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11-07-2012, 01:56 PM   #1
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Hyperfocal distance and infinity

On my Tamron 28-75 f/2.8 macro, the markers for focusing go from 0.33m to 3m, and then to infinity, with nothing in between.

I have a DoF calculator on my ad which says that when the lens is at 28mm with aperture f/8, if I focus at 3m then 0.30m in front and 0.38m behind will be in focus, but if I focus at infinity then 26.87m in front will be in focus...

So the DoF calculator says that for 28mm on f/8 the hyperfocal distance is 26.87m but my only choices are 3m and infinity... So if I set it to 3m then I'm only getting the area from 2.7-3.18m in focus, and if I'm focused at infinity then I'm only getting the horizon and 26.87m in front of it?

Also, if I'm using autofocus, when I'm focusing on anything further than 3m, will the focal point it chooses be considered "infinity"? When I focus on different things, all more than 5m away, the focus does shift *very* slightly... I thought it should stay the same?

11-07-2012, 02:15 PM   #2
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keeping in mind that I do not have this lens, it sounds like you are describing a REALLY short turning focus ring on this lens. If so, I can understand part of your confusion.

'Focus' is a specific point some distance from a lens. Focus degrades in both directions from this point. Hyperfocal distance is a somewhat arbitrary range of acceptable focus ahead and behind THE focus point determined by focal length, aperture and subject distance. Auto focus will always attempt to match THE focus point to what it thinks is the subject. And there is a continuous range of focus from 'minimum' to infinity (and often a bit past infinity to accommodate auto focus mechanisms and temperature related expansion/contraction of the lens mechanism). If the focus ring has a long turn, all this becomes a bit more obvious. On a lens with a short focus turn, I can certainly understand why it seems there are only the 3 options you describe.

As it sounds like you have a hyperfocal table for a 28mm lens at your desired aperture, by all means, use that table to hand focus so that your subject falls within this range.
11-07-2012, 02:18 PM   #3
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Its a short focus throw lens. That little tiny amount of turn between 3m and infinity covers a VERY turn on the lens but represents a lot of distance. This is why people talk about the 'focus throw' on lenses in their reviews and in asking about them. The longer the focus throw the easier, in general, it is to manually focus the lens accurately/precisely. The converse is very true in my experience, the shorter the focus thrown the harder focus is manually.
11-07-2012, 02:22 PM   #4
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QuoteOriginally posted by djmundy Quote
On my Tamron 28-75 f/2.8 macro, the markers for focusing go from 0.33m to 3m, and then to infinity, with nothing in between.

I have a DoF calculator on my ad which says that when the lens is at 28mm with aperture f/8, if I focus at 3m then 0.30m in front and 0.38m behind will be in focus, but if I focus at infinity then 26.87m in front will be in focus...

So the DoF calculator says that for 28mm on f/8 the hyperfocal distance is 26.87m but my only choices are 3m and infinity... So if I set it to 3m then I'm only getting the area from 2.7-3.18m in focus, and if I'm focused at infinity then I'm only getting the horizon and 26.87m in front of it?

Also, if I'm using autofocus, when I'm focusing on anything further than 3m, will the focal point it chooses be considered "infinity"? When I focus on different things, all more than 5m away, the focus does shift *very* slightly... I thought it should stay the same?
Are you sure your hyperfocal distance is accurate? I used DOF websites and it says hyperfocal at F8 and 28mm is 16 feet (which is about 5 meters, give or take).

Anyways - you focus on an object beyond 3m but less than 5m, then your DOF isn't going to extend to infinity. Let's use your example of 28mm and F8:

3m - Near limit: 1.87m, Far limit: 7.62m
4m - Near limit: 2.21m, Far limit: 21.1m

So until you reach your hyperfocal distance, you won't get infinity into focus. Once you reach your hyperfocal distance, infinity will be in focus. One you pass your hyperfocal distance, infinity will remain in focus while your near limit will increase.

