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07-09-2014, 04:38 AM   #1
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Depth of field and focus trouble with manual lens

Hi everyone
Can you please tell me about the depth of field conundrum I have with my Pentax-M 50mm 1:1.7 lens and K10D camera. I find it impossible to focus at long distance (infinity) and the depth of field is extremely shallow – even at f22 at close distance. Reading articles on focusing with manual lenses make me think there is something wrong with my camera ie misaligned sensor or something because the the view from the viewfinder seems OK. I have followed the focussing instructions in articles but still unable to get a good result. The lens itself is in perfect condition and the diopter is adjusted correctly.
Any suggestions?

Many thanks

Craig

07-09-2014, 04:58 AM - 1 Like   #2
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At f22 the DoF will not be shallow. Focusing near infinity, the DoF will also not be shallow. (the shallowest DoF is at wide open and minimum focus)
I would suggest you make sure you are using the lens/ camera correctly.
Camera: - Enable Aperture Ring (back of camera Menu)
- M mode. If you use M-mode, the camera will stop down the lens to the number you selected on the aperture ring. You meter by using the green button (or +/-?), or you can just input the settings without metering.
- Av mode: If you use Av mode or enable auto-ISO, the camera will always take photos wide open, at f1.7. This will mean shallow DoF. This is probably your problem. Note that all modes other than M will turn into Av with manual lenses. Even some other features, like auto ISO can change M to Av. Make sure the LCD says M if you want the camera to stop down the aperture according to the aperture ring
-Jpeg mode and digital filters: Make sure you aren't applying some odd filter to your photos that might cause such effects

Lens: - make sure the lens is mounted correctly. It needs to go on until it makes a little click sound
- make sure the aperture blades work properly. Simply look into the lens as you take a photo - do the aperture blades move quickly enough? To the right position?
- Make sure the lens mount is not bent or broken. you can get odd DoF (diagonal, shallow) if the lens and sensor are not aligned
- make sure the back element of the lens is nice and clean
Has the lens been damaged or serviced? Can you test it on another camera?

My Pentax M 50mm f1.7 was very sharp until I dropped it and had it serviced. Now its a little fuzzier. I'll attach a photo I recently took with it. As you can see, it has a reasonable DoF. (The house is in focus, the trees behind it and in front of it are not) A great thing is that it even comes with DoF scales, right on the lens!

---------- Post added 9th Jul 2014 at 14:14 ----------

Here is another photo I took with the Pentax M 50mm f1.7 recently (this one is more post-processed than the previous one). You can press M on the keyboard to see a bigger version on 500px: Bust by Stolpulus II | 500px
In this case the DoF is shallow, because I took the photo at aperture near to wide open, though I don't remember exactly what it was

Last edited by Na Horuk; 05-23-2017 at 09:10 AM.
07-09-2014, 05:39 AM - 1 Like   #3
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QuoteOriginally posted by Craig Barber Quote
because the the view from the viewfinder seems OK
QuoteOriginally posted by Craig Barber Quote
diopter is adjusted correctly
QuoteOriginally posted by Craig Barber Quote
I find it impossible to focus at long distance (infinity)
How do you know it's adjusted right? You've adjusted it with another lens? If so, does this other lens manage to take good pictures?

If your sensor would be crooked or misaligned, you wouldn't be able to take a good picture, at any distance. You would also more than likely be dealing with severe chromatic aberrations.

Also, the M50 f/1.7 has a hard stop at infinity (cannot focus pass infinity) so your lens may need to be adjusted slightly for infinity focus. I had to get my A50 f/1.7 adjusted, it was fine on my film SLR but just shy of OK on my DSLR. It took the camera store guy about 30 seconds to do it and he didn't charge me anything. A quick thing I usually do is to try to focus on a star at night and see if I can get it to show up as a point instead of a blob of light in the viewfinder.

As Na Horuk said, in any mode but M, your camera will leave the aperture wide open (f/1.7) for focussing and will only stop it down to take the shot. So even if your ring is set at f/22, you'll be focussing at f/1.7. Using M mode, the camera will not interfere with your aperture settings, so you'll be focussing at your set aperture, you'll notice right away as your viewfinder will become darker as you stop your lens down.

And lastly; without seeing an example it is hard to say if anything is indeed wrong. You may be dealing with camera shake or motion blur. The K10D still take very nice picture but you won't get the resolution of a newer camera, maybe you're expecting too much? How far is your subject? Infinity will bring everything far in focus but only to a certain extent. You may get better results in your case if you'd be using hyperfocal distance instead of infinity.

Again it's very hard to say without seeing what you're dealing with.
07-09-2014, 06:03 AM - 1 Like   #4
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All good advice from Na Horuk as usual,


I would add this however:-


I don't have your lens for comparison but I have the Super Takumar 55mm f2 and although the maximum aperture is smaller than yours, it doesn't offer f22 it only goes to f16, I think it behaves similar enough to your lens to draw some conclusions.


You say the depth of field is shallow when focussed at close distance, my 55mm f2 Super Tak, which I expect to have similar performance to your lens only has a depth of field of 4 inches at f16 when at minimum focussing distance. Whereas at infinity and at f16 that same Super Tak gives me a depth of field all the way from 10 feet to infinity.


