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03-14-2011, 09:48 PM   #1
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Q about: Hyperfocal distance + focus scales

Hi everyone,

A few months back I treated myself to the 31ltd and K5, but I have to say I need to brush up my skills, so I'm currently learning about maximising DOF for landscapes.

I've got fair understanding what the hyperfocal distance is for and how to use DOF calculators. The problem is just doing it in practice. So I'd really appreciate some help and a few tips.

Question 1

Specifically for my 31ltd, I entered the following into a dof calculator (dofmaster.com)
Camera Pentax k7
Focal length 31mm
f-stop -f/16

the DOF calculator says for me to maximise my DOF i should focus at 3.03meters.

However my 31ltd only has distance scales for 0.3m, 0.5m, 0.7m, 1m, 2m and then infinity.

How should I set my focus? Should I try to AF at approx 3.03m?

Question 2:

The DOF scale on my 31ltd is for 35mm/FF cameras, so id it of any use to me on my k5?

I'm a bit confused, I was thinking the 1.5 crop should give me larger dof than whats on the DOF scale but read elsewher that because the 31=46.5 i get less . Could someone please help clarify this?


Thanks in advance, I know I've asked alot.

03-14-2011, 10:11 PM - 1 Like   #2
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QuoteOriginally posted by vincechu Quote
I'm a bit confused, I was thinking the 1.5 crop should give me larger dof than whats on the DOF
This is a complicated subject with one simple answer...DOF is related to three things:
  • Numeric aperture (not f/stop)
  • Magnification
  • Viewing distance for the final image
Simply put, the smaller the iris opening the greater the DOF. The greater the magnification (lens + enlargement) the less the DOF. The closer your distance to the final image, the less the DOF. There is a lot of talk about circles of confusion and such (very confusing) and several online calculators if you need hard numbers, but the general relationship is above.

As an example, I would submit the avatar photo to the left of this comment. Great DOF, eh? Compare it to a larger version of the same image below:


K10D, LZOS Jupiter-9 85/2

Same lens and magnification to the sensor, but huge difference in final image size. If you pixel peep the original RAW import, the DOF is even more limited than the larger image above.

I would suggest using the optical DOF preview feature of your camera. It will give you a good approximation of what you can expect.


Steve
03-15-2011, 12:00 AM - 1 Like   #3
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If you read the lens reviews on this forum the general concensus seems to be that f4.5-5.6 is the sweet spot for the coveted 31Ltd, so your f16 could be overkill. I would think that somewhere between f5.6 and f11 you're going to get sharp from your toenails to infinity. As Steve suggests, your DOF preview is probably your best guide. (And it's certainly going to be no hardship to take lots of photos with that lens for practice. Lucky man).
03-15-2011, 12:20 AM - 1 Like   #4
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There's also diffraction to consider. The smaller an iris opening, the more that light passing through it is diffracted by the iris edges. The effect is blur. Diffraction is a function of the absolute size of the aperture opening -- f/64 on an 8x10in frame is larger than f/11 on a dSLR frame. The diffraction limit of a lens depends on the frame size of the camera it's on -- on our APS-C dSLRs, it's around f/8-f/11. Going beyond f/11 starts introducing blur. You might not see diffraction at f/16 without pixel-peeping, but beyond that, it can be noticeable in enlargements.

Which is a long way of saying: If your goal is absolute sharpness for enlarged pictures on ANY lens, keep the aperture wider than f/11. If the sweet spot is f/5, go for it!

03-15-2011, 01:12 AM   #5
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QuoteOriginally posted by RioRico Quote
There's also diffraction to consider. The smaller an iris opening, the more that light passing through it is diffracted by the iris edges. The effect is blur. Diffraction is a function of the absolute size of the aperture opening -- f/64 on an 8x10in frame is larger than f/11 on a dSLR frame. The diffraction limit of a lens depends on the frame size of the camera it's on -- on our APS-C dSLRs, it's around f/8-f/11. Going beyond f/11 starts introducing blur. You might not see diffraction at f/16 without pixel-peeping, but beyond that, it can be noticeable in enlargements.

