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09-04-2015, 11:05 PM   #1
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Pentax 645D with FA 55mm f2.8 hyperfocal distance issiues

Hi all ,my name is Rytis .I have read all the information awailable here ,in this forum regards my problem,but still having major issues with it. The story is,that im about to trade my H3D-31 with Pentax645D+55mm f2.8 and i have aweek to decide yes or no to Pentax.as im mainlylandscape+outdoor shooter, so far all goes in favour of Pentax 645D, im amaized with the camera.
Now the problem... yesterday i went to play with hyperfocal distance and all im getting is sharp foreground, about from 3 ft dawn to 20 ft from the camera.After 20 ft its all blurry as hell or in best case-notacseptable sharpnes. Ihave tryed the folowing: tripod,remote relese,mirror up, manual focus ,following:

1.set infinity on the lens on 22, camera set f22 -sharp foreground, blurry afterabout 20-25 ft.
2.set inffinity on the lens on 22, camera f16 - sharper foreground( less difraction i guess) ,but still blurry background,id say less ,than before,but still not acseptable at all.
3.set inffinity on the lens on 16, camera f16 -again, sharp foreground, blurry background..again blurry and not acseptable

The only way im getting sharp image from front to back is ,then i focus at infinity and recompose , but im not tallking about crisp sharp,im getting *acseptable sharpnes* at most...
Later that day i was shooting subjets at mid distance (6-8ft) and noticed the lens back focusing about 2 inc, close distances looks to perform with no problem. Ive noticed,that the cameras AF fine tune was +5, so just started to wonder if the lens need to be properly fine tunned.
Im thinking to hit the charts latter on, but my main worry is , is lens back focusing could affect my results at hyperfocal distance?

Hope i layout everything clearly for you to understand,
Thanks for any input
Regards

09-04-2015, 11:15 PM   #2
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At F22 you're not going to get the best sharpness due to diffraction. If you shoot F16 and focus 15 feet away, everything from 6ft to infinity will be in focus.

If you're focusing on something 6ft away there's no way to get the entire background in focus (unless you're shooting with a smartphone ).

I'd recommend that you experiment with the DOF calculator.

Adam
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09-04-2015, 11:23 PM   #3
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QuoteOriginally posted by Adam Quote
At F22 you're not going to get the best sharpness due to diffraction. If you shoot F16 and focus 15 feet away, everything from 6ft to infinity will be in focus.

If you're focusing on something 6ft away there's no way to get the entire background in focus (unless you're shooting with a smartphone ).

I'd recommend that you experiment with the DOF calculator.
to be honest i never used DOF calculators, always trusted lens markings and never had an issue, ill give it a try today,thanks.
As for f 22 -i know..i used it only for test purposes.

Thanks again,
09-04-2015, 11:26 PM - 1 Like   #4
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Also, do keep in mind that objects near the extremes of the DOF range may not be perfectly sharp. FF shouldn't have much of an effect if you just go by the distance scale.


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09-05-2015, 02:02 AM   #5
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The 55mm has a lot of field curvature near the edges, so you may not get everything in focus across the frame depending on the nature of the scene, also there's a lot of sample variation between lenses.

Never use the scales on the lens, they're based on standards used since film days, while digital sensors have a much higher tolerance for focus. Instead, use a DOF calculator, and print out a cheat sheet with various apertures and the required focus distances to get hyperfocal.

As mentioned, f/16 is the limit for this camera, every lens that I have and tested was excellent up to this aperture, but immediately dropped in sharpness as soon as I went to f/22.
As usual, hyperfocus is a function of how large you want to print the resulting image, so it's up to you to figure out if a greater DOF will provide more apparant sharpness than a sharper image.
09-05-2015, 02:24 AM   #6
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QuoteOriginally posted by Adam Quote
At F22 you're not going to get the best sharpness due to diffraction. If you shoot F16 and focus 15 feet away, everything from 6ft to infinity will be in focus.

If you're focusing on something 6ft away there's no way to get the entire background in focus (unless you're shooting with a smartphone ).

I'd recommend that you experiment with the DOF calculator.
I did, fine tune lens with a chart, then went for some shooting, Using DOF calculator im getting the same results as yesterday f16, f11 doesnt matter,from 20-25 ft into picture -all blurry including background, but once i auto focus to infinity and compose and shoot- results like day and night , sharp from front to back .
im convinced there is something with the lens...on top of that ive tryed 67 105mm f2.4 , set it up HP using dof scale on the lens - perfectly sharp bottom-top .i need to go shoot more, to decide if i can live with it... Interesting,what couses FA 55mm f2.8 to act like this...
09-05-2015, 02:43 AM   #7
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QuoteOriginally posted by Kolor-Pikker Quote
The 55mm has a lot of field curvature near the edges, so you may not get everything in focus across the frame depending on the nature of the scene, also there's a lot of sample variation between lenses.

