said if you have a specific subject in the composition that you want or need as sharp as possible then focus on that instead.

Digital hyperfocal distance is fixed based on your body and lens combination.  There are many online calculators which will help you make a table for your equipment or you can calculate it yourself.

Let me know and I'll give you an example of the equation.
 « Last Edit: July 29, 2007, 06:46:27 PM by GregW » Logged
eatstickyrice
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 « Reply #4 on: August 03, 2007, 02:58:56 AM » Reply

Thanks to all of your for your replies! I am trying to take more control of my photography these days, and have found your comments helpful.

Rick
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Bravin Neff
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 « Reply #5 on: September 06, 2007, 07:57:00 AM » Reply

I shoot Nikon 35mm film and 1.5x digital capture. The things I’m going to say apply universally, but you need to know some specifics of your camera.

The hyperfocal distance is directly connected to your circle of confusion value, which is directly related to the size of your imager. The larger the imager, the larger the circle of confusion value.

Here are the values and formulas for my cameras, where D = circle of confusion, and are easily memorized:

D = .025mm for 35 mm film
D = .020mm for 1.5x crop factor.

A Canon 1.3x camera would be somewhere about .023. A 6x7 medium format would be considerably larger. A 4x5 view camera would be way larger.

So here's the hyperfocal distance formula:

H = (L x L)/(F x D) This is your hyperfocal distance.
L = Focal Length of your lens. If you use a zoom, you need to note your setting.
F = Aperture, in F-stop values.

Your actual closest focus will be H/2, or half your hyperfocal distance. Notice that the D is in millimeters. If you want your hyperfocal distance in feet, you’ll need to convert at the last step. You’ll end up multiplying your result by 25.4 (millimeters in one inch) and then you multiply that by 12 (inches in one foot). But knowing this, you can make your life easier and just do it up front. In other words, you can convert your “D” values to this:

D = 7.62 (.025 * 25.4 *12) for 35mm film.
D = 6.1 (.020 * 25.4 *12) for 1.5x crop factor.

Thus my equation becomes:

H = (L x L)/(F x 7.62 or 6.1, depending on my format).

So lets say I’m shooting a city scape. I’ve got my 20mm lens on my FE2 35mm camera, and I’m thinking f5.6 is about right. Thus:

L = 20mm (lens focal length)
F = 5.6 (f stop setting)
D = 7.62 (circle of confusion  with feet and inch constants thrown in)

Therefore:

H = (400)/(5.6 x 7.62) = 9.37 feet. This is my hyperfocal distance. The actual focus will be roughly 5 feet in front of me to infinity.

After studying the formula, you can see the hyperfocal distance has everything to do with the circle of confusion value, which has everything to do with the imager size. This explains why Ansel Adams and his buddies named their group “F64.” This kind of f stop is unheard of in smaller formats like 35mm and modern DSLRS, but necessary to achieve hyperfocal distances on large view cameras.
 « Last Edit: September 06, 2007, 07:58:45 AM by Bravin Neff » Logged
Bravin Neff
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 « Reply #6 on: September 06, 2007, 07:57:30 AM » Reply

Sorry, double post.
 « Last Edit: September 06, 2007, 08:12:35 AM by Bravin Neff » Logged
Hank
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 « Reply #7 on: September 06, 2007, 10:21:23 AM » Reply

I'm acquainted with Bryan, and in fact took a workshop with him when this paricular book was new.  In the course of his lectures and a field exercize we spent quite a bit of time on this particular technique.

It was in fact written for film cameras and wide angle lenses, specifically his beloved Nikkor 20mm f/2.8 lens.  He was most certainly working with hyperfocal principles, even if he expressed it differently.  His point in lecture and field demos was that using this technique rather than AF and P settings freed you to simply react to what you saw through the viewfinder and around you without distracting yourself about camera technicals.  With longer lenses, or now with digital capture, both the focus point and the aperture certainly would have to be altered for best performance.

When film was contemporary to the book and even now with digital capture, his points about "story telling" and minimizing your camera concerns are both valid and valuable.  Whether he updates the book or writes another, I agree with his thesis that it's important to become so familiar with your gear that the subjects of your shots are your focus, rather than the camera in your hands.
 « Last Edit: September 06, 2007, 10:59:14 AM by Hank » Logged
seberri
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 « Reply #8 on: October 25, 2007, 02:42:33 PM » Reply

why dont  you use dofmaster
 « Last Edit: October 25, 2007, 02:42:42 PM by seberri » Logged
Argyllman
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 « Reply #9 on: November 17, 2007, 03:26:50 AM » Reply

Quote
I found this of intrest
[a href=\"http://www.trenholm.org/hmmerk/DOFR.html]http://www.trenholm.org/hmmerk/DOFR.html[/url]
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http//web.mac.com/jwrevie/welcome
simonkit
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 « Reply #10 on: November 24, 2007, 05:57:05 PM » Reply

I'd also highly recommend this book, learnt much from it - very well written !!

simon
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seberri
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 « Reply #11 on: November 25, 2007, 12:03:06 AM » Reply

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The rule for determining what objects are resolved is ultra-simple. We focus our lens exactly at some distance, D, from our lens. An object one-tenth of the way back from D, towards our camera (that is, at a distance of 0.9 D), will be resolved if it is at least one-tenth as big as the opening in our lens diaphragm

very interresting
 « Last Edit: November 25, 2007, 12:06:00 AM by seberri » Logged
simonkit
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 « Reply #12 on: December 02, 2007, 06:05:54 AM » Reply

Another excellent book by UK landscape photographer Peter Watson:

"Light in the Landscape"

simon
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