Which is a long way of saying: If your goal is absolute sharpness for enlarged pictures on ANY lens, keep the aperture wider than f/11. If the sweet spot is f/5, go for it!
A much more elegant and thorough explanation than my "overkill!"
03-15-2011, 01:53 AM   #6
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I hadn't considered that the absolute aperture size, not the f-stop, is what determines when defraction starts to be a problem. As it happens I've checked the test results for my lenses and try to use f8 so far as possible.
03-15-2011, 02:27 AM   #7
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QuoteOriginally posted by Wombat Quote
A much more elegant and thorough explanation than my "overkill!"
Hit my rep, eh?

Here's an even jucier illustration: Poke a hole in a fence. Aim the water flow from a garden hose through it, so the water doesn't touch an edge of the hole. Nice smooth flow, eh? Then aim the water so it DOES touch an edge. The water starts spraying around now, eh? That's diffraction -- photons hitting the edge of the aperture spray around, so they don't follow nice straight paths to the film or sensor frame. Spray equals blur. OK, water fight!!
03-15-2011, 05:36 AM - 1 Like   #8
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the simplest and often most effective way to maximise your d.o.f. is to focus 1/3rd of the way in to your image and not focus on some other point in the main composition,
so you are standing looking at a lake with a mountain behind. your feet ain't wet so you want a bit of that dry ground as well. you are happy with your lens quality at say f11 so thats what you set your camera to.
now when you look through the viewfinder you can see the ground as well as the lake, as the lower horizontal etched lines that you see in your viewfinder are 1/3rd up so you look for something thats on that line, set you camera on MANUAL focus and focus on that point, recompose your image and shoot. you will have an image that has as near as dam it maximised the dof. via the hyper focal distance.
you could use the focus lock, but sometimes I find that goes wrong and I have that button fixed as exposure lock which is for me much more use.
tables are great if you have lens distances marked which most now don't have or you have longggggg tape measure!!
go look at the pic I have put in the gallery as a example.
Alistair
QuoteOriginally posted by vincechu Quote
Hi everyone,

A few months back I treated myself to the 31ltd and K5, but I have to say I need to brush up my skills, so I'm currently learning about maximising DOF for landscapes.

I've got fair understanding what the hyperfocal distance is for and how to use DOF calculators. The problem is just doing it in practice. So I'd really appreciate some help and a few tips.

Question 1

Specifically for my 31ltd, I entered the following into a dof calculator (dofmaster.com)
Camera Pentax k7
Focal length 31mm
f-stop -f/16

the DOF calculator says for me to maximise my DOF i should focus at 3.03meters.

However my 31ltd only has distance scales for 0.3m, 0.5m, 0.7m, 1m, 2m and then infinity.

How should I set my focus? Should I try to AF at approx 3.03m?

Question 2:

The DOF scale on my 31ltd is for 35mm/FF cameras, so id it of any use to me on my k5?

I'm a bit confused, I was thinking the 1.5 crop should give me larger dof than whats on the DOF scale but read elsewher that because the 31=46.5 i get less . Could someone please help clarify this?


Thanks in advance, I know I've asked alot.


03-15-2011, 06:48 AM   #9
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QuoteOriginally posted by cats_five Quote
I hadn't considered that the absolute aperture size, not the f-stop, is what determines when defraction starts to be a problem. ......
Not exactly...

Diffraction spot size is determined by the ratio of absolute distance from the aperture to absolute aperture size. The Wikipedia article Airy disk - Wikipedia, the free encyclopedia ( and any basis physics source) says...

"...where x is the separation of the images of the two objects on the film and
f is the distance from the lens to the film..... x = 1.22(wavelength)f/d........
"

When the lens is focused far away the focal length f is a good measure of the distance from the lens to the film, but not for close focus. The correct distance for all cases is (focal length)(1+magnification); therefore the appropriate relationship describing diffraction spot size is:

Airy Disk radius = 1.22(wavelength)(f-number)(1+magnification)

Therefore, as magnification increases, the f-number where diffraction becomes apparent decreases inversely proportional to magnification.

For a perfect lens, diffraction for macro photography will appear when:

Diffraction limit f-number for macro = (Diffraction limit f-number for infinite focus)/(1+magnification)

As our lenses are not perfect, the situation is usually worse than indicated by the perfect lens relationship above. Logic implies that for macros the f-number indicated on the lens should not be taken below the infinite focus "sweet spot" to avoid diffraction softening for any magnification. That effectively limits the maximum useful macro depth of field.