Never use the scales on the lens, they're based on standards used since film days, while digital sensors have a much higher tolerance for focus. Instead, use a DOF calculator, and print out a cheat sheet with various apertures and the required focus distances to get hyperfocal.

As mentioned, f/16 is the limit for this camera, every lens that I have and tested was excellent up to this aperture, but immediately dropped in sharpness as soon as I went to f/22.
As usual, hyperfocus is a function of how large you want to print the resulting image, so it's up to you to figure out if a greater DOF will provide more apparant sharpness than a sharper image.
thanks for replay, so far DOF calculator doesnt help, before , using 35mm FX cameras never came across with a problem like this,even if i use scale on the lense, im avare that dof in 35mm slightly larger than on 44x33 mm sensor,but still,how to explain f16 , focus set on 4 metres and i get only foreground in focus....
09-05-2015, 04:46 AM   #8
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QuoteOriginally posted by Rytis Quote
DOF calculator doesnt help, before , using 35mm FX cameras never came across with a problem like this,even if i use scale on the lense, im avare that dof in 35mm slightly larger than on 44x33 mm sensor,but still,how to explain f16 , focus set on 4 metres and i get only foreground in focus

DOF is now equal to COC. DOF it is not calculated in a useful way by so called "DOF calculators". DOF is not absolute - many people like to think it is, but it isn't. It can be affected by tiny vagaries of manufacture, optical alignment, optical design of the lens.

In a nutshell: DOF is about the region of acceptable sharpness*. The absolute point of focus is always going to be sharper than elements in front or behind it** - all DOF scales or "DOF calculators" do is give you a loose idea of how in focus, or out of focus objects that are in front of or behind the point of focus will be.

Remember when you are looking at your images from the 645Z at 100% or 400% on screen, you will see things that will probably not even be visible on a 36"X24" inch print. Remember the optimal viewing distance of an image is based on the diagonal dimensions of the image - of course this is a rule of the thumb, some people like to look at images at closer distances than this, some at further distances. But it pays to keep this in mind when producing images for display.

*sharpness is a relative and subjective thing, and that is the premise on which DOF claculators function.
**at least until diffraction kicks in, which limits the maximum amount of sharpness on the plane of focus a scene. It is possible to stop down to the point where nothing will be critically sharp. But on a 8X10 print, who will notice?


Last edited by Digitalis; 09-05-2015 at 04:55 AM.
09-05-2015, 05:42 AM   #9
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Rather than talk about numbers all day, shoot a series of images at various aperture and distance combinations, and print out crops equal to how large said crops would be on a final print, arranged side-by-side, and then compare.

A difference that is clearly visible on screen at 100% may be completely imperceptible at regular print sizes. If you want to print really massive images that even push the limit of the 645Z's resolution, you'll be better off getting a tilt-shift lens or using a tech-camera instead of trying to brute-force your way to a greater depth of field... or do focus stacking, no one said that it's only for macro shooting.

Honesty, I never bother with hyperfocus, I just focus on the subject of interest or the plane where most of the subject matter intersects, using live view.
09-05-2015, 11:56 AM   #10
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QuoteOriginally posted by Kolor-Pikker Quote
.

Honesty, I never bother with hyperfocus, I just focus on the subject of interest or the plane where most of the subject matter intersects, using live view.
well its very personal and depends on the things you have in mind then shooting, im not saying ,i always need tack sharp image from front to back...but sometimes i do
09-05-2015, 12:55 PM   #11
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here 100% from the same scene, is this the best you can get using DOF calculater? what you think of that, is it a user error somethere?

Regards
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09-05-2015, 01:06 PM   #12
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QuoteOriginally posted by Rytis Quote
I did, fine tune lens with a chart, then went for some shooting, Using DOF calculator im getting the same results as yesterday f16, f11 doesnt matter,from 20-25 ft into picture -all blurry including background, but once i auto focus to infinity and compose and shoot- results like day and night , sharp from front to back .
It is possible that a lens element has moved internally, causing focus error. A less likely scenario is that the lens had been machined incorrectly at the rear, giving a focus error.
09-05-2015, 01:17 PM   #13
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QuoteOriginally posted by desertscape Quote
It is possible that a lens element has moved internally, causing focus error. A less likely scenario is that the lens had been machined incorrectly at the rear, giving a focus error.
Yeap,its hard to know, ive tunned lens to +5 and its ok if i focus on the subject ,Taken from various distances -its perfectly sharp
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