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09-04-2010, 07:28 AM   #1
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Front to Back Sharpness: Distance setting on front of lens

Hello all,
I had a question regarding front to back sharpness for a landscape photograph. I'm reading Understanding Exposure by Bryan Peterson and see him continually referring to "presetting the focus via the distance settings on the front of the lens" while using an f/22 aperture to ensure sharpness from a few feet out to infinity.

What exactly is meant by presetting the focus via the distance setting? Is this a feature with particular lenses? I have my Pentax K-x kit lens and a Pentax-A 50mm/1.7 and can't seem to figure out what he's referring to.

Any other tips for front to back sharpness would be appreciated.

Thanks!

09-04-2010, 07:39 AM   #2
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There's a distance scale on the focusing ring (directly above the aperture ring and DOF scale) - it's the one with feet in blue and meters in yellow- so I'm assuming he wants you to focus based on the distance and the DOF rather than by looking through the viewfinder.

Refer to the photo for details.

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09-04-2010, 09:29 AM   #3
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I believe your kit lens doesn't have this, but the 50 should. I have the same lens and it looks just like the one Adam pictured, but shorter. Same color coding - feet in blue, meters in yellow. The lenses small enough that the numbers are staggered instead of being directly on top of each other like the 135.

But note, the approach you are describing isn't perfect. For one thing, the DOF calculations are just approximations. Only the thing you are focused on is in *perfect* focus; things in front of and behind it just get progressively closer to being in focus as you stop down, and at some point, it's considered "close enough" - but usually that means, close enough for a typical print viewed from a typical distance. It doesn't mean perfect sharpness viewing on screen at 100%. Also while stopping down to f/22 does increase your depth of field, giving you lots in focus, it also decreased the overall sharpness of the lens. Plus it often gets you into territory where you need high ISO to get shutter speeds fast enough to handhold even with SR, or else a sturdy tripod to get a sharp picture at the slow shutter speeds that result form low ISO.

All this is to say, in most cases, you're probably better off not stopping down all the way to f/22 but going for maybe f/11 or at most f/16, and instead of relying on a DOF calculation or whatever else Peterson recommends to tell you where to prefocus, pick an actual point you want maximum focus to be, focus there, take the picture, examine it on the LCD, then decide for yourself if it's acceptably sharp here you want it to be. Then you can kind of fine tune the focus point and aperture by trial and error. That's one of the great advantages of digital.
09-04-2010, 09:40 AM   #4
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Here's my favorite example. This is a Lentar-Tokina 21/3.8 manual M42 lens. From the bottom up: The A/M indicator, on Manual. Then the aperture ring, set to f/11. Then the index/DOF scale. Then the focus ring, pre- / hyper- focused to 2m. (Hyperfocal is the distance you set for a desired DOF. (Prefocusing is the act of setting the hyperfocal distance.)

The DOF scale is designed for a full-frame (FF) camera. But I use an APS-C camera, the K20D, so I must make a 1+ f-stop allowance. If I'd wanted DOF to extend to infinity on a FF camera, I'd have set the infinity focus mark atop the 11 on the right-hand side of the DOF scale, which would make 4' / 1.2m as the hyperfocal. My close-focus distance would then be around 2' / 0.6m.

But for the APS-C sensor, I set the lens as shown, with infinity a bit within the f/8 index mark. That puts the hyperfocal at 2m and the close-focus at 1m. (See, over on the left side? The 1m mark is just within the f/8 mark.) So when I'm street-shooting, I know that everything between 1m and infinity will be in focus.

Suppose I set the aperture to f/8 and focus on a subject at 1m. I look at the DOF scale, see what distances are inside the f/5.6 mark. On the right side, the low/close end, it's around .75m. On the left side, the high/far end, it's around 5m. That tells me my range of sharpness, 0.75-5m, and everything closer or further will be out of focus (OOF).

So the scale lets you control DOF by adjusting aperture and hyperfocal as needed.

EDIT: As Marc said, only the point you've focused at is RIGHT in focus. The DOF is a range of acceptable focus, for most eyes. YMMV.