Last edited by newarts; 03-15-2011 at 07:05 AM.
03-15-2011, 07:26 AM - 1 Like   #10
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Adjust f-number by crop factor for DOF scales

QuoteOriginally posted by vincechu Quote
........
Question 2:

The DOF scale on my 31ltd is for 35mm/FF cameras, so id it of any use to me on my k5?

I'm a bit confused, I was thinking the 1.5 crop should give me larger dof than whats on the DOF scale but read elsewher that because the 31=46.5 i get less . Could someone please help clarify this?......
Yes. Your lens' scale is useful. Just multiply the f-number indicated by your crop-factor.

Theory indicates that for the same sized display and display resolution (like say, 8x10", 1500 pixels wide) the f-number in the Hyperfocal Distance equation should be multiplied by the crop factor.

If your full-frame Hyperfocal distance scale says 32' at f:8; for your 1.6x crop camera you should use 8*(crop factor=1.6)~ f:12.8 to get a 32' Hyperfocal distance.

Check it out with the on-line DOF calculator Online Depth of Field Calculator Change the camera type from FF (like Canon 5d) to your camera - you'll see that the Hyperfocal distance remains the same if you adjust your crop camera's f-stop up by the crop factor.

Last edited by newarts; 03-15-2011 at 07:32 AM.
03-15-2011, 11:11 AM - 1 Like   #11
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QuoteOriginally posted by vincechu Quote
However my 31ltd only has distance scales for 0.3m, 0.5m, 0.7m, 1m, 2m and then infinity.

How should I set my focus? Should I try to AF at approx 3.03m?
There is no need to set it to 3.03 meters. The practical approach (for lenses with a DOF scale like3 the FA31Ltd):

For film, set the infinity of the scale on the f/16 mark; the other f/16 mark will be at approx 1 meter on the distance scale. DOF from 1 meter to infinity (matching the calculations of DOFmaster for film)

For APSc, set the infinity mark on f/11 (one stop narrower); the other f/16 mark will be between 1 and 2 meters on the distance scale. DOF from approx 1.5 meters to infinity (matching the calculations of DOFmaster). And you will see that the focused distance is just over 2 meters (3.03 more than likely )

This is why DOF scales are so useful and lenses without them are basically bad design. Be glad that you don't have a DA-L kit lens as it even misses the distance scale.

Note:
for easy calculation,I've rounded the crop factor (1.5) to a stop (1.4 and-a-bit)

Last edited by sterretje; 03-15-2011 at 11:51 AM.
03-15-2011, 11:47 AM   #12
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QuoteOriginally posted by sterretje:
………Note:
for easy calculation,I've rounded the crop factor (1.5) to a stop (1.4 and-a-bit)
Close enough! For a crop camera, look at the lens' hyperfocal lines and add an f-stop.
03-15-2011, 06:02 PM   #13
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QuoteOriginally posted by newarts Quote
Close enough! For a crop camera, look at the lens' hyperfocal lines and add an f-stop.
I like to stay on the paranoid side of DOF -- add a bit OVER one f-stop. So if the aperture is set to f/11, prefocus so the DOF range is within f/7 or thereabouts. This is especially important if DOF to infinity is desired, to keep infinity sharpness within the realm of acceptability.
03-15-2011, 07:02 PM   #14
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Thanks everyone, rep given to all :-)

I'll try and apply all this to practice when I find some time (I have an assignment for uni due soon), possibly tomorrow.

Thanks again, much appreciated
03-16-2011, 06:05 PM   #15
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Thanks again everone, today I walked around my uni campus trying out some of the techniques you all mentioned.

I like using the hyperfocus and distance scales, but I'm not up to scratch with it just yet - will practice much more, hopefully it'll become as close to second nature as possible - will stick with it just with the 31ltd for now.

I found the easiest thing to do was set aperture and use the DOF preview on my camera - I got some insanely sharp photos this way, dangerously sharp :-) Focusing in 1/3 was pretty good too, though i had to keep telling myself not to focus on the buildings i took photos of, but rather the whole scene for it.

I would post the results but they dont look too special, mainly grass, paths and buildings, with a very miserable looking grey sky (which I'm tempted to 'shop out).

Thanks again :-)
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