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11-11-2008, 04:51 PM   #1
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K10D and AUTO ISO with manual lenses

Using a K10D, I want to use AUTO ISO with a manual Tamron SP500 Mirror lens.
The shutter speed has to be set high to stop camera shake.
TAv mode would be ideal, but choosing it sets Av mode instead.

I can get Av mode to use AUTO ISO (after pressing the green button), but the ISO changes only kick in when the shutter speed drops to 1/100th or 1/90th sec (seems to vary for no reason I can think of). That speed is too low for a 500mm lens. The camera knows the focal length of the lens because it is set at startup in the shake reduction option and it correctly sets it in the EXIF data.

So how can I set my shutter speed at say 1/500th sec and get the camera to adjust the ISO according to light conditions. The camera can set the shutter speed OK when I fix the ISO, so it should be able to let me set the shutter speed and change the ISO. Isn't that what AUTO ISO is for?

Would a different lens mount help? Is one available? I think the camera is holding back on the TAv option because it doesnt know the aperture setting on the lens. Although when you press the green button that should tell the camera all it needs to know.

The best compromise I have found is to manually change the ISO settings for each shot using the front E-Dial to keep the shutter speed close to what I want - a sort of manual TAv mode. This is frustrating when AUTO ISO promises just what I want and yet doesn't deliver.

Any suggestions??


Last edited by kevbirder; 11-11-2008 at 04:54 PM. Reason: clarification
11-11-2008, 05:20 PM   #2
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I have not tried auto ISO in a very long time, because I don';t trust it.

The first thing to ask is what is the focal length set to?

This might change the exposure curve if it is trying to set for handl hold, but how about another approach,

why not just use Sv mode and dial in the ISO until you get the shutter speed, based on metering.

As an aside, what is the metering doing? how accurate is it with the mirror. I have never tried this combo yet?

Normally for things like this, I set the ISO manually and then use green button for shutter speed, but I always consider how high to set the ISO so that I get teh shutter speed I want.
11-11-2008, 06:28 PM   #3
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QuoteOriginally posted by Lowell Goudge Quote
I have not tried auto ISO in a very long time, because I don';t trust it.

The first thing to ask is what is the focal length set to?

This might change the exposure curve if it is trying to set for handl hold, but how about another approach,

why not just use Sv mode and dial in the ISO until you get the shutter speed, based on metering.

As an aside, what is the metering doing? how accurate is it with the mirror. I have never tried this combo yet?

Normally for things like this, I set the ISO manually and then use green button for shutter speed, but I always consider how high to set the ISO so that I get teh shutter speed I want.
I tell the camera the focal length at startup - 500mm as it cannot read the focal length from the lens.

Sv mode is not available, if I choose it on the dial I get Av instead.

Metering seems OK, but highlights can get blown. I dont think that is because of the lens itself. I have to put in up to 2 stops compensation for photographing white gulls against a medium to dark background (water). This itself messes up the AUTO ISO I understand, but I haven't got far enough for this to be the next issue.

Yes I can set the ISO manually, but when you use a long lens the light conditions change very quickly. I'll explain that - moving the lens even a small amount to follow a moving target can completely change the background from light to dark or vice versa and that has big effects on the meter reading and therefore the shutter speed chosen by the camera. This means frequent manual changes of ISO to keep the shutter speed fairly constant. This is what I thought AUTO ISO would solve.
11-12-2008, 01:44 AM   #4
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I have the same problem : Auto-iso works fine for me, except that it won't take the focal length into account and go as low as 1/100...
Having both the focal length and the SR state, an easy algorithm would have been more than capable of handling manual lenses, instead of using the 1/100th floor...

Anyway, you can still trick your k10 into thinking the lens is a PK-A, by shorting both the A (tricky!) and Data pins... This way, you'll have access to the TAv mode (but make sure you select the wide-open aperture!)...
Try this : https://www.pentaxforums.com/forums/pentax-dslr-discussion/40690-p-ttl-matrix...pk-lenses.html...

