[jolepp]

I am following all of these threads closely, wanting to hit "Buy Now" as soon as possible, but not wanting problems or the hassles of returns or waiting for a repair that may never come.
...
1/30 F 2.8 115 mm about 8 ft away ISO 6400
...

Those parameters are about 2 EV so having a phase (VF) AF problem would seem likely to me; then again we don't really know, your K-5 with your lenses could work better. In a similar situation I would wait to avoid the potential hassle. I'm waiting myself to send my K-5 in for the sensor stains, one consideration being that there might be another warranty repair round-trip for the AF issue.

[Gimbal]

I gave the 18-55 kit lens (from the k-x kit) a try @50mm. For this I moved the eneloops a bit closer (30 cm from sensor plane) for less DOF. Spacing in "depth" is still 14mm ("width" increased to 14mm as well, but should be of no consequence). For tungsten light I used two 40W spots and one 28W halogen fist reflected off the white ceiling to avoid reflections from the battery surfaces (these could interfere with AF?). Led light is still the bicycle light.

#1 : tungsten : contrast (LV) AF : baseline : camera WB 3156K
#2 : tungsten : phase (VF) AF : FF in comparison to #2 : camera WB 3156K
#3 : led : contrast (LV) AF : baseline : camera WB 6970K
#4 : led : phase (VF) AF : very slight FF in comparison to #3 : camera WB 6989 K

Nicely done, but please test slightly below EV4 and at the wide end of the lens.

[jpzk]

Jolepp:

This is a question from someone (me) who has no idea about:

How do you calculate the EV from the shutter speed, aperture and ISO?

I have a little applet that does it automatically but I'd like to know how it is actually done.

Thanks,

JP

[pawel.z]

EV0 is 1.0s f1.0 ISO100

EV1 is 0.5s f1.0 ISO100 or 1.0s f1.4 ISO100 or 1.0s f1.0 ISO50

and so it goes

[jolepp]

Jolepp:

This is a question from someone (me) who has no idea about:

How do you calculate the EV from the shutter speed, aperture and ISO?

I have a little applet that does it automatically but I'd like to know how it is actually done.

Thanks,

JP

f=1 t=1s @ISO 100 is EV 0 by definition, and 1 stop change = 1 EV unit, so:

EV = log2(f^2 / t) - log2(iso/100)

The above is good with a calculator, for mental computation I like to think EVs like this, say for f=2.8 t=1/30s @ISO 1600

f=1.0 : EV 0
f=1.4 : EV 1
f=2.0 : EV 2
f=2.8 : EV 3

and from there

t=1/2s : EV 4
t=1/4s : EV 5
t=1/8s : EV 6
t=1/15s : EV 7 (strictly: t=1/16s or a bit below EV 7 but close enough ...)
t=1/30s : EV 8

and finally

ISO 200 : EV 7
ISO 400 : EV 6
ISO 800 : EV 5
ISO 1600 : EV 4

As long as one remembers where the scale starts and that larger values mean brighter it is easy enough to work these out.

[betaPhoto]

This is a question from someone (me) who has no idea about:

How do you calculate the EV from the shutter speed, aperture and ISO?

I have a little applet that does it automatically but I'd like to know how it is actually done.

Thanks,

JP

This chart from post #11 may help if you can understand how to read it.

Start with an EV, then slide to the right and pick an fStop, then slide up and pick a shutter speed and finally, slide to the left and and you see the needed ISO.

[jpzk]

jolepp, betaPhoto, pawell ...

Thanks for the info!

Much appreciated and now I can "see" what's going on in this thread quite a bit better.

Cheers.

JP

[Smeggypants]

Did a test today and my sigma 30-mm/1.4 which is normally set to +4 in the AF settings, needs less than -10 in low light EV2 conditions. At -10 it's still front focussing - I would guessm, through extrapolation it would need about a -15 in order to focus correctly. so that's -19 in total

[jolepp]

Here are some test snaps with the DAL 18-55 @18mm EV 3.2 (f=3.5 t=1.3s ISO 100). Distance from middle battery to sensor plance 27.5cm, spacing of batteries 14mm in depth and width. 40W tungsten spot reflected from white roof, led lamp shining directly on the batteries.

#1 : tungsten : LV/contrast AF : camera wb 3260K
#2 : tungsten : VF/phase AF : camera wb 3260K

#2 is front focused in comparision.