In other words as you focus very close the depth of field does become very shallow even at the smallest aperture.


Can you add some more details of the distance your focussing at, and the depth of field your getting at that focussing distance, it is possible that the lens is behaving normally and your depth of field is normal for that lens at the focussing distances your experiencing the problem.


One way ive had success with when examining depth of field that a lens gives is to photograph a measuring tape, you can read the distances of whats in focus directly off the image then.

07-09-2014, 08:06 AM - 1 Like   #5
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Sounds like you've got two problems.
The first is, as has been noted already, that your lens needs to be adjusted for infinity focus.


I'm guessing that you're depth of field "problem" isn't a problem at all. First of all, if your lens wasn't stopping down, then all of your pix would be over exposed. I'm guessing that you were expecting to see a change in the DoF as you moved the aperture ring. If you're expecting to see a change in DoF prior to taking the image, well, you won't unless you press the DoF preview button which stops the lens down to the picture-taking position. Otherwise you won't see any difference between f/1.7 and f/22 through the viewfinder or in live view as you move the aperture ring. It's only when the picture is taken or when you press the DoF preview button will the lens close down to the specified f/stop.
07-09-2014, 01:22 PM - 1 Like   #6
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QuoteOriginally posted by Craig Barber Quote
Can you please tell me about the depth of field conundrum I have with my Pentax-M 50mm 1:1.7 lens and K10D camera. I find it impossible to focus at long distance (infinity) and the depth of field is extremely shallow – even at f22 at close distance.
Regarding f/22 at close distance, the closer the focus distance the shallower the depth of field. The M50/1.7 isn't a macro lens, of course, but even so at its minimum focus distance at f/22 the depth of field is only a foot or so.

It's possible to shoot with a manual lens on the K10D (or any Pentax DSLR) in various exposure modes, but only M, B, and X modes will stop the lens down when you shoot. So if you are shooing in Av mode, say, the shot will be wide open no matter the setting on the aperture ring.
07-09-2014, 04:21 PM   #7
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You guys are fantastic!
Sorry for the delay getting back to you.
Thank you very much for steering me in the right direction. All this time I've been shooting in Av mode so I will change and shoot in M from now on. The problem is cured! I shot some photos using M and putting the camera on a tripod. I will probably buy a hand held light meter though as the exposure is a bit of guess work. I am really keen to use this lens and maybe buy some more prime manual lenses as they are much cheaper. My 50mm came from a ME-Super that I own.

Thanks again, I was nearly going to give up!

Craig
07-09-2014, 05:12 PM - 1 Like   #8
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QuoteOriginally posted by Craig Barber Quote
I will probably buy a hand held light meter though as the exposure is a bit of guess work.
I'm not sure about the K10D but in manual mode you hit the green button and the camera will set your camera for correct exposure automatically (lazy way). Also, in manual you will see the exposure bar in the viewfinder (and the top LCD) which will guide you (manual way) it's accurate enough to rely on.

If you have your manual, look at page 26, item 10 in the list for reference of where it is in the viewfinder. Page 28, item 5 for the top LCD.

This is exposure compensation indicator in most mode but in manual mode, it is your exposure bar.

And if you don't have your user manual, here is a link for a PDF:

http://c758710.r10.cf2.rackcdn.com/files/support/manual/423_1314745042-1253836402_manual.pdf

07-09-2014, 05:46 PM   #9
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QuoteOriginally posted by fgaudet Quote
I'm not sure about the K10D but in manual mode you hit the green button and the camera will set your camera for correct exposure automatically (lazy way). Also, in manual you will see the exposure bar in the viewfinder (and the top LCD) which will guide you (manual way) it's accurate enough to rely on.

If you have your manual, look at page 26, item 10 in the list for reference of where it is in the viewfinder. Page 28, item 5 for the top LCD.

This is exposure compensation indicator in most mode but in manual mode, it is your exposure bar.

And if you don't have your user manual, here is a link for a PDF:

http://c758710.r10.cf2.rackcdn.com/files/support/manual/423_1314745042-1253836402_manual.pdf
Thanks very much for your help fgaudet, I will study this. I'm only a learner at this game but finding it most enjoyable.
07-09-2014, 06:45 PM - 1 Like   #10
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QuoteOriginally posted by fgaudet Quote
I'm not sure about the K10D but in manual mode you hit the green button and the camera will set your camera for correct exposure automatically (lazy way).
"Correct" is probably a bit of an overstatement for both the K10D and K20D. Unreliable stop-down metering is legendary with those two cameras. The tendency is toward gross underexposure (2+ stops) at wide apertures. Behavior is fairly nominal at f/4 and narrower, though there is a risk of overstepping the meter's sensitivity (a historic issue with stop-down metering) at narrow apertures in dim light.

With experience and a lot of chimping, I was able to use my K10D with good success with my non-A contact lenses.


Steve
07-09-2014, 07:18 PM - 1 Like   #11
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DOF preview has to be set to "Optical" in order for the EV meter bars to turn when you activate the DOF preview lever in manual mode with non-A lenses. Then you adjust the f-stop with the aperture ring or the shutter speed with the e-dial.
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