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Last edited by RioRico; 09-04-2010 at 09:51 AM.
09-04-2010, 11:28 AM   #5
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Thanks for the replies everyone. Some of this is still a little over my head but I'm trying to take it all in :P

So I'm still trying to understand what Peterson is recommending. This is all still very jumbled in my mind so let me explain how I'm understanding it so far:

If we look back at the first image posted by Adam, there are three rings correct?

Would this be the correct way to set an f/22 aperture for front to back sharpness?

- Have the infinity symbol on the first ring overtop of the '22' marking on the second ring
- Change the third (last) ring to then have the '22' line up with the orange marker on the second ring

And then you would focus on an object roughly 10 feet away and shoot? How can you focus on the object 10 feet away if you can't adjust the focus off of the infinity marker?

Thank you all for your patience, all responses appreciated.
09-04-2010, 12:07 PM   #6
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Not exactly. First, there's what's called the diffraction limit. You lose a bit of sharpness when you're at the smallest aperture setting, like f/22. I'd stay at f/16 or wider, if sharpness matters to you. Next, both the lens that Adam shows and the one that I show are designed for full-frame cameras, those wonderful film dinosaurs. You must mentally adjust the scale by at least 1 f-stop. I take another 1/3 to 1/2 f-stop as a safety margin, to insure maximum sharpness in the range I want. Finally, once you've set the aperture and prefocused to get the DOF range you want, DON'T CHANGE THE FOCUS!

The lenses have what *look* like three rings, but the middle part with the index mark and the DOF scale doesn't rotate; it's part of the lens body, and the red/orange pointer there is the index mark.

Using your example of shooting f/22 on the 135mm lens Adam posted, you'd set the lower ring (with only f-stop numbers) so '22' is below the index mark. If you use a full-frame camera, set the top (focus) ring so the infinity symbol is over the unlabeled f/22 mark between the '16' and '32' marks on the left side of the DOF scale. Whatever number is over the unlabeled f/22 mark on the right side of the scale, indicates the other end of the DOF range. That will probably around 11m / 35 ft. The index mark then points to around 25m / 80 ft.

BUT YOUR dSLR ISN'T FULL-FRAME! THAT WON'T WORK! Instead of setting infinity atop the f/22 mark, set it INSIDE the f/16 mark. (Setting it atop the f/16 mark gives that 1 f-stop adjustment necessary for using FF lenses on APS-C cameras; setting it INSIDE the f/16 mark gives you an extra margin of sharpness.) Now the other end of your DOF range is around 25m, and your hyperfocal distance is about 50m. So you can LEAVE THE LENS SET LIKE THAT, and everything you shoot that's between 25m and infinity should be acceptably sharp.

How do you shoot something that 3m / 10 feet away? You focus on it. 10 feet is now your hyperfocal distance. If the lens is set to f/22, you look at the f/16 marks on the lens and see that your DOF range is about 9 to 12 feet. That's as close as you'll get to "front-to-back sharpness" on a 135mm lens. If you want a deeper DOF range, you'll need a much wider lens, like the 21mm I showed above. That's just the reality of optics.
09-04-2010, 01:11 PM - 1 Like   #7
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Here is something from old Pentax Super Program manual, 50mm f 1.4 used as an example.
1 picture worth 1000 words.
They do not write owners manuals like they used to. Hope this helps.

Cheers, Mike.

Last edited by Ex Finn.; 11-11-2014 at 05:50 PM.
09-04-2010, 01:29 PM - 1 Like   #8
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Afternoon Yoon,

With all the variations of equipment out there floating around now, what use to be the norm is not the norm any longer, especially if you have some equipment that is slightly different. The examples given are really good, but if your lens does not have the DoF vernier scale, then you are left wondering what is going on.

So, there are some websites that provide the same information, such as ...
They also give some excellent examples and explanations. Also, they have some tables that you can print out. However, once you get a feeling for what is going on, and that the scaling is not perfectly exact, and that the focus distance field in not all that wide, estimation will suffice 99% of the time.