11-07-2012, 03:04 PM   #5
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Ah yes, hyperfocal focusing is really hard with modern lenses - I mean particularly zoom and digital lenses. Often they don't have aperture rings, bad or no distance scales, a short focus throw..
Basically, not much you can do at the moment. You can either use optical preview (set the green button for that) and see when you get acceptable/best focus, or you can buy a prime with good distance scales. Not all primes have these. For example, the DA 35mm f2.4 is also no good for hyperfocal focusing as it has no distance scales at all and a very short focus throw.
You set the camera and lens to MF, turn the focus ring to infinity, then hit optical preview and in OVF or in LV look at the thing that is at infinity and focus towards yourself. Stop focusing when you notice the thing far away is no longer in-focus.

Last edited by Na Horuk; 11-07-2012 at 04:34 PM.
11-07-2012, 04:10 PM   #6
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Even older MF wide-angles can jump from 3m or so to infinity from one marking to the next.

Choose a couple of spots part way between the two and practice with both settings on a subject, finding something that recedes. A picket fence shot on an acute angle would be ideal. That way you can observe exactly what DOF you are achieving at each spot on the focus ring. Or focus on a subject at a couple of known mid-distances and note where the AF places the focus ring for each distance. At 28mm and F8, there's a fair bit of DOF to work with at mid-range and probably no need to be super scientific. Then make a mental note (or pencilled note in your kit bag) of these spots and their effects.
11-07-2012, 08:55 PM   #7
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When I plan to use a lens for zone focusing (usually set at hyperfocal distance) it is a wider angle and I generally choose a super takumar/smc tak. (works great w/ digital as can AV exposure mode. Otherwise (for couple wide angle zooms) I jot down hyperfocal distances on a label/card. But again they are older lenses with sufficient distance marks. (You may want to buy a super tak for this reason.)

I suggest with a lens that has inadequate distance marking you guess what part of the scene is at the hyperfocal distance and focus on it. It does not have to be very precise--you likely can tell the difference between 5 ft, 10 ft, and 15 ft (or roughly 2 m, 3.5m, 5 m).

BTW you do know that hyperfocal distance for pentax digital is a value for lens stopped down one more than table/lens value. i.e., if the lens or table has f/11 and 3 meter, it will be f/16 and 3 meter.
11-07-2012, 11:39 PM   #8
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one more thing. You said:

"So the DoF calculator says that for 28mm on f/8 the hyperfocal distance is 26.87m but my only choices are 3m and infinity... So if I set it to 3m then I'm only getting the area from 2.7-3.18m in focus, and if I'm focused at infinity then I'm only getting the horizon and 26.87m in front of it?"

You presumably meant 2.7 meter--or something like that. And the difference between 2.7 m and 3 m is trivial.

And as I pointed before--you need to adjust for the reduced sensor size--thus the 28mm lens hyperfocal distance of about 3 meter, is really for f/11 on your digital slr (unless it is FF). If you really want to use f/8, then the hyperfocal distance is about 14 ft (about 4.2 meter).

11-08-2012, 03:35 AM   #9
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Whether an adjustment to a 'table' is required depends on the DOF calculation source - the phone app/on-line DOF calculators I have used ask you to input the camera model or sensor size and the output is adjusted to suit the crop factor effect on useful DOF. A fixed table will probably be for full frame so agreed will need adjusting for the crop factor.

Now I am home, looking at both my M 28's, the markings on both jump from 10 feet to infinity while the M 20 jumps from 1.5 metres (5 feet) to infinity! So for wide angles, it's not a 'modern' lens phenomena that there is a big range unmarked towards infinity in the focus markings - its simply the lens manufacturer being aware of how quickly the DOF of WA lenses broadens past the last marked distance. For comparison at the tele end, my M*300 is marked out to 280 feet and has an enormous throw to get there.
11-08-2012, 08:35 PM   #10
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As a follow up to southlander's reply--looking at my 28mm super takumar: the feet indications are: infinity, 20, 10, 6.5, 5, 4, ... (and in meters: infinity, 7, 3, 2, 1.5, 1.2, ...). I presume the K mount m (and post m) lenses were generally redesigned for a smallest size--and thus the mechanics suffered.

I think one answer is (if one chooses to use zone focusing as a standard working method--which it likely should be for street photo.) you look at the lenses available (old and new) and pick what works best for you--the lens throw and distance markings are one of the important characteristics of the lens to be considered, along with mf vs af, lens speed, IQ, etc.
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