#3 : led : LV/contrast AF : camera wb 7006K
#4 : led : VF/phase AF : camera wb 7006K

With #3 and #4 a difference in focus is hard to find. (It seems the camera moved very slightly between shots, but this shouldn't be material ...)

[Acheron]

My FA35 f2 DAL had to be set to +4 to focus properly.

[falconeye]

How do you calculate the EV from the shutter speed, aperture and ISO?

JP,
I see your question has been answered.

Just an additional remark. EV can be regarded in two possible ways: EV of the light source and EV of the subject.

Normally, EV is given for the light source. Like with the Sunny 16 rule.

But here, we really care about the EV of the subject as this probably is what determines AF performance.

EV of the light source and subject can be considered the same if the subject reflects all light (bright white paper) and is exposed to saturate the image (100% gray).

However, the camera would expose said sheet of bright white paper to ISO norm (18% gray) which is 2.5 stops less.

Therefore, if you compute a light sources' EV value from the automatic exposure value for a sheet of bright white paper (w/o EV compensation dialed in), subtract 2.5 from your computed EV value. If Pentax talks of EV -1 as low AF limit, then they must mean light source EV.

[jolepp]

These have been taken with the Pentax DFA 100mm @EV 3 (f=2.8 t=1s ISO 100). Tungsten light from a single 40W spot reflected from white roof, led light from led bike lamp (direct, not enough power to reflect with this). Distance from focal plane to the battery in the middle 53cm, battery spacing 7mm in "depth", 14mm in "width".

#1 : tugsten : contrast/LV AF : camera WB 3162K
#2 : tungsten : phase/VF AF : camera WB 3162K

In comparison #2 is front focused.

#3 : led : contrast/LV AF : camera WB 6927K
#4 : led : phase/LV AF : camera WB 6927K

In comparision #2 is very slightly front focused.

[jolepp]

Enclosed is yet another focus test sheet. The intent is that this should be a simple and cheap aid for detecting/demonstrating front/back focus: this is basically a A4 sized document that has dotted lines to ease folding the printed sheet (with a ruler) to resemble a "stair" standing on its side where the width of the difference in depth of middle/side sections can be varied in 1/2" steps. See attached photo for a finished product with 1" graduation. I also put dotted lines near the edges: making folds here might help the sheet to keep stand on its own once folded (it seems to be doing ok even without these folded as in the picture).

I plan to follow up with some actual test snaps; I thought I might as well post the sheet used in these. This is no precision instrument, of course. Due to the paper sagging a bit the actual depth of the flat sections may vary a bit. I think it is good enough for testing / demonstrating front/back focus and better that the Eneloops I used previously in that the paper surface is not shiny which might interfere with AF when using direct light from a single tungsten bulb or such to illuminate the target.

Other attachments: .pdf: the document in a reasonably common format, .zip: the OpenOffice document in case someone wants to edit this (say, for Letter -sized paper).

[jolepp]

Enclosed is a set of photos with phase AF, DFA 100mm 1:2.8 at f=2.8, subject distance 1 m, direct light from a 'naked' 25W tungsten bulb where distance was adjusted to vary light level as seen by the camera using spot metering, most of the light should be directly from the bulb, some has inevitably been reflected from the surfaces of the room, though. It was dark outside so all light in the otherwise dark room was from the single bulb that was mounted at on a tripod at the approximate heigth of middle of the target. This was kept on the right side on the camera at about 30-40 degree angle to the line from the sensor to target.

#1 EV 4 (t = 0.5s) : camera WB 3308K : focus OK
#2 EV 3 (t = 1.0s) : camera WB 3278K : front focus
#3 EV 2 (t = 2.0s) : camera WB 3148K : front focus (somewhat worse than #2)

With the use of spot metering the area relevant to AF operation should be within 1/3 EV of the above values assuming camera meter is accurate.

[creampuff]

Did a test today and my sigma 30-mm/1.4 which is normally set to +4 in the AF settings, needs less than -10 in low light EV2 conditions. At -10 it's still front focussing - I would guessm, through extrapolation it would need about a -15 in order to focus correctly. so that's -19 in total

I had a couple of friends who had the Sigma 30mm (in Canon, Nikon and Pentax mounts) and they all encountered varying degrees of focus issues at close distances. They had to send their lenses to be recalibrated at the local Sigma service center. It seems to be pretty common where I am because Sigma charges a fee of S$80 for it. Friends who got the early batch of the Sigma 50mm in Canon and Nikon mount also faced this issue, which is easily adjustable by Sigma. Perhaps you might need to check with your local Sigma service center if you're not getting precise focus.