I use this quite a bit. Since I do landscapes, I am just going to infinity and backing off a "smidge" (a very exact and precise amount used in engineering all the time).




Last edited by interested_observer; 09-04-2010 at 01:34 PM.
09-04-2010, 01:37 PM   #9
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Hey, I thought it was "smidgen"..

Also, the optical DOF preview can give an idea of what is in focus.
Luckily the OP has an A 50mm 1.7 that does have the scale.
09-05-2010, 11:42 AM - 1 Like   #10
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QuoteOriginally posted by yoon395 Quote
Thanks for the replies everyone. Some of this is still a little over my head but I'm trying to take it all in :P

So I'm still trying to understand what Peterson is recommending. This is all still very jumbled in my mind so let me explain how I'm understanding it so far:

If we look back at the first image posted by Adam, there are three rings correct?

Would this be the correct way to set an f/22 aperture for front to back sharpness?

- Have the infinity symbol on the first ring overtop of the '22' marking on the second ring
- Change the third (last) ring to then have the '22' line up with the orange marker on the second ring

And then you would focus on an object roughly 10 feet away and shoot? How can you focus on the object 10 feet away if you can't adjust the focus off of the infinity marker?
No, I think you are thinking this is far more complicated than it is.

The lenses in question have only two rings yes can turn. One to focus, and one to set the aperture. A zoom lens of course has a third ring, to set focal length.
And you actually won't turn the aperture ring on these lenses - you will leave it will stay permanently locked in the "A" position for as long as you use the lens on a modern camera (and that includes relatively recent *film* cameras. Instead, you'll set the aperture using the dial on the camera in Av or M mode.

So the only lens ring you will ever turn is the focus ring. If you want to set a distance of 15 feet, you simply turn the focus ring until the 15 displayed on the distance scale is right above the the center line. That's all there is to it.

The numbers to either side of the center line, BTW, are the DOF markers, and you'd use those to help you choose an aperture and a focus distance if you didn't already know from whatever you read in the book what aperture and focus distance you wanted. Presumably, the book does explain that process as well.
09-05-2010, 03:28 PM   #11
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Ah yes, that helps clears things up. Yeah I was thinking there was some mystical way to get incredible sharpness. So it's not very much more effective than just relying on the viewfinder image?
09-05-2010, 05:45 PM   #12
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Well, sure, there's plenty more to it than that. Peterson isn't making stuff up. There really is a point to setting an appropriately small aperture, focusing somewhere into the scene, and relying on DOF to give decent (not perfect) sharpness throughout that zone. I personally am more of a trial and error guy than a calculate it in advance guy. But even trial and error goes far better it you understand the principles at play.
09-05-2010, 10:42 PM   #13
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I hope i am not hijacking this thread by asking about the DA14.

As the DA14 does not have a manual aperture ring, how do u pre-focus using F16? There is a Ft/m dial and a F-stop marking (which looks like 22 16 8 | 8 16 22). What do you align the F16 to?

actually i asked this question before but i am still confused.
09-06-2010, 12:50 AM   #14
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QuoteOriginally posted by raider Quote
I hope i am not hijacking this thread by asking about the DA14.

As the DA14 does not have a manual aperture ring, how do u pre-focus using F16? There is a Ft/m dial and a F-stop marking (which looks like 22 16 8 | 8 16 22). What do you align the F16 to?

actually i asked this question before but i am still confused.
Raider, in your case you'll have to use trial and error/guess. But, seeing as it's 14mm, it should have a large hyperfocal distance.
09-06-2010, 06:27 AM   #15
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"how do u pre-focus using F16? "

Just focus with optical pre-view, it will give you an idea of the actual DOF.
Set the camera to manual focus,
dial in the f stop you want,
pull and hold preview switch on, this will stop down the lens, ( camera set to optical preview) and now you can set the focus point where you want it.
This works on DA* 16-50, should work on DA 14 also.

Last edited by Ex Finn.; 09-06-2010 at 09:37 AM.
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