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07-03-2012, 08:21 PM   #1
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Depth of focus scale

I am used to using the DOF scale on manual lenses, Takumars and Pentax-A. On those I assume that using them on APS-C requires a little adjustment so I allow 1 or 1.5 stops less when setting it. I recently got a Pentax 21mm Limited and it has the DOF scale unlike most modern lenses. I shot some pictures with it using the scale set 2' to infinity (f/22) and assumed that the scale would be accurate for APS-C so I did not 'fudge' the numbers by a stop as I usually do. The results were less than impressive.

So, does anyone know if the DOF scale on the DA limited series has been adjusted to APS-C or should we use the same procedure on this as with full frame lenses on APS-C?

07-03-2012, 08:25 PM   #2
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DoF remains exaclty the same regardless of sensor size.
07-03-2012, 08:49 PM   #3
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This is how it works : a 50mm equivalent for the Pentax Q is a 8mm lens, so the DOF is as shallow as an 8mm lens for full frame or APS-C, but the field of view is narrower
07-03-2012, 08:59 PM   #4
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QuoteOriginally posted by cmohr Quote
DoF remains exaclty the same regardless of sensor size.
Thanks for the quick answer, but I was asking about the scale printed on the lens, not DOF

QuoteOriginally posted by GabrielFFontes Quote
This is how it works : a 50mm equivalent for the Pentax Q is a 8mm lens,
Thanks for the quick answer, but I do not have a Q. I was wondering if the DOF scales on the DA lenses have been adjusted for APS-C or if they are calibrated for full frame.

07-03-2012, 09:08 PM   #5
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The DOF scale printed on a lens (M, K, F or FA) is a guide for printing at 8x10 from 24x36 and viewing the image from about 10 inches or so. Note that the magnification of the final image and the viewing distance are important as well. I bet that if you print an 8x10 from your f/22 21mm lens and look at it from a little under a foot, things will look normal. It takes quite a while to get used to forgetting about pixel peeping and look at the image on screen at the size you expect it to be viewed.

Have fun!
07-03-2012, 09:08 PM   #6
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That was just an example. I think it works the same way when using Fullframe x APS-C
Of course, you're picking up a smaller portion of the image and magnifying it...so i guess you stop down for safety only
07-03-2012, 09:13 PM   #7
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Just a technical nitpick...depth of field and depth of focus are not the same thing. There is no depth of focus scale printed on the lens. Depth of focus refers to the near and far distances before and after the image sensor between which the circle of confusion is small enough for the image to be considered in focus.
07-03-2012, 09:22 PM   #8
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Hi, Gabriel - the whole DOF thing was for years predicated on the 8x10" print viewed at 10 inches, and I have not seen anyone come up with another standard. The DOF scale is meant to tell you the distance range the lens will provide an image where you cannot distinguish the individual grain/pixel (everything is sharp) at 10 inches. So working from 24x36, (1 x 1.5") you are working to an 8x magnification viewed at 10 inches. With any other size of starting image (including cropping, if any) to view at 8x10 inches (20x25 cm) then you have to recalculate to get the same smallest view element that appears to be an infinitely small point.

I don't bother. I could just use the LCD and view it at the magnification I am expecting and scroll around if I want to be really picky about the DOF. That, of course takes forever, so it is easier to just bracket the f/stop and pick the one that works out right when you have the final product.

07-03-2012, 09:43 PM   #9
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assuming the depth of focus given on an older lens to be the accepted criterion (using standard 35mm negative/full frame sensor) then if the sensor is smaller by a factor of 1/1.414 (1/square root 2) then the depth of field will be the same at 1 f stop less.

e.g., 21mm lens on full frame at f/11 has hyperfocal distance of about 6 feet, so set at that focus distance depth of focus is 3 feet to infinity. On a smaller sensor by a factor of 1/1.414 the same values apply if f stop is f/16. As the pentax sensor factor is 1/1.5 about--it is close enough to 1/1.414 that the 1 stop less works fine. So check your DA lens against older 21 mm depth of field values. Or use following rounded values (for 21mm lens on full frame):

f/8 & 8 ft, f/11 & 6 ft, f/16 & 4 ft, f/22 & 3 ft
07-03-2012, 10:35 PM   #10
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Thanks for all the info everyone, I know this is a much debated subject.