11-12-2008, 09:17 PM   #5
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I would simply go manual exposure. The exposure then will not change as the bird changes backgrounds. You are using a K10D, so you can use the exposure meter in the viewfinder (or on top if you protect the eyepiece from external light) to set the ISO to give you a normal exposure. It may appear upside down at first, but you should be able to get the hang of it quickly.
11-12-2008, 09:47 PM   #6
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I have been wondering how to do this also. I have an old Sigma 600 f8 Mirror in a PK mount that I used with my Pentax MX then Samsung GX-1S and now with my K20D. I will wait to see how this is working. Please post back if you find any other ways to make using long lenses like this work.
11-12-2008, 09:53 PM   #7
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QuoteOriginally posted by WheresWaldo Quote
I have been wondering how to do this also. I have an old Sigma 600 f8 Mirror in a PK mount that I used with my Pentax MX then Samsung GX-1S and now with my K20D. I will wait to see how this is working. Please post back if you find any other ways to make using long lenses like this work.
Here's a quick list for you.
  1. In the Custom menu set [Using Aperture Ring] to [Permitted]
  2. Set mode to M
  3. Point the lens at an average toned subject
  4. Press Green Button.
  5. If the shutter speed is too slow, increase ISO
My preferred system
  1. In the Custom menu set [Using Aperture Ring] to [Permitted]
  2. In the Custom Menu set Depth of Field Preview to Optical
  3. Set mode to M
  4. Point the lens at an average toned subject
  5. Hold the power switch over to the preview position past On
  6. Use the front edial to set the exposure to over/under/zero on the scale at the bottom right of the viewfinder.
  7. Take an image and adjust as necessary.
You could even to version 1 to get a rough shutter speed, then use version 2 from 5 to 7 to fine tune.

I grew up with manual cameras, so the scale in the k10d and k20d is just exactly what I like to see.
11-14-2008, 11:07 AM   #8
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Using manual settings is quite sensible in a lot of situations, so thanks for the suggestions.

However my actual question was to do with how to use AUTO ISO with a manual lens. This is because sometimes I am trying to grab quick shots of moving targets in changing lighting conditions.

It seems the answer is to fool the body into thinking I have a PKA mount. One way might be to actually buy an overpriced PKA mount. Another would be to attempt shorting out of the recessed pin?

I'm not sure how that works though because I guess you have to transmit the actual fstop info? In fact I can see that it really doesn't matter what the fstop is, its just the fact that with some lenses and other than full aperture setting the amount of light reaching the sensor will be reduced at the moment of taking by a certain number of stops and the camera needs to know this. In practise my lens only has one fstop anyway. It would be much nicer if I could tell the body to treat the lens as PKA rather than try to physically fool it.

11-15-2008, 12:46 AM   #9
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Kevbirder: unfortunately, that's the only way...
But you're lucky: your lens has a fixed aperture, so you can set it accurately by masking the appropriate pins (have a look at b.dimitrov's site).
11-15-2008, 04:36 AM   #10
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I am trying to remember how I had the Green button setup in AV mode ... at one point I had the green button setup in the Custom menu for MTF (it selects the best settings for the lens attached at the time.

But there was a High Speed Option in there ... I haven't tried it ... but it may adjust your ISO to allow as fast a shutter speed as possible.

I don't have my K10D here to try it though ... but someine might have an idea on that one.
11-15-2008, 10:03 PM   #11
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QuoteOriginally posted by Mechan1k Quote
I am trying to remember how I had the Green button setup in AV mode ... at one point I had the green button setup in the Custom menu for MTF (it selects the best settings for the lens attached at the time.

But there was a High Speed Option in there ... I haven't tried it ... but it may adjust your ISO to allow as fast a shutter speed as possible.

I don't have my K10D here to try it though ... but someine might have an idea on that one.
The green button will set the exposure based on the Program line that you have selected. That is where you have MTF. Unfortunately, it will have absolutely no effect on a mirror lens because you are either in Aperture priority (Av) or Manual exposure mode. Pentax has four program modes in most models of its cameras starting back in the later film cameras:
  • Normal
  • Depth of field
  • High Speed
  • MTF
The MTF mode requires an A type lens of later vintage that actually has the "good" apertures in the lens chip. The others work with any lens that has an A settting on the aperture ring.

Using the Original Poster's mirror lens, he will somehow have to trick the camera into believing that the lens is set on the A aperture setting. The suggestions for taping certain contacts are how to do it. I use, for all my birding, my one and only really long lens, the M 400/5.6, and I use it in manual mode at f/8 or f/11. In normal lighting I set the ISO at 400 and leave it there with this old slow beast. Works for me, but maybe not for you.
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