But what I want to know is whether the scales printed on the DA lens are optimized for APS-C sensors (since the lens is designed for that sensor size) unlike the older (K, M, A, F, & FA) lenses which had scales printed on them that were optimized for a 24x36 sensor.

When I use a film era lens on APS-C I automatically adjust what scale number I use for the smaller sensor. So, do I have to still do that on the DA lenses which were designed for APS-C?
07-03-2012, 10:39 PM   #11
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My point was if you take some values from your DA lens you can see if they are: (1) consistent with older full frame lens, or (2) whether the 1 stop more closed is already included, or (3) if they are inconsistent with either of these. No matter what anyone says--the real test is in the numbers on your lens!
07-03-2012, 10:48 PM   #12
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BTW at f/22 diffraction may make the pictures pretty soft. Are you sure it isn't the problem? The old rule of thumb was f/16 gave a maximum of 100 lines per mm, and f/22 gave about 70 lines per mm. With the smaller sensor and thus greater enlargement it will be worse.
07-04-2012, 08:04 AM   #13
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QuoteOriginally posted by dms Quote
BTW at f/22 diffraction may make the pictures pretty soft. Are you sure it isn't the problem?
Actually I am sure that is part of the problem. I got carried away with f/22 and things are soft, but I know how to fix that. I just wondered if Pentax adjusted the scales for the sensor the lens was designed for or if they just used the same numbers from the older lenses.

On my DA 21mm @ f/22 if you set the left mark on infinity the right mark shows about 1.8 feet.
On my A 28mm @ f/22 if you set the left mark on infinity the right mark shows just a hair under 2 feet.
Those are the closest comparison I have and the difference might be the difference in the focal length. So it seems that no adjustment was made on the scales and that to use them properly you need to fudge the numbers by 1.5 stops just like on lenses designed for full frame.
07-04-2012, 10:38 AM - 1 Like   #14
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First my apologies--I gave you depth of focus values that were adjusted for the smaller sensor. I calculated them for full frame sensor and then reduced the f stop by one full stop.

BTW the equation for hyperfocal distance, for full frame and usual criterion (circle of confusion value and print size/viewing distance--the precise values I have forgotten) is H(ft)= FL(mm)*FL(mm)/(10*Fstop). As a check if you put in FL=28mm and Fstop=22 you should get H=3.9 feet (about 4 feet), so on full frame set at 4 ft, about 2 ft to infinity is in focus, and on 1/1.5 smaller sensor the same values, except for lens at f/32.

Given the difficulty in accurately reading distances on most lenses--particularly auto focus where the total angle of rotation is reduced for faster focusing--it sounds like the DA21 has already taken into account the smaller sensor.

Look at it this way (for reduced sensor) going from FL of 28 to 21mm and f/22 the depth of focus would go from about 2.5 ft to infinity (H=5 ft), to 1.5 ft to infinity (H=3 ft), and you had about 2 ft to infinity on the DA21. And adjusting the 28mm value back to full frame sensor would brings it to about 2 feet to infinity (H=3.6 ft).
07-04-2012, 08:38 PM   #15
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QuoteOriginally posted by dms Quote
My point was if you take some values from your DA lens you can see if they are: (1) consistent with older full frame lens, or (2) whether the 1 stop more closed is already included, or (3) if they are inconsistent with either of these. No matter what anyone says--the real test is in the numbers on your lens!
Another real test is to take pictures and evaluate the results. As Canada_Rockies pointed out, depth of field scales are guidelines based on a somewhat arbitrary set of criteria. Best to make some pictures, look carefully at the results, and if necessary establish a personal fudge factor to suit your own tastes.

None of the lenses I regularly use for digital have DOF scales, so I won't comment on APS format and DOF scales. I"ve been a photographer for a long time, so I rely on experience combined with DOF preview.

For 35mm film, I tend to close down one more stop than recommended by the DOF scale. Same for medium format, where my bodies are all rangefinders or TLRs.

Most of my 35mm and digital work is done in the f/8 to f/16 range. My personal style involves a high level of sharp detail in subjects that often stretch from a few feet away to infinity. I find that for 3-dimensional subjects the benefits of increased DOF outweigh diffraction effects in terms of apparent sharpness in that aperture range.

John
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